Thursday, May 11, 2017

C85 Stroker - Which Prop Can I Use?

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. 

- or- 

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. 

Final thought:  Do you think Continental Motors never thought of this configuration?


  1. This could be used as justification per TC A-759
    1a90 on a c-85 is approved on the S7DC (S7DC only)
    Static r.p.m. at maximum permissible throttle setting:
    Not over 2440, not under 2290. No additional tolerance permitted.
    Diameter: not over 74 in., not under 69.5 in.

  2. Terry, as Don Swords was the origin of this STC, and I have spoken to him on many occasions about this I can assure you that the reason for NOT making RPM and output changes was rooted in a far simpler analysis.

    Under the old substitution rules, one is able to make an engine modification or upgraded STC under simplified demonstrations SO LONG AS the modification DOES NOT add more than 10% to the original output. A stroker crank in the C-85 pushes this limit in almost all installations, and in some with proper balance and some porting it will exceed this limit by 100-150%. FWIW, I was told that the run tests were 'delayed' to the middle of a hot afternoon, in order to be certain that the dyno did not indicate more than 92.5HP (85+8.5 HP), as that would have forced the STC review into different territory.
    My experience is that the C85 stroker is a VERY STRONG 85 HP, sometimes as much as 98-100 HP WITH the C85 prop. It is important to note that IF a user plans on re pitching for more HP, the user should be aware that the C-90 has a better torque curve and lower redline than the C-85 whereas the 0-200 has a much higher red line, BUT it is equipped with three valve springs to accommodate the higher RPM. Operating in the higher rpm range without such benefit is likely to be damaging to the valves long term.

  3. Don Swords was not the originator of the STC. Try acquired it from Aircraft Specialties under a business agreement.

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