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Old 09-29-2022, 04:57 AM
SuperCubDriver SuperCubDriver is offline
Join Date: Feb 2011
Location: Germany
Posts: 511

Peter, I see you did some serious thinking!
Note that before takeoff the idling RPM was around 900-1000 and throttle not completely closed. If I close the throttle completely my idle RPM is around 650.
After landing with the stuck open valve the idle RPM was around 1150-1200 but with a completely closed throttle. The additinal MAP must have caused cyl 2 with the open exhaust valve.
I didnīt do the testrun as of today but intend to do the next days.

If anyone can run a simulation then this could help explain things.
RV-7 flying, IO-375 w. P-Mags, 200RV Prop

RV-8 flying, IO-375, P-Mags, 74RV Prop
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Old 10-05-2022, 12:51 PM
SuperCubDriver SuperCubDriver is offline
Join Date: Feb 2011
Location: Germany
Posts: 511

So here is an update:
Meanwhile I flew 3.5 hours since the event. Today I did a 50 h inspection and oil change so this was a good opportunity trying to replicate the event.
So after a flight today just before shutdown I pulled the throttle all the way out to the mechanical stop, mixture full rich: MAP 13.2 / RPM 630 / EGT around 1050 F.
Then I removed the cowl and the #2 spark plugs to simulate the open exhaust valve, then started the engine again and saw following datas with throttle all the way back again and mixture full rich: MAP 17.0 / RPM 1150 / EGTs around 1000 F except #2. Both tests are done stationary, so no forward speed.

This looks like proof that the engine is in fact producing some power or thrust with idle power and throttle closed with one exhaust valve stuck open. The only difference during this test was cylinder #2 breathing through the spark plug holes instead through the exhaust valve, so this takes out the influence of the exhaust. I think this is a representative test and confirms the original data.
Just one more info in case somebody is wondering: Yes I did ground the removed spark plugs to protect the ignition modules of the Pmags.

Here are the screenshots:

Throttle fully closed, mixture full rich

Throtle fully closed, mixture full rich, #2 spark plugs removed
RV-7 flying, IO-375 w. P-Mags, 200RV Prop

RV-8 flying, IO-375, P-Mags, 74RV Prop

Last edited by SuperCubDriver : 10-05-2022 at 12:54 PM.
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Old 10-06-2022, 05:17 AM
Dan 57's Avatar
Dan 57 Dan 57 is offline
Join Date: Sep 2018
Location: LSZF
Posts: 1,101

Thanks for your feedback Hermann, very interesting...
- ain't the Destination, but the Journey -
YLL On Tour... the blook:

RV-6.9 #25685, slider, O-360-A3A (carb/dual Lightspeed II), MTV-12-B, HB-YLL owner & lover
RV-4 #2062 HB-YVZ airframe builder
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Old 10-06-2022, 07:25 AM
Ingo Weise Ingo Weise is offline
Join Date: Nov 2005
Posts: 58

I’m sorry but there are some wrong assumptions.
Your first paragraph about the increase of RPM contradicts the law of conservation of energy.
To increase RPM with constant TAS you have to add energy and/or reduce losses.

“Higher RPM means higher MAP, period.”
With a fixed pitch Prop, RPM follows MAP. With everything else being constant, If MAP is increased, pumping losses are reduced and RPM goes up. If additionally the fuel flow goes up, power is increased and RPM goes up higher.
With closed throttle, the pumping losses are highest and the pistons try to suck as much air in as possible. If now RPM is increased, they need more air, but because the airflow is restricted the MAP has to go down. Without any flow restriction, MAP would stay constant, but never rise. You can try this in your aircraft.

where do you measure MAP?

“But on the other hand this cylinder doesnīt produce power at all. “

This is correct, but this cylinder sucks air from the open exhaust valve. The amount of air that flows through the throttle in idle position, is completely used up by the remaining cylinders. A constant air mass is producing the same power, no matter whether it is used in three or in four cylinders.

“And - there is still the increased MAP which I believe is the key to this problem.”
Do you know the valve timing of the intake and exhaust valves? I don’t know, if this is sufficient to solve the riddle, but it is necessary.
Ingo Weise
Celle, Germany

RV-8 slowest build, D-EXKI reserved
Tail finished
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Old 10-06-2022, 02:03 PM
SuperCubDriver SuperCubDriver is offline
Join Date: Feb 2011
Location: Germany
Posts: 511

MAP is measured at the intake port of #4 cylinder, I assume this doesnīt make a big difference.
I donīt know the valve timing or valve overlap, this data is probably available somewhere. I do have a roller cam with roller tappets.
For me the testrun yesterday proofs that the stuck exhaust valve in open caused a significant power increase, it must have acted like a significant intake leak. Looking at a closed throttle plate showes only a tiny gap for the intake air to go through.
RV-7 flying, IO-375 w. P-Mags, 200RV Prop

RV-8 flying, IO-375, P-Mags, 74RV Prop
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Old 10-09-2022, 07:57 PM
Triumph1974 Triumph1974 is offline
Join Date: Feb 2008
Location: Richmond VA
Posts: 115
Default collecting sticky valve data

I am collecting data for sticky valve experiences (engine type, fuel, oil, operating data etc.) . If you interested in contributing please send an email to valvedata@protonmail dotcom and I will send you the data fields we are collecting.

Not sure what may come out of the results, but maybe it will be show up something consistent across several hundred instances.....

Glad you didn't have any other issues on the flight!

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