CAM timing vs. Ignition timing, vacuum

Tiny
GEARHEAD1963
  • MEMBER
  • 1972 CHEVROLET CHEVELLE
  • 7.4L
  • V8
  • RWD
  • MANUAL
  • 5,000 MILES
I am working with a 1972 454, 8.5:1 comp, heads just reworked, draw down tested to 22 in. New cam (factory LS5) Hyd. Cam installed to dead 0. Ignition timing should be at 8deg. BTDC but will only run above 12deg. (15 in. Vac. At 12 deg.) Runs better at 16 deg. BTDC (17 in. Vac. At 16 deg BTDC) runs best at 22 deg BTDC @ about 18.5 in. Vac. But at these settings I am having a major problem adjusting the carb. Also the engine will start but has a hard time getting up to proper idle speed, will start at about 425 RPM and is hesitant to get up to 750 RPM. If I advance the cam will it not bring down the ignition timing, (1 deg. Cam advance, 2 deg ignition retard) any sugestions would be helpful.
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Wednesday, July 22nd, 2015 AT 4:33 PM

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Tiny
MHPAUTOS
  • EXPERT
I would suggest that that you start with getting the distributor re graphed to suit the cam specs and new comp ratio that you are now using, you may have a badly worn distributor and not know it, that would be the best place to start.
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Thursday, July 23rd, 2015 AT 4:17 AM
Tiny
GEARHEAD1963
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Already have new distributor, and the engine is factory spec. My issue is with cam timing and ignition timing. I need to know weather to advance the cam or retard it in order to increase idle vacuum and reduce the ignition timing advance from 16 deg. To around 8 deg.
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Thursday, July 23rd, 2015 AT 10:11 AM
Tiny
MHPAUTOS
  • EXPERT
This was sent to me some years ago by a mate the did a lot of performance work, we were discussing a simmer issue to yours so I hope that this is of some tech help, I am not sure where the original text came from, so I can't give due credit to the author.

Effect Of Changes In Cam Timing And Lobe Separation Angle
The table in the pic illustrate how variations in lobe separation angle and cam
timing will affect the behaviour of the engine in which the camshaft is installed.

EFFECTS OF ALTERING CAMSHAFT TIMING
Advancing Retarding
Begins Intake Event Sooner Delays Intake Closing Event
Open Intake Valve Sooner Keeps Intake Valve Open Later
Builds More Low-End Torque Builds More High-RPM Power
Decrease Piston-Intake Valve Clearance Increase Piston-Intake Valve Clearance
Increase Piston-Exhaust Valve Clearance Decrease Piston-Exhaust Valve Clearance

EFFECTS OF CHANGING LOBE SEPERATION ANGLE (LSA)
Tighten (smaller LSA number) Widen (larger LSA number)
Moves Torque to Lower RPM Raise Torque to Higher RPM
Increases Maximum Torque Reduces Maximum Torque
Narrow Power band Broadens Power Band
Builds Higher Cylinder Pressure Reduce Maximum Cylinder Pressure
Increase Chance of Engine Knock Decrease Chance of Engine Knock
Increase Cranking Compression Decrease Cranking Compression
Increase Effective Compression Decrease Effective Compression
Idle Vacuum is Reduced Idle Vacuum is Increased
Idle Quality Suffers Idle Quality Improves
Open Valve-Overlap Increases Open Valve-Overlap Decreases
Closed Valve-Overlap Increases Closed Valve-Overlap Decreases
Natural EGR Effect Increases Natural EGR Effect is Reduced
Decreases Piston-to-Valve Clearance Increases Piston-to-Valve Clearance
Effects Of Changes In Cam Timing And Lobe Separation Angle

EFFECTS OF ALTERING CAMSHAFT TIMING

Advancing Retarding
Begins Intake Event Sooner Delays Intake Closing Event
Open Intake Valve Sooner Keeps Intake Valve Open Later
Builds More Low-End Torque Builds More High-RPM Power
Decrease Piston-Intake Valve Clearance Increase Piston-Intake Valve Clearance
Increase Piston-Exhaust Valve Clearance Decrease Piston-Exhaust Valve Clearance

EFFECTS OF CHANGING LOBE SEPERATION ANGLE (LSA)

Tighten (smaller LSA number) Widen (larger LSA number)
Moves Torque to Lower RPM Raise Torque to Higher RPM
Increases Maximum Torque Reduces Maximum Torque
Narrow Power band Broadens Power Band
Builds Higher Cylinder Pressure Reduce Maximum Cylinder Pressure
Increase Chance of Engine Knock Decrease Chance of Engine Knock
Increase Cranking Compression Decrease Cranking Compression
Increase Effective Compression Decrease Effective Compression
Idle Vacuum is Reduced Idle Vacuum is Increased
Idle Quality Suffers Idle Quality Improves
Open Valve-Overlap Increases Open Valve-Overlap Decreases
Closed Valve-Overlap Increases Closed Valve-Overlap Decreases
Natural EGR Effect Increases Natural EGR Effect is Reduced
Decreases Piston-to-Valve Clearance Increases Piston-to-Valve Clearance
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Sunday, July 26th, 2015 AT 3:03 AM

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