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Post  TheotherGeoff on Sat May 11, 2013 5:09 pm

It seems that most all Cox 1/2A engines have the ports at the 10:00-4:00 position instead of at the 3:00-9:00 position that would seem most logical for good power. One bypass port is blocked by the crankcase; the other is blocked by the reed housing. Wouldn't they go better with the exhausts at 12:00-6:00 and the bypasses at 3:00-9:00?

Curious Geoff
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Post  pkrankow on Sat May 11, 2013 6:59 pm

So...are we talking about the exhaust ports? 12:00 being the propeller direction, forward, and 6:00 being the back plate direction, directly backwards.

I am unsure which orientation provides the best evacuation of spent exhaust without stripping the fuel charge with. I have heard indication from people who have more experience than me that about 45 degrees, 1:30/7:30, or 10:30/4:30, providing the best performance.

From my limited number of engines I have found that 3:00/9:00 seems weaker (I have 1 engine that aligns this way), and other orientations are stronger running.

Phil
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Post  RknRusty on Sat May 11, 2013 9:34 pm

TheotherGeoff wrote:It seems that most all Cox 1/2A engines have the ports at the 10:00-4:00 position instead of at the 3:00-9:00 position that would seem most logical for good power. One bypass port is blocked by the crankcase; the other is blocked by the reed housing. Wouldn't they go better with the exhausts at 12:00-6:00 and the bypasses at 3:00-9:00?

Curious Geoff
It would seem so. To have the bypass ports equidistant from the reed at 3:00 and 9:00 seems like they would get the most even distribution of fuel. I have gone to the trouble of putting the crankcase shoulder on 1000 grit paper and knocking a quarter of a thread off to get them to align that way and have never actually seen any difference. It doesn't seem to matter.


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Post  Cribbs74 on Sat May 11, 2013 9:36 pm

RknRusty wrote:
TheotherGeoff wrote:It seems that most all Cox 1/2A engines have the ports at the 10:00-4:00 position instead of at the 3:00-9:00 position that would seem most logical for good power. One bypass port is blocked by the crankcase; the other is blocked by the reed housing. Wouldn't they go better with the exhausts at 12:00-6:00 and the bypasses at 3:00-9:00?

Curious Geoff
It would seem so. To have the bypass ports equidistant from the reed at 3:00 and 9:00 seems like they would get the most even distribution of fuel. I have gone to the trouble of putting the crankcase shoulder on 1000 grit paper and knocking a quarter of a thread off to get them to align that way and have never actually seen any difference. It doesn't seem to matter.


Really? Wow, never thought of that. Do you shim the cylinder afterwards to correct the port timing and close the SPI gap?

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Post  RknRusty on Sat May 11, 2013 9:40 pm

No, the distance is not that much different, barely measurable just to knock off a quarter of a thread's depth. The engines probably varied that much from the factory or else the ports would all be aligned the same anyway.

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Post  pkrankow on Sun May 12, 2013 7:53 am

The threads were not indexed during manufacture, so the alignment is purely random. The Special .15 MKII is the only cox engine that the threads were timed on so the ports line up in the same place on every engine.

Phil


Last edited by pkrankow on Sun May 12, 2013 3:56 pm; edited 1 time in total
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Post  Cribbs74 on Sun May 12, 2013 8:57 am

RknRusty wrote:No, the distance is not that much different, barely measurable just to knock off a quarter of a thread's depth. The engines probably varied that much from the factory or else the ports would all be aligned the same anyway.

I figured you would have to take more off than that. Time to experiment.
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Post  Cribbs74 on Sun May 12, 2013 8:59 am

pkrankow wrote:The threads were not indexed during manufacture, so the alignment is purely random. The Special .17 MKII is the only cox engine that the threads were timed on so the ports line up in the same place on every engine.

Phil

Correct, except I assume you meant .15 Wink
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Post  pkrankow on Sun May 12, 2013 3:56 pm

cribbs74 wrote:
pkrankow wrote:The threads were not indexed during manufacture, so the alignment is purely random. The Special .17 MKII is the only cox engine that the threads were timed on so the ports line up in the same place on every engine.

Phil

Correct, except I assume you meant .15 Wink

Doh! I thunk I typed that... Huh...

(fixed)

Phil
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