I have recently used IWT chamber design program plus manual formula out of the Jennings two stoke handbook . Both pipes based on 17000rpm figure but ended up longer in length by about 50mm compared to pipe i have now . Can anybody tell me how to get proper rpm figure , is it based off rpm and tourqe , or maximum rpm . Mark Dewar

There are lots of pipe design formulas, but they are all empirical. Jennings pipe designs are obsolete by modern standards. My favorite can be downloaded here: www.iwt.com.au/MOTA.HTM
The best way to design a pipe for a specific engine under specific conditions is with a simulation program. You start with an empirical design, make small changes and run the program to see what happens to the power. I added about 1/2 hp to a very good M&D Zenoah with a better pipe than the standard 2 cone M&D pipe. Their latest engines raised the intake timing along with other modifications that eliminated the difference between my pipe and the M&D pipe. I am currently trying to build a pipe for some new straight line runs with a specially modified Zenoah. I think it may be possible to approach 7 hp between 18,000 and 20,000 rpm. We'll be dyno testing some pipes on a couple of engines soon.

Lohring Miller

Sorry, I see you are using the Ian Williams program. Temperatures are an important consideration in pipe design. I've been using 550 degrees Centigrade for the latest designs. If you use too high a temperature, your pipes will be tuned for too high an rpm. You can best adjust the tuned length by changing the band section. Multi-section diffusers seem to work best at a particular design length. A smooth transition that gradually expands with a 2 or 3 degree angle from the piston to the first stage of the diffuser seems to be important as well. The elbow on the Quickdraw hot pipe does this and it is one of the reasons it works so well. As I mentioned above, engine port timing is important. It pretty much sets the rpm for peak power.

Lohring Miller

Thanks for advice Lohring i had temprature set at 600 degrees on program. I plan to build this pipe for a straight line motor. I decided to do multi stage pipe at between motocross and road bike settings on program i was after midrange - topend pipe. I am not sure if i should be a bit conservitive on rpm figure. Mark

I just wrote two articles on pipe design for the September and December 2006 NAMBA Propwash. The last one describes how we came up with the pipes we used to set the GX2 record last fall. I learn more each time i do one of these development programs. We have a big advantage in that we can build several pipes from simulation results and then actually dyno them to check the predictions. Our next effort is to get the G1 record over 100 mph, and no, no one in the world has averaged over 100 for an official record with a Zenoah powered hydro. We will be using a special M&D engine in Mike Bontoft's rigger design.

Lohring Miller

Lohring... I 've been wanting you to do some articles for MGB for some time now...Emailed you on this a couple times a little while back. I am sure alot of readers would appreciate your experience and knowledge... if you ever want to submit stuff like these articles you just did for Propwash, we could get them in MGB no problem! Would get a LOT of exposure!!! Email me if interested.... if I don't reply, it might not have made it through, so post here if need be!

Mark

I sent you an e-mail. Did you get it?

Lohring Miller

I have not recieved any email Lohring but i think you are after Marc . L

Got one Email and replied...did not get article though... please send all emails directly to marc at modelgasboats.com will be easier... tackling with Spam these days.

Lohring, have you tried experimenting with truly parabolic diffusers? The multi-stage versions approximate the effect, but what's left on the table? I know manufacturing is a bit of a problem, but I would think a decent swedger could do it. I work near one, and have thought about discussing it with them...