Tuned pipe questions

[RobC]

I'm finishing off a 1980s Wolfgang Matt Joker, which I'm powering with an MVVS90, and for the first time I'm going for a tuned pipe to keep it in period style. I've got a Genesis 90 size quiet pipe, and I'll be trying 14*8 and 15*8 APC props to start with, so I'm expecting around 9500rpm.
So the questions are - how long should the pipe be, and does it matter if I leave it overlength? I certainly don't expect the model to be underpowered, so I'm really after a clean throttle response and low noise levels.
Any hints and tips?

Bob

[TMKits]

Tuning info from Macs may be helpful http://www.macspro.com/tuning.asp

[jeffie8696]

You can probably just bolt it on and see an increase in power with the right fuel, plug, needle settings but if you are looking for the maximum power you will need to do some trimming and testing.

[nitrometan]

You must to start with a length of aprox 520 mm from exhaust flange of engine to middle of cone in the pipe.Longer pipe is not bad but sips fuel extra fast.To tune the pipe,run the motor with pipe header only.Tune for maximum speed.Than put the pipe.If rpm drops,you are way too long.If you have the same rpm or a little more,you are probably near the required length.Then short the header 5mm each step untill max rpm is acheived.Than back 5mm.For flight use a propeller 1 inch more diameter or more pitch that the propeller you used for proper pipe tuning.