MicroSquirt.com read only forum (Jan.15,2018)

Hello,

Looking for info on what size throttle body to use on my KTM 530 EXC. Will be doing a conversion to FI.
1 cylinder 510cc and currently uses a 38mm carb. Any help appr.

stonehands wrote:1 cylinder 510cc... currently uses a 38mm carb.

You've sort of answered the question yourself, as throttle body size is normally related to potential air flow and horsepower, so if you are aiming for a similar hp level to a 510cc, then you want around 38mm, however anything close will do.

A standard Husaberg 640 single from yesteryear used a 40mm Dellorto, and had 53 rwhp, yet a tuned one with 74+rwhp had a 46mm Mikuni. Another way to look at it, is check out other engines, 500cc singles or some 1000cc twins and see what they use. IIRC the Suzuki TL1000, Ducati and Honda VTR1000 were around 52mm.

It really depends on what you intend to do with the engine, but anything around 40mm and above that fits your intake rubber and fits in the space available would be adequate.

+1 You can get away with a slightly larger throttle body without compromising torque at lower engines speeds due to the superior atomization of the injector. My DR650 FI project has the LT-R450 throttle body which is 42mm. The bike had a 39mm FCR-MX carb before the FI project. I was worried I might lose torque going to the bigger TB but the dyno chart quelled my low end loss worries... the extra top end is really nice though.

Thanks guys. Read your 650 project and will be ref to it often as I go about mine. Just wasn't sure about the 140cc dif in displacement between out bikes. I will try for 38-42 range when I select. More latter have to go.

stonehands wrote:Thanks guys. Read your 650 project and will be ref to it often as I go about mine. Just wasn't sure about the 140cc dif in displacement between out bikes. I will try for 38-42 range when I select. More latter have to go.

Displacement is one thing but engine pump efficiency is another. Your KTM motor is a much newer higher RPM engine design than my DR650. You will have no problem running a 42mm throttle body on your machine.