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Manifold Pressure line: Orifice restrictor

S

Stockmanreef

Well Known Member
Today I was attempting to install the right aft baffle on my IO390 and the fitting for the manifold port I had in place was in the way of the baffle. It was in the way because it is a 45 degree angle fitting. So I pulled off the fitting, and put in a regular AN816-4D fitting. I then looked more closely at the first fitting and it was a VA-128 fitting, which is outfitted with an orifice restrictor--they are $30 from Vans.

So my question(s):

Should i just drill a small hole in an AN470 rivet and put that into the AN816-aD fitting then put the hose on? This has been suggested here on the forums. If so, what size hole?

Or put back in the VA-128 fitting and make sure that is is tightened down to the correct angle so that it is not in the way of the baffle.

thanks
 
The fitting will need that orifice restrictor, so if you can use the original one which I did, will work great otherwise you will need to either buy one or make one as you prescribed. Without that fitting your manifold pressure will be bouncing and would not show a steady reading.
 
On the other hand, the restrictor makes the indicator lag the throttle position. tremendously ? I think. At some point, I?m going to try it without the restrictor.
 
A relevant issue

I have not bothered Ross at SDS with this question, so here it is. I tapped my AFP FM150 injector servo above the servo throttle plate ,with agreement from Don, to get a very stable MAP. I put a tiny orifice in the fitting like .030 or so to protect from hose failure. However, an EI with advance / RETARD function might be slow to retard with a WOT command for a go-around style input.

So the question is , does anyone KNOW what a minimum orifice diameter should be ?
 
Another good place to pick off manifold pressure (for horiz induction engines) is by drilling/tapping a 1/8" NPT hole in the back wall of the intake plenum. The casting is plenty thick there. Remove one of the rear intake tubes (cyl 3 or 4) so that you can get all the chips out. No restrictor needed if you measure plenum pressure directly. My -6 has had this setup for 13 years now.

Also if you do like Larry DeCamp did by tapping the injector servo, you wouldn't need any restrictor there either. Just that the plumbing to the sensor is longer from there than the back of the plenum.
And if the hose ever failed, all that would happen with -2 hose size is you'd get high idle speed. That would immediately tell you there's a leak in that hose, if the erroneous MP reading didn't get your attention first.
 
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The purpose of the restrictor is more than just for a hose failure.

Some EFIS systems require the installation of one. I think it is G3X (and probably others) that will have bouncy indications if there is not one installed to act as a snubber.

There is no noticeable indication hysteresis because air has a relatively low viscosity and there is never any appreciable flow through the restriction.
 
Depending on how all of your plumbing is done you might be able to buy a NPT rescrictor fitting from McMaster. That is what I'm planning on using. Mine will be further upstream but I think that is fine from my understanding. Then you can use a standard fitting at the engine.
 
Ed_Wischmeyer said:
On the other hand, the restrictor makes the indicator lag the throttle position. tremendously — I think. At some point, I’m going to try it without the restrictor.
Click to expand...

I have an .040" restrictor with a port on the servo on my 6. Very responsive with 0 lag. I have an .004" restrictor off the #5 cyl on my 10. Also very responsive albiet a tad slower than the 6, with no appreciable lag. The lag you experience could also be your EMS. I have found the G3X to provide WAY too much smoothing for my taste and wouldn't be surprised to find some lag built into the code, knowing that the cyl intake port feed method is VERY bouncy without a restrictor.

Larry
 
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Stockmanreef said:
...
So my question(s):

Should i just drill a small hole in an AN470 rivet and put that into the AN816-aD fitting then put the hose on? This has been suggested here on the forums. If so, what size hole?

...

thanks
Click to expand...

What's the smallest drill bit you have?
 
Larry DeCamp said:
I have not bothered Ross at SDS with this question, so here it is. I tapped my AFP FM150 injector servo above the servo throttle plate ,with agreement from Don, to get a very stable MAP. I put a tiny orifice in the fitting like .030 or so to protect from hose failure. However, an EI with advance / RETARD function might be slow to retard with a WOT command for a go-around style input.

So the question is , does anyone KNOW what a minimum orifice diameter should be ?
Click to expand...

