Ron Lee

Ron Lee

Well Known Member
Recently I had lower than desirable oil pressure indications. After I verified that the sender/monitor were correct, I removed the oil screen to find metal. Essentially automatic engine teardown. My mechanic borescoped cylinders and saw something in #4 that was odd (oil streak). When he removed it it was obvious. The end cap (may not the be right term) somehow dislodged and caused significant damage to that area of the piston.

So far most parts look ok but since I will have to do an overhaul at the minimum, I am almost 100% convinced to go to 9.2:1 pistons (O-360 engine).

I am hearing conflicting info on porting/flow matching as to whether it is worth it. There may also be a camshaft that offers improved performance but at this time I have not found any info on it.

Any suggestions and sources for factual performance improvements?
 
Wouldn't do it

My buddy recently finished his RV-7A and went with 9:1 pistons. Based on all the problems it has caused, he would definitely NOT do it again. The claimed gain was about 5 HP which may help slightly in climb, but maybe 1.5 MPH in speed (if that).

As it turns out, he had piston squirters installed (these are recommended by the manufacturer, ECI, if you use high compression pistons), and the engine runs MUCH hotter than my standard engine without squirters and 8.5:1 pistons. In an effort to cool the engine, it led to more expensive oil coolers, louvres in the cowl, and cutting the cowl about 4" at the exhaust exits just to get the temps into the 200-210 range. In doing all of this, his airplane is about 10 mph slower than mine.

So in the end, all of that for an extra 5 hp and then creating extra drag to keep the engine cool offset any benefits, and it seems to actually have degraded performance. At a minimum, if you go with the higher compression pistons, I would question installing the piston squirters.

Then again, this all may just be an isolated case but I've seen several threads reporting cooling issues with higher compression pistons with squirters.

My 2 cents.
 
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Engine Stuff

Ron:

Sounds like a piston pin plug. Aluminium pin plugs frequently wear out and have been AD'd out of service, except for circumstances of repair. You might check and see what you had.

9:1 pistons shouldn't be any problem. We generally see a 6-8 hp increase in the initial bump in compression. No resulting change in TBO (all other things being equal, i.e. good assembly techniques, maintenance and operation.)

Re: porting/flow matching - don't believe the 15+ hp claims. Most you'll see is a few hp gain. It's really more about efficiency through equalization of the airflow.
 
ronlee said:
Recently I had lower than desirable oil pressure indications. After I verified that the sender/monitor were correct, I removed the oil screen to find metal. Essentially automatic engine teardown. My mechanic borescoped cylinders and saw something in #4 that was odd (oil streak). When he removed it it was obvious. The end cap (may not the be right term) somehow dislodged and caused significant damage to that area of the piston.

So far most parts look ok but since I will have to do an overhaul at the minimum, I am almost 100% convinced to go to 9.2:1 pistons (O-360 engine).

I am hearing conflicting info on porting/flow matching as to whether it is worth it. There may also be a camshaft that offers improved performance but at this time I have not found any info on it.

Any suggestions and sources for factual performance improvements?
Click to expand...

Sounds like detonation damage to me. Got a picture of the piston?
 
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If you are using a Hartzel constant speed propellor then stay with the standard compression. The increased compression ratio is a wild card in propellor life expectancy. Hartzell is certainly conservative in this area and you should be too.
 
Update

Rhonda, you are probably right about the piston pin plug. I think that mine were brass.

I have a fixed pitch prop (catto three blade). I already cut the bottom cowl and added louvers when I had CHT/Oil temp problems with the current engine. Although those helped dramatically in solving those problems, I just don't recall if they was any airspeed reduction.

An RV-8A dude added 9 or 9.2:1 pistons when he made some changes and reports no temperature issues. I just talked with him this morning about it.

I will try to get a picture of the piston posted today. No one that has seen it has suggested detonation. The consensus is that it was a piston pin plug problem.

It is interesting seeing the innards of the engine. I am also fortunate that the problem occurred here instead of over some other areas that I fly over. It does reinforce my preference to fly high and with minimal time over the Rockies with no landing options.
 
ronlee said:
Recently I had lower than desirable oil pressure indications. After I verified that the sender/monitor were correct, I removed the oil screen to find metal. Essentially automatic engine teardown. My mechanic borescoped cylinders and saw something in #4 that was odd (oil streak). When he removed it it was obvious. The end cap (may not the be right term) somehow dislodged and caused significant damage to that area of the piston.
Click to expand...

I was flying with a friend in his Tri-Pacer a few years ago and same thing happened with very low oil pressure. An inspection revealed one piston pin plug missing, a ring broken, and a large chunk of the piston was gone also. It was amazing the engine kept running as well as it did but obviously was on a short leash. Lycomings have a way of sending messages....:)
 
rgbewley said:
Ron:


9:1 pistons shouldn't be any problem. We generally see a 6-8 hp increase in the initial bump in compression. No resulting change in TBO (all other things being equal, i.e. good assembly techniques, maintenance and operation.)
Click to expand...

My understanding is that the timing is usually retarded by 5 degrees from standard if 9:1 or more pistons are installed. So, does the 6 to 8 hp increase occur with the timing retarded or left at the standard setting??

Fin 9A
 
I think it depends a lot more on the cooling setup than the 9:1 pistons. I don't think an extra 5HP worth of heat is going to make or break a 180HP engine!

We are having CONTINUAL problems getting our O-360A1A to cool in our Cessna Cardinal, just cowling and baffling design.
 
Piston squirters

Again, I think we have determined (and confirmed with the engineer at ECI) that the piston squirters are the source of the additional heat, not necessarily the 9:1 compression ratios. The catch-22 is that they recommend installing the squirters if you use the higher compression pistons.
 
Stock and here is why

With 100LL possibly going in the future ** , 8.5:1 makes more sense. As was mentioned the extra HP will give you 1.5-2 mph more speed. A new SJ type cowl and sealed pressure plenum might give you 6-8 mph. The more power the more fuel burn. Just a thought, but I would stay stock.

Porting, polishing and cams will all increase power. If running a Carb you don't want smooth intake ports. Since you intake is "wet" you need the roughness to keep the fuel atomized and not becoming big drops on the walls of the intake.

Better 4-into-1 exhaust, electronic ignition will improve power with OUT compression increase. However with HC pistons and/or cam and not changing the exhaust, ignition or intake, you will not get the true benefit. In a word it all has to work together, change one thing, change everything.



** The new gas will be unleaded and rated at 92 or 95 (called 92UL or 95UL). 9.2:1 will not run well and timing will need to be retarded at min. This of course lowers the power again. Bottom line: There is no such thing as a free lunch. You will pay for HC pistons one way or the other.
 
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