1999 Dodge Durango



March, 1, 2013 AT 6:36 PM

I am in need of a new exhaust system and am undecided as to which one to choose for best performance and economy. My choices are Gibson, JBA, Flowmaster(Force 2), Magnaflow and Dynomax. Some have 2 1/2 in pipe, some 3in should I go with 3in? I can also chose between Stainless and aluminum in some cases but I thought of using RUST-OLEUM Automotive Primer for rust protection 5$ compared to $100, any thoughts? Also note there is no emissions test in my county should I eliminate the CAT?


8 Answers



March, 1, 2013 AT 8:48 PM

People modify the exhaust to make it louder. That's all you're going to achieve and that's hardly a worthwhile goal. If you could gain any power or fuel mileage, you can be sure Chrysler would have installed those parts at the factory.

You can't remove the catalytic converters. Their efficiency is monitored by the Engine Computer through the downstream oxygen sensors. Once those are eliminated there will be diagnostic fault codes set and the computer will turn on the Check Engine light. From then on you'll never know when a new and different problem occurs. Many of them are very minor but they can turn expensive real fast if they're ignored. Also, to set any fault code, there is a long list of conditions that must be met, and one of them is that certain other codes aren't already set. Some of those codes refer to things the computer uses as a reference to perform other self-tests. Those tests won't run until the causes of the original codes are fixed. You could have a running problem with an easy solution, but if no code sets to tell you what to diagnose, you may never solve the problem until you put the exhaust system back to original and clear the codes.

As for cost, your best best is to go with the factory original if it was made of stainless steel. My '88 Grand Caravan daily driver still has all of the original pipes and catalytic converter after 25 years. The muffler corroded through at 18 years old. I went the cheap route and installed a $20.00 farm and home store special. I spent four hours crawling in granite, on my back, making it fit and building custom hangers. It lasted almost two years before that one rusted out. I just put the third one on last summer and the cost jumped to 45 bucks. In a year and a half I expect to need another one. So far I spent $85.00 and lots of hours to cobble on universal inferior mufflers that lasted six years when I could have spent $180.00 to have it last another 16 years without wasting all the time I spent on the repairs. Had I known this was going to be such an uncommonly reliable and trustworthy vehicle, and it would last this long, I would have gone for the better value from the dealer instead of the cheaper price.

All your primers and paints in the world aren't going to do anything to stop the parts from rusting out. They do that from the inside. If your current exhaust parts are original and 14 years old, they are likely made of stainless steel, and my guess is the pipes are still okay. Why replace them with pipes that won't last as long as what's on the truck right now?



March, 2, 2013 AT 2:22 PM

Wow, really! So every car is as efficiant as it can be straight from the factory? I don't think this sounds right. I believe cars are made due to minimum standards set forth by the automotive industry. I've already added more spark using e3 spark plugs and performance distributors Screamin Demon coil cap and rotor with brass terminals and a 10mm wires. I've added more air using K& N's cold air intake and a throttle body spacer(more air at a cooler temp). So now it's time for a better exhaust(better airflow after combustion). Ever try to breath through a straw? Anyway, my factory exhaust is rusted from the outside(in the city we have to park right on top of snow sometimes), so I need to protect my exhaust with rusto or get stainless I just wondered if you had any previous experience with that(guess not). Anyway thank's for your input I guess I'll have to check the specs on my aftermarket choices on my own and see what I can determine to be my best choice.

Have a great day!



March, 2, 2013 AT 2:37 PM

Oh! I forgot, as far as the CAT I'm sure all the fuel will be spent so I won't have any sensor issues the computer will sense a clean burn and better fuel economy(no issue there). Thank's again, you made me think deeper on this decision.



March, 2, 2013 AT 11:16 PM

You're misinformed on a few points but others are correct. The main one is on how the catalytic converter works. Once the engine coolant temperature reaches a certain point, typically 160 or 180 degrees, and / or the oxygen sensor reaches 600 degrees, the Engine Computer goes into "closed loop". That means it continues using all the sensors to calculate fuel metering plus it starts looking at the front oxygen sensors. The fuel / air mixture will switch from too rich to too lean a couple of times per second with the average being perfect. When it goes too lean, the unburned oxygen is stored in the catalyst, then, when it goes too rich, that extra fuel mixes with the oxygen and is burned. You can watch those sensor readings switch rich and lean on a scanner that displays live data. I use the Chrysler DRB3. A lot of independent shops have them too because with an additional plug-in card they can do emissions-related stuff on any brand of car or light truck sold in the U.S. Starting with '96 models.

You might think the computer would be happy to see the ideal mixture at all times, and while that would be the goal, that would trigger a fault code related to "fuel / air not switching properly". The computer knows the catalytic converter can't do its job that way.

