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MAP gauge with 4 port primer


Active Member
Jan 4, 2023
Good day,

My plane (Mustang 2 with Lycoming O-290-D2) has a primer line going to each cylinder. I would like to add a manifold pressure gauge (and accurate electronic tach) for testing the effects of some aerodynamic cleanup, so I can have a more accurate idea of engine power vs airspeed. However, from what I understand, the common way to do this is to cap one primer line, leaving a 3 port primer and the manifold pressure measured at one cylinder.

What I was thinking of doing is putting a tee in the output line from the primer pump, and measuring the manifold pressure/vacuum from there. Of course, I don't want the high(er) pressure fuel feeding into my MAP sensor so a valve would be added in between. The procedure would be: prime the engine, lock the primer, start the engine, then once it's running open the valve to allow the gauge to read MAP. The disadvantage would be a more complicated installation, but the advantages are:
1) allows me to prime all four cylinders, which is nice for cold weather starting
2) don't have to add another firewall penetration since the primer pump is already on the cold side
3) MAP is measured from 4 cylinders instead of one, so less need to dampen vacuum pulses

What do you guys think, is this a stupid idea? Is there some blatant issue that I'm missing?