Question for Paul McBride, the General Aviation News engines expert: I am building a Velocity and it happens that I purchased a zero time rebuilt 250-hp Lycoming IO-540-C4B5 engine. I have been told by a maintenance shop that the only difference between a C4B5 and a D4B5 is the setting of the fuel servo. The D4B5 is rated at 260 horsepower at 2700 rpm, and they claim they can adjust the servo to convert the C4B5 into a D4B5.
It appears you are very knowledgeable, and I was wondering if you could help me to verify it is as simple as an adjustment.
I would appreciate your insight in this matter.
Ruben G. Creus
Paul’s Answer: Ruben, after I read your question, I did some preliminary research into the two models you mentioned.
I’ve got to be honest with you and tell you that your mechanic is not exactly correct in telling you that the IO-540-C4B5 is the same as the IO-540-D4B5. While they may be similar, they are not the same.
However, with the limited research I did, I don’t see any serious issues if you decide to run the engine at 2700 rpm.
I will warn you that you must have the fuel servo adjusted with an increase in fuel flow in order to support the additional horsepower you’ll be taking out of the engine.
These two engine models are configured with more effective counterweights for use with a Hartzell “compact” propeller, so I recommend you do your homework with regard for which propeller may be best for your engine/prop configuration.
In a practical sense, this modification can be done; RPM increase and fuel flow adjustment. But according to Note 5 of the IO-540 Type Certificate, the -D4B5 is the same as the -D4A5 except for magneto selection, and the -D4A5 is the same as the -C4B5 except for a hybrid camshaft. What this means in another practical sense would be better asked of Mr. McBride.
Modifying engines is fine so long as the modifier understands he is disengaging the many years of engineering experience and expertise in exchange for an experimental ideal. I suspect when the novelty of modification wears off, you may be happier with the comfort of knowing some very smart guys already figured out how to make that engine reliable just as it is.
Paul is spot on, the counterweights are different in the two motors. These two motors are found in the Comanche 250 and 260.
Any engine can be converted to garbage in just a few seconds
Indeed.
Rolls Royce famously managed to coax 3000 horse power out of a heavily modified Griffon for all of 8 minutes after which the engine underwent Rapid Unscheduled Disassembly.
If you increase your rpm and soup up the fuel and air supply, you may well get more power but at the cost of reducing your engine life.
I enjoy the articles very much and find them very interesting. I have been flying for over 60 years with over 6000 hours of stickyime
and still have my 1960 Cessna 172 in perfect condition which was my daily for commuter all this time.
I have learned that PowerPoint engineering has it’s reasons for originality.
Changing things could cause serious consequences to performance. Metal stress is always lurking and can result
Unexpectedly from changes.
Lycoming is a fantastic product as it was engineered.
Perhaps changes other than PowerPlant mods are inorder to avoid unwanted surprises.
I have been a Continental user most of my flying career and have never had an unexpected surprise.
Do as you will but the envelope has an engineering reason. Fly Safely.
Another myth is that the 230 HP I0-540 from the later model Cessna 182’s can be bumped up to 260 HP by setting the constant speed prop to to allow for 2700 rpm vs the 2550 of the C-182. Not true. The 230 version has thinner wall piston pins, cast connecting rods, different counterweights, and different crankshaft as well as a need to adjust the fuel flows as described in this article. As far as I know the camshaft profile is the same so the engine should be able to develop the extra rpm but would severely overspeed the engine based on the internals as inertia rally goes up even for just 100 rpm increase. I am surprised Lycoming invested a lot of money to generate different parts and sku numbers instead of just limiting the traditional 260 HP 540 to a reduced rpm.