I am looking for (simple) information on how to calculate the required length of a tuned pipe system.
Can anyone help?

Peter,


Please scroll down in [link=http://www.tmfc.org.uk/info/pipes.html]this web page[/link]...

It has the right formulæ for the starting values to begin your testing...

Thank you Dar.

You're welcome, Peter.

When you have a good starting point, it is much easier to arrive at a good final length...


Please note, every time you change to a different prop size, that would cause RPM to change, you might need to recalculate and readjust.

Now be so kind as to give us the Ext for a MVVS 15cc. When using the 3250 pipe (for which I assume we must use the twin cone formula) what would distance be from the neck to the cone (Li minus header length)
Cheers
Piet

Piet,


The MVVS #3250 tuned-pipe is a 'cone and disk' type... Not a dual-cone.

MVVS engineering found that this type of pipe, while not giving as large a power boost as a dual-cone, is much easier to length-tune and when tuned, the boost-range is wider...

So, when changing to a similar size prop, it is often not necessary do do the tuning all over again.
For optimum performance, in very high RPM engines, MVVS did offer the #3247 dual-cone tuned-pipe, however.

The formulæ I linked does tend to err in the case of the MVVS pipe. It is often needed to tune it significantly longer than the formula suggests... Usually longer than the #3251 and for sure, the #3252 rear-exhaust header.

Using a 14x8 APC requires nearly 10 cm of aluminium tubing to extend the header for optimum length.

I think I have a improved the constant for the #3250 pipe by using a average constant of the twin disk and flat disc constants (1650+1200)/2 = 1425. I am guessing that the cone is 32.5 cm down the pipe and the header length is 15 cm. Taking Ext for my 15cc as 155 and my target RPM as 10300 with a 14X8 APC :
(1425*155 ) / 10300 = 21.44 inch = 54.5 cm
I am getting the most power with about a 7 cm extension pipe. Ad 1 cm for gaps where the pipe is in silicon pipe.
32.5+8+15= 55.5 cm
But I am not getting 10300 yet, I am only getting 10000. So :
(1425*155) / 10000 = 22 inch =56 cm
That’s creepy close
Cheers Piet