High compression Pistons

[Delta Golf]

I have a Lycoming O-360 engine with high compression pistons (10:1) but the rings have never really seated. Had new rings put in and they have not seated either and the oil comsumption remains high. (1 quart every 2.5 to 3.5 hrs)

Any suggestions before I go back to the standard compression pistons?

[gereed75]

Welcome to VAF.

Don't think your seating issues (or high oil consumption) has anything to do with the compression ratio of the pistons. It has to do with the normal seating (and consumption) things like -

Were your cylinder walls honed before installing new pistons and rings??

Are the rings compatible with the cylinder wall materials (chrome, nickel steel etc)??

What is the size and condition of your bores??

Is there another problem (worn valve guides)??? Etc etc

There are plenty of high compression engines out there with low oil consumtion numbers (good condition and good ring seating). Sounds like you need to consult with a good engine guy (A&P).

Welcome again, fly safe.

[Mel]

Were the cylinders honed when the new rings were installed?
This is a must!
Are the cylinders chrome?
Sometimes chrome cylinders are hard to break-in!
Are you running an AD oil?
Often a non-detergent oil will help ring seating.
Did you run the engine a lot on the ground before flight?
Too much ground running will "glaze" the cylinders!
Are you running the engine "hard" for break-in?
This is necessary for break-in!

Compression ratio is not related to ring seating!

[Delta Golf]

Cylinders were honed
Steel Cylinders
Using Mineral Oil
Very short ground time before take off on first flight.
Running the engine at 75% pwr or higher
At the ring change all valve guides were with in tolerance but put new ones in anyway because they were getting close.

Using a professional engine shop but just wanted to get any ideas before I give up on the high compression pistons. The cylinder walls seem to be glazing very early in the break in.

Thanks

[flyingriki]

Originally Posted by Delta Golf

...but just wanted to get any ideas before I give up on the high compression pistons.

You're asking and not listening. See what Mel said, he's usually right. Not the compression ratio.

[Delta Golf]

flyingriki
If it is not the high compression then tell me what the problem is. I covered all of Mels suggestion.

I've talked to my the engine guy about all the other high compression engines that don't have high oil comsumption. All we can come up with is the higher temperature and pressure in my engine in my airplane is causing the cylinder walls to gaze before the rings seat.

Before I fly the next time I want to make sure I've covered all the bases on what else might be causing this so I don't end up with glazed cylinder walls and rings that don't seat.

Sometimes I miss what is right in front of me, so tell me what I'm not "listening" to.

[Mel]

When I rebuilt my O-320, I changed from 7:1 to 9.5:1. My oil consumption is about 1 qt every 16 hrs. and has been for over 800 hrs.

[Delta Golf]

Mel
That is the kind of oil consumption that I want with my engine. All kinds of things running thru my mind as to why I don?t have that. Thought the CH temperature might be to high and caused the glazing but they are all within limits and I?ve checked the probes to make sure they are right. Going to re-check before next flight.

Seems like I might the exception and not the rule as far as high compression and high oil consumption goes.

[rocketbob]

Originally Posted by Delta Golf

All we can come up with is the higher temperature and pressure in my engine in my airplane is causing the cylinder walls to gaze before the rings seat.

That is completely wrong. High cylinder pressures help break in rings, but in addition to that there are a lot of variables to the hone job which must be carefully done by the engine guy in order to be successful. I can tell within 10 seconds of examining a cylinder bore if it will use oil or not after it comes back from a cylinder shop. And it depends on the crosshatch pattern and the grit of stones used. If he's using a ball hone and not a Sunnen, run Forest run. Too little of roughness, the cylinder walls will not retain oil. This is why chrome cylinders use oil, the channels are too thin and shallow to retain oil. Too much and you'll get too much heat on the cylinder wall thus causing glazing and premature ring wear.

None of the engines I've built use oil. I've even had cylinders with larger-than-spec ring gaps due to tolerance stack-up that don't use oil.

You must allow the engine to cool in between runs per the ECI procedure to prevent glazing. Most of the breakin occurs in the first minute or so.

If you pull a cylinder and they're glazed they'll have a tan appearance to them, like coffee with cream in it.

[badmrb]

I've got the o-360 with 10:1 pistons. Bought plane couple years ago with around 700 hours and burning about a quart every 4 to 5 hours. It had a couple cylinders getting below 70 on comp check so had new rings put in and honed cylinders. Ran straight weight for about 50 hours and ran hard.... Burns a quart every 12 to 13 hours now.

Just adding my 2cents for another data point.

As a side note, I thought the high comp pistons caused higher temps also, had to really watch it in a long climb. Last year at annual I spent some time with the sillycone and sealed the baffles to engine seams. Also added a couple washers at the rear baffle on right side. Now I can leave full power in, set trim for 120kts and climb as long as I want on a 100 degree day with no problems.