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HC or LC ?

cgoldy

I'm New Here
Not RV related - sorry. But would love your opinions

In my Glastar Sportsman is a hotted up 200hp I0-360 built by Performance Aero Engines. 200 hours only. There have been a lot of problems with these engines where they fitted forged 10:1 pistons without the appropriate bore clearances. I found stuck exhaust valves, scored barrels and gudgeion bushes honed not bored. Hence I am up for a tear down and at least new pistons, rings, valves, guides, gudgeon bushes, and big ends just for the **** of it. Nothing indicates that we need to split the case but I don't trust the PAE so I am going the whole hog.

My question is, should I put High comp pistons back in?

The motor has been ballanced and polished and is reported as being 200hp and it is my cross country speed machine that was supposed to replace my RV6 which I am regretting I ever sold.

If I go for the standard comp cast pistons (I think that is 8.5:1) I will loose that Hp but might get a more reliable engine?

And are there any recommendations of piston suppliers?

Col Goldy
 
An air show pilot friend of mine told me to go w 8.5/1 compression, and that if you needed extra power from time to time, to set the governor to 2800 rpms. You don’t need to run it that fast all the time, but it’s there if you need it. 10/1 pistons will not make tbo.
 
What's the cost of AvGas down under? Any likelihood of auto gas in your future? That could make the choice for you.
 
The trick for more power is getting air in and out of your cylinders. If you can get them ported and the openings (exhaust and intake) matched, you will breathe easier.

8.5:1 will let you burn premium auto fuel, if you need.
 
Carl,
You were fortunate to get those hours out of it. My air show friend is Rob Holland, and my A&P friend that I also consulted was a Red Bull Air Race mechanic, and I got the consensus that with 10/1 you are building a hand grenade. More times than not, it won?t make tbo. There are always exceptions to the rule. Robs engine goes 700 hours before it gets overhauled.
 
I've worked on a couple of RV's that had 10:1 pistons in them, and they were beginning to show signs of needing an overhaul between 600-900 hours. Lots of blowby and dark oil.

My opinion, for what it's worth, is that for the normal flying that is done in the RV's, meaning non-airshow work, the lower TBO overhaul on the 10:1 pistons is not worth any extra power they might provide.

Vic
 
My opinion, for what it's worth, is that for the normal flying that is done in the RV's, meaning non-airshow work, the lower TBO overhaul on the 10:1 pistons is not worth any extra power they might provide. Vic

+1, and do recall we're moving into an era of changing fuels.
 
Col - see my post over on the Sportsman forum. You'll see I'm firmly in the same camp as the folks posting above. High Compression = High Headache factor, on a number of fronts.

BTW, in a big country like Australia, or Canada, with lots of rural areas, getting Avgas can be a real problem. The lack of 100LL in many outposts is one of the principle reasons I stuck with the straight 8.5:1 compression ratio.
 
In my opinion, the key benefit to higher compression is efficiency. Generally, the higher the compression, the more power produced with the same fuel/air charge. However, that comes with some penalties. Detonation risk is a good bit higher with 10:1 and you'll need to be more cautious of when you lean and how much. You may also be forced to retard timing in some or all operating modes and that gives back some of your "free power."

Increased compression will increase cylinder pressures (ICP), which are also influenced by timing and air fuel ratio. You will be seeing higher ICPs than what the engine was designed for and therefore have smaller margins before these pressures create damage or increased wear. That damage is more likely to be increased wear and not catostrophic failure.

HC requires more diligence in operating parameter to achieve longevity. The margins on timing and air fuel ratio are thinner and you have to stay on your game to not "bust" these margins and reduce longevity. This is easy with a fully programmed EFII setup, but not as easy with Pmags and a Red mixture knob.

I really wanted the efficiency of HC, but was unwilling to give away the margins for pilot error as well as pretty much eliminating the option of running auto fuel if gas prices skyrocket again.

I don't know how common custom grind cams are in the lycoming world outside of racing and whether or not you have one. However, cam grind will influence dynamic compression and a custom cam with 10:1 static compression could be producing the same dynamic compression as a stock cam with 8.5:1. This may be worth investigating given the "performance" orientation of your engine build.

Larry
 
Last edited:
Thanks everyone. I also posted this question on the Supercub, Glastar, Beech and Lanceair forums. After all the wonderfully incitfull comments I have decided to go for 8.5:1.

It seems that that's the best chance of achieving 2000 hour TBO and a little less critical when it comes to engine management. And someone summed it up pretty well I think re the fact that the engine is already port and polish balance and machflowed.

Other comments that stuck in my mind were.

"Building with HC pistons and your building a hand grenade"

"10:1 and your TBO will be 1000 hours.

"If you want more HP get more cubic inches"

Have had 4 recommendations for combustion technologies as a piston supplier.

I pulled the motor out and removed the sump and rear case to check the oil pump. The oil pump was new the latest spec and all the gears were new. On close inspection of the cam, crankshaft and big ends for wear we decided not to split the case. All workmanship and parts inspected were A1. The cam was new. It seems that piston clearances and maybe valve guide clearances were the culprit. Have since discovered that the Engine was manufactured on or before 2004 so most likely before the shop went sour. Some one reported that between 2005 and 2009 he was flooded with orders and farmed out the experimental work to outside contractors. I can't verify that but it sounds plausible.

A lot of work to give me piece of mind but well worth it.


Thanks again.

Col Goldy
 
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