The International Cessna 170 Association

Hi all...
Anyone on here know anything about putting higher compression pistons in an O-300 in a C170B? Is it “legal”. Does it create dependability issues? I’m looking at an aircraft to purchase that had it done about 100 hours ago. Not sure if it was “official” or bootleg. Thoughts? I just read on another site where guys have used C-85 pistons...the post said they are sligtly taller upping the compression slightly and giving 15-30 more hp. ???

I have “seen” the previous mentioned pistons fitted . It seemed to work well but oil temperatures were a little higher necessitating a external oil filter with a fin pack fitted to the oil filter .we will not talk about the legality of the mod as it was done NOT in the USA .but the extra engine power was welcomed and about 250 hrs was done on the engine before it seized DUE to sabotaged. This was done about 10 years ago and continental was talking at one point making it a STC . This never happened and a change of management at the company ,the matter was “forgotten”. But there is a 170B in Thailand thats been flying for many years with this configuration .
If you want extra power and performance obtain a STC and fit a lycoming . This makes a real Fun machine .

From my research, the C85 pistons generally won't "bolt in" and require the tops to be chamfered slightly.
The aftermarket pistons are lighter, balanced better, and work well with many, many flying in the EXP/HB arena.

As mentioned, there is no STC to put these on our planes...but the EXP/AB guys who do this generally also open up the exhaust (no mufflers) and advance the timing to 28-32* and claim about 10-11% more HP, so about 160HP out of a C145/O300.

TFA170 wrote:From my research, the C85 pistons generally won't "bolt in" and require the tops to be chamfered slightly.
The aftermarket pistons are lighter, balanced better, and work well with many, many flying in the EXP/HB arena.

As mentioned, there is no STC to put these on our planes...but the EXP/AB guys who do this generally also open up the exhaust (no mufflers) and advance the timing to 28-32* and claim about 10-11% more HP, so about 160HP out of a C145/O300.

This is not intended to be "argumentive" at all...., only a contribution to the statement...

The O300/C145 engines only develop 145 horsepower when they are turning 2700 RPM. Therefore, a claim that some modification increases output some/certain "percent" is a dubious claim if that is claimed to result in some/certain HP.... because that claim would also have to accomodate a stated RPM at which that HP was developed.In fact, Continental/TCM themselves managed to increase output to 175 HP and all they did was turn the engine up to 3400 RPM. The problem THEN became that of propeller efficiency-losses because the prop would exceed the speed of sound. The solution to THAT problem was...... install a reduction gear-box between the engine and prop and call it a GO-300.
That engine STILL could not produce claimed HP because again, it involved a fixed-pitch prop and the engine never made rated RPM. Their next attempt to get that HP was to mate the engine to a Constant Speed prop... and THAT WORKED fine! Except that it was vary RARE McCauley C-20 prop which came out with an AD note requiring expensive modification to the prop (truncated threads) and only two shops in the world was approved to do that mod (one in Canada and one in Germany) and the airplane did not perform all that much better than the basic 172 in cruise flight and the engine lost 600-hours of recommended TBO. To top that off, the 20% (claimed) increase in HP made the engine fairly unreliable in the hands of average pilots.

In aviation .... EVERYTHING is a compromise. You can't get something for nothing. What one has to decide is.... is the trade-offs worth it?

Until I win the Lottery I'll remain satisfied with 8 gph @ 120 mph and $75/hr direct ops costs with the world record-holder for endurance... a legal O300.

gahorn wrote:
This is not intended to be "argumentive" at all...., only a contribution to the statement...

The O300/C145 engines only develop 145 horsepower when they are turning 2700 RPM. Therefore, a claim that some modification increases output some/certain "percent" is a dubious claim if that is claimed to result in some/certain HP.... because that claim would also have to accomodate a stated RPM at which that HP was developed.

Agreed. I think it is generally claimed at rated HP RPM, in this case, 2700. And since a HP curve isn't linear, it's difficult to assess exactly how much increase, as either a hard number or a percentage, exists at a more usable RPM.

But if we assume we're getting approximately 120 HP out of our C145/O300 at 2450, and we use the 8-11% (which also is assuming it's an across-the-board percentage increase, which it's likely not), we'd have somewhere between 9-14HP more for takeoff. Will that make much difference? I'm not certain it'd be very noticeable. And the engine wouldn't be legal in our planes...so a fair amount of legal risk for very little gain.

A legal constant speed prop would go much farther in terms of usefulness...

In the early-years of the V-tails Beech used a Beech-Roby electric prop. A rheostat controlled an electric servo that controlled the blade-pitch. (In later years someone even came up with an improved controller that allowed the pilot to select a particular RPM and the electro-servo would re-pitch the blades continually to cause it to behave as a CS prop.) I'd love to have even the earlier, simple electric option on our 170s. One could pitch it fine for takeoff and then coarse for cruise, or anywhere in-between. While we're day-dreaming ...maybe we should also wish for a wet-bar... and a flight attendant... like Mickey Rooney...
https://www.youtube.com/watch?v=i415QwSj0Og

and the rest of the story...:
https://www.youtube.com/watch?v=xsu3hiP ... 415QwSj0Og

Increasing the compression ratio increases power output at all MP/RPM. If you know the before and after CRs you can look up the power increase on various automotive web sites. For example, if you increase the compression ratio from 7.0 to 8 (I made that up, I don't know the CR with C-85 pistons), 145 hp turns into 149hp.

Not that I think using C-85 pistons is a good idea, just saying that you can calculate the power increase.

In that example that effort would be an increase of HP in the range of 2% and result in an insignificant performance increase with a resultant increase of fuel-consumption and reduction of TBO and reliability. Not worth it, IMO. A pitch-change of the prop would accomplish similar results for less money and is already legal. (Of course, any change is a compromise...like all things aviation.)

Lightspeed Engineering's 9.4:1 pistons would give you around a 5% bump in horsepower, up to 152.8 hp, using the calculator above. Since we have to burn 100 octane anyway it would be nice having a higher compression engine but one can only dream at this point as they are for experimental aircraft. Now combined with an MT constant speed propeller ... .

n2582d wrote:Lightspeed Engineering's 9.4:1 pistons would give you around a 5% bump in horsepower, up to 152.8 hp, using the calculator above. ... ... .

... that would be at 2700 RPM (where an O-300 makes 145 HP)..... and that is the problem.