Why did this work: Using a Holley timing retard device.

Theron

Theron

Member
I was having trouble getting enough advance on my mud truck's engine (350 Chevy with Vortec heads, 670 holley vacuum secondary) to pull minimum vacuum for my class (15'' @ 900 rpm) while still safely staying away from detonation.

On another racer's recommendation, I used a Holley 8987 Step Retard controller for my MSD box to drop the timing back 7 degrees. I then advanced the distributor 7 degrees to bring the timing back where it was tuned. The vacuum reading went from 13'' to 15'' @ 900 rpm and the idle rpm increased. I was able to close my throttle plates at idle by a significant amount. Power stayed the same. Everything seems to run fine, but then this truck essentially goes from moving around, to WOT for 6 seconds, and then back to moving around.

Question 1: Why did this work?

Question 2: Can I use this method to close the throttle plates more on my chevy 383 stroker where I have trouble with getting the throttle plates closed enough to be on the idle circuit of the 770 street avenger? The 383 is a much more radical engine and has problems at part throttle.
 
I'm confused. Retarding the timing slows the engine. Advancing speeds it up.

To close the throttle plates more to be on the idle circuits of the carb, you have to add bypass air,
usually done by drilling holes in your throttle plates. But once you go too big, you are screwed.
You can add air by creating a vacuum leak. I do it by using an adjustable valve in the rear port of
the carb usually used to connect to the power brakes.
Set your idle screw for "squares" on the transition slots, and adjust your idle with the valve.
 
So, on the 770, if I square the throttle plates, it won't run. If I advance it far enough, I can lower the idle, the plates are close to square, and it'll run.

This is what it seems (and I don't quite know enough about now an MSD box works to figure out the truth):

Advance the engine mechanically by advancing the distributor.
Retard the engine back to where it was electrically.

Observed result: Mechanically advancing the distributor + electrically retarding the timing = more vacuum and a higher idle.
 
Theron said:
Mechanically advancing the distributor + electrically retarding the timing = more vacuum and a higher idle.
Click to expand...

If you added 7 degrees, and subtracted 7 degrees, then the net change is zero.
So I don't know why it works either.
Did you verify this with a timing light?

Try this:
With the transfer slots "squared", create a vacuum leak by pulling a 1/4" ID vacuum line off (manifold vacuum).
The engine should speed up and hold idle. The more radical the cam, the bigger the leak you need.
 
Yes. I verified this with a digital timing light on the mud truck with the holley 670.

Fully warmed up, 900 rpm, 13 vacuum reading. Steady. Timing set to 8 degrees, all in at 3200, 36 degrees.

Advanced the timing by 7 degrees, verify on the light. Set the MSD timing device to 7 degrees of 'retard', verify with light. No net change in timing: shows 8 degrees and 36 as above.

Measured the vacuum at the same port: 15.

Verified by the rule of three: repeat it 3 times over 3 separate days.

I've read through all of what Grumpy has posted since I've been haunting this forum. Do I understand it all? Nope. Not even with degree in engineering (Chemical, not Mechanical).
I am no closer to understanding how this works.

Do you know how the MSD device changes the timing? It obviously doesn't mechanically rotate the distributor... but what is the mechanism that it uses to change the timing? Does it literally tell the distributor to trigger differently in the same way that it would if you physically twisted it and changed where it triggered?
 
Brian thinks your mechanical advance is hanging up. If you are all out racing, lock out the advance.
 
I am going to plot out my advance curve tomorrow. Not all out racing. This is mostly a street truck driven hard with 2x per year on the track.
 
Theron said:
Yes. I verified this with a digital timing light on the mud truck with the holley 670.

Fully warmed up, 900 rpm, 13 vacuum reading. Steady. Timing set to 8 degrees, all in at 3200, 36 degrees.

Advanced the timing by 7 degrees, verify on the light. Set the MSD timing device to 7 degrees of 'retard', verify with light. No net change in timing: shows 8 degrees and 36 as above.

Measured the vacuum at the same port: 15.

Verified by the rule of three: repeat it 3 times over 3 separate days.

I've read through all of what Grumpy has posted since I've been haunting this forum. Do I understand it all? Nope. Not even with degree in engineering (Chemical, not Mechanical).
I am no closer to understanding how this works.

Do you know how the MSD device changes the timing? It obviously doesn't mechanically rotate the distributor... but what is the mechanism that it uses to change the timing? Does it literally tell the distributor to trigger differently in the same way that it would if you physically twisted it and changed where it triggered?
Click to expand...

Well, you've got me stumped.
I have an MSD Tech Notes booklet. I will check that for you.
 
http://documents.msdperformance.com/8987.pdf

Answer:
The Controller also has a very accurate magnetic pickup compensation circuit built-in and will hold
the timing steady throughout the entire rpm range of the engine. This is why there will be a 4° change
in the engine timing when the Control is installed.
 
Last edited:
I notice that it does not say *which way* the timing changes. I will simply buy another controller and experiment. If I were really gutsy, I'd pull the controller out of my mud truck to experiment, but I'd rather that vehicle not change during race season.
 
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