Where do you hook to your '72 350 stock engine to check how much vacuum you have? Thanks.

 

Old photo. Just below the vacuum break.

 

Any manifold (has vacuum at idle) port except the one for the vacuum advance. You should be able to feel it with your finger.

 

Why?

 

You will want your vacuum advance hooked up when determining how much vacuum the engine makes. Assuming it’s adding timing at idle.

 

Doesn't make a difference. Your checking for engine vacuum, not timing. The distributor vacuum advance is adding timing to the engine, not vacuum.

 

I am quite certain that my L48 makes more vacuum at idle with the extra 12 degrees of vacuum advance, than it does with the vacuum can disconnected.

The point is that if you are measuring something, don't change let your measurement affect the thing you are measuring.

I bet the OP doesn't need the brakes while testing vacuum at idle, though, so that big T on the intake manifold or the port at the base of the carb can be used, too.

 

Tell that to quantum physicists

 

I asked Grok to diagram an appropriate experiment. It did not disappoint.

 

ahhh, ignition timing affects vacuum. It makes a difference.

 

No where in OP original post did he mention anything about ignition timing. So, it does not make any difference.

 

I could go measure the engine vacuum of my car right now. Plug it into any port (manifold, ported, pull the vac advance can out, whatever). It would read 0 inHg, because the engine isn't running.

The OP didn't mention that the engine would be running during his test, so I guess that doesn't make any difference either.



In all seriousness, if the vac can's manifold vacuum port is the ONLY port available (and it surely is not), then put a Tee in it. That way you can still test both with and without vacuum advance. I assure you there is a difference. A Tee is the vacuum advance line is bad practice for a running engine, but for a test it's fine.

 

Kansas123

What are you trying to assess? Power brakes / automatic transmission or engine timing?

PB/AT uses manifold. I expect your 1972 distributor is using a ported signal.

Manifold vacuum signal is from a source BELOW your carburetor's throttle plate. That low pressure is created by a running engine's pistons working against a partially closed throttle. Maximum vacuum is going to be at idle. As throttle opens, vacuum decreases and ignition advance from your distributor's vacuum can also decreases. Practical application is the "2 inch rule" where you want your vacuum can to continue to provide ignition advance as throttle opens up (engine rpm increases) and ignition advance transitions to mechanical advance. .

Ported vacuum signal is from a source ABOVE your carburetor's throttle plate. That low pressure is created by air rushing through your carburetor. Much like vacuum source for a siphon paint spray gun or a nozzle on a sand blaster. Maximum signal is going to be at high engine rpm as more air rush past that port.

So, that test for inches of mercury when the vacuum can "begins to move" can indicate two different things. Manifold "begins to move" indicates point where vacuum can STOPS adding ignition advance. Ported "begins to move" indicates point where vacuum can STARTS to add ignition advance.

This OEM transition from manifold to ported, I believe, was to address emissions by increasing engine temperatures at idle/low rpm. Seemingly it worked too well, as we have this hodge podge of sensors that work to provide signal to reduce engine temperature if they get too high.

Problem is, plug and play with distributor vacuum signal sources. Plug your original distributor ported source into a manifold source and you'll get more advance at idle/low engine rpm - but maybe too much. You've also removed high speed ignition advance provided by that vacuum can. Hence you ought consider increasing mechanical advance for performance at higher engine rpm.

Mapman

If your question was about power brakes - disregard the above!

 

To assess total vacuum with engine runing. To increase vacuum to maximum by adjusting idle mixing screws. (Sorry, I've been away sick)