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WOT cruise MAP fluctuations

MK77

Well Known Member
Was doing some high speed cruise testing at altitude with WOT and I noticed that the MAP would fluctuate. The MAP line has a pressure snubber installed and these are not small, cyclical oscillations, the MAP will jump from 22" to 26", then back to 22" for a bit and then down to 19", etc. Seems to only happen at WOT and high RPM (2700). When I run at say 2400 RPM, the MAP stays consistent. Anyone else ever seen this type of behavior?

Matt
 
If the fuel flow and RPM aren't moving with the MAP, it's an instrumentation issue. You won't see swings of that magnitude from the typical small vacuum oscillatoins due to the pickup on one cyl near the valve. Likely a gauge failure. You can confirm that at altitude, as MAP is limited by the atmospheric pressure. So, if you are showing 26" at 8000', your gauge is failing.

Larry
 
What kind of gauge do teh both of you have (OP and Ed, since Ed is having the same problem)? It is a bad ground or a bad gauge.

Vic
 
I had an unstable and widely variable MAP reading on my IO360 parallel valve withMAP coming off the number 3 cylinder. I put a small gas filter in the MAP line before the gauge and it acts as an accumulater to damp out presure pulses. I now have steady MAP readings
 
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I had an unstable and widely variable MAP reading on my IO360 parallel valve withMAP coming off the number 4 cylinder. I put a small gas filter in the MAP line before the gauge and it acts as an accumulater to damp out presure pulses. I now have steady MAP readings

I have a G3X touch (for about the last 2 years), and I see an oscillation in MAP when I run at 2400 rpm. I don't see any oscillation at my typical cruise rpm of 2300. I believe it is a sample rate/signal conditioning problem, combined with likely resonances going on in the pressure line from the engine to the sensor. On top of this, the instantaneous pressures in our MAP lines are most likely quite noisy (literally also!), given the intermittent nature of valves opening, shock waves, etc.

I like Brad's idea of the gas filter to dampen pulses, and I plan to try this!

Perhaps someday I'll hook my data acquisition system up directly to record with high sample rates the actual voltage coming from the MAP sensor. I've seen similar oscillations in the fuel pressure indications.
 
I have a G3X touch (for about the last 2 years), and I see an oscillation in MAP when I run at 2400 rpm. I don't see any oscillation at my typical cruise rpm of 2300. I believe it is a sample rate/signal conditioning problem, combined with likely resonances going on in the pressure line from the engine to the sensor. On top of this, the instantaneous pressures in our MAP lines are most likely quite noisy (literally also!), given the intermittent nature of valves opening, shock waves, etc.

I like Brad's idea of the gas filter to dampen pulses, and I plan to try this!

Perhaps someday I'll hook my data acquisition system up directly to record with high sample rates the actual voltage coming from the MAP sensor. I've seen similar oscillations in the fuel pressure indications.

I saw fluctuations when mine was installed in #3, but the swing was only an inch. I put a solid metal piece in the tube with a #60 drill hole in it. That stabilized it nicely and this method is more effective than an accumulator. When I put the bendix servo on, I drilled a port at the throttle plate for a vacuum feed where it belongs.

Larry
 
I saw fluctuations when mine was installed in #3, but the swing was only an inch. I put a solid metal piece in the tube with a #60 drill hole in it. That stabilized it nicely and this method is more effective than an accumulator. When I put the bendix servo on, I drilled a port at the throttle plate for a vacuum feed where it belongs.

Larry

Larry, about how long was the pin with the hole? Length of the small hole would seem to be an important variable. Also, was it near the engine end? I have a restrictor in mine also, but it?s a fair bit bigger than a .040? hole.
 
Larry, about how long was the pin with the hole? Length of the small hole would seem to be an important variable. Also, was it near the engine end? I have a restrictor in mine also, but it’s a fair bit bigger than a .040” hole.

Restriction needs to be smaller than you would think. Pin was about an inch long and about half way between source and sensor.
 
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MAP Fluctuations

FWIW: A orafice in the MAP system is a common way to damp out oscillations. Some older cert planes had a adjustable orafice in the back of the gauge. If trying to fix needle fluctuations you could close down the orafice with a adjustment screw. If you closed it down too much the MAP gauge response to throttle movement would be sluggish.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
Yep, G3X (non-touch) so don't think it's a gauge issue. Either a pressure fluctuation issue at high RPM (seems more stable at lower RPM) or a signal issue (grounding, noise, etc). I was up at 8k' when I did the test so the fact it would jump up to 26" MAP is definitely wrong.

I'm going to experiment with the plate with the hole and see if that helps vs the snubber.

Thanks!
Matt
 
This was what I discovered when my MAP gauge was acting up. This is the restrictor at the back of cylinder #3. A quick soak in MEK and a little work with some safety wire put everything back in to service.

I’m not really sure why this would gunk up since it’s a vacuum line.... but it does.

o7F.png
 
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When I experienced this issue it was a vacuum leak. The leak was at the sensor where the hose connected to it. Thrimmed off about two inches off the hose and re-attached to the sensor and problem disappeared.

Sometimes harmonic vibrations at certain RPM cause just the right combination to cause leaks to be more pronounced. I suspect this could be what?s happening to you. Mine also was more erratic at cruise settings.
 
Our Gen One EICommander has a manifold pressure gauge function and we discovered that when running it next to a full blown EFIS, some EFIS manufacturers would smooth the MAP readings with software and some would not.

Since we were working the ignition advance readings, we did not smooth the readings because we wanted to be able to tell if our customers had put some kind of restrictor in the line.

It turns out that Manifold Pressures are not always negative. When the intake valve slams shut, you get a short duration, high intensity, positive pressure wave. In other words, any electronic igntion that is using the manifold pressure to set the timing, needs to have a steady manifold pressure.

One easy way to do that is put some type of restrictor in the line, as close to the pickup point as possible. Remember, the MAP gauge is reading pressure, not volume.
 
Yep, G3X (non-touch) so don't think it's a gauge issue. Either a pressure fluctuation issue at high RPM (seems more stable at lower RPM) or a signal issue (grounding, noise, etc). I was up at 8k' when I did the test so the fact it would jump up to 26" MAP is definitely wrong.

I'm going to experiment with the plate with the hole and see if that helps vs the snubber.

Thanks!
Matt

It's not possible to generate 26" at 8K, so the snubber is not your issue. It's an instrumentation / sensor / wiring issue.

A vacuum leak would also show MAP readings higher than normal at idle and low power settings and their differences from actual should be bigger at those levels.

Larry
 
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