The International Cessna 170 Association

Chris at Airworx just posted this gem on their Facebook:

“Airworx has just gotten approval to install high compression pistons in the Continental O-300-D engine. This will be for 1 engine that can be used as a test bed for a future STC. and will still be a certified engine. We anticipate an increase of horsepower from 145 to approximately 180 with this modification. Airworx is always looking to support our aviation family with new processes.“

When I reached out to him he said that the hope was that the STC would extend to all variants of the o300 including the C145. We shall see.

I know that rumors of sneaking c85 pistons into the o300 have been around for years. It would be amazing if a legal avenue opened for increased o300 power. In a world without 80 octane avgas 7:1 compression is a joke.

Figured some of you might be interested and want to reach out to Chris and help show him that the market for a fire breathing o300 exists.

Having my curiosity aroused provoked me to visit their site and found that to investigate Airworx requires FB participation, …So I will not be spending much time there. However, I must say that the reply to a potential customers’ post (which chided Airworx’s publication of a competitors email-address on FB)…. ended my curiosity.

Airworx wrote : “… I really don’t know who you are or why you feel the need to follow our post and respond with your negative and snide comments. Maybe your drunk, a keyboard warrior hiding behind a cats face, or just stupid!….”

Doesn’t sound like the kind of place with which I’d want to do business.

I'm not a fan of Mark Zuk either. Airworx has a website : https://airworxaviation.com/

My engine is there now getting a major overhaul. I sent him an email to get more details, but the comments on Facebook indicate that the upgraded engine won’t be able to be run on mogas. The test engine hasn’t been built yet from what I gather.

What is the compression ratio they will be using? I can't find it said anywhere. One can predict what octane fuel by experience. But until it is said and done you don't know.
I can say from one engine builder the builds race motors for airplanes 9-1 comp pistons are fine to run 87 octane . No knock / detonation.
Will be very interesting to see what they come up with.

Increasing compression by installation of “taller” pistons certainly can be done… but that method also reduces displacement.

Actually, taller pistons don't decrease displacement. Displacement is the volume of air that the piston pumps through the cylinder: bore area times crank throw. Clearance volume above piston travel affects compression ratio, but not displacement.

DaveF wrote:Actually, taller pistons don't decrease displacement. Displacement is the volume of air that the piston pumps through the cylinder: bore area times crank throw. Clearance volume above piston travel affects compression ratio, but not displacement.

So if space in the cylinder is reduced due to a taller piston…less air is displaced.

GAHorn wrote:

DaveF wrote:Actually, taller pistons don't decrease displacement. Displacement is the volume of air that the piston pumps through the cylinder: bore area times crank throw. Clearance volume above piston travel affects compression ratio, but not displacement.

So if space in the cylinder is reduced due to a taller piston…less air is displaced.

The stroke of the piston is the same, therefore the same displacement.

Displacement = Stroke x Area x (# of cylinders)

GAHorn wrote:

DaveF wrote:Actually, taller pistons don't decrease displacement. Displacement is the volume of air that the piston pumps through the cylinder: bore area times crank throw. Clearance volume above piston travel affects compression ratio, but not displacement.

So if space in the cylinder is reduced due to a taller piston…less air is displaced.

No George. Same amount of air is displaced regardless of the height of the piston. Stroke is determined only by the crankshaft. The top of the higher piston starts higher and ends higher; the stroke is the difference between the two.

cessna170bdriver wrote:

GAHorn wrote:

DaveF wrote:Actually, taller pistons don't decrease displacement. Displacement is the volume of air that the piston pumps through the cylinder: bore area times crank throw. Clearance volume above piston travel affects compression ratio, but not displacement.

So if space in the cylinder is reduced due to a taller piston…less air is displaced.

No George. Same amount of air is displaced regardless of the height of the piston. Stroke is determined only by the crankshaft. The top of the higher piston starts higher and ends higher; the stroke is the difference between the two.

Miles I am struggling to follow this discussion, in a rough algebraic formula please display how you consider the volume of the combustion chamber when calculating compression ratio.

Think about it this way: at the start of the intake stroke there is no air in the cylinder, so at the end of intake the only air in the cylinder has been pulled in by the piston, and its volume is bore area times stroke. The engine breathes in multiples of displacement volume.

Google "does piston height affect displacement" and you'll find more than enough references on the subject. The definition of displacement is bore area times stroke.

CR = (Vd + Vc)/Vc , where Vd = cylinder displacement and Vc = clearance volume at top of cylinder. That's the static CR, not including valve timing, intake efficiency, etc.

DaveF wrote:Think about it this way: at the start of the intake stroke there is no air in the cylinder, so at the end of intake the only air in the cylinder has been pulled in by the piston, and its volume is bore area times stroke. The engine breathes in multiples of displacement volume.

Google "does piston height affect displacement" and you'll find more than enough references on the subject. The definition of displacement is bore area times stroke.

CR = (Vd + Vc)/Vc , where Vd = cylinder displacement and Vc = clearance volume at top of cylinder. That's the static CR, not including valve timing, intake efficiency, etc.

What is the proper term to use when defining (Vd+Vc)?

I’m having trouble with understanding this also. If the taller piston is installed … then the amount of air inside the cylinder at the bottom of the stroke is less than if a shorter piston were used. Is this considered “swept” volume?

GAHorn wrote:I’m having trouble with understanding this also. If the taller piston is installed … then the amount of air inside the cylinder at the bottom of the stroke is less than if a shorter piston were used. Is this considered “swept” volume?

If only the height of the piston is changed, then on the intake stroke of the engine the same volume of air/fuel mixture is drawn into the cylinder as before. Yes the distance at the bottom of the stroke from the piston to the top of the cylinder is shorter, but that only affects compression ratio. The key is that once the intake valve opens the same volume increase occurs drawing in new air/fuel because the stroke (and therefore displacement) didn't change.