I have just bought a MVVS 91 with 3251 header pipe and 3250 tune pipe. I want to prop it for about 9600 RPM. How long distance from outlet manifold to the end of the tune pipe cone do I need ?

Piet,


The #3250 is a cone-and-disk type tuned-pipe.

There are models you can run on the Internet, that would give you a good starting number...

My experience is that you would have to add ~5" (12.5 cm) to the standard header length.



As your MVVS engine gains more time under its belt (up to 10 hours), the RPM will go up a certain amount, making it necessary to shorten your setup to a degree to achieve optimal performance.

Thanks for the reply Dar. That make it about 64 cm (15+36.5+12.5)
Taking the Exhaust Port Timing as 140 degrees My calculations came to 61 cm. I hoped that it was wrong because this would move my CG back quite a bit. Maybe I should let it rev to 10800. Would it be happy running at 10800 RPM?

Piet,


This could be a good idea.

This engine does not like high RPM very much, but under 11K on the ground is not excessive.

Hi!
You can of course use the original length of the exhaust pipe!!! Just prop the engine using either a 14x6 ,15x4W or 16x4W APC for whatever plane you intend to use it in. 16x4W APC works best in a You-Can-Do .60 plane. 14x6 in a scale ww-2 fighter and 15x4W in a Great Planes GEE BEE R-2.
If you intend to use it in a pattern type of plane there a many variants that work (Which I have never tried) 11x8,11x9,11x10,12x6,12x7,12x8 12x9 etc. Most of the time APC make the best props. But the British RAM or German Graupner Cam-prop (not the older gray variety) are equally good.