When the converter is working properly, the gases coming out will be carbon dioxide and water vapor. That vapor is what corrodes exhaust system parts from the inside and is why most original systems are made of stainless steel. Most mufflers also have a tiny drain hole on the bottom in the back where you'll see water dripping from. The important thing to understand is that when the converter is doing its job, the rear, or "downstream" oxygen sensor will detect a slightly rich condition for a long time, perhaps as much as a minute or more, then it will detect a lean condition, also for a long time. (They don't really detect rich exhaust; they fail to detect a lean exhaust. That's different because oxygen sensors only detect that; oxygen, not fuel). The Engine Computer causes the mixture to go too rich, then too lean, then it expects to see the results of that from the upstream oxygen sensor. That switch rate, as I mentioned, is about twice per second. Next, it expects to see a switching rate from the downstream sensor of maybe once or twice per minute. That's what it takes to prevent setting a code.

When the catalytic converter begins to lose its efficiency, less change takes place in the composition of the exhaust gas. What goes out of the converter starts to look more and more like what's coming in. The switching rate, or frequency, of the downstream O2 sensor picks up and may approach four times per minute, or six times, then more. Based on that higher frequency, the computer knows not enough change is taking place inside the converter. At a pre-programmed point, the fault code "catalytic converter efficiency" will be set and the Check Engine light will turn on.

You are proposing to remove the converters but there are only disadvantages to doing that. You can not remove the downstream O2 sensors because that will be detected by the computer right away. With no converter to change the makeup of the exhaust, the downstream sensor will read exactly the same as the upstream one, and it will switch rich to lean at the same times. That will instantly set the "efficiency" code. Knowing it can't rely on that sensor to see the results of some of the self-tests the computer runs, those tests will be aborted and other potentially minor problems will go undetected. Those problems almost always affect performance, fuel mileage, and emissions, so where is the gain in power or mileage you're seeking?

You have to understand the world the manufacturers live in is extremely competitive. People buy one minivan over the other brand because this one has two extra cup holders. That truck has five more horsepower. This car is rated at one more mile per gallon. Why do you think years ago one manufacturer came out with an engine that was just a couple of cubic inches bigger than their competitors? If they could legally advertise one more horsepower or one more mile per gallon, they would do it because they know that translates into more sales. Now you are going to come along and improve on what a team of researchers and engineers couldn't do. You may succeed in meeting one of your goals but there is going to be a trade-off. I could take the 340 out of my '72 Challenger and build it like a NASCAR 355 and get 850 horsepower from it, ... For perhaps as much as 500 miles, then it's worn out. I'm sure that's not your goal to rebuild your engine every 500 miles.

By the way, for that larger diameter exhaust you think is going to make a noticeable difference, look at the size of the pipes on an NHRA top fuel dragster. They burn over ten gallons of nitro methane and all the air to go with it, in less than 700 revolutions of the engine in four seconds. They're approaching hydrolock by stuffing so much liquid in the engine, but it all comes out as a gas. You don't see any liquid dripping out. That's a HUGE pile of gas they're trying to get out. Calculate how long it takes you to burn ten gallons of gas, then tell me you need to get 1/100th the volume through your exhaust system. If you don't think the manufacturer would go with a larger exhaust system, regardless of the tiny additional cost, to get more power, you can be sure the guys who totally rebuild an engine after every four-second race would spend the money, ... If it would help.

A better example is my buddy owns a body shop and he specializes in rebuilding smashed one and two-year-old Dodge trucks, mostly diesels. One of his first ones was a '99 dually diesel that he regularly used to drag around a three-car hauler with no problems, power-wise. He decided after more than 100,000 miles that he wanted more power so he added a "chip" with three levels of power increase. The first thing he promptly did was tear up the transmission. Can't haul many cars around that way. What did he gain?

I helped him three years ago rebuild an '06 dually that was hit real hard in the front. Part of my job was to install a chip on that one too. After he took it out for its first test drive and the clutch was slipping under wide-open-throttle, a local fuel injection service company identified the previous owner had installed an over-size turbo and over-size injectors. He had the CAPABILITY to produce unusable horsepower, so what did he gain? Sure, it launches like a Viper, but who drives like that? Just because the injector CAN squirt more fuel in a given period of time doesn't mean they do.

You also have to understand diesels work differently than gas engines. With gas, too much or too little air is a bad thing. Only the perfect mixture results in the best fuel mileage, the best power, and the lowest emissions. Diesels are wide-open to air all the time and will take in as much as possible. Power and speed are controlled by the amount of fuel squirted into the cylinders. By packing the air tighter, a turbocharger dramatically increases its temperature so they run it through an intercooler to bring it back down. Normal air temperature has a cooling effect on the intake valves that you don't want to lose. Because all that air that got stuffed in under pressure expands a lot when it gets burned, the exhaust system has to be big enough to accommodate that without restriction. That's why they use 4" diameter pipes. Because air compresses, the volume of the air going in stays the same, and is a lot lower than what goes out. The MASS or weight of the air is the same going in and going out, but not the volume, so the intake side can be smaller than the exhaust side. You may think that's like sucking through a straw, but it's a pretty big straw that is more than sufficient for the engine's needs. Since going bigger wouldn't affect emissions, and it would barely impact the cost, don't you think the manufacturer would have done it if there would have been some value to it?

