Modifying SUs
Turbo Mini Forums

Looking through Vizards book at modifying the butterfly and spindleon SUs, it shows knife edging the butterfly.
Only thing is it doesnt say which side to knife edge, it confused me because the edges are not square, theyre chamfered to the shape of the carb if you know what I mean.
Any ideas?
Also someone mentioned to me it was good to polish the butterfly but someone else said it was better to bead blast finish it so fuel doesnt 'pool' on it.
Again, any ideas?

Paul

I seriously doubt it!

i woulndt try knife edging the closing edge as your likely never going to get it to perfectly seal when fully closed Paul

i doubt very very much fuel would pool or collect on the butterfly tbh, but stand to be corrected

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

Butterfly shaft is easy. Remove one half.

Doing the shaft, but am surprised by Vizards claims of CFM on fly cutting and knife edging the butterfly itself, not that I doubt them.

This guy said that the fuel droplets could stick to a polished or smooth butterfly but would come off the bead blasted one, on the other hand the other guy said the air would pass over a polished one better than a bead blasted one.

I dont know, theyre made smooth as standard

Edited by paul wiginton on 17th Dec, 2009.

I seriously doubt it!

how much of a difference is it going to make if you smooth or standard, however ever bit helps

I find it interesting. The topic arose when someone said "Whats the point of producing loads of boost if you cant deliver it all through inefficiencies of a carburettor" so Im looking to make it as good as possible

I seriously doubt it!

Keith spent a fair bit of time flowing his twin SU's earlier in the year; performance went up but this was in connection with other mods. I'm sure it was the yellow bible he used

Cutting shaft half :

I dismantled the lever on the side in order to have the half shaft with the threads "on top" if that makes sense.

At standart the half shaft with the threads is "under".

So at partial load, there is only the 2 bolts heads to be an obstruction.

Anyway, I asked about this mod a while back and people didn't show much interest ...

Rusty by nature

I modified my 1.1/2" carbs pretty much as the Vizard book. I knife edged the butterfly to leave the sealing edge as Joey says, and with the spindles I kept both sides, but took them down to about .040" and gave them an aerofoil shape. I also blended the body of the carbs where the vertical bore for the dashpot meets the vertical bore of the throat of the carb. I stopped short of profiling the bridge of the carb or building a ramp upto the bridge out of epoxy.

These mods, along with raising the compression gained me 16bhp on the previous build with everything else identical, so it is fair to asume it does make a difference. The only down side was that to torque curve was not as smooth as it was previously. Interestingly I ended up using the same needles as the previous year, I can only asume the airflow through the carb was better and was pulling up more fuel.

I've got some pictures I'll try and dig up and post.

Keith

14.914 sec 1/4 mile @ 91.38mph, NA 1293cc on 1.1/2" SU's (all steel and glass shell)

The first two pictures show the modified bodies. I left the outer edges as machined so that where the piston travels it still sealed at the edges. In my mind it can only help air flow. We spend so much time porting the head to then leave sharp edges in the body of the carb!





You can see the difference in area the modified spindles take up in the throat of the carb. The screws were cut down and counter sunk so that the were within the profile of the spindle.





Looking down the carb, inlet and in to the head.




I'm sure you'll make a neater job of it than me Paul!

14.914 sec 1/4 mile @ 91.38mph, NA 1293cc on 1.1/2" SU's (all steel and glass shell)

Nice job Keith, thats just what Im looking to do, cheers for the pics

Paul

I seriously doubt it!

what did you use to modify the spindles ??
good job by the way !

on booosttt

Did you chamfere the piston too? Thanks for the pictures - they give a very good impression of the difference.

Cheers,
Jan

[X] nail here for new monitor

To modify the spindles I just used a file and a vernier to make sure they were identical (as near as possible). As it is brass (I think?) it did not take to long. I took them down to the thickness first and then formed the edges to get the profile.

I didn't chamfer the piston on these carbs, because at full lift they were level with the throat of the carb. Vizard mentions that on some carbs they protrude into the throat by about an 1/8" and so would be of benifit to do.

Keith

14.914 sec 1/4 mile @ 91.38mph, NA 1293cc on 1.1/2" SU's (all steel and glass shell)

If you want better air flow get the air to twist.

Own the day