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

AX-O

AX-O

Well Known Member
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?
 
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
 
Anywhere in the intake system after the butterfly and before the intake valves should give you the same reading.
 
A little OT

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! :eek: 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?
 
I had a WESTACH do the same thing in my Piper Warrior. I finally removed it and tossed it....
 
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 [email protected], 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
 
Intake tube

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

Peter
 
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.
 
PCHunt said:
Not trying to hijack this thread, but I have an interesting issue with my MP gauge...
Click to expand...

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.
 
Axel and the other gent that asked for the file:

Having a little connectivity issue from my location...posting from my iPhone. As soon as I get the computer back online, will send it to you. Apologies for the delay.

Cheers,
Bob
 
I would think this is possible with FI, but I really don't know. My engine originally had a vacuum pump--and back up vacuum from the manifold. I assume the procedure was, if the vac pump failed, throttle back to where MAP was something over 4 psi less than ambient and switch sources. The pickup was an approx 3/8" thick plate just above the carb with a hose fitting. I machined a reducer to allow about a .020" hole and used that as my MAP pickup point. I suppose those plates are available and are a bolt-on item. No drilling/tapping needed.

Bob Kelly
 
AX-O said:
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?
Click to expand...

Not quite. It takes time for the pressure to equalize, subject to mechanical constraints. Things move from areas of high pressure to low. Its better to imagine the pressure seeking to equalize. The cylinder must have a lower pressure than the manifold in order for it to draw fuel/air in.

Implied in you question is whether the two sources will have the same reading. Probably not, but it does not matter unless you have a device other than your brain using that info.

I had no idea there was a class of manifold gauge that tied into the primer port. Eeeeewwwww. That's what I get for being a renter.
 
PCHunt said:
Not trying to hijack this thread, but I have an interesting issue with my MP gauge: ...

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

Possibly the aneroid (absolute pressure reference) is stuffed or it was built as a pseudo Manifold Absolute Pressure (MAP) gauge, measuring gauge pressure and interpreting it as MAP.
 
Toobuilder said:
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.
Click to expand...
TOOBUILDER, when you say at the back of the instrument, are you referring to the westach mentioned above? I just installed a westach because my MP sensor was acting weird and reading 5” lower than what it should. But the needle just bounces soo much is almost not readable.
 
I’ve installed restrictors in the MAP line to reduce bouncing MAP readings.

Axel- Bill Beaton’s cold air intake had a couple plugged ports at the rear of the plenum that we used for MAP & we drilled another port just behind the servo (for water methanol injection). If you find a spot on your plenum that looks to be at least 1/4” thick, should be ok to drill for a MAP port.
 
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