Hi all...

Anyone on here know anything about putting high 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?

Phil

Call Dan Ellis up in Wasilla, he knows about this
602-538-3162

I did...thank you. He was very kind and informative!

So what came of the conversation? Just interested to hear some others thoughts on it.

I am equally interested in this conversation, if your willing to share it

Mapleflt

Yes, I too would like to know.

Count me in as curious as well.

Thanks.

Frank

Dan said that there are a number of airplanes up in Alaska running with higher compression pistons. However there is no STC for it and so your engine becomes Experimental. He stated that the original pistons gave 6.5:1 compression (sounds really low to me...?) and that the higher compression C-85 pistons yield about 7:1. He thinks maybe 15 more horsepower. He said he has also run 9:1 compression pistons in the O-300 and it gives close to 200 hp. The pistons come from Lycon in CA and are about $250/ea.

I’m looking at a 170B where the pistons were changed to the higher compression (not the 9:1) and the owner says he would never go back to the original after experiencing the performance difference. He’s flown 100 hours with them. I’m concerned about reliability and the fact that if I ever sold the airplane I would either have to change them back or really limit my potential buyer pool. The airplane is a ‘54 with Horton STOL, 26” Bushwheels, Cleveland double pucks. Owner has all logs and the plane has no damage history. I have an 80x42 prop I can buy used that I’d like to put on it. The plane needs a panel redo but for $38K it might be a decent buy. Paint and interior look like about a 7 from the pics. Planning on driving to see it in a few days.

BirdyinBOI wrote:Dan said that there are a number of airplanes up in Alaska running with higher compression pistons. However there is no STC for it and so your engine becomes Experimental. He stated that the original pistons gave 6.5:1 compression (sounds really low to me...?) and that the higher compression C-85 pistons yield about 7:1. He thinks maybe 15 more horsepower. He said he has also run 9:1 compression pistons in the O-300 and it gives close to 200 hp. The pistons come from Lycon in CA and are about $250/ea.

I’m looking at a 170B where the pistons were changed to the higher compression (not the 9:1) and the owner says he would never go back to the original after experiencing the performance difference. He’s flown 100 hours with them. I’m concerned about reliability and the fact that if I ever sold the airplane I would either have to change them back or really limit my potential buyer pool. The airplane is a ‘54 with Horton STOL, 26” Bushwheels, Cleveland double pucks. Owner has all logs and the plane has no damage history. I have an 80x42 prop I can buy used that I’d like to put on it. The plane needs a panel redo but for $38K it might be a decent buy. Paint and interior look like about a 7 from the pics. Planning on driving to see it in a few days.

I'd say be happy you found an honest guy who was willing to be transparent about it all. The "illegal" piston mods are often a hush-hush affair.

Has anyone ever gathered data on what the cylinder pressures are with pistons as high as 9:1? And how that might compare to the stock pistons and other known uses of the -300?

BirdyinBOI wrote:Dan said that there are a number of airplanes up in Alaska running with higher compression pistons. However there is no STC for it and so your engine becomes Experimental. He stated that the original pistons gave 6.5:1 compression (sounds really low to me...?) and that the higher compression C-85 pistons yield about 7:1. He thinks maybe 15 more horsepower. He said he has also run 9:1 compression pistons in the O-300 and it gives close to 200 hp. The pistons come from Lycon in CA and are about $250/ea.

I’m looking at a 170B where the pistons were changed to the higher compression (not the 9:1) and the owner says he would never go back to the original after experiencing the performance difference. He’s flown 100 hours with them. I’m concerned about reliability and the fact that if I ever sold the airplane I would either have to change them back or really limit my potential buyer pool. The airplane is a ‘54 with Horton STOL, 26” Bushwheels, Cleveland double pucks. Owner has all logs and the plane has no damage history. I have an 80x42 prop I can buy used that I’d like to put on it. The plane needs a panel redo but for $38K it might be a decent buy. Paint and interior look like about a 7 from the pics. Planning on driving to see it in a few days.

I think someone is getting their numbers mixed up.

The O-300 makes 13% more power than a C-125 because they increased the stroke (displacement), compression as well as RPM redline.

Increasing compression alone will increase efficiency of the engine but not it's power output. You'll burn less fuel for a given power setting. This will make the engine feel stronger but it's not going to increase power by 25% to 200hp @2,700RPM. If it was that simple, why did TCM develop the GO-300? (Because to increase power at a given displacement, you need to increase RPMs)

How much of that "never go back" feeling is comparing a tired run out O-300 to a fresh overhauled O-300.

I am yet to see actual dyno numbers comparing engines but changing compression ratio alone does little to nothing to increase power output at a given RPM.


C-85
HP: 85 @ 2,575
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-125
HP: 125 @ 2,550
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-145/O-300
HP: 145 @ 2,700
Bore: 4.0625
Stroke: 3.875
Compression: 7.0:1

Bagarre wrote:

BirdyinBOI wrote:Dan said that there are a number of airplanes up in Alaska running with higher compression pistons. However there is no STC for it and so your engine becomes Experimental. He stated that the original pistons gave 6.5:1 compression (sounds really low to me...?) and that the higher compression C-85 pistons yield about 7:1. He thinks maybe 15 more horsepower. He said he has also run 9:1 compression pistons in the O-300 and it gives close to 200 hp. The pistons come from Lycon in CA and are about $250/ea.

