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-   -   Vacuum level in over-run (http://forums.pelicanparts.com/911-engine-rebuilding-forum/1056994-vacuum-level-over-run.html)

JoeMag 04-05-2020 05:40 AM

Vacuum level in over-run
 
Anyone happen to know rough vacuum level our motors pull in over-run (ie: higher rpm with foot off gas, like brake zone).

I have vacuum sensor on intake manifold but looks like i put lower limit as "zero" in AIM setup versus negative number.

Dr J 04-05-2020 12:07 PM

I get about 0.15 bar minimum which is about 25 in of mercury vacuum

JoeMag 04-05-2020 02:57 PM

Thanks... I assume you mean 0.15 bar absolute (-0.85 bar gauge)?

jpnovak 04-05-2020 06:14 PM

What cams? The more overlap you have the higher pressure (less vacuum) your engine will pull. the number below will depend on intake plenum volume under throttle, idle speed (throttle closure and bypass), temperature, base atmospheric pressure (weather related) and altitude of use.

From tuning a lot of EFI systems I can offer the following anecdotal numbers based primarily with ITB setups.

A 98-102 type cam will usually just barely break 40kPa ( think high-30s).
A 108 deg cam will be in the low 30s to high 20 kPa range.
A 112-114 deg cam will pull in the 15-18kPa Range.

You can convert to psia, psig, torr, mtorr, mbar, ATM or bar from here depending on what units you plan to use. :)

Dr J 04-06-2020 11:38 AM

Quote:

Originally Posted by JoeMag (Post 10812461)
Thanks... I assume you mean 0.15 bar absolute (-0.85 bar gauge)?

Exactly, absolute pressure. That is how I calculated the value in Hg 0.85 x 30 = 25.5


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