Manifold pressure max 29” on Scout?

Skymarc

Skymarc

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Aug 29, 2021
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Canada
Just installed a CGR-30P by EI.

When I ordered the CGR-30P I sent a copy of the POH restrictions to EI so they put the red lines in the controller.

Im also removing the mechanical manifold pressure gauge as it will show MP in the CGR-30P.

Scout POH shows a maximum of 29” manifold pressure but the mechanical MP gauge never had a red line.

29” is way too low as now the alarm on the CGR-30P shows a red light on every take-off. Why does the POH shows a max of 29” MP but it never had a red line on the mechanical gauge?

I dont believe 30” of manifold pressure will over boost the engine.

Can I get feedback from the group or ACA why there is a max of 29” in the POH?
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Maximum manifold pressure is equal to outside ambient pressure, which varies by weather conditions and altitude, so a red line at a specific value would be meaningless. That's why your mechanical gauge never had one.

Manifold pressure should really be called manifold suction, since you are really measuring the partial vacuum created in the manifold by the engine trying to draw air past the throttle. At WOT, you are letting the engine have as much air as it wants, so the pressure in the manifold should be equal to or close to the pressure outside. At lower power settings, you are restricting air flow to the engine, so the resulting manifold pressure is less than atmospheric pressure. That is why at idle your manifold pressure might be set to 10 inches, while the outside ambient pressure is 30 inches. What your gauge is actually telling you is that you are choking off your engine air flow enough to create 20 inches of vacuum in your manifold. Even the word "throttle" means to restrict.

Unless you have installed a turbocharger in your Scout, there is no way to "over boost" a normally aspirated engine.
 
Skymarc said:
Agreed, but why does the POH states max MP of 29”?
Click to expand...
Dunno, but seems kind of silly. 29 is about what you should usually see when taking off at sea level in standard temperature and pressure. If you see substantially less than that, you might abort a takeoff. But what would you do if you exceeded it? Nothing. 29 is more of a goal than a max.
 
I have the same problem with the MVP-50P in my 8KCAB. I don't see anything from Lycoming which supports it. I wonder if someone saw this statement in the Type Certificate Data Sheet for the engine and .....
"Engine models IO-360-C1F and F1A are eligible for turbocharging and under these conditions the following additional limits apply: Intake air manifold pressure max. 29 in. Hg. absolute, exhaust back pressure max. 32 in. Hg. absolute at inlet to turbosupercharger."
 
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Ed and I get to fly on those nice high pressure days. Guess the newer birds have to stay on the ground until a cyclone hits - or at least until the pressure drops to 2899. Don't see that often around here.
 
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