twin 90 pipe length - CMB 91RS

[Shane Bastick]

Just wanted some recommendations on a good starting point for pipe length on a twin 90 rigger, in this instance involving twin 91RS engines, and CMB 91RS black pipes and or AA90 pipes

thanks

Shane

 

[Tom Foley]

Start at 11-3/4 - 12 " , get the package in perfect harmony , then shorten 1/4' at a time . i think you will end up at 11-1/2 or so for a good reliable fast setup .

 

[Shane Bastick]

Thanks Tom,

given I have 2 different sets of pipes, would there be any preferance bewteen CMB's own 91RS pipes or using the parabolic style AA pipes

 

[Danny King]

91RS run great with Mac 13cc or 15cc pipes. 11.5 is a popular number but I have run them at 12.5" and ran great.

Do what Tom said and you'll be good to go.

I don't like the AA pipe on the 90s or 101.

 

[dwilfong]

I liked the 13cc MACS pipe with the stinger opened up to .470 at 11 1/2 great running and easy to tune.

the parabolic pipes don't run as good on the RS eng in a rigger for some reason.

 

[simen gjørstad]

I run mine at 13" very easy on the pipe and good speed.

 

[Rudi Arnesen]

I run the same as Simen

 

[Shane Bastick]

Hi Guys,

I found out all the pipes I have lying around, I also have

2 x New improved bar stock AB67 pipes from CMDi

The new Billet AB 67-80 pipe is machined from solid barstock.

Would these work on the CMB 91RS engines, see pic attached

I'm just trying to avoid buying another set of pipes like the MAC 13cc's,

 

[dwilfong]

Shane those are great pipes. But thy are to small for a RS91 I tried the AB 67-80 black pipe and it will not run good at low speed. Great on the top end but will die at low speed.

There are some that will give you good $ for those pipes as thy work killer on 80's

My advice is sell them and get the 13cc MAC pipes and open the stinger to .470

Even better maybe you can trade them for the billet AB 90-1.01 pipe.

 

[KenBeller]

My understanding is that the standard way of measuring from the CL of the head to the fat point is easy but not correct. The echo comes off the reduction end of the cone so the proper way is to find the mid point between the fat section and end of the convergence as it necks down at the stinger and measure this to the piston edge. So a pipe with a gentle convergence will be longer than one with a short or severe convergence even though the fat point could measure the same using the old methode. All this came from extensive dynometer testing done by my engine guy over two years. It makes sense but does complicate the matter. If you start playing with different pipe designs that all measure the same using the old way you will get different performance. Perhpas this explains why.

ken

 

[dwilfong]

The standard for measurement comparison for pipe is from the plug to the weld or fattest point.

This is just a "reference" for swapping lengths as so every one is on the same page.

As far as calculation for design of a pipe it is not used for any thing more than a standard reference that all can under stand.