Cox exhaust port orientation

Is there a "sweet spot" to locate the exhaust ports on the Cox engines? They seem completely ramdom from the factory. Should be able to locate them by milling the case.
Or does it just not matter?
Thanks,
Rob Johnson
Orange,Ca.

LAST EDITED ON Mar-29-04 AT 09:15 PM (CDT)
 
Rob,
I guess I am curious also. Obviously if you change the height of the cylinder, you end up changing the timing. For example, if you shim up the cylinder (to put the exhaust sideways), that would increase the port timing (it is open longer) and increase the compression (if you left the glow plug gasket the same). If you mill the case, that would decrease the port timing (as well as lower the compression.

I am guessing that shimming would tend to increase the high end rpm, milling may make it able to turn (slightly) larger props.

I am also guessing that the random orientation is due to a random thread cutting, not a variation of case or cylinder length.

I ahve no idea whether it is preferable for the exhaust to be at right angles (or not) to the crankshaft.

LAST EDITED ON Mar-29-04 AT 09:35 PM (CDT)
 
I think I'm in way over my head. I spent almost two hours talking to Tony (in Corona) (thanks Tony!) about Cylinder Blueprinting on Sunday. He had ALOT of great info. We never got to exhaust port orientation.
Did offer to teach me to fly using stunt kites!
Learning to Fly without breaking up my Fat Akromaster?? What a deal!
Rob Johnson
Orange,Ca.

>For example, if you shim up the cylinder (to put the exhaust
>sideways), that would increase the port timing (it is open
>longer) and increase the compression (if you left the glow
>plug gasket the same). If you mill the case, that would
>decrease the port timing (as well as lower the compression.

You have it backwards inserting a shim will lower compression since it moves the head further away from the cylinder top, while milling the case will increase the compression as it does the opposite. T.D. engines use sub port induction, raising the cylinder will increase the amount of sub port induction while lowering it will decrease it.

You are correct. The mind is going!

We used to turn the cylinder so the exhaust openings were fore and aft. In land lubber usage they were placed so they face the front and rear of the engine. This placed the two ports equal distance from the venturi. We're tallking TD .049/.051 circa 1970-75. This would give an extra 800 plus rpms on 25% nitro (Missle Mist) and a 5.5X 4 prop. I learned this from Al Culver when I was in Pocatello, Idaho. He used to beat us in our 1/2A racing, then he finally broke down and showed me his "secret". Real easy to do, but the rpm increase was very noticable. We placed the crankcase on a glass with 400 paper, added oil and did a slow figure eight and kept trying the cylinder until the exhaust ports faced front and rear. It did increase the compression slightly, but at 6,000 msl, it sure helped. It helped to buy an engine with the ports very near what we wanted. The number of degrees the cylinder had to turn was not much, maybe 45 degress tops, usually much less. Remember the TD has two inlet ports, and I believe the Baby Bee etc have only one, but may be full of it here. It's been awhile and Cox and Estes had/have so very many models now.

Kustom Kraftsmanship used to sell shims specifically for adjusting cylinder height and/or port orientation.

Sad to say, you need to do some multiple part selection to get the optimum setup. The ideal piston and cylinder combo occurs once in a gazillion times. Yes the ports need to be fore and aft as stated above, but then you need the correct "deck height" or tdc position of the piston top compared to the top of the cylinder seating for the glow head.

And, sadly, I don't know what that is.

Assuming someone more knowledgable will give us that distance, then you need to hunt for a combination of cylinders that will stop at the right spot, have the deck height correct, and still fit correctly all at the same time.

Lots of luck. Buy a Norvel and be done with it.....

LAST EDITED ON Mar-30-04 AT 06:39 AM (CDT)
 
>Sad to say, you need to do some multiple part selection to
>get the optimum setup. The ideal piston and cylinder combo
>occurs once in a gazillion times. Yes the ports need to be
>fore and aft as stated above, but then you need the correct
>"deck height" or tdc position of the piston top compared to
>the top of the cylinder seating for the glow head.
>
>And, sadly, I don't know what that is.
>
>Assuming someone more knowledgable will give us that
>distance, then you need to hunt for a combination of
>cylinders that will stop at the right spot, have the deck
>height correct, and still fit correctly all at the same
>time.
>
>Lots of luck. Buy a Norvel and be done with it.....

Hi Larr... I know......
The piston(at TDC) is supposed to be dead even with the seating shelf.
That's the reason for the cylinder shims that Joe Klause used to
sell. That position is far more important than exhaust cutout
orientation. Headspace is product of squishband depth and number of
.005" copper glowplug washers used. Set Headspace for amount of NITRO
and glowplug life expected...This was extremely important in 1/2A
Combat. Dale Kirn and Dave Williams showed me how to setup a T.D.
for Combat and 1/2A speed... I tried it and I liked it. OH YES,
I spent many hours,days,weeks and months, looking for correct fitting
T.D. Piston and Cylinder matches.
Tony

In all this, nobody (myself included) has mentioned WHY the port position has an effect. The reason, as I understand it, is that with the exhausts front and rear, the bypasses are free flowing up the sides of the crankcase, and are not blocked by the crankshaft and backplate or reed housing. There might also be some sort of supercharging effect due to the crankshaft spinning the mixture and throwing it up one bypass.

I agree, though, that bypass timing and compression ratio are bigger effects than port position. But if you can arrange to have it all and a good fit too, well, you have a winner!