Hi guys!

We have a 1953 C170 in our family with O-300 engine here in Finland. We are flying it with floats and skis.

Now we are thinking about a little boost for the engine. Does anybody have any experience about high compression pistons in O-300? Ten years back there was a one 170 in Finland with hc pistons, but the funny part is that nobody knew before they overhauled the engine with normal pistons. Suddenly all the power was gone.

We are probably planning to install those pistons in the shadows so at this point STC is not necessary. Availability and reliability after the modification are the big questions.

Our plane on the frozen gulf of Finland last week:

Can't help....but need to say...Sweet looking 170!!!

The old story is to use C-85 pistons with the heads chamfered for clearance.
Same bore, less stroke so the domes of the pistons are taller off the wrist pin - in an O-300 that means higher compression.
Increasing compression will increase BMEP which will increase engine efficiency but probably not a huge horsepower bump.

Also, higher compression will need different timing on the mags lest power's arch nemesis Detonation comes a knocking.

I don't know of anyone who has ever done it (just talked about it) so i have no idea what gains could be expected.

It would be an obvious mod to any mechanic peeping with a bore scope down a spark plug hole as the top of the piston is smaller than the bore.

Thanks for the replies!

I have heard rumours that the gains would be like 15-20hp, that would make it somewhere near 160hp. Even that would make a noticeable different, especially on floats.

Any experience from these: http://www.nfspistons.com/store/p26/NFS ... _to_1.html

I have had conversations with a local (excellent) mechanic (not an A&P..) who has installed the NFS ( I think..) pistons in an O300. At first, with the new pistons in the old cylinders, oil consumption was high (quart in 3 hours or so). Power was good. He decided to install new cylinders and this solved the oil issue. I was told that cooling and mag timing became a lot more critical, probably due to the increased BMEP. He is happy and would not consider going "back". IIRC he said that he thought he was in the 160 hp range. Besides the legality issue in a certified plane, take this one step at a time, and be prepared for a little R&D. I probably have forgotten or left out details: you are welcome to PM me and maybe I can find out more..Good luck.

I had a neighbor in remote Alaska, that had the high compression pistons in his 0300 170. It worked really well he hauled supplies into his backcountry store. At the time I had the same airplane 1955 170B 0300. For comparison he took me for a ride, there was a big difference in performance. He also had a borer prop 80x42 . They convert 0300s to airboat engines with the high compression pistons. That is where I would look .

3tracks wrote:I had a neighbor in remote Alaska, that had the high compression pistons in his 0300 170. It worked really well he hauled supplies into his backcountry store. At the time I had the same airplane 1955 170B 0300. For comparison he took me for a ride, there was a big difference in performance. He also had a borer prop 80x42 . They convert 0300s to airboat engines with the high compression pistons. That is where I would look .

I think the 80x42 prop had more to do with the performance difference than a bump in compression.

We're talking about going from 7:1 to maybe 8.5:1 at best. That might get you a 10% power increase after you adjust timing and possibly re-jet the carb.
Back in the day when I was building Harley Sportster motors, compression ratios were talked about a lot but even 10:1 pistons alone don't do much without the right cam/exhaust/carb/timing setup.

Not to go the curmudgeon route but, I always wondered why Continental didn't go this route? Swapping pistons for a 10% power gain seems like something TCM would have wanted to do. Maybe because they wanted to keep it running on 87 octane fuel? 8:1 would require 100 octane to be safe in all conditions but that's not a real problem today as all avgas is 100.

If I was overhauling an O-300 and didn't need to worry about the FAA, I would seriously look into slipping those pistons in and getting the tuning right. The cost and effort at that point is worth a possible 10% gain. I don't know if I'd bother tearing down an otherwise good motor just to do this.

Bagarre wrote:.... Maybe because they wanted to keep it running on 87 octane fuel? 8:1 would require 100 octane to be safe in all conditions but that's not a real problem today as all avgas is 100.....

87 octane mogas isn't. And the new UL94 (?) by Swiftfuel or whoever isn't either.
I know people who run 150hp 320's instead of bumping them to 160 just so they can use 87 octane mogas without worrying.

hotrod180 wrote:

Bagarre wrote:.... Maybe because they wanted to keep it running on 87 octane fuel? 8:1 would require 100 octane to be safe in all conditions but that's not a real problem today as all avgas is 100.....

87 octane mogas isn't. And the new UL94 (?) by Swiftfuel or whoever isn't either.
I know people who run 150hp 320's instead of bumping them to 160 just so they can use 87 octane mogas without worrying.

But 91 moGas is ok in 160 hp 0320 per the Peterson STC. We can't get 87 with ethanol up here so 91 it is.

I'm curious on this topic as I have just acquired an 0-300 and a 172 to put it in. More HP is always nice...

What I meant with the 87 octane comment was directed at the late 1950's or early 1960's when TCM might have considered further development of the O-300 motor. Today, it's a non issue.

Thanks for the replies! I'm going to make some phone calls now. Let's see what happens. I'll keep you posted!

I need to check MOGAS compatibility with these mods. Avgas prices are about 12$ or more per gallon in Finland. Mogas is half the price, so trying to keep that option available. Normally using Shell V-Power with 99+ octane rating.

The best way to get a lot more horsepower in a 170 is to chuck the Continental in favor of a 180hp O-360 Lycoming! Or maybe, if you're hooked on 6 cylinders, a 210 hp IO-360 Continental, or maybe shoehorn in an O-470! Or maybe chuck the whole thing in favor of a 180 or 185. Just musing.

Cary

Cary wrote:The best way to get a lot more horsepower in a 170 is to chuck the Continental in favor of a 180hp O-360 Lycoming! Or maybe, if you're hooked on 6 cylinders, a 210 hp IO-360 Continental, or maybe shoehorn in an O-470! Or maybe chuck the whole thing in favor of a 180 or 185. Just musing.

Cary

That's going to cost a little bit more than a few pistons installed on the down low.

From what I've heard, the C85 pistons got you a touch over 9:1 and 165ish HP. Remember 10% Increase on 145 is 14.5 or just about 160HP. HELL of a lot cheaper than the O-360 route, but I don't know how you keep the oil cool.

Oil cooler, Oil Filter Cooler, and putting the cooling air exit where it isn't being hammered by high pressure air.

EZFlap wrote:Oil cooler, Oil Filter Cooler, and putting the cooling air exit where it isn't being hammered by high pressure air.

No provisions on an 0-300 to run an oil cooler... that was the reason I came up with the diverter system to run one with an oil filter.. currently I have that STC on hold though and have not manufactured any in a year or so...

Brian


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