What I should have said was at 3000 rpm the vacuum gage indicates 10"hg.
    vac. At wot 4600 rpm it indicates 0"hg vac. The vacuum goes down
    linearly from 10" to 0" with the corresponding increase in throttle
    setting from 3000 rpm to 4600rpm.
    Picture of compression gage and slide throttle attached.
    A/c is an RV-7A
    Correction, it is a 2.175 to 1 psru from Real world solutions model
    RD-1/B.
    John Prendergast

    This is the wrong way of saying it. Manifold pressure is specified
    in absolute pressure terms. The word "vac" as in 0"hg vac is not used.

    So what is the pressure in terms of inches of mercury at 4600 RPM WOT?

    Paul Lamar ...No rotor no motor.


Manifold pressure is aproximately 30" hg at 4600 rpm wot.
For those who are using a vacuum gage  for example at 2000 ft altitude
and the manifold vacuum gage reads 8" hg:the usual conversion is:

30" (aproximate sea level pressure) - 2" ( 1" per 1000' altitude) - 8"
(vacuum reading) = 20" hg manifold pressure

The  0" vac read at  4600 rpm wot sea level is 30" - 0" = 30" hg pressure
The 10" vac read at 3000 rpm sea level is 30" -10" = 20 "hg pressure
Sorry I don't have a pressure gage for a direct reading.

John Prendergast

This is essential if you are doing anything with a p-port. Direct read out
in inches of Hg of your manifold pressure. Apparently it is also essential that you build
a vacuum reservoir can big enough so the pressure pulses are averaged out.

I have no idea what John's p-port engine is doing. Too much conflicting information.
If the vacuum reservoir can was large enough the vacuum reading WOULD NOT pulsate.
I have seen and heard many p-port engines that run fine at idle or above.
Many were built in the 1980's and 1990's with slide throttles. Some with
electronic fuel injection. Notably the Lemans winning four rotor engine
had slide throttle and EFI. Mazda had the car at Seven Stock a year ago or so
and I heard and saw the engine start, idle and run. Every year at Seven Stock
there are several p-port engine that start, idle and run fine.

Un fortunately this is a bad picture but we had no trouble reading the manifold
pressure gage on the Stuska dyno when we had the p-port engine running with
Tracy EFI system.

Paul Lamar ...No rotor no motor.

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