The International Cessna^® 170 Association

The C-85 pistons have been mentioned before but I've never seen these for-sale.

The unknown reliability and reasons for no approval have also been mentioned here as well.

It's a shame. Something like this approved would make an overhaul almost fun.

Personally, I'd be a little more interested at about 8 1/2 instead of 9 to 1 compressoion ratio.I know of engines, both O-200 and O-300, running with chamferred-top C-85 pistons which are estimated to bump the horsepower up to 110 in the O-200 and to 160 in the O-300. They seem to run trouble free and thrive even using mogas.

Aryana,
When I installed the 210 HP Continental IO-360 engine in my '52 B model,my fuel burn was the same as the O-300 engine at the same airspeeds. Without a change in the configuration of the airplane it uses the same HP to accomplish a common air speed. The specific fuel consumption, that is the pounds of fuel per horse power per hour, is practically the same on both engines, about .42#/HP/hr.

Well, of course, we all know the reason it's not certified.....because it's not tested, and is not proven to reduce the reliability and safety of our engines.

Who knows what the increased compression will do to our crankshafts? (You know...the ones which are getting so rare?....especially thinking of the 8-bolt cranks?)

Who knows what the increased pressures will do to piston pins?, or valves? Increased HP means increased heat. Will the valves be capable of dissipating that heat? Will the oil temps worsen? Would the increased HP actually need an added oil cooler?
Will CHTs increase? What about exhaust systems and cooling baffles? Will they need modification?

What is the new red-line? (The 175 HP GO-series engines do fine at higher rpms, and the cyls and pistons are the same...not higher compression. Will the domed pistons introduce new cylinder head failures?)

Nice dream. I wish it was so simple.

But I fly at night a lot with my family aboard. And insurance (not just aircraft, but personal life insurance) will likely be invalidated with my deliberate act of illegally modifying my aircraft.

No thanks.

Much better to get more performance with legal (and proven) means, IMO.

A very good point. If the O-300 could be modified to make 170HP reliably by changing out/beefing up some components, why did TCM go thru the bother of making the the GO-300 for 175HP?

I think if it was remotely possible to do with the same reliability, we'd all be flying behind 170HP O-300's from the factory cuz they would have been all over it in 1945.

Does compression increase horse power or does it just increase efficiency? I thought it increased efficiency so, for more power you'd still have to turn more RPMs or burn more fuel.

I would be most worried about what the result would be if you had a stuck valve with that piston shape. would it strike the piston and destroy the engine like on some of the older hondas when the timing belt sould snap?

Would it change you engine into an interference engine? That is what I was thinking of. I couldn't think of the correct term

bagarre,
The three things in an engine that determine the horsepower are DISPLACEMENT, RPM, and BMEP. BMEP is Brake Mean Effective Pressure, that's the average working pressure in the cylinder on the power stroke. Raising the compression ratio will raise the BMEP thus raising the Horsepower at any given RPM.
Displacement is fixed, but RPM and BMEP are variables controlled primarily by the throttle on fixed pitch prop equipped airplane engines. With a constant speed prop, the prop governor primarily controls the RPM by varying the prop pitch, and the throttle controls the BMEP.

Forgive me if this is a dumb question but I'm not up to speed on all the STC's for this engine. Would it be legal to polish the intake ports to obtain a few more HP? Anyone done this or is it more of a nudge nudge wink wink subject? The reason I ask is I have an almost new (30 hrs) C-145 sitting in a crate which will be going in my plane in April. Since I operate on floats any extra power is welcome.

Alex, The polishing and porting is a no harm improvement but any modifications to the engine will be evident in an accident investigation. That is where the lawyers will find out what you have done. If you can avoid that step then your okay to do it. WINK WINK. The performance gains by these modifications are minimal. It also lowers the reliability ratio of the TBO. I used to be a hot rodder but in the aviation world the only place in Aviation where this type of mindset is applicable is in Reno. Up in Canada, I would bet you the Canadian Fuzz would have a field day with you. In this litigious society I would only dream about doing these things if I was in a populated area. Then again in an unpopulated area I would only dream of making modifications also. Doug

CF-HEW wrote: Would it be legal to polish the intake ports to obtain a few more HP?

Legal aside, it would be of no benefit. The O-300 is a slow turning engine, the intake velocities are not fast enough to require porting and/or polishing.
In fact, the course walls of the intake are BETTER for slow turning engines as it allows the fuel air mixture to tumble across them.

It's like balancing your lawn mower blade to with a gram scale to get a better cut lawn.

Bumping the compression in my Harley to 9.5:1 DID make a noticeable power increase but, I also swapped the cams out at the same time.
Also, when I blew the motor, I just had to pull over to the side of the road.

Horsepower in an engine is determined by Displacement, RPM, and BMEP only. Port polishing will only increase power if it changes one of the three mentioned above.

Horsepower in an engine is determined by Displacement, RPM, and BMEP only. Port polishing will only increase power if it changes one of the three mentioned above.

The theory behind port/polish is the same as bigger/more valves; decrease the resistance of moving the fuel mixture to the cylinders allowing more volume thus increasing BMEP.

But, just like bigger/more valves or a bigger carb or anything else that increases intake efficiency, it requires very high reving engines to make use of. Even then, the difference is tiny. But in races where winners are defined by .01 seconds, it's worth it but even then it's the LAST thing you do to a motor for more power.

The O-300 (or any airplane motor) doen't turn fast enough to make use of this technique.

Thanks for the replies guys, I kind of figured it had something to do with racing. The guy asking me if I thought about polishing the ports is an ex race car driver. He sounded adamant doing this would increase power a bit and result in cooler temps as the engine doesn't have to work as hard to draw air/fuel. I thought I'd ask around anyways if there was a legal means to do it. Sounded like an easy way to gain a little power. I guess someone would have an STC out if it was worth doing. I think i'll just stick to my plan to buy a seaplane prop and reduce weight as much as possible.