C-85/O-200 conversion
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wanttaja
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Not sure what you mean by C-85 to O-200 "conversion"; you can install some O-200 parts, but the engine remains a C-85 and you don't get any more horsepower.
Harry Fenton is the Fly Baby engine guru, here's what he says:
Unfortunately, there is no STC for just O-200 cylinders. However, the basic C-85 and O-200 cylinders are nearly identical, except for valve springs and minor details. If you have a set of O-200 cylinders, you might want to see if you can have a cylinder shop convert the O-200 cylinders to a C-85 configuration. If you do this, then there are no major changes to the engine. Just bolt the cylinders onto the engine.
By the way, installing the O-200 crankshaft, rods and cylinders does not increase horsepower on a C-85. If legally installed and legally run to the limits detailed by the STC, the parts change provides no increase in engine horsepower. The increase in legal horsepower is an Internet wives tale or at least a continual re-telling of incorrect information.
Yes, when the crank, rods, cylinders and piston are bolted the O-200 are bolted to the C-85, the C-85 becomes mechanically similar to the O-200 in crankshaft throw and displacement. However, the C-85 is legally limited to 2575 rpm whereas the O-200 is rated at 100 hp at 2750 rpm. The difference in rpm limits the modified C-85 to 85 hp. I think that most customers report that the engine runs stronger, which is probably true. But, if you think about it, the primary reason to convert to the O-200 parts is because the C-85 is runout and in need of a rebuild. Of course, once the engine is rebuilt with any sort of new parts, the previous “tired” engine is going to feel like a “new” engine with lots of power.
http://www.bowersflybaby.com/tech/fenton.html#O-200_Cylinders_on_a_C85
Ron Wanttaja
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wanttaja
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With an STC it would be legal, but I am not aware of one.
Harry Fenton again:
Be aware that the C-85 pistons are not legal to install into a certified engine....
The p/n for the C-85 piston is 40327. The piston needs to be modified with a ¼” wide, 45 degree chamfer at the piston crown to clear the edges of the cylinder combustion chamber. However, the cylinders vary, so some experimentation may have to done to get the right fit. The mod bumps the compression ration from 7:1 to about 7.8:1 and theoretically will add about 4-5 hp. The C-85 pistons will make a bit more power, but the change is not a major gain.
http://www.bowersflybaby.com/tech/fenton.html#c85_pistons_in_o200
Harry did talk with someone who was trying for an STC for putting the pistons in an O-200....
STC projects are a fairly big project, especially for a project which will affect engine performance. Typically, some sort of calibrated dynamometer will be required to obtain baseline data and to measure the enhanced performance. Typical parameters which the FAA will want to document is horsepower, torque, CHT, EGT, and then calculations for torsional load on the engine. All of the data will require an engineering plan along with the supporting documentation. Overall, a pretty big project.
http://www.bowersflybaby.com/tech/fenton.html#STC_Piston
Ron Wanttaja
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Me either.
birdus
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Jay Williams
How is it possible that an increase in displacement results in no increase in horsepower? The only way I could imagine would be a decrease in compression ratio. The C-85 has a compression ratio of 6.3. Could the O-200 conversion really have lower compression than that?
Grum.Man
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Because HP is more influenced by RPM than anything. The small increase in displacement will not make a measurable HP difference if kept to the old RPM limit. The TQ will be higher for sure which is probably what most people are feeling. Or the more likely reason is that people ignore the RPM limitation and run it to o-200 specs.
If it makes more torque at the same RPM it is making more power. Rated horsepower may not change however, depending how things are set up.
Tom-D
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Using C-85 pistons rises the compression ratio to 9/1.
the 3, 0-200's I have done for the Very EZ produced 140 HP, at 2700.
That concluded the fuel injection, electronic ignition, and large valves.
all 3 engines self-destructed about 1000 hours.
the 3, 0-200's I have done for the Very EZ produced 140 HP, at 2700.
That concluded the fuel injection, electronic ignition, and large valves.
all 3 engines self-destructed about 1000 hours.
Im curious, what failed in those engines? When you say “self destructed” are we talking catastrophic with rods hanging out of the case or something repairable?
I should ask my friend who raced o-200s at Reno what they expected the lifespan of the engine to be. 1000 hours doesn’t seem unreasonable for an engine closer to the edge.
birdus
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kyleb
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Drake the Outlaw
I think the truth on these engines is that they are stronger than the stock C-85. The people who have them are generally very pleased and speak to the increased power. However, for certification purposes (and the purpose of the STC), they can't be advertised that way. Maybe the power increase falls inside the "rated HP +/- X%" window these engines are certified under.
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Timbeck2
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dafuq? 
kyleb
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Drake the Outlaw
dafuq?
LOL. Fixed it.
Grum.Man
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This is my point, a large TQ increase only nets a small HP increase if no other variable changes.
wanttaja
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Being contrarian here....
They take a tired, worn out C85 and install new pistons, cylinders, and a crankshaft. It's likely they're going to sense the engine being stronger than before, no matter whether the parts were originally meant for a C85 or O-200. PLUS, after spending the time doing the work they'll probably *never* admit that it wasn't stronger.
It make sense that it's got more HP and *is* stronger, but I'd like to see some actual data.
Ron "The plural of 'anecdote' is not 'data'" Wanttaja
kyleb
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Drake the Outlaw
No need to be contrary. Aren't you going from 190 cubic inches to 201 cubic inches (~5% increase) with the conversion? That's where the additional power comes from.
wanttaja
En-Route
I don't doubt there's "some" improvement. But you have to wonder how much of the owner's perceived improvement is due to having a newly-overhauled C85 replacing a tired engine, regardless of where the parts came from.
Again, I'd like to see test data vs. the "It's stronger" assessment of an owner with a significant monetary investment in it BEING stronger....
Ron Wanttaja
kyleb
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Drake the Outlaw
Confirmation bias. All us aviators do it. Bo>RV-10>Comanche>Mooney>Bo... Whatever you own (or recently purchased) is clearly superior.
d.grimm
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My C-85 has the O-200 crank STC and was run on a dyno when complete.
92 HP at C-85 RPM's.
Dave
92 HP at C-85 RPM's.
Dave