The factory spec for my 502 motor says...
"set the ignition timing
to 4º before top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected
and plugged. This setting will produce 26º of total advance at wide-open throttle (WOT) when using the HEI
distributor P/N 93440806."
It's given in page #4 here.

In another place on the same site, it says...
"Ignition timing: 36 degree total @ 3600 rpm, with vacuum advance disconnected"
It's given here...

Why are they saying 26º @ WOT (26º+4º=30º) and 36º @ 3600 rpm in two places?
What should I follow?

 

Did you notice the next sentence?? I ran into this when I bought my ZZ4. We discussed it here on the forum, no real consensus. Are you going to run the advance canister or not?? On my ZZ4 the specs say to start with 10 degrees and you'll get 32 total at 4000 (no vac canister..) Thats about where I am at (52 degrees with the vacuum connected)

....The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve....

Rob

 

Less mechanical advance means less HP/torque produced by the engine. Less HP/torque means that the manufacturer has less warranty claims. GM did the very same thing when they established the idle advance settings. It's to their advantage to 'weaken' the engine performance....not to yours.

 

I am running the vacuum advance now, but yeah... GM recommends disconnecting it and plugging it while setting the timing.

 

I agree, they may do whatever they can to reduce the warranty claims.
But my point is.. they are recommending different total timing in two different literature. Which one is correct?

 

Actually, they specify LEAVING it disconnected and plugged, even after the timing is set...

Rob

 

the vacuum advance GM used on that distributor has 20 degrees of advance which is too much, we use a ac/delco D1370a vac advance can which has 10 degrees of advance. Then i would try 12 to 14 degrees of initial timing (with the vac advance disconnected).

I hope this helps Henry @ oles carb

 

rajsid, the long block specs just state that when you set the initial to 4 degrees, the dizzy will give you 26 total at WOT using the 806 dizzy. That is in the "break in" procedure.
WOT could be anywhere from idle to redline, no particular RPM called out, so I don't really know how to interperate that. The 806 dizzy apparently gives you 22 degrees of mechanical. You can add more initial to get more advance if you need it, and the motor lets you.

36 degrees at 3600 is a clear statement of a specific timing curve. A phonecall to verify might be good.

 

Are you running with it disconnected and plugged?
I've never tried that.

 

It's 36* total. So what should I set the mechanical advance to in order to get a 36* total at 3600 RPM?
Do I not worry about the initial timing?

 

No I have it connected.

My ZZ4 specs call for 10 degrees initial, 32 total @ 4000 rpm with vac plugged, which is just about what I have.

With the vac canister I get about 52 degrees.

Rob

 

this spec is a mistake in all the literature for some bb crate engines and long blocks and has been so for many years, makes one wonder who edit's all this stuff....you should set your timing at 36-40 degrees total and running a vac adv is your choice ( i dont want to get into a vac or mechanical debate here).....the 4 deg initial is a mistake...good luck....

 

36 - 22 = 14
If the motor has trouble turning over at 14 initial, back off initial till it turns over easily.
I second the vacuum advance comment earlier. The motor can use a lot of advance at light throttle (lean mixture) on the highway. This is what vacuum advance does for you.

 

I have to bother you once more.
Where did you get this from? -> "The 806 dizzy apparently gives you 22 degrees of mechanical"

So, to set the total at 36*, all I have to do is bring the car up to 3600 RPM and set timing at 36*... correct?
And ideally, this should produce an initial timing of 14*... Correct?

 

No bother, and sorry it wasn't clear. I got the 806 number from the page #4, step #4 of the document you posted. It's the last 3 digits of the distributor number referenced in the instructions, that's all.
Also on page #4, step #4 they state that with 4* initial, you will see on the balancer 26* total. Subtract the 4* initial that the owner sets, from the 26* total you get out of it, leaves you with 22* mechanical advance that the distributor can deliver without altering it mechanically.
Use different springs, and weights if needed to attain that spec. Thats all in their break-in instructions on page #4.

Set the initial timing to 14* at idle and snug the distributor clamp bolt. Then bring the RPMs to 3600 while using a timing lite. At 3600, the advance should be 36*. If it's less than 36* you can use lighter gage mechanical advance springs to bring the mechanical in closer to 3600. You want to see the timing increase smoothly through this RPM range (idle to 3600) and reach 36* at the same time the engine reaches 3600. If it reaches 36* at say, 2800, it has "come in too soon", and you need to use heavier gage springs (or just go "up one spring, you don't have to use pairs) to delay full advance until 3600 RPM or as close to it as possible.
There are also different weights included in a distributor re-curve kit as well as different springs, but I am satisfied with the facory ones.
I don't think it would be greater than 36* but if it is you can also back off the initial to less than 14*, as long as you wind up with 36* at 3600.
Sorry if it's long winded

 

Thanks SH-60B,
It's clear now...