The International Cessna^® 170 Association

I am studying the Mountain and Canyon Flying Training Manual published by McCall Mountain/Canyon Flying Seminars LLC. I am trying to calculate takeoff horsepower available at different Density Altitudes. In order to this correctly I must determine what my available takeoff horsepower is at sea level. The 170 Owner's Manual states 145 b.h.p. is available at 2700 RPM. I have the 7653 prop installed which results in 2300 RPM at full throttle takeoff. So I assume I am producing something less than 145 b.h.p. I figure at 2300 RPM I am producing approximately 85% of the b.h.p. produced at 2700 RPM or approximately 124 b.h.p. Does this make sense or am I completely off base?

Makes good sense to me.
Its been discussed on this forums several times before. Our motors only make 145 @2700rpm, which you do not get on take off.

Since most airplanes with a fixed pitch prop are not equipped with a MAP gauge, the above formula is likely to be of little use. I've ben rather closely associated with airplanes for about 72 years and this is the first time I've ever heard of that formula.

Measuring with a Micrometer and cutting it with a chain saw. Works every time.

"Monty" (Jim Montague) over at the Swift group is an experienced engine man. I'm not denigrating anything he said in that response.
But the response is NOT what you think it is. That "48" engine response is merely a "handy" way to determine 75% power if you don't care to look at the power charts. How can that info be useful? Well...if you're cruising along and want 75%...well, then remember that handly little rule.

BUT.... that little technique has NOTHING to do with determining how much power you are making with any OTHER rpm/MP relationship. The "48" rule has no official status. It's only a device dreamed up by someone who discovered it to be a "handy" "rule-of-thumb" relationship.

The O-300 Operators Handbook (found in the Members section of our Forums) has the power charts for these engines.
Those charts will tell you how much horsepower is being developed by your engine. Those charts also show you that at S.L., the 2230-2350 RPM the TCDS allows our engine/McCauley prop combinations for static takeoff power equals about 118-120 HP. Plain and Simple.

If you want more HP out of this engine, you're going to have to turn more RPM. (That's what they did when they put a gearbox on it and called it a GO-300.) With this engine and a fixed pitch prop, that means you have to repitch to flatter pitch....or supercharge the engine. If you repitch from standard to a lower pitch you'll lose gas mileage and (eventually) top cruise speeds. If you supercharge it, good luck getting it approved.

Hope that helps.

gahorn wrote:If you supercharge it, good luck getting it approved.

You'll have better luck getting the MT electrically controlled Constant Speed Prop approved and at $15,000 it would cost less too

Thanks for all the comments. I will use 120 for my calculations.

bagarre wrote:

gahorn wrote:If you supercharge it, good luck getting it approved.

You'll have better luck getting the MT electrically controlled Constant Speed Prop approved and at $15,000 it would cost less too

YEs...but you'll still become horsepower limited at altitude, while supercharging will help overcome the altitude losses.
There used to be a guy in Pagosa Springs who had a turbo-normalizing set up he sold to Bonanzas and a few others. Don't know if he ever tried it on O-300 engines. I doubt it. Most owners of most airplanes using this engine realize it's an economy airplane...not a high-performance one.

Most all of my flying is below 1,500' agl so a super charger would be wasted on me.
However, yesterday density altitude over here was -1,900' and the 170 was REALLY fun to to fly.

You could also raise the engine horsepower by installing bootleg pistons and raise the compression ratio to bring up the BMEP.

blueldr wrote:You could also raise the engine horsepower by installing bootleg pistons and raise the compression ratio to bring up the BMEP.

Where would you install the gauge?

Some of the Cub guys use nitrous oxide for a little takeoff boost. And when the engine blows up it will be a good time for an upgrade.

I know of one stock engine that has been running for years with a bootlegged set of modified Continental C-85 pistons. The owner thinks he is getting about ten to fifteen HP boost, and he claims that it seems to digest 100LL fuel a little better though he normally runs on mogas. I would immagine that some of the formula one racing guys would know what the compression ratio is with the modified pistons.
The engine of which I speak is running in a Swift, not in a C-170, though I don't see where it would make any difference. I really dont know what the Swift uses in the way of diameter and pitch on their prop, but I would guess thet they strive for about the same power numbers as the C-170.

If I did anything to boost HP on an O-300, and didn't worry about being legal, I think the hi-compression pistons would be the way to go. I just wonder why no one ever did an STC on it back when it was easier to get one approved. These engines have always been in need of more HP. Increasing compression is the logical choice, anything else adds complexity. Are the cylinders so weak they couldn't take it?

I've wondered about that as well.

If adding 8:1 or 8.1:1 pistons would have that much of a boost in power, why did Continental spend so much effort trying to develop the GO-300?

One guess could be that they wanted to stay with 80/87 octane or whatever was more available at the time.

Higher compression requires higher octane.