High Compression Pistons-a bad idea ?

[Aviaman]

I?ve seen O-320 engines with pistons that give a 10:1 compression ratio. This is supposedly done to get 180 hp from the engine. To me, it seems a bad idea since that puts more stress on the cylinders and case that the O-320 was designed for. Also that high compression leaves less margin against detonation.
Do I worry too much? What do you guys think?

[Mike S]

John, welcome to VAF

[F1R]

In general, the more HP you crank out of a given displacement, the lower the TBO. A 150 HP low compression 0-320 can often go 3000 -4000 hours before a rebuild is needed - just ask VLAD who flew a 3000 hour example across The Bering Sea and back- and virtually every other place stateside. The 160 HP versions generally don't go much past 2000 -2200 hours.
10:1 and 180 HP out of a 320.... good luck

[jrs14855]

Facts:
Wide deck 0 320 crankcases are all similar or the same as far as strength goes. There are several variations in cylinder studs vs thru bolts.
O 320 wide deck cylinders are essentially the same as 0 360 except for total cylinder length of 1/2" more for 360.
Some of the connecting rods are stronger, most are similar except for dimensions.
A properly built 10-1 0 320 should last at least 2000 hours which is way more than most people will ever fly with the exception of Comrade Vlad.
However if you put the engine in an airplane like the Pitts and run it 3500 r/m, you are likely to wear out the rod bearings in 700-1000 hours. Not a big deal because the TBO for aerobatic Lycomings is 1200 hours. Changing the rod bearings can be done without splitting the case.
Think of 10-1 as poor mans supercharger. It allows you to get 75% power at a slightly higher altitude. I can still get around 75% power on my 0 320 10-1 non RV at around 10,000' .
Having said all that my current project will get a 360 but with 10-1 pistons. 10-1 pistons are only good for about 10 hp total on 320 or 360. 180 hp 320 has MANY more mods to get 180 hp.

[flyboy1963]

welcome!
....I have to make this one observation whenever someone talks about 'all that power' and 'stress' and TBO etc.

the 180 hp 0-320 is going to be at 2700 rpm, right? How many hours of the TBO is spent at that actual power setting?
average flight?, 60 minutes.
take-off power....., before dialing back the black and/or blue knob? maybe a half-minute?
So you are really stressing all that stuff for about 5 or 10% of it's life....somehow, I don't think that is going to cut the TBO in half!
( meaning, somehow, you are DOUBLING the wear on the innards?)

Yes, detonation and other concerns are valid, so you need to control the ADVANCE, not just the C/R.

lots of smarter folks than me will guide you in the right direction, but most will likely say, why hot-rod an 0-320, when it's cheaper and easier to do a similar rebuild of an 0-360, using light accessories.

[RV6_flyer]

The modern present day parallel valve 320 and 360 use the same crankcase. The parallel valve 320, 340, and 360 all have the same bore, cylinder head, valves, and valve springs. They all get more horsepower from more displacement by stroking the engine. (Different crankshaft and connecting rods.)

The problem with the parallel valve engine using more compression is the cylinder head temperatures go up. At some point, one makes more heat than the cylinder heads can dissipate. The heat CAN lead to shorter engine life. The guys getting 200 HP or more out of the parallel valve 360 will tell you that their temperatures are higher WHEN they are making more horsepower.

The way you will get 180 HP out of a 320 engine with 10:1 pistons (or without) will require that you run the engine above 2,700 RPM. Start thinking about 10:1 pistons and 2,900 RPM.

[jrs14855]

With the 10-1 pistons in my 0 320(Wittman Tailwind), I have had no significant issues with temps including engine break in in 100-120 degree ambient temperatures. First few hours I did have to pull the power back to 24" and increase the climb speed. I initially set the mags at 22 degrees. That no doubt helped the temperature situation. I have a plenum and cowl flap, smaller than normal inlet and outlet. Below about 60 degrees the oil temp runs too cool.
I do not run above 2650 on the Whirlwind GA prop. I may eventually try another fixed pitch prop that will run 2900 full throttle at low altitude. RPM doesn't hurt the Lycoming as long as you keep the temps under control.

[Mel]

I've got about 850 hrs. on 9.5:1 pistons in my -6. Everything works great and temps are all within "normal" range.

[flyinga]

I'd consider future fuel options. In future it may be difficult to find a suitable fuel for a 10:1 engine.