Piston Question?

[BlakeB]

I currently own a RV8 with a 0-360 A1A. The guy I bought it from had 7.3. to 1 pistons in it. Not sure how much horse power it produces and I'm thinking about changing them to 8.5 to 1 or 9.1 pistons. I have a 3 blade catto prop. The plane perfoms great, it just seems to be a little sluggish on take off. I'm not an engine guy, but my understanding is my engine is derated right now and is very capable of producing 200hp. Any advise would be greatly appreciated!

Thanks,

Blake

[Bob Axsom]

I have the 8.5:1 pistons and I am interested in more HP. Contacted Barrett Precision Engines, Inc. in the Tulsa area and was told he could put in slightly higher compression pistons while keeping it carburetor fed and increase the horsepower by 7 or 8 but that is still far below 200 hp. He said to go higher would require fuel injection. The company advertises on this website and they will answer this type of question candidly.

Bob Axsom

[Ron Lee]

When I rebuilt my O-360 I went with 9.2 range pistons and notice improved take-off performance. Fuel burn is also higher.

I may be wrong but it seems the 200 HP O-360s are often the angle valve version.

[X18]

Lycoming used to have an O-360 low compression engine that was rated at 170 HP. It was built for 80/87 octane avgas, similar to the 150/160 HP difference with the O-320 engine. The lower compression pistons would allow for the use of regular unleaded auto gas if you desired.

Jim

[N941WR]

Blake,

You mentioned that your -8 "seems to be a little sluggish on take off."

This could be more your prop or empty weight.

What kind of 75% power cruise numbers are you seeing at 8500' DA?

I ask because my 135 HP O-290 powered RV-9 goes up at over 1400 FPM w/ two on board and full fuel. Solo I'm off the ground in around 250 feet.

That's why I think your prop might be biased towards cruise and/or your -8 is overweight.

[BlakeB]

I'm using a 3 blade catto prop. The plane came with the prop. Once you get over 100mph it perfoms awesome. Maybe your right, you can only do so much with a fixed pitch prop. The extra HP sure sounds attractive.

Blake

[rvbuilder2002]

Originally Posted by BlakeB

I'm using a 3 blade catto prop. The plane came with the prop. Once you get over 100mph it perfoms awesome. Maybe your right, you can only do so much with a fixed pitch prop. The extra HP sure sounds attractive.

Blake

I'm not saying that part of what you are seeing as far as performance goes, isn't related to slightly lower power output, it is probably the result of you having a fixed pitch prop set up for good lower altitude cruise.

This is a compromise of using a fixed pitch prop.

As already mentioned, only the angle valve IO-360 is rated at 200 HP. The O-360A1A is only rated at 180 HP with 8.5:1 compression. Even if you change the pistons you still will not ever produce 180 HP with the fixed pitch prop. You need to be able to turn up to full rated RPM to get full power. This can only be done with a constant speed prop.

[MrNomad]

When we ordered our Superior O-360 kit, we specified 7.5 pistons so that we can run auto gas. Superior claims that it will develop 168-172hp.

Other folks have told me their Superior with 8.5 pistons burns auto gas just fine but detonation is sometimes difficult to detect outside of a controlled environment without appropriate instrumentation.

I'm in no hurry (I'm used to a C150) but prefer to send Chevron as few $$ as possible.

[asav8tor]

That engine will probably last a long time.

I would get a C/S prop before changing the pistons.

[breister]

Agreed with the previous two posters - get a C/S prop and keep the low compression (or, at least don't go over 8.5). The T/O performance will be hugely improved with just the prop, and you won't see appreciable extra improvement from the extra 10hp that higher compression pistons would bring. Meanwhile, you are future-proofed against any new legislation limiting 100LL. Comparing the costs of the two changes, your bang-for-the-buck is good for the prop and not so good for the pistons (and that not even counting the increased wear on the engine).

For those worried about detonation in very high compression engines (over 8.5:1), short of the much anticipated anti-knock system from a not-to-be-named vendor you can also simply switch to electronic ignition and retard the timing a touch. For reference, my 10:1 IO-320 uses 20BTDC instead of 25BTDC. The hotter spark of these systems will prevent excessive power loss from the retarded base setting, while the timing advance features of these ignitions will assure you are still getting better-than-MAG performance at altitude / reduced power settings. My limited understanding is that anything over 10:1 is a racing configuration, and may need even higher octane than 100 to prevent detonation.