The International Cessna^® 170 Association

In these rather intricate calculations for the various comression ratios, which connecting rods are being used? Is there a difference between C-85 and C-145 rods? I suppose there is, but I have no idea what it might be.

blueldr wrote:In these rather intricate calculations for the various comression ratios, which connecting rods are being used? Is there a difference between C-85 and C-145 rods? I suppose there is, but I have no idea what it might be.

I don't think anyone mentioned changing connecting rods, so I assumed that only the pistons would be changed. What I took from Bruce's post was that the wrist pin hole in C-85 pistons is 1/8 to 1/4 inch further from the top surface than the wrist pin hole in O-300 pistons, effectively making the C85 piston that much "taller" than the O-300 piston. That would mean that only that difference would affect compression ratio when C-85 pistons are used in a C-145/O-300.

Miles

blueldr wrote:In these rather intricate calculations for the various comression ratios, which connecting rods are being used? Is there a difference between C-85 and C-145 rods? I suppose there is, but I have no idea what it might be.

N9149A wrote:...if you swap a C-85 piston for a 0-200/C-145/0-300 and do nothing else you are increasing piston travel measured at the top of the piston at TDC at a minimum .125" and maybe as much as .25". That is one heck of an increase in compression either way.

I assumed that only the pistons would be changed. What I took from Bruce's post was that the wrist pin hole in C-85 pistons is 1/8 to 1/4 inch further from the top surface than the wrist pin hole in O-300 pistons, effectively making the C85 piston that much "taller" than the O-300 piston. That would mean that only that difference would affect compression ratio when C-85 pistons are used in a C-145/O-300.

Miles

johneeb wrote:
Why didn't we have Excel when I was in grade school?

That's what I used, only I used the wrong cells to compute the new C.R.'s in my first post. Garbage in, garbage out...

Miles

Bruce,
in your third paragraph above you mention "increased piston treavel" with the C-85 pistons. Not true. Piston travel is controlled only by the crankshaft stroke.
The piston head would raise higher in the combustion chamber, thus raising the compression ratio, but it also would not go as low at the bottom of the stroke. There would be no change in displacement.

Miles,
In your calculation of the compression ratios, did you allow for the bottom of the stroke difference? The sweep volume would remain the same.
Your calculation of the higher of the two examples seems a little too high to me. It also seems to me that a 1/4" piston top raise is excessive.

C-85 pistons will not change stroke but do increase compression. If you do use C-85 pistons in a 0-200 or 0-300 they will need a bevel machined at the top to allow valve clearence. I have flown an 0-300 with 9 to 1 pistons and the difference over stock is very pronounced on take off and rpm increase. With higher compression the engine sounds more like a 0-470 or a hot rod ( I happen to like the hopped up 0-300 but it probably isn't worth the trouble ).

blueldr wrote:Miles,
In your calculation of the compression ratios, did you allow for the bottom of the stroke difference? The sweep volume would remain the same.
Your calculation of the higher of the two examples seems a little too high to me. It also seems to me that a 1/4" piston top raise is excessive.

Since John Barret's calcs agreed with my second try out to two decimal places, I'm fairly certain it's right. You are right that the swept volume (displacement) doesn't change (because neither the bore nor the stroke change), but the total cylinder volumes at the top and bottom of the stroke both decrease by the volume taken up by the "taller" piston. The compression ratio for a 1/4" taller piston seems high because the volume taken up by the taller piston takes a larger percentage of the volume at the top of the stroke than it does at the bottom of the stroke. Since that smaller volume is in the denominator (on the bottom) of the calculation, the result gets larger. It's entirely possible to fabricate a piston that would take up the entire clearance volume, which would make the compression ratio infinite. In other words, the compression ratio gets bigger much faster than the increase in volume taken up by the piston.

The 1/4" is probably not real, just the outside range that Bruce could remember. The real C-85 piston is probably somewhere between 1/8 and 1/4" taller than an O-300 piston resulting in an increase in compression ratio to somewhere between 8.44 and 10.79. IF the real number was 0.1615" taller, the new compression ratio would be 9.

