Vacuum

[steve from staten island]

How much vacuum should a healthy stock 383 make? How much does timing affect vacuum and why? Thanks

 

[andyf]

The 512 in my Coronet has about 15 inches of vacuum at 850 rpm idle. That is with a 246 hyd roller cam. I run 25 degrees of advance at idle which really improves the idle quality and the manifold vacuum.

 

[PurpleBeeper]

Hmmmm…… I bet a stock motor would be near 20" Hg vacuum....just a guess. As for the timing, again another guess, as the motor reaches its optimum timing (& air/fuel mixture too), the pistons will move "regularly & even" with the carb nearly closed, creating the highest vacuum....again, a guess. Good question...anybody else?

 

[mopar 3 B]

Any where between 18 & 21 at idle.

 

[WileERobby]

What cam is in it, is it installed straight up, advanced, or retarded ?

 

[steve from staten island]

What cam is in it, is it installed straight up, advanced, or retarded ?

Stock cam for a 383, 2 barrel engine. Timing is slightly advanced

 

[moparedtn]

If it's truly stock, I'd want at least 15Hg at idle but I agree with mopar 3B - 18 is more like it.
Yes, advancing timing will sometimes achieve more vacuum, but that's not where you want to start when diagnosing
low vacuum at idle - it's more of a fine tuning method on a known good engine.

 

[Ranger16]

If I remember correctly, mine is running about 18.

 

[steve from staten island]

What exactly makes vacuum low.

 

[Bicylindrico]

My 242/247 @ .050" camshaft idle vacuum is about 10" @ 1000 RPM. I am running a slant six adv canister to pull in 10 degrees timing at that idle vacuum.

The engine will run much more efficient with a functional vacuum advance allowing less air/fuel for the same RPM. Idle vacuum is more dependent on valve timing/reversion more so than ignition timing but increased idle speed will increase vacuum somewhat.

 

[beanhead]

What exactly makes vacuum low.

Not having a good seal on the cylinder during the intake stroke. In other words when that piston is moving down, and the intake valve is open, the only place it should be able to draw in air is through the carburetor. So, low vacuum can be caused by something like the exhaust valve being open longer (bigger cam/overlap), or a leaky valve/intake manifold/carb base.

 

[WileERobby]

Stock cam for a 383, 2 barrel engine. Timing is slightly advanced

I meant cam Timing, not ignition. Nevertheless, you should easily pull 18

 

[Ranger16]

What exactly makes vacuum low.

I think a large cam overlap will cause low vacuum as well.