Exhaust pipe counting

[Martin27]

Hm, just thinking how big mistake I made… cylinders to mu Flying Squirrel were changed to long stroke but I made 27 exhaust coffee pot and pipes. During the process of making I find out is different hight of the exhaust ports in the cylinders. Short stroke 19 mm, long stroke 24 mm. Ok, I did this, but somehow I didn’t think farther and still used the tube of the diameter 38 mm. So this gives me the space in the cylinder 1220 mm2 and 962 mm2 at the end of the tube. Long stroke tube is 2″ diameter, so there it would be 1810 mm2. But of course, this is 2 to 2, so this is the diameter (surface) for the rest of the tube. If someone know a bit more about this counting, I would be happy to have some information what about I should expect? I don’t have the engine finished yet, so cannot try now. But could it work? Is there different running or is it complete disaster? Just to know if it make sense to try it or remake it right away…
Thanks

[Bob Mather]

Hi Martin 27,
As you have realised the pipe size to the exhaust port must be the same size as the port. Any step down here will result in a big loss of performance. This pipe should not taper down, but remain the same cross sectional area all the way to the coffee pot so that the exhaust gasses can escape as quickly as possible. Any back pressure at this point is bad and any restriction or back pressure should be at the open back end of the exhaust pipe.
Hope this is of some help,
Regards,
Bob Mather.

[Martin27]

Hi Bob,

thanks for the answer. Ok, as I thought, so little bit of work extra 🙂

One more think, how big influence has the final pipes? I made the coffee pot and the pipes by original drawings by John Underhill, there are two pipes, each 32 mm OD. This make 29,6 mm inner diameter (space 688 mm2) with short pipes at the coffee pot on which are the pipes mounted with ID 27 mm (572 mm). Is this enough? Just thinking if there is Ok to have pressure, what the stream in the coffee pot could do, if it’s going out from both tubes or if the pressure doesn’t allow to split the gas to both tubes and each cylinder is going to one pipe? There could be solution to remake the short pipes going out of the coffee pot to outside mounting of the pipes, so there could be at least 29,6 mm diameter and not smaller.

On the drawings are also overlaps of the exhaust pipes inside of the coffee pot. In my original product, I made it as it’s on the drawing. I thought it has probably some influence. Is it correct if the pipe inside is ending by strait cut or should it be cut together with the inner surface of the coffee pot?

Thanks a lot, best regards,

Martin

[Bob Mather]

Hi Martin,
The new mounting is the best as it causes no restriction to gas flow using 32mm pipe. Cut back the exhaust pipes from the cylinders, they do not need to stick far into the coffee pot. The exhaust gasses will flow down both exhaust pipes from both cylinders. The 32mm outlet pipes would be better cut to follow the curve of the coffee pot wall for better gas flow. Do not cut the 32mm pipes too short as the 29.6mm pipes need to push in at least 45mm to stop leakage. A much better alternative is to use the same diameter pipe as the exhaust pipes,29.6 mm, take a short length of this pipe to the local exhaust pipe fitter and get the ends expanded to form a socket to fit over the over the 29.6 mm exhaust pipes, a neater job and less likely to leak.
Regards,
Bob.

[Martin27]

Hi Bob,

perfect, thanks a lot for all informations. Just last question, is important the length of the tubes between the cylinders and coffee pot? I don’t remember exact lenth from the drawings, by they’ll be bit longer than 1929/30. Pipes has the length by the John Underhill drawings, so hope this should be fine.

Thanks, best regards,

Martin

[Bob Mather]

Hi Martin,
Sorry I cannot help you with the pipe length cylinder to coffee pot. The John Underhill drawing size should be alright. I have never had a Scott with a coffee pot exhaust. I have done a lot of experimenting with Scott exhausts with a front manifold,pipe and silencer resulting in quite a big power gain after just over 20 exhausts.
Regards,
Bob.

[Martin27]

Hi Bob,

no problem, I think John’s drawings will be good. Thanks a lot.
Btw, it would be interesting to see how you developed your manifold, pipe and silencer, so if you have some picture, it could be great to see it.

Thanks,

Martin

[NIngram]

@Bob Mather wrote:

Hi Martin,
Sorry I cannot help you with the pipe length cylinder to coffee pot. The John Underhill drawing size should be alright. I have never had a Scott with a coffee pot exhaust. I have done a lot of experimenting with Scott exhausts with a front manifold,pipe and silencer resulting in quite a big power gain after just over 20 exhausts.
Regards,
Bob.

Can I have your 19th please….

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