CMB 67 Header Length

[Bill Simmons]

Lookng for some advice. I am building a 1/8th scale Hydroplane and I have a CMB 67, (Blue top) engine with the CMB exhaust pipe. The exhaust header is about 5 inches long and I was hoping to cut it down so that the exhaust pipe would not extend past the end of the boat. Before I do this, I wanted to see if there is any performance issues with doing this? This is my first build so want to make sure I do it right. Is were a certain length that I should have? Thanks for any and all help.

Pictue of engine and exhaust I have.

 

[SayMikey]

The longest length ive seen is 12 and 1/2 inch from center of glow plug the the bump on the pipe. If that helps

 

[Bill Simmons]

The longest length ive seen is 12 and 1/2 inch from center of glow plug the the bump on the pipe. If that helps

Thanks, this does help.

 

[FloridaScaleBoater]

Yes, you will be cutting the header down in length.
You are looking for a measurement of 11-1/2 to 11-3/4" as measured from center of cylinder along the pipes length to the fatest point of the pipe. Depending on boat weight and altitude you will be running. Pipe could be longer or shorter.

 

[RaceMechaniX]

Leave enough header length so at 12-1/2" center of plug to center of belly you still have about 1" overlapping.

 

[Wasted wages]

If your pipe doesn't slide over the header, there are a few ways to expand the inlet of the pipe... I just use a 1/2 drive socket set and 12" extension... find a socket that is close to the inlet ID and drive it in with a dead blow hammer.. repeat with a larger socket if needed... use lots of spray lube and work in small steps...

Once you get the slip fit, coupler material will last forever..

I'm sure there are others that do it differently but this has always worked for me and costs nothing..

 

[timothy johnson]

That pipe slides over the header and telescopes nicely. You can trim the header so that the overall header length does not go beyond the beginning of the cone, if you do that be carefull to leave at least an inch or so of header going into the pipe. Great pipe by the way.

 

[Mike Cathey]

Lookng for some advice. I am building a 1/8th scale Hydroplane and I have a CMB 67, (Blue top) engine with the CMB exhaust pipe. The exhaust header is about 5 inches long and I was hoping to cut it down so that the exhaust pipe would not extend past the end of the boat. Before I do this, I wanted to see if there is any performance issues with doing this? This is my first build so want to make sure I do it right. Is were a certain length that I should have? Thanks for any and all help.

Pictue of engine and exhaust I have.
View attachment 310286

11 1/4" from the back of the motor to the weld on the pipe

 

[Ray Sametz]

Cut my headed at 5 degree angle, rotate 180 degrees and had Tom Foley weld it. It now points straight back to keep pipe under deck. This pipe expander work great to fit pipe over header.

 

[Bill Simmons]

Many THANKS to everyone. This all helps!

 

[Steven Kinney]

11 5/8 from the glow plug to the weld on the pipes. I personally have never ran a CMB Pipe but would assume somewhere around there would be a good starting point.

 

[Bill Eilers]

You have lots of good pipe lenth info. But I think it's important to not insert header to far into the divergent cone. At your desired pipe lenth header shouldn't extend to deep. (Deep enough to use all of the pipes strait section is good)