540 NA MEFI 3 Intake Vacuum Question
 

I have 540 stroker motors,500 EFI intake & fuel systems, MEFI 3, Crane 761 cams. less than 100 hrs. Vacuum at idle is 11-12 (800' elevation). Fuel pressure with the regulator vacuum hose attached is around 38psi, around 43 with it removed. When I increase RPMs to 2000+ (in neutral) the vacuum goes to around 18, fuel pressure with hose attached is high 30's. Question: I thought that vacuum is highest at idle because the throttle body is closed and vacuum drops as it opens. Therefore the purpose of the regulator vacuum hose is to lower pressure at idle and let it increase at higher RPMs. If that's incorrect and vacuum increases at speed, what's the purpose of the regulator vacuum line?

You have it mostly correct.

More load = less vacuum = higher fuel psi.

Where you are off is checking it in neutral. Many motors will increase vacuum with more throttle in neutral. Why ? No load which means very little throttle opening for the rpm.

Example: Very little throttle needed to reach 5000rpm in neutral. A lot of throttle in gear, onthe water to get to 5000rpm.

Thus the higher vac when not under a load. Throttle is closed a ton more than when under a load.

Got it, thanks.

SB nailed it perfectly.

And just to amplify his thoughts in my early MEFI tuning days I had an engine on a stand working the idle parameters, AFR and such. I would raise the rpm a little to do some other tuning but the MAP went from 55 kPa at idle to less than 40 kPa as the rpm increased (higher vacuum). I quickly realized without a load on the motor my off idle tuning would be limited.

Bottom line is your situation is normal.

Good information, thanks again-