The same is true with a gas engine but to a lesser extent. Your air intake system will already allow you to go over 100 miles per hour at less than wide-open-throttle. Do you need more than that? Your exhaust system will handle that too. There's a reason the guys at the auto parts stores ask you your engine size when you buy replacement exhaust system parts. The manufacturer knew larger pipes were needed for the needs of larger engines under the most extreme conditions, so that's what they came with. There's that numbers game again. If a 2 1/2" exhaust system meant they could advertise five more horsepower than if they used a 2 1/4" system, you can be sure they would have done it.

The biggest misconception comes in with your desire to cool the incoming air. That is exactly opposite of what we tried to do since forever. First you must understand liquid gas does not burn, period. Fire safety instructors do a demonstration where they throw a lit match onto a pan of gasoline, ... And it puts the match out. It isn't until that gas starts to vaporize that the vapors will burn. Even then, the entire pan of gas doesn't explode. Only the vapors burn as they boil off over quite a long period of time.

The problem we had with carburetors was they were designed to make liquid gas vaporize so it would burn in the cylinders. There isn't time for the burning vapors to heat the liquid droplets and vaporize them so they too burn in the cylinders. Instead, the heat makes them vaporize in the exhaust system. It doesn't produce any power there. Every carburetor was carefully calibrated to the specific engine and they almost always had two circuits, an idle circuit and a high-speed circuit. Those were the only two points that could be adjusted for a perfect mixture. In between they had to dump in too much gas. That extra gas would be wasted, but going a little too lean would cause an objectionable hesitation or stumble.

The additional problem came in when the engine was cold. That prevented the gas from vaporizing properly. Two things took care of that. The carburetor usually sat right above an exhaust passage in the intake manifold or there was engine coolant circulating near it to warm the base of the carburetor. That solved the vaporizing problem once the engine was warmed up. For the cold engine we needed a choke. By blocking air flow, engine vacuum sucked in more fuel. That did not result in more power. The goal simply was to pour in so much raw fuel in hopes a large enough percentage would vaporize to make the engine run smoothly. A large percentage of that fuel went out the exhaust, wasted. Cold air killed fuel mileage and greatly increased emissions. It did nothing to increase horsepower. Now you want to go back to what we tried for decades to solve.

Fuel injectors are more efficient in vaporizing gasoline because they spray it in much smaller droplets. That increases the overall air contact area. There still isn't time for 100 percent of it to vaporize so the amount of fuel commanded by the computer has been carefully programmed for many variable conditions, and one of them is air temperature. More fuel will be commanded when the air temperature is colder. Again, that does not translate into more power. A very specific amount of fuel is needed for each cylinder, and the extra just goes out the exhaust without contributing to the production of power.

The purported goal of cold air intake systems is to condense the air so more can be packed into each cylinder, just like turbochargers do. In every engine other than a Chrysler, the weight of that air is measured by the mass air flow sensor, and the computer uses that to calculate how much fuel to use. Chrysler is the only manufacturer that has been able to make an engine run right without a mass air flow sensor. They use the speed-density system with just a map sensor that measures intake manifold vacuum to calculate engine load. That sensor also measures barometric pressure just before engine start-up. Higher barometric pressure means more air pressure pushing the air into the engine. Colder air temperatures means more air by weight going into the engine. Both of those factors dictate how much fuel is needed to create the perfect mixture. If your intake system cools the air, more air goes in, you have a lean condition. That is detected by the O2 sensor, and the computer commands more fuel to correct that mixture. What you have gained is more engine speed. You must either let off the gas pedal, or if the engine is idling, the computer will close the automatic idle speed motor to restrict incoming air flow around the throttle blade. At that point you have gained nothing except dumped more unvaporized fuel into the exhaust system. You'll have increased emissions, increased fuel consumption, and no change in power. The only way you would get more power is if you packed more fuel into the cylinders to go with that extra air. You already have the capability to do that, but that only occurs at wide-open-throttle. Unless you're on the racetrack, how often do you drive at wide-open-throttle?

The companies that sell cold air intake systems and larger exhaust systems are selling an illusion. Colder air means more air. The computer adds more fuel and you get more engine speed. I do the same thing with my minivan by pushing the gas pedal further. I get more air, more fuel, and more speed. What WILL change is how far you have to push the gas pedal to start getting that power. You'll get the power sooner, but you won't get more total power at the high end. That's where the illusion comes in. For both of us, the only restriction to engine breathing that matters is the throttle blade. Everything else has less restriction. The best you could hope to do is totally remove the fresh air intake system and the entire exhaust system. That won't give you an increase in anything until you eliminate the throttle blade.

As for the rust issue, parking on snow is less of a problem than driving in salt. I live in Wisconsin, the road salt capital of the world! Since my ten-car garage is filled with, ... Uhm, ... Much more than cars, my daily driver '88 Grand Caravan sits in the snow-covered driveway all the time. In fact, it has never been in the garage since it was new. The original muffler lasted 18 years, and after 25 years, all the other pipes are still original and in good shape. I DO have a '93 Dynasty that I painted the muffler on when it was new, but not to prevent rust. That was to mask the big silver blob hanging down in the rear. The cheapy replacement mufflers for my Caravan get loud after about a year from the internal baffles rusting away, then the housing rusts through after two years. All three rusted from the inside first, not from the outside. In fact, due to its location, it rarely even gets hit with the slop with the salt in it. Painting your exhaust parts might make you feel good but it's not going to extend its life. Instead of wasting your money on paint, put it toward a stainless steel system. Had I bought the $180.00 stainless steel muffler from the dealer seven years ago, I would have gotten a much better value. At the time I had no idea my van would be so uncommonly reliable and that it would last this long. So far I've spent $105.00 on four mufflers, and each one required crawling around in granite, on my back, for four hours to make them fit and to make custom hangers. What did I gain?

