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Run times with Various Heads Empty Run times with Various Heads

Post  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
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Post  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
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Post  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.
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Post  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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