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 Vacuum/MAP gauge readings for tuning? 
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Joined: Sun May 12, 2013 5:21 am
Posts: 332
Location: Michigan
I know some of them but figured it'd make a good discussion.

What do you look for to tune your cars with a vac gauge? I'm interested in the driving readings more myself but always interest in other views.

Sometimes you ask the wrong question, but it still leads to the right answer.

Wed Jul 27, 2016 7:46 pm

Joined: Thu Mar 28, 2013 6:09 am
Posts: 728
Good Idea :thumbr:
Is there any difference with street driving with an auto trans vs a standard trans??

Also to take it a step farther same idea but with a drag application.

Fri Jul 29, 2016 8:16 am
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
The more you use a gauge the better you will understand what you see. With hot rod engines having such variety of displacement, valve timing and compression ratio, as well as all the other variables, there is no specific "do this" or "do that" about intake vacuum. You will just need to learn by doing. In general, more radical cams will have less vacuum at idle and low RPM part-throttle. Carburetor restriction vs. HP (air flow) will be evident in the WOT vacuum.

In general, at idle and part throttle, the higher vacuum, which is less manifold pressure, you see at the same power output at the same RPM, idle RPM or cruise on the same stretch of level road at the same RPM, the more efficient the engine is converting the air (less pressure, remember) to power. At low and intermediate load, a change of A/F or ignition timing that provides the same power with less manifold pressure (more vacuum) is almost always the most desirable.

Sat Jul 30, 2016 1:22 am

Joined: Sun Jun 23, 2013 1:41 pm
Posts: 375
Location: Phila. Penn.
Wow, just saw this. But looking at the date I know why I missed it. :)

My car came with a factory vacuum gage :D and I do like having it. When there's opportunity on the interstates I glance at where it is on what seems to be slight uphill, and flatter sections where I can back off the throttle a wee bit with no loss in speed. I try to do the same on the local highways but a) that's not usually possible b) as far as the carb goes, at 60+ I'm more confident that when under load its on the main circuit.

A couple other things I've done specifically related to tuning is watch the vac (or better glance at it and log the MAP) when slowly accelerating from a stop. And the other thing is to use it for figuring out when there are flat spots during part throttle accelerations. Squeeze the throttle 'til 14" and it goes good, do the same at 12" and so forth. One reason I started messing with the datalogger and got on the Innovate forum was a dead spot that had developed around 10".

Thu Mar 16, 2017 9:05 am
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