How to pick up manifold pressure for the P-Mag

[Tomas J]

I'm installing dual P-mag's in my RV-4 and thinking of the best way to get the manifold pressure. I'm thinking of installing a DISCHARGE NIPPLE AN4022-1 in cylinder #3 or #4 primer port, and use a AN FITTINGS AN824-2D TEE to split the pressure for both ignitions. Or is it better to give the ignitions manifold pressure from separate cylinders, use both #3 and #4 and skip the tee?

But what about the fluctuates in pressure? Will they affect the ignitions? Can the discharge decrease that effect? Is there an alternative and better place to get manifold pressure on a Lycoming 320 with Marvel carburator?

[TS Flightlines]

Tomas----we pull MAP from cylinder 3, route the hose (generally a -3 hose) to a dual tee for the PMags and the MAP sensor. -3 hose because the stem ID is .079 and 'generally" doesnt need a restrictor fitting for the EFIS MAP readings.
Flying on alot of different experimentals.

Tom

[Doug Rohrer]

I have an O-320 on my -9A. I used a -3 line off #4 cylinder into a precision needle valve screwed into the standard Van's manifold block on the firewall. The output from the block goes to a brass tee fitting to split the tubing going to the dual Pmags. The purpose of the needle valve is to provide a variable orifice to smooth out the pulses going to the Pmags. Before installing it on the manifold I blew through the valve and set it to only let a small amount of air through. Not very scientific, but I was concerned taking the vacuum signal right behind one intake valve would send strong pulses to the Pmag which would confuse it. I wish Lycoming provided a vacuum port on the intake manifold that would automatically combine and average intake pulses. I wish I had machined one in when I built the engine...

[calpilot]

pUT A "t" IN THE LINE FROM YOUR MANIFOLD PRESSURE GAUGE OR TRANSDUCER, THEN TO BOTH P MAGS. GIVING THEM THEIR OWN MP FROM SEPARATE CYLINDERS HAS CAUSED ISSUES WITH TIMING FLUCTUATION. rEGARDS,

dar GARY

[Scott Hersha]

Tomas,
Like Tom, I had a fitting off cylinder 3 with #3 braided line (TS Flightlines), going to a T fitting on the firewall, which supplied the MAP line, and the dual Pmags. My fitting on the #3 cylinder was a steel restrictor fitting, although I don’t know if that was necessary. I used small aluminum lines with flared fittings for much of this (I think it was 1/4”, but not sure), but when it got near the Pmags, I transitioned to the silicone tube supplied with the Pmag kit. I used a nylon barbed fitting with a clamp to split the pressure source on that silicone line to the two Pmags.
It worked well for my with no noticeable fluctuations.

[Scott Hersha]

Here’s a pic of my install. Braided line to an AN fitting/aluminum line - to a T (blue on the firewall manifold), feeding the MAP sense probe, and also to the Pmags. The nylon T in the silicone line to the two Pmags isn’t visible.

[pa38112]

T.S. Flightlines makes a hose for this - very nice profesional looking install...

[gasman]

A simple brass fitting to replace the manifold plug (pipe thread) with 1/8" barb, and use 1/8" vacuum line from auto parts. The "T" can be as simple as a "T" from the irrigation section of ACE hardware or any hardware store.

CHEAP.....SIMPLE.....AND LIGHT.

[rv8ch]

I used one of these:

https://www.ebay.com/itm/1PC-Air-Tee...-/251886532058

http://www.rv8.ch/manifold-pressure/

[TS Flightlines]

Mickey and others, using the brass tee is fine if you use a 306 rubber hose from the cylinder. It doesnt work with teflon, 1 because of the OD/ID differences, and 2 because of the stiffness of the teflon liner, it doesnt seal well with barbs. Rubber does give, expands and then contracts and seals the gap in the barb between the root and the peak. Teflon, being stiff, doesnt collapse to fill the void.

Since most of our clients doing this install are using teflon, we came up with this dual tee for 2 PMags/Sureflys. Can be done alot of different ways, but the object lessen is to NOT have a MP leak which creates other issues.

Tom