enginehistory.org

[rkammlott]

Hello, i have question that has puzzled me for a while. heres a example. i have a Jacobs 755-9 radial. its only rated at 245 hp. it seems like a large cubic inch wise engine compared to a car engine. it pulls a large aircraft through the air faily easily. why are they rated so low and how is it determined. why is there never a torque number in the specs ? even some radial with close to 1000cid with a blower section can only muster up 450 hp?

[rwahlgren]

Aircraft engines are rated like industrial type engines are, that is continuous HP, except for max Take off power. Automotive car type engines are usually just run up fairly quick on the dyno and brought back down fast, so their HP rating is more like burst power, at least for performance applications.
Another factor is the aircraft engine designer desired to eliminate the posibility of detonation, the best way they knew to accomplish that was keep the compression ratio low. Most radial aircraft engines run in the 6:1 area compression ratios. And most automotive engines nowadays run in the 9 to 10 :1 area. Then there is the thing of heat rejection, they didn't want to over rate an engine that was desired to have it last for awhile and be dependable, more power means generating more heat and durability issues. The cooling fin area would set the maximum amount of heat that engine design was meant to handle. The other option for generating more HP is to increase the crankshaft speed, and is one way the reno racers add cheap HP. But then as speed increases those already heavy master rod assemblies get real weighty.

An engine in a car cruising down the road may use a maximum of 20% of its max rated HP if even that much. In an aircraft the power demand is much higher, I'm not sure but I think its in the 75 to 80% area. Maybe someone else knows for sure.


If you have the HP figure, and the RPM then you can find the torque.

HP = (torque in lbs ft)*(RPM)/5252