Miles

Johneeb was saying something like having a computer in school......wish we would have had calculators let alone a computer,I don't know if I can remember how to use a slide rule anymore either

Miles
Now that you have that all puuted out, @ the comp ratio of 9 what would the ponies be?

bluelder were you talking about the cylinder and all or just the piston?


W.

Ok I dug out the C-85 pistons which was right next to the 0-300 pistons. Viewing the picture the C-85 is on the left and the 0-300 on the right. The pistons are aligned with the a wrist pin between them. You can see the C-85 piston is .125" taller than the 0-300. Though it doesn't make any difference the bottoms of the pistons are actually even though they don't look that way in the picture because of the angle I took the shot trying to show the 1/8" differences at the top.

Now that we know the actual difference what would the new compression be?

W.J.Langholz wrote:Johneeb was saying something like having a computer in school......wish we would have had calculators let alone a computer,I don't know if I can remember how to use a slide rule anymore either

I saw my first electronic calculator as a senior in high school in 1973. We still had to learn the slide rule in freshman Basic Engineering class, but were allowed to use non-programmable calculators. Laptops now are required equipment for both of my daughters in college, one in engineering, and one in nutrition. I suppose most slide rules are either in attics, musuems, or the garbage. , (However I do still keep an E6-B in the airplane and use it from time to time.)

W.J.Langholz wrote: Miles
Now that you have that all puuted out, @ the comp ratio of 9 what would the ponies be?

W.

Sorry, but I retained more geometry than thermodynamics.

N9149A wrote:...You can see the C-85 piston is .125" taller than the 0-300. ...Now that we know the actual difference what would the new compression be?

8.44:1

Miles

If the C-85 cam is more "powerfull" (for lack of a handy term).... then wouldn't it be a simple matter to use that in lieu of an O-200 cam? (I would certainly be curious to compare the lobes on an IO-360 cam with an O-300)

gahorn wrote:If the C-85 cam is more "powerfull" (for lack of a handy term).... then wouldn't it be a simple matter to use that in lieu of an O-200 cam? (I would certainly be curious to compare the lobes on an IO-360 cam with an O-300)

Yes George but that would not be legal as there is no approval to do it and you would still have a 0-200 with it's limitations which probably are not as liberal as the C-85 limitations. For one thing there is no approval to hang a 0-200 on my Clipped Wing Cub but there is the C-85. For another the static rpm limit of the C-85 is 2525, red line is 2600 rpm. So you can prop the C-85 so that it develops it's full potential. The 0-200 like our C-145 rarely if ever develops it's rated HP because it can not be legally propped to do so at least on takeoff and climb.

Now if you are talking theoretically or for experimental use you would be right.

An engine with a compression ratio of 7:1, and ignition timing of 28 degrees BTDC was probably pushing the limit of the 80 octane fuel that was recomended for it in 1948. Higher compression, advanced timing and better air flow can really 'perk' up an engine even at low (2700) RPM's. The 0-300's are an antique design that probably would not take too much of a performance increase due to the increased stress on the bearings, rods and the flanges on the cylinders that hold the cylinders to the the crankcase,,, But,,,, with the better fuel available now which should prevent detonation I do believe you can get at least 180 HP and be totally dependable. Keep the compression to 8.5 to 9.00 to 1, bump the timing to 32 degrees BTDC, install an oil cooler then re-route the intake air thru a custom manifold so that it stays cool (think density altitude)(why heat the incoming air after the carb ice?), clean up the ports (think smoother airflow, Not larger ports, the ports around the valve guides are Horrible and very restrictive) any of these modifications will add power to any engine thru the entire operating range with no need to spin the engine faster, though I bet it would Want to rev much better especially on take-off,,, bet it would use less fuel at cruise RPM too !
my opinion
Dwain

Dwain your are absolutely right. These engines are crude by todays automotive standards. There is plenty of room for improvement which doesn't necessarily mean loss of reliability.

Sure is nice to dream about it anyway.