How to Set Your Timing for Peak Performance
How to Set Your Timing for Peak Performance
How to
Set Your Timing for Peak Performance
(Non-HEI)
by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto
Repair & Restoration
Broomfield, CO
Rev. B 4-18-01
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.
Questions, Comments & Technical
Assistance
If you have questions or comments regarding this article, or
if you notice any errors that need to be corrected (which is quite
possible since I’m writing this from memory…), please feel
free to drop me an e-mail. Also, if you need any technical assistance or
advice regarding this process, or other maintenance issues, feel free to
contact me:
lars.grimsrud@lmco.com
