Question for Paul McBride, the General Aviation News engine expert: I would like your thoughts about the following: One of my customers has a Piper PA-28-235 that’s coming up for an overhaul. He would like to convert his engine, a Lycoming O-540-B, to a high compression engine to make a 250-hp engine.
I know the spray boys have done this in the past with their PA-25-235 aircraft.
I searched for an STC for this conversion but haven’t been able to locate one. I don’t know if one exists or not.
Last thing: I just wanted to let you know how much I enjoy your column in General Aviation News. Keep up the good work.
Jim Kjeldgaard
Fremont Aviation
Paul’s Answer
First of all, Jim, I’d like to thank you for your kind comments regarding the joy you get from reading my column. I can only hope the column serves to educate our readers as well and maybe even dispel some of the controversial subjects that come up when folks are hangar flying.
I’d like to add a little caveat here while we are talking about hangar flying. I would highly recommend that for any subject that comes up during one of these round tables of knowledge that it is essential to read the manufacturer’s publications, such as the Engine Operator’s Manual and the Pilots Operating Handbook, for the aircraft. Most maintenance or operational subjects will be covered in the respective publications from the manufacturer.
Now to your question: I’m aware that the conversion of the Lycoming O-540-B series 235-horsepower engines has been done in the past. I also did some searching for an STC and had no success either. But I know there are STCs out there to do this upgrade conversion.
I believe that an FAA authorized repair station also could possibly make this conversion using FAA Form 337.
I’d suggest checking first with your local Flight Standards District Office (FSDO). They may be able to offer guidance as to where to find more information about this conversion.
When this conversion is made, they remove the 7.20:1 compression ratio pistons and replace them with the 8.50:1 compression ratio pistons. While still maintaining the rated 2,575 rpm, that should give you 250 horsepower.
The one important thing to keep in mind is there would also be a need to adjust the carburetor’s fuel schedule upwards in order to support the increase in horsepower.
As the old saying goes, there is no such thing as a free lunch!
I’ll counter that by saying the two least expensive things you can put in your engine are gas and oil.
Paul, I read your response to the question and in the final sentence found the nugget of gold… I was just contemplating an auto gas STC. Your comment made me rethink that issue… I will continue to fill up with 100LL and add TCP. The risks of vapor lock and turning myself into a crispy critter because of static electricity aren’t worth it.
Also, note that the fuel requirement will go from 80/87 to 100LL …but since only 100LL is available….no change. just a placard change at the tank fillers.
The displacement is the same with the new pistons, so I don’t understand having to make any mixture changes ?
And, the CHTs can run a bit hotter, so baffles should be checked.
Increasing the CR improves the “fuel specific power” of an engine. The pounds of fuel/HP/hr is reduced when the CR is raised.
The increased power comes from the increased efficiency the higher CR provides.
This means more power is produced from the same amount of air and fuel.
The displacement volume is not changed but the compressed volume is reduced so
the actual working volume of the engine is reduced slightly which would call for a very small reduction in fuel required.
You need to increase the fuel rate as a higher CR creates more heat. While these are “Air Cooled Engines”, they also cool with fuel, especially at higher throttle settings. That’s why the carb has an enrichner valve, aka, economizer valve, in the carb. It enriches to cool with fuel.
JS, When the CR is raised, the engine has a higher expansion ratio during the power stroke. This has a cooling effect that is much greater than the extra heat during the compression cycle. Overall the engine will run cooler at the same power output with higher CR. This is why Diesel engines run much cooler.
yup, true….who knew.? with higher thermodynamic efficiency, extracting more work from the combustion process leaves less heat energy in the exhaust gases.