I've had too much time on my hands the past couple days and starting thinking about the O-300. Everyone always talks about how they are smooth, reliable engines but just not enough power. How bout we change it up a little. Not sure I'd ever do this but it is fun to talk about, IMO.

A google search will bring up some info about installing C-85 pistons in an O-300 with claims of netting 165-180hp. True or not, who knows.

The GO-300 is similar to the O-300, same stroke and bore but has a 7.3:1 vs the O-300 7.0:1 compression ratio. Additionally the GO max rpm is 3200 and produces 180hp. The O-300 max rpm is 2700 and produces 145hp. I've been staring about the power charts for these two engines and I think they follow pretty close to the same curve. Does this mean if the O-300 is ran up to 3200rpm it will produce ~180hp? Possibly. Even if it did who knows how long it would stay together at that rpm.

O-300


GO-300


How bout we combine the C-85 pistons with the O-300 and run it at say 2900 rpm max, I wouldn't want to turn a prop any faster than 2900. I wonder what HP would be produced.

Now let's take the custom pistons Bob Barrows designed to go in his C-85/O-200 hybrid and put them in the O-300. Bob says the compression ratio is 8.8:1 in his engine and he run 93 octane fuel. I don't know if the crank stroke is the same between the O-200 and the O-300 but if it is then how much power will the O-300 produce with a compression ratio of 8.8:1? Bob estimates somewhere around 100-110hp for his C-85/O-200 at 3000rpm. The custom pistons are also lighter so they will reduce the stress on the crank; maybe this will help durability when running the O-300 at higher rpm.

Next we look at the O-300E. Apparently this is a rare engine that is identical to the O-300C but with the addition of a governor pad, in front of #6 cylinder I think, and crankshaft provisions to supply governor oil to the propeller. Problem here is the "rare" part. Where we going to find a O-300E? According to the parts manual the C and the E crank will interchange but I don't know if that means the C crank can be made to supply oil to the propeller. I know Bob can modify Lyc O-360 fix pitched cranks to allow the use of a CS prop, maybe he could do the same here. The prop flange on the C crank is an ARP 502 type 1 which if google isn't lying is the same flange used on the IO-360 so finding a prop to fit the flange isn't a big deal. The parts manual says all the cases are interchangeable so I don't know how that will work because I'm sure very few cases have the governor pad. I can't imagine it would be possible to machine the pad into a case that doesn't already have one. This will be the tough part.

So if it all went together, which it won't, we'd have a 180ish HP O-300 with a CS prop. Sounds like fun eh...probably just me that gets a kick out of stuff like this.

Note: dry weight is listed at 268lbs.

Someone is going to say, "just install an O-360 and be done." Yep, probly the easiest way to go.

I think it's a great idea Whee! I would try to honor the existing redline, unless you could find some racers who have spooled these engines up a little higher that have advise. Maybe the Swift guys have some ideas too? Prop to engine dynamics can be an elusive beast that can bite real hard. Any GO 300 references are kind of irrelevant, as the crank/gearbox/case config leaves the top end as about the only thing that these two engines have in common. I know that racers turn IO-550s up around 3000rpm at Reno. What airframe do you have in mind for this? I have heard of the C85 pistons in the O-300. The teller swore that it would outclimb an O-360 powered 170 with an 8048 prop. There are plenty of these engines available around the $3000.00 mark on Barnstormers.

It would be ideal to stick with the 2700rpm redline but with lighter pistons I think a little more would be ok. Might have to do some asking around to see if anybody has experience spooling these up. I know guys spool the C-85 way up without problems. Does Continental have a reputation of overbuilding the bottom end?

You might be right about the Go-300 references. I thought it could be relevant because the top end is common and the power charts looked very similar, almost the same except the Go-300 shows up to 3200rpm. I'll see if I can post them.

For an airframe I'm thinking a Mexican Maule AKA: Bearhawk

Just bolting on different pistons doesn't always work too well.
Gotta know if there's any differences in piston pin height, piston dome height, ring types, etc. Then there's rod length that's matched to the stroke and piston pin height, and stroke length... Lightened pistons may be good, but cranks are balanced with rod and piston weight in mind.

But hey, it may work out just fine.

I was told by an engine builder in Central California, that if you take C 85 Pistons and put them in O-300 cylinders, preferably something like Titans and Polish the ports, for every point of compression ratio you increase you gain 15 hp +15 hp for the port polish. You got to gouge out a little meat on the top of the piston so it doesn't bend your valves at top dead center. Then if you do a little grinding on the camshaft you can pick up about another 10 hp according to this builder. Problem is you no longer have a certified engine. He also said that the bottom end of that engine is pretty darn strong. Do it

UngaWunga wrote:Just bolting on different pistons doesn't always work too well.
Gotta know if there's any differences in piston pin height, piston dome height, ring types, etc. Then there's rod length that's matched to the stroke and piston pin height, and stroke length... Lightened pistons may be good, but cranks are balanced with rod and piston weight in mind.

