Ported or Manifold vacuum

40grit

40grit

Member
As you guys can see by my join date I do more lurking than talking. I modified a Chevy distributor 6 or 7 years ago and have always run ported vacuum from 390 Holly. The engine is a fairly well built '51 Merc with Crower Cam, 1.6 in chevy valves, heads, headers etc. Is this correct or should I be running manifold vacuum? I have tried to research the issue but find both opinions. I see Bubba recommends no vacuum and do not doubt his opinion but since mine is set up for vacuum I would like to be sure it is set up for best performance. Thanks for putting up with the newby :confused:

John L.
 
40grit said:
...mine is set up for vacuum I would like to be sure it is set up for best performance.
Click to expand...
You want to use straight manifold vacuum.

Set your maximum mechanical advance at 20°, then adjust your vacuum canister (assuming it it adjustable) to add 8° in high-vacuum regimes like cruise and idle.

You'll cruise at 28° which will make your engine happy, but the instant you go to WOT (Wide Open Throttle) the vacuum drops out and you're back to 20°, which is ideal for WOT.

The higher advance at idle will let your engine idle cooler, too.

Works very well that way.
 
Richard in Florida said:
You want to use straight manifold vacuum.

Set your maximum mechanical advance at 20°, then adjust your vacuum canister (assuming it it adjustable) to add 8° in high-vacuum regimes like cruise and idle.

You'll cruise at 28° which will make your engine happy, but the instant you go to WOT (Wide Open Throttle) the vacuum drops out and you're back to 20°, which is ideal for WOT.

The higher advance at idle will let your engine idle cooler, too.

Works very well that way.
Click to expand...

Thanks Richard. The engine has always run cool though it does run warmer since I installed the hood. If I drive it to Nashville I try to hit Monteagle Mountain before the heat of the day. It is about a 4 mile grade at 6%. The engine will walk it in 5th gear as long as I don't let the rpm's drop. That is doubly surprising since it has a 3.25 rear end with the 5 speed.

I am driving to the Shades of the Past this weekend and will change it to manifold vacuum. I have total advance set between 26 and 28 degrees already.
 
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