91 with 3250
 

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 ?

RE: 91 with 3250
 

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.

RE: 91 with 3250
 

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?

RE: 91 with 3250
 

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.

RE: 91 with 3250
 

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.