FMF Expansion Chamber Exhaust install.

midnight_rider

midnight_rider

New Member
Hey everyone,.. I been looking into different expansion chambers that get sold through distributers and they all look real tacky, I've seen and even thought about fabbing up my own , I got the FMF now and all I have to do is cut off the threaded end of the pipe, , cutoff the pipe on the stock exhaust and curve the pipe to the E/C , I know that these pipes are tuned to be mounted directly to the motor but is that 2 inches extra I have to make up to fit the bike and the expansion chamber fonna make any difference in anything ? ... I doubt it but I thought I would ask,.. it looks killer on the bike... cant wait to weld it up, hang it and check it out !!.shft.
 
Yes, it will make a difference! What the diff will be is hard to tell.. Back in my RD350 racing days, adding 2 inches to the headpipes moved the "powerband" UP in the rev range.. WAY UP! What it does is change the exhaust port timing.. You do know that the Schnurle Pipe, "expansion chamber", actually performs the work of an exhaust valve right? The frequency and timing of the resonant exhaust pulse is controlled by the shape of the pipe.. A company called "Circle F" can supply you with all the cones needed to build a custom pipe.. But I don't know if anyone has done the proper research to find what combo (diam and length) works... One of the things on my list to do..
 
Uh I don't think that is correct, the longer the pipe the better the timing is for lower rpm, takes longer for the pressure wave to travel the longer distance. The shorter the header the less time taken for the pressure wave to travel so better suited for higher rpm.
 
And then there is the Grubee "no header" pipe, so too speak....it seems to give a broad powerband range with improved low-end as well as a tad bit more top-end benefit. The formulas are there but you can always experiment!
 

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yea scotto those are great pipes believe it or not. thats the one i think is def the best for theese bikes
 
Thanks for the info guys...

I take adding say3" for a header is no problem then , that's good to know.

Scotto , nice bike man, I'll take pics of mine when it's all done , it should look and perform better... once that is done it's jackshaft kit time.
 
if you wanna make it simple do like me take the expansion chamber of a 49cc dirtbike/pocketbike, install it upside down and weld the original muffler on and there you have it.
much better torque and gained some more rpmPICT0019.JPG

PICT0021.JPGxct2
 
Ghost0 said:
Uh I don't think that is correct, the longer the pipe the better the timing is for lower rpm, takes longer for the pressure wave to travel the longer distance. The shorter the header the less time taken for the pressure wave to travel so better suited for higher rpm.
Click to expand...

When ya raise the roof of the exhaust port (increase exhaust port timing) what happens to the powerband? Does it move down in the rpm range?
 
scotto- said:
And then there is the Grubee "no header" pipe, so too speak....it seems to give a broad powerband range with improved low-end as well as a tad bit more top-end benefit. The formulas are there but you can always experiment!
Click to expand...

Hi Scotto, what is the diameter of that Grubee pipe at the fattest point? Thx...
 
talltommy46 said:
When ya raise the roof of the exhaust port (increase exhaust port timing) what happens to the powerband? Does it move down in the rpm range?
Click to expand...

i have a 66cc engine were i ported the intake and exhaust wider, cleaned up the burrs in the transfer ports and slightly enlarged the transfers in the case halfs, i then made a .100 inch spacer under the jug, raising everything equally (all ports raised the same amount) then chucked up the jug in the lathe and took .100th off the top, this raised the ports as high as they could go without creating any over lap in the intake side, the results are it has much more low end power and the mid range has a hit to it. very little was lost from the top end. i can now run a 36 tooth sprocket with out the low speed bogging i got from the stock porting.
i've also found that the shorter you can get the intake manifold the better they run. the pocket bike expansion chambers are better than a gutted stock pipe but you need a pipe about twice as big for best power. the head pipe is going to be what ever it comes out to to get the pipe to fit as long as you don't go crazy and make it 2 feet long it will be ok.
 
camlifter said:
i have a 66cc engine were i ported the intake and exhaust wider, cleaned up the burrs in the transfer ports and slightly enlarged the transfers in the case halfs, i then made a .100 inch spacer under the jug, raising everything equally (all ports raised the same amount) then chucked up the jug in the lathe and took .100th off the top, this raised the ports as high as they could go without creating any over lap in the intake side, the results are it has much more low end power and the mid range has a hit to it. very little was lost from the top end. i can now run a 36 tooth sprocket with out the low speed bogging i got from the stock porting.
i've also found that the shorter you can get the intake manifold the better they run. the pocket bike expansion chambers are better than a gutted stock pipe but you need a pipe about twice as big for best power. the head pipe is going to be what ever it comes out to to get the pipe to fit as long as you don't go crazy and make it 2 feet long it will be ok.
Click to expand...

