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I'm going to make a pair of 90 degree exhaust headers for my dual Mc101s / RLV blimp pipes.

My 3 main concerns are:

1. Size of the flex connection tubing.

2. Smoothness of the passageway all the way from the cylinder to the pipe.

3. The kart is a Hartman, so the left header will be close to the seat - this is part of why I need to make the headers myself.

Regarding size, it's my understanding that 1-1/2" o.d. tubing is pretty standard for the tubing of the header itself, and the flex piece is usually 1-1/2" or 1-5/8" i.d. I've heard conflicting opinions about bigger being better vs. 1-1/2" o.d. header tubing being the magic optimum best size. Seems to me the reasonable possibilities are 1-1/2", 1-5/8", and 1-3/4" o.d. tube for the header. Which is best?

In a conventional setup with a header, corrugated flex tube, and pipe, the transitions at both ends of the flex are not optimized for smoothness. I would think maximum smoothness all the way from the cylinder to the beginning of the divergent cone of the pipe would be best. I was thinking of trying to accomplish this by using all the same size rigid tubing with slip fit connections.

I'd appreciate any thoughts you might have on the above.

Phil,

Here are a couple general rules of thumb on pipe and header design that may help.
First, the combination of any given engine, header and expansion chamber produce a form of Helmoltz resonance that is one of the factors affecting engine performance. Assuming your expansion chamber design was optimized for a certain engine using one particular header diameter, a change in the header diameter would consequently require a re-design of the expansion chamber. So, assuming you are using one of the Gary Hartman Blimp or RLV variations as a pipe you will want to stick with the head pipe diameter they commonly used at the time.
Second, when designing the header setup a good rule of thumb is to cause the first cross sectional change to occur as far from the piston as possible. Any change in cross section will produce a reflective wave and early waves create problems in a finely tuned expansion chamber exhaust system. If you know you are going to set your head pipe length at say 9.5" from the piston to the start of the divergent cone it would be ideal to have the only break in the system occur at that transition. The header should be one continuous tube that ends 9" from the piston face and there should be a 1/2" section of the same diameter tubing welded to the small end of the divergent cone so that when a piece of flex is used to join the two parts the exhaust wave does not see the step up in size to the ID of the flex. On most header like the popular Kendick models the flex slides over the actual header tube and also inside a "flex cup". The end of the pipe should be configured the same way such that the inner tubes of the header and the pipe nearly touch and the flex cups are separated by at least a half inch to allow some alignment flexibility and some movement when subject to the bouncing around from on track activities.
Is it possible that a larger header tubing diameter can work better than the standard Kendick 1.5"??? Sure, but only if extensive development of the chamber goes along with the header tube diameter change. That development takes a lot of testing so I would stick with the standard size tube and use flex rather than straight rigid tubing as the connector. The best fitting and most durable flex around today can be bought at your local electrical supply house. It is the stuff made for underground conduit use and has a thick light gray flexible plastic covering that has to be stripped off. I forget the actual dimensional call out on the material but is is a standard size and you just take a header with you and make sure the ID of the flex fits close to the OD of the inside header tube.
Hope this helps.
Steve O'Hara

Steve,

Thanks very much for your reply.

I do already have one header, which I think is fairly standard, and pretty crude. The header tube is 1-1/2" o.d. The first transition looks like it was made by putting a mandrel bent tube in a bench vise and closing down on it. At the moment, it's too dirty to get meaningful photos. I'll clean it up and post photos.

The RLV blimp pipe has a 1-3/4" i.d. inlet sleeve. Rod Verlengiere says 1-1/2" is the magic number for the header tube size, though he was not sure whether 1-1/2" is the i.d. or the o.d.

Phil