To get back to your original questions: "So every car is as efficiant as it can be straight from the factory?"

Yes, and I've stated why. It takes a specific number of BTUs of energy to move a specific weight at a specific speed, and a specific rate of speed increase. You don't improve on that by making the engine consume more fuel, no matter how efficiently that is done. What you CAN reduce is air flow resistance and friction. That's why all manufacturers have huge wind tunnels and chassis dynomometers. That's why cars have hood stickers instead of hood emblems, and it's why windshields hit your forehead, ... Less wind resistance.

"I believe cars are made due to minimum standards set forth by the automotive industry".

I can't imagine what you're thinking to come up with that, and I can't imagine what you think you're going to design better. There will always be trade-offs and compromises. A '66 Buick Wildcat was so huge you needed binoculars to look in the mirror and see the tail lights, but they got 23 miles per gallon too with that real heavy car. The trade-off was you could die from breathing the exhaust. Do you really want to go back to those days? You can suck on your tail pipe all you want, but all you're going to get is bored.

You'll have to be more specific when you say "minimum standards". Compare your truck to one from the '60s or '70s. Heck, drive a '93 or older Ramcharger, then tell me you prefer that ride quality over your Durango's. Chrysler has always been the leader in innovations that actually benefit car owners. They were first with alternators, (they copyrighted that term), air bags, anti-lock brakes, (1969), lockup torque converters. No clueless politician was involved with any of those things. Look at the fit and finish of any Chrysler product from the '70s, then look at any new car or truck today. It took a friend weeks of massaging on one of my Challengers to make things fit half as well as on any new car. THOSE were minimum standards. No manufacturer can stick with minimum standards and stay in business. They are all in constant competition with their competitors to be the best, not just good enough. If you believe otherwise, start your own car company and see how many you sell.

"I've already added more spark using e3 spark plugs and performance distributors Screamin Demon coil cap and rotor with brass terminals and a 10mm wires"

You can't get more spark unless you change the laws of physics. What you CAN get is the capability of achieving a higher spark voltage if that is what is required to jump the spark plug's gap. Once that voltage is reached and the spark occurs, there can't be any further increase in voltage. We used to pull one spark plug wire off to create a much bigger gap, then we'd watch on the analyzer screen to see how high the voltage could go. The typical original ignition coil could produce around 18,000 to 20,000 volts. If a spark plug required 8,000 volts, that was all that was developed, and no more. (Think of a dam on the river. If it's ten feet high, the water has to reach ten feet, then it will run over. The water in the lake behind it will never get higher than ten feet even if it rains a pile. If it does rain a lot, there will be the CAPACITY for the water to go higher, but only if the height of the dam is increased. Increasing the dam height is the same as increasing the spark plug's gap). If the gap on the plug wore or was adjusted too large and it required 21,000 volts for the spark to jump, there would be no spark if the coil could only develop 20,000 volts. THAT is where having the higher capacity becomes a benefit. Your higher capacity can overcome some misfires caused by the spark plug or wire, but if you're looking for all these tricks to result in some improvement, you have already looked at the basics, so now instead of lighting off the fuel / air mixture, you're REALLY lighting off the fuel / air mixture. With your original ignition system, all you did was create pressure in the cylinder to push the piston down. With your new system, you're creating pressure to push the piston down. A stronger spark won't result in more power unless it is overcoming some other problem. That's where I'm assuming you don't have some other problem that needs overcoming. With all the work you're going through, you would have checked that already. Power is a factor of the amount of fuel, the fuel / air ratio, compression ratio, and where the crankshaft is when the piston gets pushed down, (ignition timing). As long as that fuel ignites, spark is not a factor in determining how much power is produced. To think otherwise would be to think a forest fire's severity is determined by the size of the match used to start it.

Where you'll really see the benefit with higher voltage ignition systems is when something was done to make that mixture harder to fire. One of those conditions is high compression. We had high compression engines in the '60s, around 10.5 to 1, and they ran quite nicely with that old technology. Race engines often are around 12: 1 and require a reliable spark. The hardest ones to fire are when the mixture is too lean. Remember Chrysler's Lean Burn ignition systems of the mid '70s? That was another first for the industry. A wide-scale use of a computer to control ignition timing. With a lean fuel mixture, the molecules of fuel are further apart and it is much harder for the flame front to jump from one to the next. That system was no "minimum standard". That was a real attempt at getting better fuel mileage. They got a bad reputation because old-school mechanics didn't know how they worked, but by today's standards they were so terribly simply a high school student could understand them.