Hiya camlifter! Good post.. This is what I like most about this stuff... All those old tricks are useful again.. Yes I remember shimming the cyl base upward.. CZ guys used to do that alot.. New dirtbikes are super techno whizbang these days, but IMO they've taken a huge part of the FUN out of it! My vintage/evo bikes are too valuable to be hackin on em any more.. So here I am, looking for that experimenters thrill.. The possible combos and results are endless, I love it...
 
talltommy46 said:
When ya raise the roof of the exhaust port (increase exhaust port timing) what happens to the powerband? Does it move down in the rpm range?
Click to expand...

it's not even worth messing with porting. I have done it and always wish i hadnt. i have one ported motor now it rips hard but not too reliable. my new combo consists of a complete stock pk 80 with my own exp chamber billet head and intake that has been my winning combo lately
 
flybytaco said:
it's not even worth messing with porting. I have done it and always wish i hadnt. i have one ported motor now it rips hard but not too reliable. my new combo consists of a complete stock pk 80 with my own exp chamber billet head and intake that has been my winning combo lately
Click to expand...

Sure, I have 2 coming now.. I'll mount the stocker on my 29 for familiarization, and diddle the extra.. Jugs are cheap.. And this motor is just dyin to be dinked with! Cheap hobby, and fraggin a few motors is always fun, yes? I haven't gotten mine yet, due sometime this week.. When I'm not here reading threads, you'll know they have arrived, lol! I'm already getting an idea of what I'll do with it.. Gettin it set up with a small Mikuni & K/N will be early on the agenda.. Adjustable ign timing will be top on the list as well, possibly new ign system.. Shave the head, blend chamber.. Weld up a pipe, (first of many) LOL! I'm doin this for kicks man! Its all part of the fun for me, lol!
 
talltommy46 said:
Hi Scotto, what is the diameter of that Grubee pipe at the fattest point? Thx...
Click to expand...

The outside diameter of the pipe is 3-1/4" X 3", incuding the welded seam on the 3-1/4" measurement....I'm not too sure of the inside dimentions as I have not cut one open, yet. It's pretty thick 12gauge steel I think....you do the math...laff

EDIT: I would guess about 3" inside diameter at the fattest point.
 
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talltommy46 said:
Sure, I have 2 coming now.. I'll mount the stocker on my 29 for familiarization, and diddle the extra.. Jugs are cheap.. And this motor is just dyin to be dinked with! Cheap hobby, and fraggin a few motors is always fun, yes? I haven't gotten mine yet, due sometime this week.. When I'm not here reading threads, you'll know they have arrived, lol! I'm already getting an idea of what I'll do with it.. Gettin it set up with a small Mikuni & K/N will be early on the agenda.. Adjustable ign timing will be top on the list as well, possibly new ign system.. Shave the head, blend chamber.. Weld up a pipe, (first of many) LOL! I'm doin this for kicks man! Its all part of the fun for me, lol!
Click to expand...

Don't get too excited, these little engines are cheap and that's what they are....get the idea?

Your liable too blow more motors in a hurry, than have success with any....believe melaff

But please don't let that stop you....it hasn't me!!! Cheers!

scotto-
 
Thanks for the replies on my thread guys , it's definately a wealth of info for sure,..I doubt I will do any porting ext,.. this is the 3rd time I have heard that doing all that and shaving the head isnt gfoing to be very practical as far as being reliable,... I got the pipe on and only had to make a header about3-4" long { with the bends} ,I didnt have a chance to take it out for a good rip , but the ride home from work wasnt too bad,... I have a new chain on order as the one that is on it got bent up like a spagetti noodle on the ride this morning... lucky enuff I was in the parking lot at work... tomorrow the new chain will be on and I'll post some pics of the pipe on there.
 
Here are some pics of the pipe on the bike, it looks really cool and I get lots of compliments or hear people say " wow what a sick looking bike !!!"

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Good lookin' bike and I like the pipe.....just watch your leg on that sucker, it'll hurt ya. Burns hurt!

Nice build, cheers!
 
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