Are Higher Compression Pistons Worth It??

[Finley Atherton]

I have an 0-320 from Aerosport with the higher compression 9:1 pistons. I got the higher compression pistons to hopefully give me a little extra ?oomph? on takeoff from my short farm airstrip at 4,000 ft AMSL. I am starting to wonder if I will in fact get any extra performance. The LSE Plasma III is retarded by 5 degrees from standard as per the LSE instructions for engines with compression over 8.7:1 and the engine data plate shows the timing at 20 degrees rather than the standard 25 degrees (I assume this is the setting for the single mag). So, am I gaining extra power with the higher compression pistons , but then losing it with the more retarded timing???

Fin 9A
Ready for first engine start and weighing

[osxuser]

Hard to tell without a true dyno of both configurations. The power gains of a higher compression ratio doesn't account for timing changes obviously.

[gmcjetpilot]

The double edge sword, compromises as always. If you are willing to run more advanced timing and lower you detonation margin or get better gas, than you will gain HP.

My thought is 95UL is coming sooner than later. No matter what your political ideology is, Green house gases and global warming and all, we will probably lose 100LL at some point. When you get to 9:1 you are starting to lose your detonation margin. The day I burned a HC piston in a O235 was the day I said no thanks to HC pistons in future planes. Also per the prop thread, there is no testing of props with High compression engines.

[zav6a]

I installed 9/1s in an IO-320 and gained 5 knots.

Did not do it for the speed but to regain some lost efficiency associated with altitude. High altitude airport and typical enroutes over 12k.

At altitude, little chance of detonation. I richen up, throttle back, or run way lean when down low.

[fodrv7]

Finley,
The LSE Plasma III has auto Ignition Advance.
I purchased the optional LED read out with mine and on the cruise it typically shows 32? Advanced.

Pete.

[Kevin Horton]

Originally Posted by Finley Atherton

I have an 0-320 from Aerosport with the higher compression 9:1 pistons. I got the higher compression pistons to hopefully give me a little extra ?oomph? on takeoff from my short farm airstrip at 4,000 ft AMSL. I am starting to wonder if I will in fact get any extra performance. The LSE Plasma III is retarded by 5 degrees from standard as per the LSE instructions for engines with compression over 8.7:1 and the engine data plate shows the timing at 20 degrees rather than the standard 25 degrees (I assume this is the setting for the single mag). So, am I gaining extra power with the higher compression pistons , but then losing it with the more retarded timing???

I've been on the road, so please excuse the late post.

The compression ratio is a major factor in determining the thermal efficiency of the engine - i.e thermal efficiency is the ratio of the mechanical power produced by the engine to the energy that is available in the fuel. For an Otto cycle engine like our Lycomings, the theoretical maximum possible thermal efficiency is equal to:

1 - 1/(CR^(K -1))

where CR = compression ratio
K = ratio of working fluid's heat capacity at constant pressure to its heat capacity at constant volume. This is almost exactly 1.4 for air. So, this formula becomes:

1 - 1/(CR^0.4)

The actual thermal efficiency that is achieved is much lower than the theoretical maximum, due to friction, heat loss during the power stroke, etc. But, if we change the compression ratio, we can use the above formula to get a pretty good approximation of the change in power, assuming no other changes are made.

P2 = P1 * (1 - 1/(CR2^0.4)) / (1 - 1/(CR1^0.4))

where CR1 = compression ratio of engine 1
CR2 = compression ratio of engine 2
P1 = power of engine 1
P2 = power of engine 2

For example, 150 hp O-320s have a CR of 7. 160 O-320s have a CR of 8.5. If we plug those values in the above formula, it predicts a power of 159.5 hp for O-320s with a CR of 8.5.

If we assume that an O-320 with a CR of 8.5 has a power of 160 hp, one with a CR of 9 should have a power of about 162.7 hp, if no other changes are made.

How much of that extra 2.7 hp would be lost by retarding the timing 5 degrees? Hard to say, but I'm betting the total change in performance (i.e. increase CR + retard timing) would be very small.

[Finley Atherton]

Originally Posted by fodrv7

Finley,
The LSE Plasma III has auto Ignition Advance.
I purchased the optional LED read out with mine and on the cruise it typically shows 32? Advanced.

Pete.

Pete,
Good point, but the fact remains that the timing on you higher compression engine will be 5 degrees retarded as compared to a standard?? compression engine. If you show 32 degrees in cruise , I presume (I stand to be corrected) a standard engine would show 37 degrees. I am guessing your HC engine at 32 degrees could be producing similar power to a standard compression engine at 37 degrees?? So my point again. Is there any worthwhile performance increase with HC pistons given that the timing has to be retarded? I suspect Kevin (post #6) is correct.

Fin 9A
Just did the first start. Sure runs sweet on the LSE Plasma III

[Finley Atherton]

Originally Posted by zav6a

I installed 9/1s in an IO-320 and gained 5 knots.

Did not do it for the speed but to regain some lost efficiency associated with altitude. High altitude airport and typical enroutes over 12k.

At altitude, little chance of detonation. I richen up, throttle back, or run way lean when down low.

Duane,
Did you retard the timing when you installed the HC pistons and do you run electronic ignition or two mags.?

Fin

[fodrv7]

Originally Posted by Finley Atherton

Pete,
If you show 32 degrees in cruise , I presume (I stand to be corrected) a standard engine would show 37 degrees.

Not So Fin.
There is NO mechanism in the Magneto to change the timing from where it is set.
In a car (pre-engine managment systems), the distributor had a set of fly weights for centrifugal (RPM Dependent advance) and a tube to the Manifold for Vacuum (MAP) Advance. So the timing changed continually with RPM and Throttle position.

No so on a aircraft Magneto. It's where you set it.

The Plasma III on the other hand, elcetronically computes and changes the ignition timing with RPM and MAP.

Pete.

[Finley Atherton]

Originally Posted by fodrv7

Not So Fin.
There is NO mechanism in the Magneto to change the timing from where it is set.
In a car (pre-engine managment systems), the distributor had a set of fly weights for centrifugal (RPM Dependent advance) and a tube to the Manifold for Vacuum (MAP) Advance. So the timing changed continually with RPM and Throttle position.

No so on a aircraft Magneto. It's where you set it.

The Plasma III on the other hand, elcetronically computes and changes the ignition timing with RPM and MAP.

Pete.

Spot on Pete. Although I am not sure if we are on the same wave-length here! From memory you have a high compression engine from Aerosport. If you have the LSE crank sensor then Aerosport will have installed the lead trigger magnet in the flywheel at 40 degrees BTDC rather than the 45 degrees used in engines with less than 8.7:1. So I would imagine that all the LSE RPM/MAP mapping in your engine (and mine) would be 5 degrees less than standard?? Also the data plate on my engine (and I would guess yours) specifies the timing at 20 degrees rather than the standard 25. I am going to check the timing on my single mag today and I am guessing Aerosport has set it at 20 degrees as per the data plate.
(Let me know if you ever plan to land at Armidale to stretch your legs).
Fin.