Restrictor in the manifold pressure line?

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.

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

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.

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.

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

yakdriver said:

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

Click to expand...

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

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

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.

What, where, and how?

elippse said:

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.

Click to expand...

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

Thanks!

N941WR said:

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

Click to expand...

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/tonyboytoo/RV3BMiscellaneous/photo#5144540091900150882

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

Thanks for all the replies.

I fully understand what the restrictor does, and if I had steam gauges I fully understand why it is necessary, but with P-mags and the ACS I doubt it is, since it is trivially easy to smooth in the software which lies between the instantaneous reading, and what is represented on the screen. I would have thought Dynon would do this but someone said 'no'. Thanks.

Anyway, that aside I realised I did in fact have one of the VANS oil/fuel restrictors left over, and since I had to dissassemble the manifold that all the hoses arrive at, on which the transducers are mounted, I put it into the end of the line from the engine manifold. The reason I did this is because I am concerned just how lean the engine would run on that cylinder if something ruptured, so it is the engine I consider I am protecting.

N941WR asked where to install it. If you want to take a look at the third picture down (double click it to enlarge), it is at the end of the black hose coming from the primer port on #3. (In fact if you have the VANS manifold pressure kit I think they provided a part with a restrictor.) In the picture the nearest position on the manifold is the oil pressure, the middle the fuel pressure and the furthest the engine manifold pressure, so you cant actually see the relevent connector. But it is the same as the nearest, just a 45 deg black steel connector with a restrictor but otherwise like the blue one in the foreground. This is a -4 so the transducers are in an unusual place. yours are/will be probably on the firewall.

How to make one I will leave to others who have done it. I happened to have one.

Thanks for all the inpus. Cheers!

Steve Sampson said:

...but with P-mags and the ACS I doubt it is, since it is trivially easy to smooth in the software which lies between the instantaneous reading, and what is represented on the screen. I would have thought Dynon would do this but someone said 'no'. Thanks.

Click to expand...

Steve,

The P-mags is the reason I want to put a restrictor in my MP line. I've been watching that MP fluctuate for 125 hours now and it occurred to me that the P-mags are digital and depending on their sample rate and IF they smooth the MP internally or not, might impact my timing.

Thus my desire to put a restrictor in the line. Just now I center drilled a AN470AD4-6 rivet with a #56 (.046") drill as described by Tony Bingelis on page 40 in his book ENGINES and will install it later today.

OK I'm back from the airport. The drilled rivet trick worked great!

The MP is now stable enough to read. Score another one for Tony B.