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  #1  
Old 08-07-2018, 11:11 PM
mountainride mountainride is offline
 
Join Date: Jan 2018
Location: Golden, Colorado
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Default High Compression Pistons IO-390

I see there is an option to the order the high compression pistons from Lycoming on the IO-390. 95% of my flying will be above 5000' DA as I live in Denver, Colorado. For a pilot based at altitude why wouldn't I go with high compression pistons? It is a negligible cost difference. Would my TBO really be any different if I am never close to full power? Interested to here what people on VAF think about this option.

-Jason
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  #2  
Old 08-08-2018, 07:46 AM
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DanH DanH is offline
 
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Location: 08A
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Quote:
Originally Posted by mountainride View Post
I see there is an option to the order the high compression pistons from Lycoming on the IO-390. 95% of my flying will be above 5000' DA as I live in Denver, Colorado. For a pilot based at altitude why wouldn't I go with high compression pistons? It is a negligible cost difference. Would my TBO really be any different if I am never close to full power? Interested to here what people on VAF think about this option.

-Jason
Ok, a few notes...

The philosophy underlying 390 development was to use cubic inches rather than significantly increased compression to deliver an honest 200+ HP from an angle valve four.

Note the 390 already has slightly increased compression compared to a 360.

Increased HP at a given altitude will require more baffle pressure drop, and/or increased heat transfer, or an acceptance of higher CHT.

With increased compression, you'll really want to pay attention to timing advance values. If you go with increased CR, stay away from EI's which don't allow control of the advance map.
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  #3  
Old 08-08-2018, 08:39 AM
Robert Anglin Robert Anglin is offline
 
Join Date: Mar 2008
Location: houston, texas
Posts: 900
Default Agreed.

I agree with Dan on this one. There is some grain of use to what you are wanting here. I can only give you an example of what we did and why we did it, with some qualification. How long an engine will last is very much a subjective thing. How you run it, how long you run it there and how well you treat & service it. It is very true that if you take an engine and raise the compressing in order to get a higher BMEP so that you can make more power out of the same engine, "Then" go out and race it hard as an example, it will be shorter lived. You can take that same engine and moderate the stress exposure and it will last a longer life. It boils down to what you want that engine to provide for you and how much you want to pay. A good analogy may be like an Indy car. You or your backers may be willing to 125- 160,000 dollars for an engine that will last a week or two or you may take that same engine in a stock configuration in a factory car and have it last 15-20 years.
Now, just like you and this question, we like to go high and have a little more power on take-off. We don't have a 390, we don't need it for the type of flying we do and like to do. We have a "P" Valve 360 with a hollo crank and no "Dampers" on it. We put experimental cylinders on it to give it more and better air flow. Then went with 9:1 pistons so that we would be on the line between being able to use 93 no E or 100 LL., more fuel choice if you like.
This kept the weight down as low as we could for a power plant that could provide a good amount of Bhp.. Power to weight if you will. We started out at a factory rating of 191 Hp. and with a little more work we are somewhere a few over that, by looking at the fuel flow rates, but not by much. The big reason we did this was to have that extra to pull a full 1800 Lbs. off the runway and into the air as fast as we could. Then we throttle back and only need 75% or more very rarely while fling below 10,000'. But we can if the winds are favorable go up to 18-19,000' and go WOT and get 150-155 KTAS and take advantage of that tail wind if it is there. Some times those winds coming out of southeast TEXAS are as much as 50-60 Kts. blowing from west to east up at that level. I find that we don't need to run much over 55% down at the lower levels. So we can expect that our power plant will go to full TBO and maybe a little more, if we service it well. We do have some fun from time to time and the aircraft will do 185-190 Kts down low at full throttle, but I also know that if I did that all the time. That this engine would not last as long as I want it to. Everyone has to do what they like if they are to enjoy it the most. Hope this helps a little. Yours, R.E.A. III # 80888

Last edited by Robert Anglin : 08-08-2018 at 08:43 AM.
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  #4  
Old 08-08-2018, 09:18 AM
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DanH DanH is offline
 
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Forgot two...

The standard 390 has no propeller restrictions of which I'm aware. Given a compression increase, you're on your own.

Future fuel remains a wildcard.
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