[Erik_H]

Still fiddling around with my new distributor and found another issue, although I fully expected this one honestly. The vac advance can that they swear only adds 10 degrees actually adds 25. With everything plugged my initial is at 11, I plug in the vac hose and it jumps to 36 at idle.

So I guess I need to braze in a little stop plate right? I have read and read that one thread so I know it should allow only 1/8" of movement right?

I'm assuming that I would only want around 10 since I'm at 32 total. Wouldn't 57 degrees advance crusing down the highway be pretty bad? What is the actual preferred amount at part throttle cruise (i.e. initial + mechanical + vacuum)?

[72LuxuryLeMansLa.]

Originally Posted by Erik_H

Still fiddling around with my new distributor and found another issue, although I fully expected this one honestly. The vac advance can that they swear only adds 10 degrees actually adds 25. With everything plugged my initial is at 11, I plug in the vac hose and it jumps to 36 at idle.

So I guess I need to braze in a little stop plate right? I have read and read that one thread so I know it should allow only 1/8" of movement right?

I'm assuming that I would only want around 10 since I'm at 32 total. Wouldn't 57 degrees advance crusing down the highway be pretty bad? What is the actual preferred amount at part throttle cruise (i.e. initial + mechanical + vacuum)?

You need to post engine specs for someone to get specific about timing. This is from Jim Hand's book:

"Each engine combination varies in regard to "correct" timing, and the ideal timing will have to be found by driving. In very general terms, most Pontiacs like 10-18 degrees of initial timing. Most engines can use 20-25 degrees of additional mechanical advance, and also 14 degrees of vacuum timing. My 473-ci engine with slightly over 10:1 compression likes 11 degrees of initial timing, 19 degrees of mechanical advance timing, for a total mechanical timing of 30 degrees. In addition, at the steady 2,900 rpm it will have an additional 14 degrees of vacuum timing, so the combined timing at that constant rpm would be 44 degrees. The instant the throttle is opened slightly, the vacuum timing will begin to go away, and at full throttle at or above 3,000 rpm, the timing immediately reverts to the mechanical total of 30 degrees."

Karl

[GoatDr]

I think you need to find your initial timing and then your total timing without vacuum first. Without the vac. attached, rev the engine to 3000 rpm and see what you total timing is. This is your actual total timing, initial + mechanical. Vacuum is just added in on top of that at varying amount depending on vacuum. Pontiacs seem to like from 32-35 degrees of total timing depending on the heads, etc. The total with vacuum would then be 57-60 with 25 degrees of vacuum.

[chiphead]

I had to fabricate a limiter plate for my MSD RTR dist to limit the vacuum advance. It fits over the screws and limits the travel. Typically you want 8-14* of vacuum advance on a hot street motor.

[Tim Corcoran]

How does the car drive like that? Is there pinging or any porblems? 32 degrees at idle is not excessive, it's actually very reasonable depending on the combo. Additional timing at idle and under light load is great for fuel efficency and a smooth idle with good vacuum. As a starting point set "total timing" at 34 degrees without the vacuum advance hooked up then test drive the car. The vacuum can needs to be matched for your engine vacuum. Many factors including converter stall, camshaft specs, vehicle weight, rear gears, your altitude where you do most of your driving, your induction system, static and dynamic compression, engine operating temperature, outside air temperature all will effect how your engine will react to a specific timing curve.

[Schurkey]

Originally Posted by Erik_H

The vac advance can that they swear only adds 10 degrees actually adds 25. With everything plugged my initial is at 11, I plug in the vac hose and it jumps to 36 at idle.

Connecting the vacuum hose advances the timing. Advanced timing generally increases the engine speed.

Increased engine speed then activates the centrifugal advance...which increases the engine speed some more.

You may not have 25 degrees of vacuum advance. You might have 20 degrees + 5 of centrifugal. Or 15 + 10.

[Nicks67GTO]

I had the same deal with my Tomahawk HEI. I found that i had the distributor can hooked to manifold vacum. I put it on ported vacum and it changed a lot. Then i bought one of the advance limiting pieces from a PY member, dialed my timing back like is required using that limiter and whalla. I have 14 initial, 35 total + 15* of vacum advance. I had 25-26* prior to that.

[Erik_H]

Thanks guys, I believe I just need to fabricate a limiter. I can then experiment to see how much is too much. Somewhere between 10 and 15 sounds reasonable.