This issue has been a source of dilemma or misunderstanding for some. I get a lot of questions on this.
Legally you must use the same propeller as a C85 and adhere to the same RPM limits as a stock C85 because the C85 stroker STC does not change the Certified engine rating.
On paper, it is still a C85.
I am not aware of any FAA approved power curve for the STC modified stroker configuration. When requesting such from Don's Dream Machines or Aircraft Specialties (the STC holders) I have been turned away but was told that "this engine is much more powerful than a stock C85". I've even been told just how much power the engine produces on the dyno.
Of course we all understand that because of the physics of engine performance regarding displacement theory (RPM/bore/stroke) more power will be produced. Don't get me wrong. I love the guys at DDM and ASSI, but I must ask: What good is it to know the engine makes 108 hp on the test stand if you have no legal authority to operate it that way? My issue as a DER is that without a certified power chart, there is no legal avenue to approve any propeller that might maximize the performance of this engine configuration. The approved data to go there has never been developed as far as I can tell.
In my opinion, the FAA and STC Applicants did us all a dis-service by not requiring an official engine calibration test to re-rate the modified configuration. I am sure the reason this wasn't done is because doing so would have required the STC applicant to run 150 hour endurance test followed by 150 hour durability test to prove all parts out to the higher rating. I get that this is a money issue. But the flying public deserves to at least understand the realities they face.
So we (the industry) are left with 2 choices.
1. Legally run the same propeller as a stock C85, not knowing exactly how much power you have, but at least assured you have equal or better performance than the stock C85 and have the comfort of staying legal.
2. Illegally running another propeller that optimizes the capability of the engine. Climb and cruise fast not having (Approved) assurance that the engine parts are going to survive to TBO. Be mindful that if you choose to operate this way, the STC holders, prop makers, and engine overhaulers are not likely to warranty an engine or propeller with apparent fatigue issues. Beyond this, I know that insurance companies and defense attorneys use expert witnesses who can tell when they see evidence of over-stressed and prematurely fatigued engine parts. All I am saying is... choose wisely.
I believe there are many folks out there running illegal configurations, without knowing it... or without an understanding of the issues. I hope this article helps some of you out.
Do I think the stroker engine is a good thing? Well I do. It seems to be a most efficient engine. I would just appreciate having the legal avenue to use it to its full potential. The saving grace here is that these little 4-banger Continentals seem to be made of long-lasting bulletproof parts. And generally they're used on small-very forgiving airplanes affectionately thought of by many as just "barely" capable of killing you.