I was asked by a guy today if I knew why the O-550, Carburated, engine had a maximum Manifold Pressure listed and marked on the gauge. He couldn't remember the exact limit, but thought it was 27". He didn't state whether it was a time-limit max or an absolute max. The engine is in a C-182. I realize this may be a little vague question based on the information I was given. But, I told him I would do my best to find him an answer. Thanks in advance.

Jughead

Hmmm, since it didn't come from the factory with that engine, that MP limitation would have to be either part of the TC for the engine itself, or in the STC to convert to that engine. The FAA type certificate for the O-550 does not even mention manifold pressure limitations (probably because the stated "27-inch" MP limit would routinely be exceeded at a low-altitude airports, even before starting the engine).

I believe someone has made a mistake. Either your friend is wrong, or the STC itself has a typo in the limitations section. Get with the STC holder to have this clarified.

PS - I understand (source - my A&P/IA) that if the gauge markings are not as required in the TC (as modified by the STC), the aircraft is technically not airworthy, and those markings must be corrected before he could sign off the annual inspection. Check with your IA to confirm that, since I'm not an A&P/IA myself, do not play one on TV, and did not sleep at a Holiday Inn last night...

I don't have a lot of confidence that the guy that asked me the question had his facts straight. I explained that the engine see's greater MP every time it was shut down at normal atmospheric conditions. I also explained that MP is really Manifold Suction. The only two area's in the inductions system where suction or air movement volume might be a problem was the induction hoses on the intake manifolds and the air filter. I was looking at some of the STC's myself and didn't see where they had the restrictions he mentions. Thanks for your reply.

Jughead

THIS IS JUST A GUESS. I do not know which STC was used for this installation, nor do I have or operate an aircraft with a 550 but I believe it has the restriction to limit the maximum power output of the engine and that this was done to simplify the STC certification process. The 550 is certified to produce a minimum of 300 hp to TBO but may produce up to 315+ when new. The FAA has approved lower power increases without a full flight test program, think rudder control at takeoff power for example, or required fuel flow without a engine driven fuel pump if a gravity feed carburetor fuel system (the "straight" model Cessna 170 requires an engine driven pump do to fuel line size which is smaller than the 170A and 170B, the 180 hp conversions to the 170A and 170B do require the engine driven pump to meet the fuel flow requirements). Anyone operating an aircraft modified by an STC or field approval would be well served by reviewing the limitations section of the approval basis. For example my 180 hp 170B is no longer authorized to operate in the utility category (no spins).

Tim A&P IA

If indeed there is a MP limit on the subject aircraft's tach, I believe Tim just hit the nail on the head.

Many of the STCs that install a 520 or a 550 in a 180/182 specify a lower hp limitation than the engine itself is capable of delivering. For example, my old 180 would have got the Kenmore O-520 conversion, which derates that engine to 270 hp. Some of these engine conversions limit power to 260. Usually, this is done by limiting prop rpm, but the 550 already has a 2700 rpm limit, so the STC holder may have had to limit MP to meet the HP limitation.

As Tim noted, with much more flight test, the STC holder may have been able to use all 300 hp in that conversion, but it would require a LOT more testing, and would be very likely to run into some issues at that much hp.....like not enough rudder/vertical fin, etc.

But, as Tim notes, tell him to look in the STC paperwork and in the flight manual supplement (!!!!) that came with the modification.....there SHOULD be a FM Supplement, I hope.

Maybe.

MTV

The Texas Skyways STC for the O550 does have a MP limitation but I thought it was 26."

I'd bet Mike and Tim are right, it's about ease of certification but I'll offer a different possibility. The O550 from Texas Skyways uses a carburetor off a different model engine that is modified to work on the 550; I'd guess it's a O470 carb. It's possible that even after being modified the carburetor just can't flow enough fuel at the higher manifold pressures causing the engine to go lean and putting it at risk for detonation.

Well, my suspicions were correct, and many of the replies you all posted were right on. This engine is actually an IO-550 Texas Skyways conversion. (I'm not sure how one owns an airplane and isn't sure what engine is in it?) He sent me copies of the STC. Yes, the engine was derated, thus the 26" MP limit. I will have to do more research if the derating was accomplished due to aerodynamic factors as discussed above, TBO extension reasons or something else such as the fuel flow concerns.

Thanks for the great replies. I hope others have learned from this discussion as well.

VR

Jughead

Now that the facts are in, I'll share an observation. What I've found interesting here is that the engine horsepower could have been limited by setting the RPM lower, but MP was specified lower instead. The clear advantage is that 100% rated horsepower is available to 3 or 4000' density altitude depending on losses in the induction system. That's cool. You're getting the full benefit of an IO-550 as soon as you climb out a bit. You could open the throttle slightly as you climb from SL until you reach an altitude where WOT nets not more than redline MP.

jugheadF15 wrote:Well, my suspicions were correct, and many of the replies you all posted were right on. This engine is actually an IO-550 Texas Skyways conversion. (I'm not sure how one owns an airplane and isn't sure what engine is in it?) He sent me copies of the STC. Yes, the engine was derated, thus the 26" MP limit. I will have to do more research if the derating was accomplished due to aerodynamic factors as discussed above, TBO extension reasons or something else such as the fuel flow concerns.

Thanks for the great replies. I hope others have learned from this discussion as well.

VR

Jughead

If I recall correctly, the 182 has been flight tested with 260 hp. So, limit the HP to 260 and the modifier probably wouldn't have to go through nearly as much flight testing. The Kenmore O-520 conversion on the 180 goes to 270, I believe, but it's only available on later model (H and later???) airplanes.

MTV

Pinecone wrote: The clear advantage is that 100% rated horsepower is available to 3 or 4000' density altitude depending on losses in the induction system.

I haven't flown this engine, but I would suspect you could exceed 26"MP at low DA's. Having to limit the MP to 26" would produce less than 100% available horsepower.

Jughead

Pinecone wrote:....the engine horsepower could have been limited by setting the RPM lower, but MP was specified lower instead. The clear advantage is that 100% rated horsepower is available to 3 or 4000' density altitude depending on losses in the induction system. That's cool. ....

Pretty clever of Texas Skyways to do it like that.
Kinda reminds me of how Cub Crafters placarded their O-340 as 180hp for 5 minutes, 80hp continous.
Doing it that way it made their Carbon Cub a better fit with the LSA power/fuel weight formula.

JugheadF15, the way I understand it, the O-550 is derated to 285 HP by limiting MP to 26". To comply, you'd reduce throttle at sea level, but as you climb, you can increase the throttle to maintain 26" and 285 HP. No loss of HP for the first 3000' to 4000'. That's what I meant by 100%.

Pinecone,

I see what you meant by that now. Thanks for your input.

Jughead

Interesting, as I am looking at a 520 going in a 210, found one that has pitch limits on the prop and RPM limitations for nothing more than prop noise!! Nothing to do with power!! perchance would limiting the MP do the same for noise?

My understanding is that it is the tip speed that makes the noise, usually approaching or exceeding Mach-1. Seem to remember best power is to stay under .92 or a bit less. Several sites out there with tip speed calculators. I ended up at .87-.89

There are some who used to call the excess, "The Backcountry Anthem."
Used to be impressed with it until after researching the subject.


Chris C