[Bwing96]

Been surfing ad's and found a couple of -6's I may be interested in. Both have the 0-320 Lycoming. One claims 185hp with 10:1 pistons the other is 150hp. Both relatively low time and speed seems to be about the same. The 185 claims a bit less fuel consumption. Would there be any big advantage or reason to avoid one over the other?

 

[walkman]

can't burn car gas in the high compression engine.

seems like an optimistic number for just higher compression pistons, but maybe it is...

 

[SHIPCHIEF]

High compression engines will have higher stress on the rings & valves.
Our neighbor has a 10:1 O-320 in an RV-6. He enjoys it, but seems to have more cylinder maintenance.
Rebuild a few cylinders @ $500 a pop, and 8.5:1 or 7:1 looks pretty good.

 

[chaskuss]

Besides the issues mentioned by walkman, 10 to 1 CR pistons are originally used in helicopter engines. Most of those only have 1000 hour TBOs, rather than the 2000 hour TBO of a 150 or 160 HP O-320. Those 10 to 1 pistons will create more heat in the cylinder heads, which reduces the fatigue life of the aluminum head of each cylinder. You pay for that extra power with reduced cylinder life. Listers who are using 10 to 1 CR pistons can comment more authoritatively on actual cylinder life in an RV.
If that 185 hp rating is real, the owner probably had the cylinders "ported" when the compression upgrade was done.
Increased compression improves torque. That is the most likely explanation for the slightly better fuel economy in the 185 hp engine. It could also be due to a more aerodynamic airframe. [better build quality or the other plane is not rigged properly]
You also want to find out what the total hours are on each engine, as the crankcases tend to crack much more easily after 6,000 hours. Also find out if the engines are using the older narrow [Lycoming refers to them as "standard"] deck or the newer wide deck crankcases.
FYI, you can increase the 150 hp to nearly 160 hp by swapping the 7 to 1 pistons for the 8.5 to 1 pistons used in the 160 hp engines.
Charlie

 

[jrs14855]

Engines

With the 10-1 pistons bearing wear is more of a concern than cylinder problems. The 10-1 piston engine should have piston cooling nozzles installed. Make sure this has been done. Cylinder flow work will do little or nothing for power. 185 hp is wildly optimistic. 170 hp is closer to reality.If you operate at r/m's above 2700, then you will get over 170 hp. If 160 hp pistons are installed in a 150 hp engine it becomes 160 hp, not NEARLY 160 hp.

 

[az_gila]

Besides the issues mentioned by walkman, 10 to 1 CR pistons are originally used in helicopter engines. Most of those only have 1000 hour TBOs, rather than the 2000 hour TBO of a 150 or 160 HP O-320. Those 10 to 1 pistons will create more heat in the cylinder heads, which reduces the fatigue life of the aluminum head of each cylinder. You pay for that extra power with reduced cylinder life. Listers who are using 10 to 1 CR pistons can comment more authoritatively on actual cylinder life in an RV.
If that 185 hp rating is real, the owner probably had the cylinders "ported" when the compression upgrade was done.
Increased compression improves torque. That is the most likely explanation for the slightly better fuel economy in the 185 hp engine. It could also be due to a more aerodynamic airframe. [better build quality or the other plane is not rigged properly]
You also want to find out what the total hours are on each engine, as the crankcases tend to crack much more easily after 6,000 hours. Also find out if the engines are using the older narrow [Lycoming refers to them as "standard"] deck or the newer wide deck crankcases.
FYI, you can increase the 150 hp to nearly 160 hp by swapping the 7 to 1 pistons for the 8.5 to 1 pistons used in the 160 hp engines.
Charlie

The Helicopter TBOs are not quite as bad as you say, except for the vertical mounted versions...

http://www.caa.si/fileadmin/user_upload/pageuploads/AD-NOTE/AD-2006/100_sb_SI1009AS.pdf