This tech paper will discuss setting the timing
on a Chevy V8. This procedure also applies to other GM V8s.
The procedure outlined here differs from the Service Manual, and is based on my
years of experience doing this work in the quickest, least painful, most
economical way while keeping the level of quality high. It is recognized that
other people will have different methods of doing things, and may disagree with
specific methods and procedures that I use.
How to Set the Timing
When you think about it, setting the timing at idle speed makes no
sense at all: You don’t operate your car at idle, and timing changes as the
rpm changes. Fact is, the timing spec at idle speed is provided as a simple way
for most people to set the timing, and is not a good procedure for optimum
performance.
Small block Chevys (and most other GM performance V8 engines)
perform best when the total timing (full centrifugal advance plus the initial
timing setting with vacuum advance disconnected) is all in by 2,500 – 2,800
rpm and is set to 36 – 38 degrees. If you have an adjustable timing light,
this is very easy to check. If you don’t, you need to scribe a 36-degree mark
on your harmonic balancer. Here’s how:
Measure the circumference of your harmonic balancer using a sewing tape measure
(or other flexible tape measure). Get it as accurate as you can. Take this
measurement and divide by 10. The number you get is the distance to 36 degrees.
Measure this distance CLOCKWISE from your existing harmonic balancer timing mark
and place a clear mark on the balancer.
Remove your distributor cap and rotor. Remove the 2 centrifugal advance springs.
Install the rotor and the cap (without the springs). Disconnect the vacuum
advance.
NOTE: This procedure cannot be used on the HEI ignition systems. Removal of the
springs will cause an artificially over-advanced condition that will never be
achieved with the springs in place. You can use the basic technique described in
this paper with the HEI units (setting timing up to 36 degrees), but to check
total timing, you must install a set of soft springs. You cannot remove the
springs altogether. With the soft springs in place, rev the engine until the
centrifugal advance is pegged out. Adjust for 36 degrees total. Then re-install
your original springs.
Start the engine. It may kick back a little due to the advance coming in
immediately without the springs. If you’re using an adjustable timing light,
set the light to 36 degrees advanced. Now rev the engine just a little while
observing the timing marks with the light. It shouldn’t take much rpm to peg
out the advance without the springs installed. With an adjustable light set at
36 degrees, align the stock timing marks with “0” when the timing is
“pegged out.” With the non-adjustable light, align your new 36-degree mark
with “0.” Rev the engine a little to make sure the timing will not advance
any further. Shut it down.
Pop the cap and rotor and re-install the springs. Put everything back together,
but leave the vacuum disconnected. Start it up. For future reference, make a
note of the timing setting at idle. This is your new curb idle timing spec. Now
give the engine a few quick rev’s past 3,000 rpm and verify that the full
timing (36 degrees) is coming in. If it’s not, you need to change to a softer
set of springs until you get full 36-degree advance before 3000 rpm. (NOTE: A
stock set of springs will usually not allow full centrifugal advance to come in
before redline rpm. If you have stock springs installed, don’t rev the engine
beyond its limits to try to force full advance in.)
Shut it down and hook up the vacuum. Now do a road test.
The 36-degree 2500 rpm advance curve is optimum for performance, but may require
premium fuel. Lug the car around, and punch the throttle at low rpm while
listening for detonation (“engine knock”). If you’re getting any audible
knock, you MUST retard the timing. Retard the timing in 2-degree increments
until engine knock stops. Engine knock will seriously damage engine components
if not corrected. If you get no knock, you may see slightly improved performance
at 38 degrees total timing. This is particularly true if you’re running at
high altitude.
If you have no engine knock under acceleration, but the car “chugs” or
“jerks” at cruising speed (light throttle application), you are getting too
much vacuum advance on top of the mechanical advance. You may need to change out
the vacuum advance diaphragm with an adjustable unit available from aftermarket
sources. Adjust these units so that you get the most vacuum advance possible
without any “chugging” or “jerking” at cruise speed.
Your timing is now set for best possible performance. Make note of the new
setting, and use this for your future tune-up work.
Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO
Special Thanks!!
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