throttle bodies

Fast Eddie said:

For best performance the carb / throttle body should be bigger than the port that’s closer to the valve. The bigger carb will then ‘force‘ more charge through the smaller port, thus increasing charge velocity, thus increasing cylinder filling.

Of course, this is a gross simplification and there are many, many, variables in play.

If the ports are 36mm, I would personally be looking at 39mm carbs (well, that’s exactly wot I done on my Seeley) it’s only a 1.5mm difference in radius.

Whether or not the manifold connecting the two works best with a smooth taper, a sharp taper, or a step, can only really be ascertained via experimentation.

This is all on an ‘old’ Commando motor, but as examples of the unpredictable variables, and experimentation, I swapped out the genuine FCR velocity stacks for some stubs designed to take pod filters… shudda been a backward step, but I gained nearly 2bhp. Then I played with different ‘reverse cone’ ends on the 2:1 pipe to reduce noise, this shudda been a backward step too, but I gained another 5bhp. So experimenting with peripheries, even ones that ‘should’ have caused reductions, actually gained me 7bhp, and all in a couple of hours!

39mm FCRs on an ‘old’ Norton:

View attachment 110710View attachment 110711

Click to expand...

Fast Eddie said:

Theil sells a big bore kit. I know he does much more, but one thing that is clearly not his strong point is communication / promotion !

I was talking to him a long time ago about the 2:1 that BritTwit is patiently waiting for. He went through serval designs but basically could not get it to perform right, I’m guessing he may have canned the idea now, or at least put it to the bottom of the priority list. This shows the guy is serious in his work and has integrity, he could easily have pushed them out to the market irrespectively.

Click to expand...

City Garage said:

Frame is a wee bit different

Click to expand...

City Garage said:

like everything else. Supply and demand....not enough demand. We looked at doing the exhausts like SBN since we have Cone Engineering in our backyard but the math doesn't work unless you sell a bunch.

Click to expand...