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05-01-2007, 11:22 PM
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Join Date: Mar 2006
Location: Denver
Posts: 20
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Compression question
Not trying to start a fight, but what is the difference and pros and cons of the various compression engines.
Isn't it like 8:1, 9:1 and 10:1?
From what I understand 10:1 runs harder and reduces TBO. Low compression can run on lower octane, but produces less power.
What do people take into account when buying an engine?
Is it better, for example, to buy a hi compression 360 for 200+ HP or get a mid compression 390 with the same power?
Newbie question but curious.
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05-01-2007, 11:52 PM
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Join Date: May 2005
Location: Santa Barbara, CA
Posts: 1,324
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Compression
Perfectly good question. The short answer is what you said, higher compression makes more power. Up to a point. More compression means more forces on the engine internals, pistons, rods and crankshaft. There is also more opportunity for detonation. In the typical air cooled aircraft engine there can also be a cooling issue because you are making more power and therefore more heat.
John Clark
RV8 N18U "Sunshine"
KSBA
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05-02-2007, 06:00 AM
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Join Date: Mar 2005
Location: Dallas area
Posts: 10,762
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When I rebuilt my 150 hp, (7:1), I went to 9.5:1. The engine has more power but is not nearly as smooth. The Catto 3-blade brought back the smoothness, but if I had it to do over, I would not go beyond 9:1.
__________________
Mel Asberry, DAR since the last century.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
<rvmel(at)icloud.com>
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05-02-2007, 11:04 AM
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Join Date: May 2006
Location: Baton Rouge, Louisiana
Posts: 152
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Quote:
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Originally Posted by Chewie65
Not trying to start a fight, but what is the difference and pros and cons of the various compression engines.
Isn't it like 8:1, 9:1 and 10:1?
From what I understand 10:1 runs harder and reduces TBO. Low compression can run on lower octane, but produces less power.
What do people take into account when buying an engine?
Is it better, for example, to buy a hi compression 360 for 200+ HP or get a mid compression 390 with the same power?
Newbie question but curious.
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Many people think that higher compression means more power but that's not always the case. It can quite easily mean less power as can be realized in lower durablility and in some cases roughness. It also requires a change in timing to gain the better Hp or better durability. What higher compression means is a more efficient conversion of fuel to power. It also means nigher internal cylinder pressures which can relate to lowwer power in some cases and in all cases reduced durability.
Walter
(remote form Bucharest, Romania)
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05-02-2007, 01:46 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,477
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<<Is it better, for example, to buy a hi compression 360 for 200+ HP or get a mid compression 390 with the same power?>>
The 390 is a little heavier, but I'm betting it will make 210 minimum and last longer too.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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05-02-2007, 03:12 PM
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Join Date: Nov 2005
Location: tulsa, Oklahoma
Posts: 178
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The IO-390 will make 210 HP @2700. The IO-360 angle head, on a good day will make 205 IF 10:1 pistons are installed. But as Walter said, the higher the compression ratio, the higher the cylinder pressures, the higher the cylinder pressures, the higher the bearing load (rods particularly). Thus less durability. The IO-390 weighs 9 lbs more than the IO-360 and makes this 210 with standard compression.
Allen
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