Ive heard rumors that people have installed c-85 pistons in the O-300 and got it up to about 170hp. Does anyone have any knowledge of this? I know its not approved but I also heard that it may be soon??? I would like to know if the performance gain is realy that high. Any info, rumor or not, would be great!

Did it with a C-85 using O-200 crank and a Ritter cam.

https://www.backcountrypilot.org/forum/ ... stons-1521

Gump

That's an old Formula One trick. Lots of Swifters with 145's have done that, undercover of course. I doubt it will be "approved" soon or ever. A possible alternative is another source of high-compression pistons: Light Speed Engineering. Klaus Savier's most prominent product these days is probably his electronic ignition systems, but he also sells high-comp pistons for the O-200 that should also be applicable to the O-300.

http://www.lightspeedengineering.com/Se ... istons.htm

Forget the paperwork for a moment, the legality is obvious and is not quite as relevant as other issues. Putting race engine modifications into a passenger / back country airplane.... without addressing the limitations and extra precautions that were acceptable and necessary in racing.... may not be the smartest thing.

Putting hi-comp pistons in makes more power. No question about that. It also puts more stresses on several internal parts of the engine. It makes a lot more heat.

A mechanical issue with an engine in a back country airplane may have more serious consequences than your average bug smasher flying around over farm fields. There was a reason the race airplanes went to Reno in a trailer.

I'm NOT saying that higher compression will or will not cause a problem in the O-300. The engine will take mild power modifications without too much complaining. We did horrible things to the O-200 in F-1 and never threw a jug. I melted one once but that was not because of compression. The big point is that the power gain needs to be balanced against any reliability loss, the use of automotive fuel, the cooling capacity of the baffling... and whether or not you are flying a single seat airplane in a Nomex tuxedo with medevac rotors turning nearby.

One guy that needs to be involved in this discussion is Harry Fenton, formerly of Slick/Unison, and a well-known expert on the "small Continental" engines.

When you look at Klaus' webspage that Hotrod linked to, you will notice that he mentions several things of interest. First, he calls out 100 octane fuel, which not all back country airplanes run on all the time. Then he says that near-perfect cylinder bores are expected. If you know Klaus as well as I do, and he uses the word "perfect".... you're talking about engine building and constant maintenance at a far higher level than most workhorse airplanes get. That is exactly how Klaus achieves most of the miracles he is known for.

Another item of interest is that his hi-comp pistons are paired with his ignition system. The ignition advances and retards the timing in a way that prevents detonation and high compression problems in conditions where there would otherwise be a risk of engine damage.

Again this is not intended as "the sky is falling!" if you boost the compression on your O-300. Just don't throw the pistons in without carrying out a proper analysis and test, and assessing the risk/reward.

If you want a bolt-on power increase without that much time/risk/effort/testing, then put one of Klaus' ignition systems on the engine. Same paperwork issue, but better power, better fuel burn, etc.

As a former O-300A/ C-170B owner, I met guy here in Alaska who had done this with his O-300/ C170B, and so I was pretty interested to get his opinion. The only downside he indicated was having only one (complicit) mechanic he could ever bring the plane to for engine work. Not too bad for scheduled annuals, but it could really pose a problem if you were traveling and needed engine work at a different shop. That, and the fact that his insurance wouldn't pay out, if he even had any. (I didn't ask.) He figured it was putting out 165-170 Hp, and said that it was quite a noticeable upgrade.
Interesting to learn about, but not in my comfort zone. I went to an STC'd Lyc IO-360, and a constant speed, and have had no regrets. Admittedly costlier.

You have to chamfer the top of the 85 pistons to clear the 300 heads. Even so, on some cylinders, the bottom of the top ring of the 85 piston will hang on the top of the 300 steel cylinder barrel. Rare, but it does happen. Depends on cylinder serial number. Be careful.

85 pistons will give 8.68:1 compression ratio in an O-300.
9.5:1 pistons will give a 9.76% power and torque increase over stock throughout the rpm band and will run fine on premium car gas.
9.5 pistons will produce 159 hp from an O-300 at rated rpm.

EZFlap is on the right track here. Harry Fenton's website takes a bit of searching but he has all the info you could want on this topic. The website is http://bowersflybaby.com You will read for hours and be amazed at the resources there. Good reading!

The Cessna 150 Tripod website also has a lot of good information applicable to the 85, 90, and O-200 Continentals, as well as the O-300.

I've got several hundred hours on the 9.5 Lycon pistons running on mogas and like them a lot. Not for certified though. Great for experimental engines.

Jim, did you adjust you timing or something to run those 9.5:1 pistons? The 8.6:1 Continentals seem to have a small detonation margin on mogas so I'd imagine the 9.5:1 pistons would put it right on the edge with stock timing.

I timed at 28 degrees. The 9.5's were not close to detonation.
At 10, I would pull a jug every couple of hundred hours to check (which I also did with the 9.5's).
At 10.5, I would be very uneasy.
I was running near sea level, but never turned over 2850 rpm on climbout, nor more than 3100 in level flight (and not for long periods of time). I ran premium mogas with alcohol (used brass float and steel needle and seat in the carb).