I’m looking at a 170B where the pistons were changed to the higher compression (not the 9:1) and the owner says he would never go back to the original after experiencing the performance difference. He’s flown 100 hours with them. I’m concerned about reliability and the fact that if I ever sold the airplane I would either have to change them back or really limit my potential buyer pool. The airplane is a ‘54 with Horton STOL, 26” Bushwheels, Cleveland double pucks. Owner has all logs and the plane has no damage history. I have an 80x42 prop I can buy used that I’d like to put on it. The plane needs a panel redo but for $38K it might be a decent buy. Paint and interior look like about a 7 from the pics. Planning on driving to see it in a few days.

I think someone is getting their numbers mixed up.

The O-300 makes 13% more power than a C-125 because they increased the stroke (displacement), compression as well as RPM redline.

Increasing compression alone will increase efficiency of the engine but not it's power output. You'll burn less fuel for a given power setting. This will make the engine feel stronger but it's not going to increase power by 25% to 200hp @2,700RPM. If it was that simple, why did TCM develop the GO-300? (Because to increase power at a given displacement, you need to increase RPMs)

How much of that "never go back" feeling is comparing a tired run out O-300 to a fresh overhauled O-300.

I am yet to see actual dyno numbers comparing engines but changing compression ratio alone does little to nothing to increase power output at a given RPM.


C-85
HP: 85 @ 2,575
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-125
HP: 125 @ 2,550
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-145/O-300
HP: 145 @ 2,700
Bore: 4.0625
Stroke: 3.875
Compression: 7.0:1

So if compression doesn't change HP, why does an 0-320 only run 150hp unless you put higher compression pistons on it? They put you to 160. No RPM change. I know the math, but higher compression does give higher hp.

A1Skinner wrote:

Bagarre wrote:

BirdyinBOI wrote:Dan said that there are a number of airplanes up in Alaska running with higher compression pistons. However there is no STC for it and so your engine becomes Experimental. He stated that the original pistons gave 6.5:1 compression (sounds really low to me...?) and that the higher compression C-85 pistons yield about 7:1. He thinks maybe 15 more horsepower. He said he has also run 9:1 compression pistons in the O-300 and it gives close to 200 hp. The pistons come from Lycon in CA and are about $250/ea.

I’m looking at a 170B where the pistons were changed to the higher compression (not the 9:1) and the owner says he would never go back to the original after experiencing the performance difference. He’s flown 100 hours with them. I’m concerned about reliability and the fact that if I ever sold the airplane I would either have to change them back or really limit my potential buyer pool. The airplane is a ‘54 with Horton STOL, 26” Bushwheels, Cleveland double pucks. Owner has all logs and the plane has no damage history. I have an 80x42 prop I can buy used that I’d like to put on it. The plane needs a panel redo but for $38K it might be a decent buy. Paint and interior look like about a 7 from the pics. Planning on driving to see it in a few days.

I think someone is getting their numbers mixed up.

The O-300 makes 13% more power than a C-125 because they increased the stroke (displacement), compression as well as RPM redline.

Increasing compression alone will increase efficiency of the engine but not it's power output. You'll burn less fuel for a given power setting. This will make the engine feel stronger but it's not going to increase power by 25% to 200hp @2,700RPM. If it was that simple, why did TCM develop the GO-300? (Because to increase power at a given displacement, you need to increase RPMs)

How much of that "never go back" feeling is comparing a tired run out O-300 to a fresh overhauled O-300.

I am yet to see actual dyno numbers comparing engines but changing compression ratio alone does little to nothing to increase power output at a given RPM.


C-85
HP: 85 @ 2,575
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-125
HP: 125 @ 2,550
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-145/O-300
HP: 145 @ 2,700
Bore: 4.0625
Stroke: 3.875
Compression: 7.0:1

So if compression doesn't change HP, why does an 0-320 only run 150hp unless you put higher compression pistons on it? They put you to 160. No RPM change. I know the math, but higher compression does give higher hp.

Should have said no appreciable increase in power.

Going from 7:1 to 8.5:1 goes from 150 to 160hp which is a 6% increase at red line. (and requires 100 octane to avoid detonation)
Even if the curve was linear (which is isnt) that would mean going from 7:1 to 9:1 would take 145hp to 158hp. Still a far reach from 200hp or the 175HP the GO-300 makes at 3,200 RPM.

Bagarre wrote:

BirdyinBOI wrote:I am yet to see actual dyno numbers comparing engines but changing compression ratio alone does little to nothing to increase power output at a given RPM.

C-85
HP: 85 @ 2,575
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-125
HP: 125 @ 2,550
Bore: 4.0625
Stroke: 3.625
Compression: 6.3:1

C-145/O-300
HP: 145 @ 2,700
Bore: 4.0625
Stroke: 3.875
Compression: 7.0:1

I'd rely on actual dyno numbers, or the physics behind it and not depend on Continentals rating as even the numbers you show above demonstrate more HP from compression/stroke.