If you really want a reliable and strong spark, go back to that NHRA dragster. The ignition systems produce around 40 amps of current when the spark occurs. A single misfire will result in the engine exploding, as we see once in a while, so they have to be reliable. Then, the spark plug electrodes are consumed halfway through the run. The engine fires on hot spots and detonation for the second half of the run. As you can see, there are always trade-offs. I suspect you don't want to be replacing spark plugs each time you hop into the truck and buzz off into the sunset.

"as far as the CAT I'm sure all the fuel will be spent so I won't have any sensor issues the computer will sense a clean burn and better fuel economy(no issue there)"

I think I covered that enough. Now you'll understand what the computer is looking for from all of the oxygen sensors. It's not looking for a "clean burn". It's looking for the changes in the readings under various specific conditions. It won't see those changes with no converter, and it won't see them with no sensor to report them.

The biggest thing I ever did to increase power and fuel mileage was to install a gas mileage camshaft. I did that on a '69 318 engine that I put in my '78 LeBaron station wagon with that Lean Burn system and lockup torque converter. The profile of the lobes is different. That sets the "personality" of the engine. In this case the profile for better fuel mileage was the same as what was used in motor homes. Retarding valve timing one or two degrees, (which is a real lot), increases low-end torque, and yes, I could squeal the tires all over the place! The trade-off was I needed to be sure I had enough room to pull out and pass someone on the highway. Motor homes need a lot of low-end torque to get out of their own way at a stop sign, but once they're up to highway speed, they remain fairly steady and maximum power is not needed there.

I also had an old '78 Monaco police car that was just the opposite. That 440 went from 0 to 60 just like any 318, but from 60 to 90 it about tore the seats off the hinges! That camshaft profile was advanced one or two degrees to provide acceleration on the high end for pursuits. Both cars were real fun to drive but they acted entirely differently.

If you don't want to go through the work of replacing the camshaft and lifters, look into making a change in the sprocket. On my older V-8 engines there is a woodruff key between the camshaft and sprocket to set its position. Chrysler's Direct Connection racing program offered offset keys with 2, 4, or 6 degrees offset. Two degrees will make more of a noticeable change than anything else you're trying to do, ... Unless the computer somehow gets involved and does something to counteract that change. You DO have a camshaft position sensor but that only synchronizes injector firing times. That can be off a lot and you'll never notice it. An offset key can be installed either way to advance or retard the camshaft. If you think of tightening the timing chain on the side that pulls on the cam sprocket, that would advance it a little. Think of "T" for "tight", "T" for "top end", and "T" for, ... Uhm, ... "Advanced". An advanced camshaft will increase power at high speeds. If the timing chain was loose, the slack would have to be taken up before the camshaft would start to turn. That makes it late. Remember "L" for "loose, "L" for "late", and "L" for more low-end torque. I'm sure there will be some ramifications to the level of emissions, but common sense dictates it should not be significant.



March, 3, 2013 AT 1:06 AM

Ok, so your telling me this is a crock?

The Screamin' Demon Coil allows you to open up the spark plug gaps to a whopping.065". The larger plug gap leads to a more complete combustion of your fuel mixture, which in turn provides more horsepower, better gas mileage and cleaner spark plugs. The coil features a brass terminal, which is more conductive and resists corrosion longer than the aluminum terminal on the stock coil.

I also cant quite understand how the computer won't adjust "less fuel"(efficiency not power) when the chamber recieves more spark(longer duration and larger arc) and more air(slightly cooler and more dense). Also doesent a less restrictive or free flow exhaust alow for the emissions to be almost vacuum sucked away when you add a one to one header for cylinder configuration? If so shouldn't a larger pipe facilitate the larger volume of combustion gases for when I do pound down on the throttle?

Doesn't the cat burn off the "unspent fuel" in a factory configuration?
Doesn't it also restrict flow?
What is the sensor wireless or made of titanium to resist all the heat?

From this site
"How a catalytic convertor works"

The catalytic converter was developed in the early 70's when severe air pollution was prevalent, but wasn't made mandatory by the US government until 1976. The catalytic converter or "cat" for short is one of your vehicles main emission control devices. The name was derived from the operation the catalytic converter performs; it utilizes a catalyst and converts the unburned gases to burned gases, "catalytic converter". The catalyst is heated to excite the chemical (the catalyst its made of), this burns the unburned fuel making the catalyst even hotter furthering the effect (thermo reactor). Because of the extreme temperatures the catalytic converter produces the catalyst material can crack and come apart clogging the outlet port of the converter causing low power and stalling.

I mean, it all seems like common sense!
Sorry I know you have spent a lot of time answering but I just can't understand why so many ppl auto mechanics included and especially mod their cars if none of this stuff does anything but make noise and look pretty, oh, and waste fuel to boot.



March, 3, 2013 AT 12:35 PM

I didn't say it's a crock. I'm saying there's no free lunch and anything you gain is going to be met with a trade-off. It all depends on what you'll be losing and whether that is significant to you.

