KT100 - SR-Y Pipe - Flex length suggestions

I realize there’s a very similar topic here but I didn’t want to hi-jack that thread. I’ve found it very difficult to find flex length suggestions for KT100 using the SR-Y pipe. I think it’s because WKA adopted it and then moved away from it pretty quickly. Our local group still runs them and I’m new enough to karting that I just purchased a pipe used from someone and mounted it the way they had it.

I’m finally getting past the large scale tuning items with the KT100 and I’m getting down into the details more. In checking today my total flex length (piston face to end of flex) is currently 9.75 which is .25 pas the min. I’m absolutely going to put together a flex section that is the min and test it but was curious if there is any testing info / history here that anyone could share? I’m over weight in my class and getting eaten up at the end of the straights and I’m curious if my longer than min flex is part of that equation (weight is the big factor but every little bit counts).

Also curious if anyone has insight on why when the SR-Y pipe was introduced by the WKA that it’s intended flex length was 8.5 and was later changed to 9.5?

Thanks for any insight.

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Hello Andy.

I used to run FYSR back in the day and I do not remember a flex length but I can find out when I see Haddock.

The good news is that I have a header and flex piece that I ran. The flex measures 4 inches.
Being too long on the flex will hurt the engine up top as you described.
Measure your flex piece first and we will go from there.


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My flex length is 5.25 inches. Not sure we’re talking about the same pipe. I think you’re referring to the formula Y - SR pipe seen here:


The pipe we have to use is the SR-Y pipe that was the WKA substitute for the 4 hole can for a couple of years:


Ah. I thought we were talking vintage stuff.

I’ll get with haddock and get the answer for you- he was the one that wrote the rules and continued to do so for the class in Vintage where they run the Current SR Y exhaust in Yamaha Limited.

I run, and our club still races that pipe. I have three flex sections cut to min, max and middle flex legnth lengths, and sometimes swap if I feel the need.

Can’t speak to flex tuning for a weight disadvantage directly, buy you may want to try the max legnth flex. More bottom end may help you pull off the corners a bit better and minimize that speed difference at the end of the straights, but the legnth of the straight probably influences that more.

I run the longest length flex at my local track, and I seem equal to other karts. Previously on the shortest length I was giving up time coming out of corners. Subtle, but noticable difference.

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@Andrace What is the min and max length you are allowed to run? We have only a min specification at 9.5" (from the face of the piston) but no max with our club.

Last week I had several sections cut that I will test back to back in a test section last week, but any data I’ve got going in makes the testing that much more focused.


Not much of a tuning window, only 1/2" from min to max. Pretty sure we’re talking similar flex, min was 9.25" and 9.75" if memory serves me.