What are the advantages to a high compression conversion of my aircraft’s engine?

Question for Paul McBride, the General Aviation News engines expert: I’m not even considering doing a conversion, but I am curious to know: What is the advantage to doing a high compression conversion to a Lycoming O-320 (from 150 hp to 160 hp)?

I suppose the obvious answer is 10 more horsepower, but my natural inclination, given the name “high compression,” would be improved fuel economy.

But what other advantages are there squeezing 10 more horsepower? Is there an improved useful load? Higher cruise speed?

I scoured the “net webs” and haven’t found any definitive answers other than back and forth pontifications about the use of mogas.

If it’s purely fuel economy, is there a sufficient fuel savings to justify the cost or is it just bragging rights?

I drive a white mini van if that tells you about where my thoughts are about being bold and sporty.

Thanks in advance,

Jeffrey A. Hughes

Paul’s Answer: Jeffrey, thanks for submitting your question. I’m certain there are many others out there who have been curious about the same question.

First of all, there is a simple statement that sums up one part of your question, and that is, “people are willing to pay for horsepower.” In some cases, it comes from converting an engine by using higher compression pistons.

This next statement is very important before even beginning to think about doing an engine conversion, especially on Lycoming O-320 series engines. There are certain models of the O-320 series that CANNOT be converted to high compression, so before any money is put on the table, make certain that the model you want to convert is an engine that can be converted.

By providing the specific engine model, such as an O-320-E3D, this will provide all the information needed to determine whether or not the engine can be converted.

The engine I used as an example, O-320-E3D, is one specific model that cannot be converted to a high compression configuration because of the main bearing configuration.

One other thing must be considered before increasing the horsepower on any engine installed in a certified aircraft: Does the FAA Type Certificate for your aircraft permit the use of a higher horsepower engine?

In many cases where a more powerful engine was installed, it was approved under a Supplemental Type Certificate (STC) issued by the FAA.

If you have a high-time engine in your aircraft, there is no doubt that it just isn’t making the horsepower that it did when it was new. My feeling is that by simply overhauling this engine and bringing it back to closer tolerances, you’ll see an improvement in performance and you’ll think you’ve gotten an additional 10 horsepower without spending any extra money.

Engines are rated by the measured horsepower on a dynamometer and the FAA Type Certificate Data Sheet notes the rated horsepower of any given engine. These engines are run in a test cell using straight exhaust stacks and before accessories, such as an alternator or aircraft induction system, are installed.

Once the engine is installed in an airframe along with the accessories, the accessories contribute to what is commonly known as “installation losses,” which actually reduce the total rated engine horsepower by some amount.

If the engine is converted to a high compression version, the fuel consumption will be greater to some extent simply because you are taking more horsepower out of the engine and you’ve got to put more fuel in the engine to support the additional power. Remember: There is no such thing as a free lunch.

I enjoyed your comment regarding you driving a white van and know exactly where you’re coming from.

And to show you that I do understand, just forget about converting your engine to a high compression version. The best advice I can offer is fly your aircraft as frequently as possible, change the oil and filter as recommended by the latest revision of Lycoming Service Bulletin 480, and enjoy what you’ve got.