You're confusing two unrelated things when you assume a bigger spark equals more power, or rather, a bigger spark allows you to tune for more power. GM made news with their High Energy Ignition (HEI) systems in 1976 when they specified a plug gap of.060". We had never seen that before but we had also never seen a stock ignition coil that could easily develop 45,000 volts. That brought on some new problems we had to learn the hard way, but those cars didn't run any better or worse than any other car. They didn't get any better fuel economy, and they didn't develop any more power than in previous years. Those distributors could be dropped right into an older engine with very little modification. People did that to do away with having to periodically clean and adjust the breaker points. Beyond that you could not tell any difference. You may have had a bigger and hotter spark, but you had the same amount of fuel in the cylinder and the same amount of compression. A squirt of fuel is going to produce a squirt of power regardless if it is ignited with a match or an atom bomb. It doesn't matter what initiates the burn. The atom bomb is just going to be certain there's no misfire. What your coil is going to do is overcome some potential misfires. The power you develop is in the fuel, not the spark. If you think otherwise, lets all put 100,000 volt coils on our engines and we won't even need any gas.

"The larger plug gap leads to a more complete combustion of your fuel mixture, which in turn provides more horsepower, better gas mileage and cleaner spark plugs."

That doesn't make sense. The spark is done and over with in an instant. Once it ignites the fuel, it's up the mixture and the quality to determine how well it continues to burn. The spark only ignites the small amount of fuel near the electrodes. The rest of the fuel continues to burn all the while the piston is moving down. It's not one quick explosion. That spark is long gone and forgotten while the fuel is still burning.

It's not the spark that keeps the electrodes clean. That sounds like advertising jargon. Clean fuel and detergents, and the proper heat range keep the plugs clean. Every time there's a spark, a few electrons are removed from one electrode and deposited on the other one. That's why they get rounded off over time. A spark does not like to jump from a rounded anything. It always prefers a sharp point, as in the sharp point of lightning arrestors on barn roofs. The higher current from a hotter spark means the electrodes will wear faster, and that by itself can promote misfires. The plugs will still be clean, but no more so than normal.

Did you ever see the guy at the car show demonstrating his gadget that went on the distributor cap to increase gas mileage? I saw that as late as the mid '90s but it was around in the early '70s too. I had one. It was nothing more than an air gap plugged into the coil wire. The theory was the coil would have to develop a higher voltage to create a spark that would jump the spark plug's gap AND the gap in this unit. With a higher voltage came a higher current when the spark occurred. For proper operation it had to be assumed the coil could develop that needed voltage. Rag newspapers and magazines are full of all kinds of instant gas mileage and horsepower wonders, and I'll bet that thing is still available somewhere. Don't you suppose if any one of them really worked, people would tell friends, and they'd be lining up to buy them? And you know for darn sure if increasing horsepower was as simple as bolting something on, the manufacturer would do it.

I understand your thinking about engine breathing, but I think you're confusing bigger as better. Chrysler's racing manuals spell out how to make your own headers custom-tailored to the application. They tell the exact pipe diameter to use and the length of each runner. They never say "bigger is better". If it was, every hot rodder would have six-inch diameter pipes. The fact is the length and diameter of the pipes determines the characteristics of how the engine performs and where its power band is. Smaller runners don't reduce horsepower. They move the rpm where the most horsepower is developed. If it helps, think of driving down a two-lane road. Hard to go fast if there's lots of traffic. Now think of driving on a four-lane expressway. You can give 'er holy smoke, but that isn't going to help you drive into your one-car garage. You can turn your intake and exhaust systems into expressways, but the intake manifold and throttle body are the one-car garage.

Also consider that there needs to be some exhaust system back pressure. In a race engine the goal is to get the spent stuff out as quickly as possible. I know this is hard to visualize, but if they used huge pipes with lots of volume, all that exhaust would just sit there and pile up. Since the engine is basically an air pump, the pistons have to push their exhaust out AND push out what's in that big pipe. By using a 3" diameter pipe the gases have to rush out faster, and that momentum helps to pull the exhaust out of the engine. It's like when you suck up a strand of spaghetti. It's more effective if you keep your mouth closed. The disadvantage in the race car is that momentum pulls out the exhaust gases but it also draws out some of the fresh fuel and air that just came in through the intake valve. That fuel gets wasted but there is no exhaust gas left in the cylinders, just fresh gas and air resulting in more power. Power is the main concern. Fuel economy is a distant second, even in NASCAR.

We do the opposite with our street engines when we use EGR valves. Those dump inert exhaust gas back into the intake to take up space in the cylinders under certain conditions when we don't need more power. That reduces fuel consumption. So here again, it is always a trade-off. You think the engine has to work too hard to push the exhaust out, and a few years ago I would have agreed with you. Once I started teaching, one of the requirements was I had to go back and work in industry every few years. One place I worked was in a friend's race engine machine shop. Those guys know all the tricks, but with increased power comes increased fuel consumption. There's no free lunch.

The catalytic converter is not nearly as restrictive as you would think. I did too at one time. The fact is if you look at the housing, it's twice the diameter as the pipes it's attached to, and half of the catalyst is air space. It has to be to have the greatest contact area for the air and hydrocarbons to be absorbed quickly. Where they got their undeserved reputation for being restrictive was when we still had leaded gas in the '70s and people used it in cars with converters. That lead stuck to the catalyst and over time it plugged the air passages. My friends and I used to tear the converters off our cars in the '70s and early '80s. Then we were disappointed to find we didn't get any more blazing horsepower like we had hoped. All we got was dirty.

