Run times with Various Heads

Iceberg Wed Oct 07, 2020 10:54 am

Just was curious. Does anyone know.

I know that the head inserts with OS Number 8 runs slower than cox high compression head by about 3-4 K RPM. I know the Merlin 2019 head runs almost the same as the original Cox head.

I'm in quarantine right now as a precaution so can't get out for actual comparison test hence the question to you.

The head adaptor that is slower by 3-4 K does it use less fuel? I mean does it pick up run time due to the lower RPM? I never tested that before. Normal runs for 17K are like 2.5- 2.75 minutes per tank. Might the slower head run time achieve an additional 60+- seconds of "slower" run or at least proportionally to its comparable high compression head run?

Anyone have any stats or experience to share?

Thanks
Iceberg

davidll1984 Wed Oct 07, 2020 11:12 am

It make sense To me as the engine wil breath faster and stronger tink it can afect the flight time

aspeed Thu Oct 08, 2020 5:38 pm

Common knowledge is that a tighter combustion chamber gives the best mileage. Smaller place for the fuel to fit. More efficient as well. In practise, a smaller prop is used, so it revs more which wastes fuel. I would think that a huge load prop with a TD head would give the best run time---if it will run reliably. Sometimes they will just flop back and forth. I have not tested it though. Just my thoughts.

dckrsn Thu Oct 08, 2020 5:57 pm

@aspeed wrote:Common knowledge is that a tighter combustion chamber gives the best mileage. Smaller place for the fuel to fit. More efficient as well. In practise, a smaller prop is used, so it revs more which wastes fuel. I would think that a huge load prop with a TD head would give the best run time---if it will run reliably. Sometimes they will just flop back and forth. I have not tested it though. Just my thoughts.

I absolutely concur aspeed. I've had engines that would do the back and forth flop due to over compression. I'd use an electric starter on them, and the they run like a scalded something(my cat's in my lap). Same goes for under compressed engines(clapped out).
Granted, I wouldn't make a habit of starting with an electric starter, without the usual prop plate/case shim in place.
Bob

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Run times with Various Heads

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