Here is some of the math.
HP = RPM x tq /5252

The C-125 at 2550 is 257.5 ft/lbs. (125 = 2550 x 257.5 / 5252) so If you can maintain that up to 2700 rpm you get 132.4HP (132.4 = 2700 x 257.5 / 5252)

The C-145 at 2700 is 282.1 ft/lbs (145 = 2550 x 282.1 / 5252) so the C-145 at 2550 would be 137HP (137 = 2550 x 282.1 / 5252 )

Horsepower is a function of work over time, that's why RPM is important. You can make 200HP at 1000rpm with 1050 ft/lbs of torque or 200HP at 10000rpm with 105 ft/lbs. In the latter case you do 1/10th of the work, 10x faster.

Since our airplanes need 2500-2700 rpm, you can get HP by more torque or more rpm (which requires a gearbox). Torque comes from cylinder pressures leveraged by the crankshaft stroke. More compression = more cylinder pressures because there is more volume of air and because to get more compression you also usually end up with more stroke.

Many calculators on the internet show around an 5-8% increase going from 7:1 to 9:1. Other places say 3% per compression point.

If you want the real math, it's here: http://web.mit.edu/16.unified/www/SPRIN ... ode25.html

Notice figure 3.11, going from 7:1 to 9:1 brings up the efficiency a little bit.

Anyway, at the end of the day, I could believe getting into the 155-158HP range with 9:1 pistons, but 200HP? No, that's a WAG, and completely outside of the laws of physics.

Is it worth it? Well, that's 25ftlbs more torque and according to more back of the napkin math, that would buy you around 60 static RPM.

If you are looking for takoff and climb performance, I would guess the 8042 prop performs way better than the extra HP, and it's legal too.

Another option is STOL kit, or light weight panel. I know someone with a stock c-145 that has his 170 down to 1230lbs on 29" bushwheels. It works just fine.

Bagarre wrote:Going from 7:1 to 8.5:1 goes from 150 to 160hp which is a 6% increase at red line. (and requires 100 octane to avoid detonation)
Even if the curve was linear (which is isnt) that would mean going from 7:1 to 9:1 would take 145hp to 158hp. Still a far reach from 200hp or the 175HP the GO-300 makes at 3,200 RPM.

Agreed, more RPM is the easier way to make HP as long as stuff doesn't come apart. Only trouble is that you need to use a gearbox, and those aren't easy to build/maintain. Ask any helio pilot.

Great intel, thanks

Mapleflt

BirdyinBOI wrote:Dan said that there are a number of airplanes up in Alaska running with higher compression pistons. However there is no STC for it and so your engine becomes Experimental.

He is absolutely right about that. If you really want to get this plane back on the up and up, you technically would at least have to do an overhaul after installing and running high compression pistons and if you want to follow the letter of the law, that engine would not be able to be "certified" again. Now I'm all for taking full advantage of all the gray area that is written into the regs and if I was running a O-300 in a 170 I would honestly be very inclined to bump up the C/R for some free power but it is important to understand the legal consequences. I feel confident the extra stress is "probably" built into the margins of the components of the engine and obviously the o-300 is very capable of putting out a lot more power but if you are using parts that are not certified its not "legal".

Old air racing guys would run the o-300's well over 3500rpm, it redlines at 3200 rpm in the GO-300 although there are some subtle differences in the rotating assembly, notably the piston pins are a little larger presumably to handle the increased inertial weight of the pistons moving faster in the bores.

I'm always interested in the conversation related to running higher c/r pistons.

I don't know but I have to imagine this would also torpedo an insurance claim if it was discovered.
-DP

Back to purchasing said aircraft.

If it were me, I would pass unless there was some other factor that made it worth while like price or condition.

If the selling party was willing to put stock pistons and rings in it, I might still buy it as long as the log book simply said replaced pistons. Personally I would be fine with a legal engine that only had the high compression pistons for 100 hours as long as everything looked good when the low compression pistons were installed. I wouldn't care if it wasn't completely overhauled.

If the airplane could be purchased for $20k, I'd overlook the pistons, and probably just run them myself knowing the cost to make it right at sell time was paid upfront.

As for buying a 145hp C170, they are a decent airplane in my opinion, as long as following is observed:

1. It's a 2 place airplane. The only time you will use the rear seat is for pavement to pavement short hops with kids in the back and long runways. If you can get one under 1300lbs, put 375lbs of people in it and 150lbs of gas, then they work okay.

2. It needs a climb prop to work well. The 8042 works well.

3. You need to pay attention to length of landing zone. This airplane will get in WAY shorter than it will get out.

schu

Thanks all for your great comments. I’m gonna pass on the plane. It’s my first airplane to own and I had a funny feeling about the pistons. I want to find one that needs an 8042 prop and a Sportsman STOL and that’s about it. I wanna fly, not rebuild.

denalipilot wrote:I don't know but I have to imagine this would also torpedo an insurance claim if it was discovered.
-DP

Has anyone ever heard of a claim being denied for an unapproved mod, even if the bent metal had nothing to do with the mod?