Another point to ponder is Chrysler uses the MAP sensor as the main factor in calculating the engine's fuel needs. It measures manifold vacuum very precisely and the Engine Computer converts that to engine load. If the air filter has some restriction, that will be designed into the programming to result in the best fuel economy and lowest emissions. Manifold vacuum is formed by the pistons pulling in air against the throttle blade and the entire air intake system. If you lower that resistance, there will be less to pull against so vacuum will go down. When does vacuum normally go down? When you open the throttle. What happens when you open the throttle? More fuel goes in. In effect, you're tricking the MAP sensor and the computer into thinking you're accelerating so it's going to inject more fuel. That would correlate with more air. With less restriction you get more air, and you need more fuel to maintain the correct mixture.

Now, to be fair, the computer is going to learn very quickly that it's injecting too much fuel. That learning is what they're designed to do because no two sensors are ever exactly alike. While the MAP sensor is the starting point in the fuel calculation, and it has the biggest say in what's needed, it's the upstream oxygen sensor that will tell the computer how to fine tune that calculation. That table of calculations is called the short-term fuel trims (STFT), and those become part of the long-term fuel trims (LTFT). Those numbers can be viewed on a scanner so you can see whether the computer is adding or subtracting fuel from what the engineers programmed in. The point is, if you reduce the intake air restriction, and the lower vacuum results in more fuel going into the engine, the computer is still going to correct that. The problem is the only place you're possibly going to see any gain is at wide-open-throttle.

As for the heat the oxygen sensors must withstand, they have electric heaters built in to get them up to 600 degrees as soon as possible because they can't rely on the exhaust gases being hot enough to do that. Police cars used to often go to "open loop" when they idled too long at crash scenes because the sensors dropped below 600 degrees and didn't function there. I don't think I'd care to be an oxygen sensor, but they seem to be quite happy in the environment they live in.



March, 3, 2013 AT 10:36 PM

Ok, so what do you think I have done by adding these replacements? Besides lighten up my wallet and gave a more custom look under the hood?

Also! Just to be clear you think from header to exhaust I should only replace what needs replacement and only with factory parts. Correct?



March, 4, 2013 AT 1:29 AM

What parts to be replaced can be a difficult judgement call. When it's my cars, I always go the cheapest route and replace just what's needed, but that can translate into a lot more time to fabricate parts to fit. I have plenty of time. Mechanics are very conscious of time because they have to charge for it. When I worked for a Sears Auto Center in the '80s, we looked at two things. The first was how long was it going to take to tug and poke on the pipes to get the new muffler to fit properly, and how long were the old parts expected to last. If we could bend a tail pipe with a pliers, for example, it may not be leaking yet but it would be before that new muffler rusted out. It was a better value for the customer if we chopped off the muffler and tail pipe together and put both new parts on.

Many exhaust systems come from the factory welded to insure there's no squeaks or rattles from incorrect orientation of any parts. Very often those parts were not standard sizes and no replacement muffler fit perfectly. To address that they started using about a dozen standard mufflers instead of about the hundred commonly-sold parts, and we had to use an expander to fit the inlet and outlet tubes to the factory pipes. That's another case where going with the larger muffler and a matching new tail pipe took less time and could be a better value. Those replacement pipes were real heavy and excellent quality, but they would never last as long as stainless steel. Those had just started showing up on a few models. For those we would go through the extra work to make the new parts fit so we could leave any original stainless steel parts on the car. We were not concerned with how long that new muffler would last because we would replace them later for free. That was a marketing strategy. We gave away a 20-dollar muffler and five dollars labor, but made it up by charging $5.00 for two clamps, but that got the customer back into the store to buy other stuff.

You have to decide how much you want or need to replace. If this is still the original muffler, you likely have stainless steel and the pipes are probably in fine shape yet.

I don't know about the Durango, but there were dual exhaust kits available for the full-size trucks, if you want to give it a custom look. They did make the exhaust louder but that was due to the muffler that was part of the kit. The one in particular that I remember melted the flexible wire harness going to the ABS sensor in the rear axle, and it rattled against the frame. My task was to diagnose the ABS problem but I ended up fixing his rattle too. On that truck there were still two front oxygen sensors, one for each side of the engine, then just a single catalytic converter and a single downstream oxygen sensor. That's where the kit started with a Y-pipe to branch out to two tail pipes.

With that single-into-two system, for every two engine revolutions there were 8 power pulses going into the exhaust, then half of that gas went to each side. They needed to have a louder muffler to get the desired sound but you can tell the difference compared to a standard dual exhaust. If you look at the firing order of all V-8 engines, you will see they do not alternate left and right side evenly. Some Fords are different and all Fords had different numbering, but the physical order the cylinders fired were the same as on GMs and Chryslers: 1, 8, 4, 3, 6, 5, 7, 2

The odd numbers are on the driver's side, so the power pulses going down the exhaust was left, right, right, left, right, left, left, right. Each exhaust pipe had two power pulses in a row, then a gap between the pulses. That's what gave it the rumble that we associate with dual exhaust.

