Restrictor in the manifold pressure line?

[Steve Sampson]

I have two systems that will want to read manifold pressure, a carburated O-320.

The systems are a) an AFS3400 EFIS and b) P-mags. Nowhere in the installation manual for either does it say to put a restrictor between them and the manifold.

I feel vaguely uneasy about this. Can anyone see a reason why there should be? If a leak develops will it do more to the engine than cause it to run lean on two cylinders?

Thanks.

[yakdriver]

If you don't put a restrictor in the line you will get erratic readings on the guage. Don

[Steve Sampson]

Don, thanks for a quick reply, but why a) would the software not take care of that, and b) AFS not ask for a restrictor? Thanks for your interest.

[Norman CYYJ]

One of the reasons for putting as restrictor in the line is to prevent rapid oscillations in the meter readings. These are caused by the intake valves opening and closing on the intake stroke. And of course to prevent running lean if the line should become disconnected or break. Same goes for oil and fuel pressure lines, prevents a large immediate loss of fuel or oil and smooths out the readings.

[yakdriver]

Steve, My Dynon didn't mention putting in a restrictor but without the restrictor I got some real wild fluctuations. Put a restrictor fitting in and it works perfect now. Don

[Jamie]

Originally Posted by yakdriver

Steve, My Dynon didn't mention putting in a restrictor but without the restrictor I got some real wild fluctuations. Put a restrictor fitting in and it works perfect now. Don

Same exact story for me! I would put one in...I just used the 45 degree steel one that Van's sells.

[scard]

Or make your own. Not hard at all. (somewhere on my build log).

[elippse]

The restrictor acts like an RC low-pass filter. The restrictor provides resistance to the flow, as with a resistor, and the volume of line beyond the filter acts as the capacitor. Typically about .01 to .02 works well. Too small and it becomes very smooth but doesn't act quickly, whereas too large and you get the pulses. Consider this: when the intake valve first opens at low MAP, you actually get flow from the cylinder, which has been exhausted to ambient pressure, back into the intake tubes which are below ambient. Then, as the piston goes down, flow again comes back into the cylinder. That's why you'll often get more MAP needle bounce at part throttle than at full throttle.

[N941WR]

Originally Posted by elippse

Typically about .01 to .02 works well. Too small and it becomes very smooth but doesn't act quickly, whereas too large and you get the pulses.

Could you post some pictures of what you used, how you did it, and where in the line you placed the restrictor?

Thanks!

[Tony Spicer]

Originally Posted by N941WR

Could you post some pictures of what you used, how you did it, and where in the line you placed the restrictor?

Bill,

On the near side of the scat hose you can see the 1/8 copper line plugged into the Dynon MP transducer.

http://picasaweb.google.com/tonyboyt...40091900150882

Solder the transducer end of the copper line shut. Use only enough solder to seal the end. Too much and the drilling gets tough. Chuck a #80 drill (.014") in a Dremel and crank up the speed. Drill one hole thru the solder, and no more MP fluctuations.

Tony