You are right about all this stuff and it is already considered. Crank/piston balance is important but at least we aren't going the other way...heavier pistons.

Hi Wee
Something to consider would be to use Bobs pistons, port and flow the heads, tuned headers, SDS engine control and fuel injection system, you would have an honest 200hp. Some guys are at 195 with the carberator pkg. Also talk to Gene down at magnum motors in Texas about his Go-300 and o-300 exp packages

whee wrote:

UngaWunga wrote:Just bolting on different pistons doesn't always work too well.
Gotta know if there's any differences in piston pin height, piston dome height, ring types, etc. Then there's rod length that's matched to the stroke and piston pin height, and stroke length... Lightened pistons may be good, but cranks are balanced with rod and piston weight in mind.

You are right about all this stuff and it is already considered. Crank/piston balance is important but at least we aren't going the other way...heavier pistons.

Very true. But increasing compression ratios tends to mean taller piston domes, so you need to do some test assemblies with putty to see valve clearances, as another poster has said.

Interesting that these old pushrod engines are limited to <3K rpms. I wonder what the rpm limitation really is. Lifters, piston speed (lol), engine efficiency due to stone age design... If a port/polish job will add 10-15hp, then its most likely the later.

UngaWunga wrote:Interesting that these old pushrod engines are limited to <3K rpms. I wonder what the rpm limitation really is. Lifters, piston speed (lol), engine efficiency due to stone age design... If a port/polish job will add 10-15hp, then its most likely the later.

I wonder also. I know very little about the O-300 but I have some experience with the C-85. I was told by a reputable engine builder that the C-85 with O-200 crank will produce 130hp at 3200 rpm but the valves start to float at that rpm. That seems to indicate that it isn't a crank or piston speed issue but a valve train speed issue. I also talked with a guy that built a C-85/O-200 hybrid for his formula1 race plane. Different rods and pistons but had a modified factory O-200 crank; idle speed was 3500rpm Doesn't really apply here because it is so modified but it is interesting.

I once talked to a guy who worked at the Anchorage Aircraft Certification Office who told me that the C-85 piston mod could be made to the O-300 legally without a field approval or anything. He told me that the way that the paperwork was written on the cylinder assemblies, you could just substitute these parts. I wanted to do that with my old engine, but never ended up following through on it. This would be worth looking in to for one of the 170/172s here on the site if you are planning to top your engine anytime soon. I would love to hear a PIREP.

The key for me to really consider this engine on a Bearhawk is the CS prop. I'm going to bet finding an O-300E would be near impossible.

Guess there is always the MT electric CS prop.

Whee,

Interesting idea! I have an O-300D in my 170B and like it but would appreciate the extra horsepower. I might look into the C-85 pistons situation when my time comes for a top end. One idea that hasn't been touched on yet for an experimental application would be fuel injection for more consistent EGT/CHT and ability to run LOP. Since that seems complicated you might want to consider a repurposed Continental IO-360. I have no idea what they weigh or cost, but it is your money and it sounds like fun.

whee wrote:A google search will bring up some info about installing C-85 pistons in an O-300 with claims of netting 165-180hp. True or not, who knows.

My info is secondhand, from talking with the former owner of one of these jobs. He saw a definite performance boost, but not close to 180hp. More like half of that.
-DP

BeeMan wrote:Whee,

Interesting idea! I have an O-300D in my 170B and like it but would appreciate the extra horsepower. I might look into the C-85 pistons situation when my time comes for a top end.

Not legal, unfortunately.
-DP

To be honest that is what I expected. If it was legal we would probably all have converted a long time ago.

denalipilot wrote:

BeeMan wrote:Whee,

Interesting idea! I have an O-300D in my 170B and like it but would appreciate the extra horsepower. I might look into the C-85 pistons situation when my time comes for a top end.

Not legal, unfortunately.
-DP

If the FAA ever passed the Non-Commercial category that the EAA proposed we would be able to modify our planes without field approval or STC. Then we would be able to legally put C-85 pistons on our 170s if we wanted.

I think this would be a cool way to go for a bearhawk, if I were doing it I would put on a prince prop because it's light and you could have them pitch it any way you want for the type of flying you want to do

Bhawk wrote:I think this would be a cool way to go for a bearhawk, if I were doing it I would put on a prince prop because it's light and you could have them pitch it any way you want for the type of flying you want to do

Are you disappointed with your new Catto?

Doesn't the 175 geared engine turn a way higher rpm? Its an O300 too isn't it?

The IVO magnum electric prop would also make a good choice for the modified o-300. There is 1500hrs on the 275hp Bearhawk we built and no trouble with the prop.