When should I achieve highest RPM?

I own a 1962 Piper Colt with a O-235-C1B engine. My operator manual says that I should have 115 hp at 2800 RPM. At what point would I achieve highest RPM? On takeoff? Level flight at full throttle?

I never see that high of RPM even in level flight at full throttle. On takeoff, I see about 2300 RPM max. I recently had the carburetor overhauled by a professional. When we removed the carburetor, we found someone had used a vacuum pump gasket for the carburetor mounting gasket. It was restricting the intake opening about .08 inch per side of the opening (incredible what one finds on a airplane!). I thought after we found that and had the “”new”" carburetor installed with the correct gasket, I would see higher RPM, but I don’t. I’m quite certain my throttle is opening wide also.

I would appreciate any thoughts you have regarding this. I haven’t spoken to my local mechanic yet on this, but plan to soon, if I can catch up with him.


Scio, Ore.

I certainly have to say you’ve asked a good question and I had to smile a little when you mentioned what you found when you removed the carburetor.

First things first, and that is to answer your question regarding when you should see the highest engine RPM considering all things are as they are supposed to be: You should see the highest RPM at full throttle at sea level. According to the FAA Type Certificate for the Piper PA-22-108 using the Lycoming engine model O-235-C1B, this should be 2600 RPM. OK, I know you’re going to say, but my Operator’s Manual says it’s rated at 115 hp@2800 RPM, which you pointed out in your email. I assume you’re referring to the Lycoming Operator’s Manual and not the Piper POH? After I did some research of the FAA Type Certificates, I’ll agree with you that this specific engine is certified at 115 hp@2800 RPM according to the FAA Type Certificate E-223 Revision 19 of Dec.17, 2003, issued to Lycoming. However, it appears when Piper chose this engine it elected to reduce the RPM to 2600 RPM for this particular airframe application. According to the Type Certificate 1A6 Revision 34, dated Aug. 7, 2006, issued by the FAA for the Piper PA-22-108, the maximum RPM is 2600 RPM. I believe what Piper did was to reduce the RPM to 2600 to accomplish the 108 hp and certified the aircraft that way for some reason.

As an aside, the later version of the Lycoming O-235 series, like the O-235-L series and a few other Lycoming models, were FAA certified at multi RPM and hp ratings to meet certain airframe manufacturers’ requirements. When Lycoming certified the O-235-L, it certified it at 118 hp@2800 RPM with alternate ratings as follows: 105 hp@2400 RPM, 110 hp@2550 RPM, 112 hp@2600 RPM, and 115 hp@2700 RPM. It looks to me like Piper was just ahead of the curve when it certified the PA-22-108!

While I was checking out the FAA data, I jotted down some additional information that may be of interest to you regarding the specific prop used on the PA-22-108. The TC Data Sheet calls for a Sensenich M76AM-2 propeller with a diameter not over 74 inches and not under 72.5 inches. This is also where you find the static RPM specification. In your case it is to be not over 2450 RPM, and not under 2200 RPM. Remember this information is all part of the aircraft Type Certificate Data and not the engine Type Certificate.

Let’s talk briefly about what you found when you took the carburetor off your engine. I guess I can say it really doesn’t surprise me, even though it frustrates me every time I hear something like this.

It’s things like this that make troubleshooting difficult because you normally wouldn’t think to look for such a thing. Maybe the person who installed the vacuum pump gasket instead of the proper carburetor gasket thought the hole size was close enough and thought it would work just fine or maybe the owner at that time couldn’t afford the cost of a new carburetor gasket.

I can’t begin to tell you some of the strange, weird, and sometimes downright dangerous things I’ve heard from folks about their aircraft while standing behind the Lycoming booth at air shows, such as last month’s AirVenture. I guess I should have made notes about some of these during my many, many years of doing that event, not to mention all of the other shows and seminars where these “”tidbits of technology”" seem to rise to the surface. I’ll bet I could have written a best-selling book titled “”You Ain’t Gonna Believe What They Do to Their Flying Machines.”"

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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