Vacuum level in over-run

[JoeMag]

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]

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

[JoeMag]

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

[jpnovak]

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]

Originally Posted by JoeMag

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