You mean if your hose fails at the very moment you're putting WOT in? I guess that would be worse case, but if you're landing you're likely very low power, down below any MAP advance, possibly even below the 25 baseline on the RPM scale. The hose falling off would be like having WOT always, which you'll probably be looking at 0 MAP advance unless you're at altitude or a higher airport, in which case you wouldn't be in the detonation area anyways.
IMO, the MAP hose failing for SDS fails safe.

lr172 said:
I have an .040" restrictor with a port on the servo on my 6. Very responsive with 0 lag. I have an .004" restrictor off the #5 cyl on my 10. Also very responsive albiet a tad slower than the 6, with no appreciable lag. The lag you experience could also be your EMS. I have found the G3X to provide WAY too much smoothing for my taste and wouldn't be surprised to find some lag built into the code, knowing that the cyl intake port feed method is VERY bouncy without a restrictor.

Larry
Click to expand...

I had that .004" restrictor from mcmaster-carr on the #5. The MAP gauge lag was tremendous, IMO. Took it out and tried nothing and it works fine. No pulsing on my GRT.
 
Needle Valve?

I am trying something different on my -9A. I installed a brass needle valve instead of a restriction fitting in the manifold line at the firewall junction block. The engine has not been started yet, so I can't say for sure it works. The plan is to adjust the valve as needed to reduce the pulsations in the line to something that works well with the Pmags. I am starting with the valve just barely cracked open to simulate a very small orifice. Has anyone tried this?
 
Interesting idea. I think that I will stick with the rivet approach. I might try squeezing the rivet a bit to make it fit better in the hole.

Hopefully I can drill through after squeezing it.
 
I think I'd just try again with the original fitting, and see if I could get it clocked around so that it's out of the way.

What's the maximum angle of rotation allowed on an NPT fitting? Isn't like 3/4 a turn or something ridiculously high like that? Seems hard to believe you couldn't get enough of an additional turn to get it out of the way of the baffle.
 
Restrictor

Sorta related and some of us have steam gauges. Some manifold pressure gauges on certified planes have a adjustable orifice in the instrument itself. Access thru the fitting the vacuum hose connects to and adjusted with a small flat blade screwdriver. Done this a couple times to cure bouncing needle.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Stockmanreef said:
Today I was attempting to install the right aft baffle on my IO390 and the fitting for the manifold port I had in place was in the way of the baffle. It was in the way because it is a 45 degree angle fitting. So I pulled off the fitting, and put in a regular AN816-4D fitting. I then looked more closely at the first fitting and it was a VA-128 fitting, which is outfitted with an orifice restrictor--they are $30 from Vans.

So my question(s):

Should i just drill a small hole in an AN470 rivet and put that into the AN816-aD fitting then put the hose on? This has been suggested here on the forums. If so, what size hole?
Click to expand...
Yep. I had a mechanical MAP gauge in my first RV-4. I was getting oil and fuel residue on the inside of the gauge glass. I re-routed the line with a service dip and put a restriction in (drilled rivet). You want a smallest hole you can drill straight (0.040 or #60 or less should be doable). You are not flowing air only pressure. Also have the line go down to a low spot then go back up to the sender or fire wall penetration.

I had to get the mechanical MAP gauge full of oil/fuel stuff cleaned. It worked fine after. What gets in there is some fuel and oil vapors. If you have a wide open line where sender/gauge is lower it will get gunk in it over time. My MAP was low in a sub panel.
 
I currently have a 0.008? restrictor that I bought from Mcmaster. Is the group thinking that I should drill that out to 0.040?? It would be relatively easy to do at the moment. My plan is to use the G3x system if that provides a relavent data point.
 
MAP hose failure? If you use a quality product, route is correctly, secure it
it will probably out last the airframe.

As for orifices, the normal restrictor is .040. WE we told that the EFIS has a page that allows you to dampen the pulses for the pressure readings. True of not? dont know. BUT----WE've made the MAP hoses from both -3 and -4 hoses and fittings. The -3 fitting stems have an ID of .079 after crimp, and it doesnt seem to be an issue. The Vans OE installs use a -4 hose, and some use the restrictor fitting, and some dont. So its hard to say how YOUR install will react. It WOULD be great if the EFIS companies would put a dampening thingy for the pressure pulses---if they dont have one now. Would make like easier for everyone.

Tom
 
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