Can I get manifold pressure out of somewhere other than the cylinders?

[AX-O]

I am converting my o-320 B2B to fuel injection and adding a cold air sump. My cylinders are the old style and only have provisions for primer ports on the bottom. I will be using the primer ports for the fuel injection. My question is, can I get manifold pressure out of anywhere else other than the cylinders? I don?t want to take the cylinder off and have it tapped for the fitting. Although I am running a fix prop I would like to have the information. Also, sometime in the future I may replace my mags. I think all those high speed ignition systems require MP.

I have a buddy that is taking his MP out of his intake plenum. He told me that when the valves opens inside the engine the pressure in the plenum is the same as the cylinders. Is that true?

[jsharkey]

As the name suggests the manifold pressure is the pressure in the inlet manifold. It is an indication of how open the throttle is and therefore how much air is flowing into the engine and how much power it is generating. I assume therefore that in principal you can measure it anywhere between the throttle butterfly and the intake valve. Not sure what other effects come in to play though.

Jim Sharkey

[Mel]

Anywhere in the intake system after the butterfly and before the intake valves should give you the same reading.

[PCHunt]

Not trying to hijack this thread, but I have an interesting issue with my MP gauge: It is a direct reading gauge, with a small copper line going from the gauge to one of the cylinder primer ports. I have an O-320, carb, and FP.

On the ground the gauge reads normally: ie: low (12" or so) at idle, and ambient (29"or so) at full throttle.

But when I climb to altitude, say 8,000 feet, the gauge still reads 29" at full throttle! If I throttle back, the MP gauge begins to read lower.

PS: I looked, but there is no turbo anywhere in sight.

Being a very minor problem, I haven't tried to troubleshoot it yet, but this thread got me wondering?

[gasman]

I had a WESTACH do the same thing in my Piper Warrior. I finally removed it and tossed it....

[rvmills]

Axel,

I'm finishing up an install of a new ElectroAir EI, and the install manual has a small section for those that don't have existing provisions on their engine for a manifold pressure line (to allow hooking up a MAP sensor line for the EI...and it also discusses adding a Manifold Pressure guage a bit as well).

I have that portion of the ElectroAir Install manual in electronic format, so if you have an eye towards changing to an EI later, and want to take a look at the file so you can do this portion of the MAP line in a way that facilitates easier going later with the EI install, PM me your e-mail address, or e-mail me at rvmills@sbcglobal.net, and I'll shoot it to you.

It lists multiple solutions (one being the primer line, which I know you don't want to use, but others as well). Others include drilling/tapping the cylinder or the manifold for a 1/8" NPT fitting (I know you also don't want to pull the cylinder to drill, but it does talk about the manifold, a course of action confirmed by Mel and others above). In each case the install manual recommends tapping off at or near one of the rear cylinders to shorten the length of the line to the EI's MAP sensor, which in the case of the ElectroAir is mounted on the hot side of the FW, and for Lightspeed, I believe is on the brain box itself (at least it was on my old Plasma I...not sure about the newer II and III).

Happy to send it your way if you'd like.

Cheers,
Bob

[kiwipete]

I've seen one installation where someone welded a fitting to one of the intake tubes and used that.

Peter

[Neal@F14]

Taking the MP from the plenum probably gives you a smoother and better reading of the average of all the cylinders in the engine, rather than taking the MP from immediately right next to just one cylinder's intake port.

[Toobuilder]

Originally Posted by PCHunt

Not trying to hijack this thread, but I have an interesting issue with my MP gauge...

At the back of the instrument, inside the fitting, there will likely be an adjustment screw. This is nothing more than an adjustable bleed and it is meant to dampen out the pulses from the cylinder. Loosen it up, and the needle will "bounce" with the intake valve; close it off completely, it won't move at all. I'd look for some dirt or other junk in there. Clean it out and adjust it so the needle doesn't bounce, but will also follow the throttle reasonable quickly.

[PCHunt]

Thanks, I'll try that!