Exhaust Flexes

[Mr X]

I recently asked somebody what the flex does in front of the exhaust they replied with something along the lines of;

Changing the length of the exhaust flex, changes the tuned length of the whole exhaust This is apparently the RPM the exhaust works most efficiently at.

By going longer I was told it is more efficient at lower RPM (giving more power down low but loosing a little up high ) and by making the flex shorter is more efficient at higher RPM (looses power down low but gains it up high).

He also mentioned something about when changing the length of the flex it can be effectively like changing gearing 1/2 tooth on the rear.

Is this True as I have no idea but he sounded like he was telling the truth? Also why change the flex can't you just change gearing?

[John]

Looks good to me.

You should first gear to the track. That is find the gearing that suits the track the best to provide your faster lap times.

Then tune the pipe to that gearing. You should have an assortment of flex in your toolbox at varying lengths to allow you to fine tune the exhaust length.

As you indicated the tuned exhaust length will suit a specific engine frequency (RPM) and so if you improve the lower RPM you will impact on the higher RPM and visa versa.

Keep a log of your setup experience for each track so that you can use it as a guide.

[karter #94]

That was a very good question MR X i had alway wondered that myself and spoken to a few ppl and thay have all said the same as you and John

[jock]

i recently changed the length of my flex and it made a big difference. i race on a short track with tight corners, i made the flex 40mm longer and found my motor pulled alot better out of the corners.

[link77]

Interesting question for sure...
I have tried various length's on my 125 fireball...with the kart strapped to the dyno at work...results went against what i thought would happen. The best power reading was with basicly no flex and the pipe butted against the header...26kwatw from memory,will print of the results next week and post them up.
Around 30mm gave the best spread of torque and power.
The longer the flex we tried the more power we lost...? interestingly changing the A/F ratio's didnt produce anywere near the results i thought they would...? isnt that the main tuning of a 2 stroke?
This is not to be takin as a tuning advice! results on dyno's do not relate to results on the track as drivability/weather and track conditions all effect outcomes...was just interesting to see the results as was allways curious what it would do on the rollers lol

Also isnt the flex length dictated by AIDKA? or is it just the header pipe? will try and find the rule for it...
not that anyone ever seems to have there engines checked...but thats another thread.....