shorting a shorty!!

[john stu]

i was looking for slightly more bottom end power from my 2002 cr250 and was thinking of shorting the shortly silencer and i was wondering if anybody has tried this and what was the out come? (except for more noise). i only ride moto tracks where noise is not an issue (and next to yz450f's who would notice anyway!!) and the bike has all the usual mods like Eric gorr porting, reed valve, sst pipe, ect. i probably won't do it but i am curious if anybody has tried this and does it work? and how much did they remove from the length.

yes it would be better to just buy a new pipe made for more bottom than the sst but its cheaper to just get out the hack saw.im not looking for way more bottom end just a little.i probably won't do it but i was just wondering if anybody has tryed it and did it work.

 

[steve125]

John, don't waste your time on this idea. Ive done a ton of dyno testing with silencers and no real gains to be had here.

All this hype on how short silencers add bottom is not true. The dyno sees it as an over-rev issue and thats about it.

But on the track short silencers can fool a rider as they generally "hit" harder. This "hit" is not extra bottom, but to some it may feel like it.

So use silencer lengths to tune the power delivery and in some cases add over-rev. Short silencers= "hit" long= "smooth". On the over-rev it can go either way, really need a dyno for that and the difference between is small.

Try shimming out your pipe a few mm's at the exhaust flange or advancing the timing 1mm thats if your running race fuel. But both of these mods may hurt your top end over-rev a bit.

 

[bclapham]

Steve:

whats the deal with stinger diameter? take the stock RMs and the FMF shorty- the shorty for this bike is around 2-3 mm more diameter. I remember reading something about why it is important to run the shorty on the big bore 2 strokes on Erics web site, but i cant remember why. i might try my stocker next week- i was worried about it maybe pinging a bit more, but if the bike is quieter, at least i will be able to hear it, LOL!!!!

 

[steve125]

Bruce, from what ive been able to gather from my testing, a larger core silencer helps top end and over-rev. But I also believe that it's engine and pipe specific. It's all about back pressure really, Eric feels the big bores benefit from the shortys and FMF fattys. This is due to a flow increase with the larger pistons.

I have had no problem running aftermarket 125 pipes with my big bores and I always use shorty silencers. But some pipes are more prone to detonation than others. This is due to a too strong return wave or a small stinger that causes excessive back pressure. Some FMF SST's fit into this catergory and don't work well with big bores IMO.

 

[bclapham]

Eric feels the big bores benefit from the shortys and FMF fattys. This is due to a flow increase with the larger pistons.

it seems to make "brucie-logic" when i think a bigger piston, should need a bigger center section pipe and a bigger hole to let the gas out the end!!!!!

i might do some experiementing next weekend.

 

[DEANSFASTWAY]

John Stu if you want a little more low end on your Cr why not advance the ign timing a bit , if you havent already . Its free , or a thin cyl base gasket . sure you could throw some time and labor at it and make it rock but I dont know your limitations or maybe youve already tried these . Seems a couple years ago everyone was trying to shorten their silencer, alot of loud bikes. I remember my new 94 YZ250 with a 12 inch silencer and a 2inch dead air space at the end of the silencer . That bike ripped and put the power down and was quiet.

 

[john stu]

i think i will try advancing the timing thanks guys for the advise. how much should i advance the timing ?

 

[FlyingBob]

"I remember my new 94 YZ250 with a 12 inch silencer and a 2inch dead air space at the end of the silencer . That bike ripped and put the power down and was quiet."


"2inch dead air space" How did that work?

 

[DEANSFASTWAY]

BOB The silencer had like 10 inches of packing and then it had a divider wall and the next 2 inches had no packing just open area, the bike was pretty quiet.

 

[DEANSFASTWAY]

John If you take off the ign cover Theres 2bolts holding the stator plate on. the lower bolt has a plate with numbers on it . Theres a small phillips screw that you remove to flip the plate around to get the advanced setting . Use good fuel. You might also want to check the power valve actuator cable adjustments. Good Luck

 

[john stu]

The power valve cables are new and adjusted and checked regularly and I run VP c-12, thanks for the tip on flipping the plate I have not even taken the cover off yet but after flipping the plate how much do I move the adjustment? is it a fixed postion after flipping the plate?

 

[DEANSFASTWAY]

John theres the fixed plate and theres a number system for degrees of advance . You wont be moving it much . On some modded motors sometimes a tuner might just scrap the plate and se thre timing with a dial indicator, but youll just want to advance it a bit.

 

[CR_Dave]

I have always added the V-Force mod to my CR250 and it helped a lot on the low end. Also, when I run the short "arenacross" type tracks, changing the rear spricket a tooth or two helps.

On an older 2000 Cr125, I added a spacer to the exhaust flange, that helped the low end too.

 

[110 octain]

hey steve125 thats not what the EXPERTS say at fmf factory racing
concedering theve been making silencers since 1971 i (and any 1 with some coman scence)should probably listen to them

 

[marcusgunby]

110 octain maybe you should think how fmf are making money-its in there interests to get you to buy a pipe even if you dont really need one, ive rode some good fmfs and some bad ones.Like steve says, unless you have a really restrictive silencer like a enduro bike has, you wont get any big gains from the silencer.The std 04s are especially good IMO.

 

[elf]

Shorter silencer = more noise and thats about it. Very helpful when racing against 4 strokes though, at least you can hear your bike ;)

 

[john stu]

i tryed advancing the timing and i must say i did not think i would be able to tell a differance but it was pretty noticeable,acutly it made so much of a differance that powerband moved to a lower rpm than i was looking for (it lost a lot of over rev even though it gained a lot down low especialy in throttle responce) so i retarded the timeing half way back to stock (maybe 1mm back) and i like it!! thanks for the advise.i will do some more back to back tests to see where i like it best thanks again