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.
