Q: Are there any practical, appropriate and compliant modifications that can be made to a 150-hp, narrow-deck O-320? For example, can the compression ratio be tweaked a bit, or are there aftermarket exhaust systems with less back pressure, or is there any porting/polishing possible to effect a tad more power to the propeller?
A: Gordie, I wish you had provided me with a specific engine model so that I could offer more specific information in response to your question.
There is a possibility that certain models of the O-320-series engines may be converted to a higher compression ratio piston. Generally speaking, most O-320 series 150-horsepower engines, except the O-320-E series, may be converted to a higher compression.
This conversion would increase the compression ratio from 7.00:1 to 8.50:1. Any conversion of this type should be completed in accordance with an approved FAA STC.
As you mentioned, there are aftermarket exhaust systems that claim to increase engine horsepower, but I have no experience with these products, so I hesitate to comment.
There is no doubt that any improvement in helping the engine breathe better will result in an increase in horsepower, however it may be very difficult to actually document or detect any increase in power during normal operation.
From my experience, I’ve been told that if you take a 2,000-hour engine and put it through a factory overhaul or a good field overhaul using the engine manufacturer’s new limits on all components, the result when flying the aircraft is like the engine has gained horsepower.
It stands to reason that any high-time engine may be a bit down on power, so anything new will appear to be a more powerful engine. This certainly makes sense to me.
There is one thing that you must keep in mind. Your O-320 series engine, if installed in a certified aircraft, has an FAA Type Certificate stating what horsepower the engine is rated at for that particular airframe engine combination. Therefore, if the TC states the engine is rated for 150 horsepower, you must keep the engine at that specific rating.
If any modification you make allows the engine horsepower to exceed that rating, then the aircraft would not be in compliance with the FAA Type Certificate.
My suggestion is for you to do some additional research focusing on specific engine modifications for your specific aircraft. Please don’t be misled by all the hangar talk or information provided by aftermarket product suppliers.
Check it out for yourself before spending any money. I guarantee you’ll be a lot better off and have peace of mind that you did it right.