FINALLY - Vacuum Advance Dilemma Solved

[DanEC]

Originally Posted by jwd

WRONG. I suggest you re-read the article you posted by John Hinckley and pay attention to the third paragraph.

It shouldn't pose any problems at idle but where you do have to be careful is in a highway cruise situation with a fast centrifugal advance curve. Cruising at 2500 rpm with 36 to 38 deg inital and centrifugal advance all in, and 18 to 20 deg vacuum advance can cause some motors to develop a miss or repetitive light stumble from basically firing too early before TDC. My old, aluminum head BB Chevy developed this problem and I had to modify the vacuum can to limit total vacuum advance to about 10 eg which fixed the problem.

[jwd]

Originally Posted by DanEC

It shouldn't pose any problems at idle but where you do have to be careful is in a highway cruise situation with a fast centrifugal advance curve. Cruising at 2500 rpm with 36 to 38 deg inital and centrifugal advance all in, and 18 to 20 deg vacuum advance can cause some motors to develop a miss or repetitive light stumble from basically firing too early before TDC. My old, aluminum head BB Chevy developed this problem and I had to modify the vacuum can to limit total vacuum advance to about 10 eg which fixed the problem.

I hope you meant 6-8 initial. The only problem I've ever seen from having too much advance at cruise speed is a surging situation. Initial timing, mechanical curve/total timing and the characteristics of the vacuum can (" of vacuum that it starts and at full and the total amount of advance) all have to work together. After you've tuned a few hundred, you get the hang of it.

Jim

[DanEC]

Originally Posted by jwd

I hope you meant 6-8 initial. The only problem I've ever seen from having too much advance at cruise speed is a surging situation. Initial timing, mechanical curve/total timing and the characteristics of the vacuum can (" of vacuum that it starts and at full and the total amount of advance) all have to work together. After you've tuned a few hundred, you get the hang of it.

Jim

No - total 36 to 38 initial and centrifugal advance - please read again. That's what my old 427 Chev runs when all the advance is in. Keith Craft ran my stroked 428 up to 40 degrees total advance (initial and centrifugal) on the dyno. That surging sensation is firing the ignition too far in advace of TDC.

[jwd]

Originally Posted by DanEC

No - total 36 to 38 initial and centrifugal advance - please read again. That's what my old 427 Chev runs when all the advance is in. Keith Craft ran my stroked 428 up to 40 degrees total advance (initial and centrifugal) on the dyno. That surging sensation is firing the ignition too far in advace of TDC.

Okay, I was reading it as 36-38 initial plus centrifugal advance. Now it makes sense. I know what causes the surging. When I tune a car and encounter it, I change the vacuum can to one that had a little less total advance. Usually only a reduction of a few degrees is needed.

Jim

[DanEC]

Originally Posted by jwd

Okay, I was reading it as 36-38 initial plus centrifugal advance. Now it makes sense. I know what causes the surging. When I tune a car and encounter it, I change the vacuum can to one that had a little less total advance. Usually only a reduction of a few degrees is needed.

Jim

Oh - yeah, I can see now how it could be read that way. I understand.

My BB Chev runs low vacuum due to the big, old-school cam and I had to purchase a special vacuum canister that starts to pull in at 6 inches vacuum. There might be one out there like it with less total vacuum advance but it was easier to make a little bracket that mounted over the arm and had a tab that stops the rod from advancing the entire way. That way I reduced the advance that was kicking in while cruising on the road and stopped the surging. Cruising down the road that surging/missing can be really annoying even if you understand what is going on.

[RET_COP]

I agree with mreid. The mechanical secondaries with a light stick car and good power is always better. The rpm's rise quicker and the car action is very responsive up and down the rpm range. I went from a VS to MS in my 500hp SBF and the difference was very noticable. I like the control I have. The VS no doubt was smooth as silk but it was not as fast and responsive under my foot. This is my experience anyway. Maybe that is why detroit used them from the showroom floor.
If I'm not mistaken (sorry to bring this up on this forum but...) the L-88 vette came with a Mechanical Sec. 850 Holley. This was not for the average joe driver.
Lou

[jwd]

Originally Posted by RET_COP

If I'm not mistaken (sorry to bring this up on this forum but...) the L-88 vette came with a Mechanical Sec. 850 Holley. This was not for the average joe driver.
Lou

Yes it was an 850 CFM Holley but it had vacuum secondaries. Maybe the engineers at GM should have asked mreid what to use since their years of racing experience and dyno. testing must have been wrong.
Like I posted earlier, NO manufacturer used mechanical secondary carbs. on a production car, even limited production ones destined for the track. Not Ford, GM, Chrysler or even Shelby. There is a reason for that.
Jim