As another passing point of interest, we had a former rodeo champion turned Olympic-caliber down-hill skier, turned stock car champion in my city who had just gotten an invitation to go to NASCAR when he was killed in a freak racing accident a week later in the early '80s. We still have memorial races for him every year at his home track three miles from me. At one point he was racing Dodge Challengers with 355 c.I. $25,000.00 engines. One week he blew the engine up in one of the early races. The next week he blew up another one while leading the feature race. What they found to be the cause was his ignition system.

Any time you have current flowing through a wire, it sets up a magnetic field around it. That is how kids make electromagnets with a nail and a battery, and is the basic part of an alternator. The other half of the alternator, or any generator, is when you pass a wire THROUGH a magnetic field, you induce a voltage in that wire. The current makes a magnetic field, and the magnetic field can make current flow.

If you look at any Chrysler V-8 engine as it came from the factory, the four spark plug wires on the left side snap into a holder welded to the rear top of the valve cover. The cylinders are numbered 1, 3, 5, 7 front to back, but the spark plug wires were snapped into that holder, 5, 1, 3, 7. People have a tendency to to rearrange them to keep the wires straight and perfectly parallel to look pretty, and that's what caused the two engines to blow. When the spark occurred for cylinder number 5, the current flow through that wire set up a magnetic field that induced a voltage in the number 7 spark plug wire. Thanks to his enormous ignition coil, that voltage was high enough to fire that plug just as that piston was starting on its way up on the compression stroke. That pounded the piston down backward with enough force to crush the connecting rod. All that was needed to solve that was to separate those two spark plug wires by a few inches, and do the same thing on the other side.

I'm not suggesting that is going to happen to your engine, but it shows how simple things can have unknown consequences. You and I will never get to know even a fraction of what the engineers have run into and solved when they design an engine.

I made a passing reference previously to a problem with GM's HEI ignition system that we learned the hard way. It was customary to unplug one spark plug wire with the engine running to create a gap so big the spark couldn't jump it. That was to show on the engine analyzer the maximum voltage the ignition coil could develop. If some of the spark plugs were firing at 15,000 volts and the coil could only develop 16,000 volts, you had better do something or that customer was going to start having misfiring problems soon. As soon as we did that on some GM systems, the engines died. With a coil capable of developing 45,000 volts, that spark is going to go somewhere, and if it can't go through the spark plug or your hand, it will find a way to ground through the rotor under the distributor cap. That was called "punch-through" and an arc like that always leaves a trail of carbon behind. Carbon is an electrical conductor so from that point on, since electrical current always looks for the easiest path, it went through the hole in the rotor instead of the more difficult spark plug gap. The rotor actually became shorted. By the way, standard radio noise suppression spark plug wires use a fiber string, not a metal wire. The string is impregnated with carbon. That's what conducts the current.

Okay, enough story hour. My fingerprints are getting worn down! As for what you have gained, if something were to happen that made a spark plug misfire due to insufficient voltage, which is not that uncommon, an ignition coil with the potential to reach a higher voltage will overcome that misfire. If you get a tank of gas that is harder to ignite, which includes higher octane blends, you will be much less likely to notice a running problem since the hotter spark is going to do a better job. Where I suspect you will notice it the most is starting the engine in really cold weather. Battery voltage is drawn down because it's a chemical reaction and those slow down in cold temperatures, and the engine oil is thick so the engine needs even more power to turn it over at a time when the battery has less power. With lower voltage feeding the ignition coil, it will still be able to develop the voltage needed to fire the plugs.

The downside, if you could call it that, besides magnetically coupling multiple spark plug wires, is to get higher current out of the coil, you have to put more current into the primary. That current comes through the automatic shutdown relay which also powers the injectors, alternator field, oxygen sensor heaters, and the fuel pump or pump relay, in some applications. That higher current is going to degrade the contacts in the ASD relay faster than normal. That relay may only last 49 years instead of 50!

You also run the risk of developing a misfire if the spark can find an easier path to ground. Usually that's under the spark plug wire's boot and along the side of the plug. That can occur when it gets wet with oil or water. It's more likely to occur with a stronger coil but the coil is not the cause of the problem. In that case, if there would be a misfire due to the plug, it's going to misfire with the original coil too. High-quality silicone wires will reduce the chances of that happening. It's not what's conducting the current; it's the insulation around it that is containing that high voltage. THAT is one place where you may be right about "minimum standards". All manufacturers squeeze their parts suppliers relentlessly to cut every penny possible. It's just a matter of what are they willing to cut/?

Ford made huge news in the mid '70s when they decided to leave off four 5-cent grease fittings on their ball joints to save 20 cents per car. That saved them millions of nickels per year but at a cost of separating parts that led to loss of control, crashes, and lawsuits. They have never figured out the blow that caused to their reputation, and they are still doing that to this day on their front-wheel-drive cars, with the same results. Even the cheapest aftermarket parts are a huge improvement in quality. That gives a new high, ... Or low, to "minimum standards".

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