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D.I.Y Modification to a GPX250 Exhaust

Discussion in 'Modifications and Projects' at netrider.net.au started by gpxkermit250, Jun 30, 2008.

  1. Sometimes when Im parking my bike at shopping centres, in the city or where ever I get random people coming up to say g'day or for a chat. Mostly are bike owners, ex GPX riders.

    One bloke said to me "If you want your GPX to go harder and sound better, all you need to remove/break the exhaust baffle/blaffler. Just get a big long screw driver, stick it down the pipe and whack it really hard untill you can feel it push through"

    I thought he was just taking the P*ss a bit, however a bit of internet browsing seems to show a few references to this, but just not in a such a 'Do it yourself with a hammer' way.


    Can anyone shed some light on this for me? Would love for my bike to sound louder at no real cost, but not really too keen on thrashing him with a hammer and screwdriver.
  2. Just drill holes! Where the flat bit is on the back of the exhaust, just drill a few around. Start with two on each can at a smallish size, then increase size and number as you go.


    Will make it sound a bit nicer. From memory I put in about 4 mediumish holes on each can. It won't make it obscene unless you do the hammer trick and rip apart the whole inner baffle - we're just making a few holes in it.

    You may lose 0.0001hp due the lack of back pressure but 250 has so little its not noticable unless you drill ALOT of holes/massive ones.
  3. nice one... jee, I didnt think only a few holes would make such a difference?!

    So the hammer and screwdriver 'procedure' is valid, but just a little bit too much noise your saying? I guess once its done, thats it.. no going back. With a few screw holes, you cant undrill them, but its not to extreme.

    I was thinking about the time I sell it when updrading, and whether or not people will see the holes and freak out, but I doubt it.

    Ok, so 4 holes in each can, at medium size and Im assuming that I drill around the 4 screws that hold the exhaust together! ha!

    Thanks for the tip. Will let you know how I go when I can get my hands on the old mans drill set.
  4. My dad did this on his GPZ400 (grey import). It was whisper quiet in stock form. It goes a lot better, according to dad, up high in the revs, but it isn't even much louder than it was stock. It's still a quiet bike. He did try to remove the endcaps to remove the baffle/drill holes in it, though that didn't work, so he took an old bit of steel rod, sharpened it and hammered it into the mufflers. I tried the screwdriver one on my own bike, an old thumper, and it made it pretty loud! (Not nearly as loud as some of the modern WRs, CRFs etc) though my muffler was already aftermarket, so was already louder than stock. I gained low end and mid range on my thumper, dad gained top end on his inline four.

    It won't hurt your bike, and it'll make it sound a lot better, and best case is you'll get a little more power.

    Actually, it may be reversible (with the screwdriver method), if you can actually take the baffle plate out of the muffler, cause if you drill it, you've removed the metal, so can only really fill it in with a weld or something, but if you punch a hole, it isn't really removing any metal, it's just poking a hole in the plate and then bending it out of the way, so theoretically, you could flatten the plate back out with a hammer from the backside, only if you can get the plate out on its own, that is.

    But once you do it, I doubt you'll wanna go back to stock. Maybe you could get a muffler(s) (how many cans do the 250s have?) from a wrecker and try it with that first? If you don't like it, you can put your old one back in, if you do, you can keep the stock one in case its too loud for rego, etc. or if you sell it and want it back to stock.
  5. The sound difference of debaffling (is that a word?) a GPX is minimal. When I had mine I just drilled out a few of the pop rivets on the end. They are easy to replace when its time to sell and the buyer doesn't even need to know.
  6. before you do, check with your local registration authorities, and read this http://www.mccofnsw.org.au/a/80.html if you're in NSW. Although the requirement for exhaust system LABELLING has been dropped, the EPA parts are still in force. You may render your bike un-roadworthy and be liable to fines and penalties.
  7. ha ha!! the things we do to our bikes huh!!

    Funny, I just rang my riding buddy to ask him about it and he rattled off about 5 backyard dodgy's that he attempted on his old RS 125 to squeeze that extra bit of noise and power out of it.

    I was delighted to hear him say that after tampering with his baffle, his RS went from a stock sounding 125 to a Ryobi Whipper Snipper on Nitro.. nice...

    Well, thanks for your replies. Will get busy with the drill on Wednesday night and post a note back to let you know how it went if your at all interested....

    cheers, Greg
  8. Good luck, just don't forget to start small, you can get louder easily, but backwards isn't quite so easy :D I suggest you do a few holes, ride around the block, think, then decide :) I say this because I know a few people who have gone over the top and hated the sound because they took massive leaps and bounds in terms of drilling.
  9. hmmmmm, yep, I would quite easily be one of those people.. I just get so damn excited and dont stop to think sometimes...good advice

    If I drill out the rivits, will the inners of the exhaust rattle around? or are they more for show?

  10. My testing officer said to us if we get a bike thats quite then to do that and take out the inner baffle to give it more sound and bit more grunt haha.
    Then she said its also illegal cause it then doesn't meet EPA guidelines and if pulled over and asked to take the bike to a defect station they point a heat gun at the muffler and read the temps and thats how they know. When its in place the exhaust gases circulate around inside and would give a different reading.

    I doubt you'd get pulled over, cops are more interested in cars.
  11. #11 L0Ki, Jun 30, 2008
    Last edited by a moderator: Jul 13, 2015
    Hey Phizog, does the zzr250 engine sound like a boxer engine if you drill the holes in the exhaust. Does it osund like the two brothers can does?

    Reference: 1YmNHnrWHVs[/media]]Two brothers exhaust

    Seriously all thats missing is the blow off valve noise. :p[/url]
  12. ok, so I drilled 4 holes in each pipe, in between the 4 existing screws.

    The wholes are about 7mm in diameter. Nothing to savage to begin with.

    At idle, there is now a popping/reverbe kind of noise which wasnt there before and at medium to high Revs, yes, it is a little louder but the tone/pitch of the sound is still very much that conversative and sensible noise that you get from a GPX250. For 5 minutes of work at no cost, its a bit of fun and Im happy with it.

    Im going to drill even bigger again and follow the same process on the weekend. (Ride around for a day, see what I think and then re asses).

    Will keep going untill I either run out of room to drill or get to the point where bigger holes wont really do much more.

    Thanks for all the advice and comments on this topic. : )
  13. I ended up drilling 4 x 10.5mm holes in the back of each pipe in between the existing screws.

    At idol or any revving up to about 5k rpm its sounds heaps louder and different (lots of reverbbing, popping/more throaty.
    When on the bike, at high revs it sounds a little deeper and heaaaaaaaaps louder.

    Will keep me entertained for ages. Glad I did it, no regrets, (was nervious at the time. Sitting there with the drill and bit thinking 'what the fringin hell am I doing this for??!') and pretty easy to rivit up come sale time if I need to at no real cost.
  14. Sounds good mate, a must for any gpx :D
  15. Hey mate, good for you. I'm guessing the popping sound is due to lack of back pressure in the exhaust now. Do you have any pictures of what you've done? I'd like to check it out.
  16. yep, I reckon so too. I think I may have lost a little bit of power, cant tell really. It just feels like it take a little bit longer to Rev higher than it did before? No biggy though. Its really minimal and I will get used to it. I spent most of Saturday with the visor up listening to the difference across all Rev ranges and deliberatly driving through tunnels and under bridges etc. Love it..

    Yep, will take a few pics when I next ride on Wed and post them/email them whatever.

    I was a bit of a Baboon though when it came to drilling. At least one of the holes out of each side is a little lop sidded. You cant tell if your not looking for it, but when you see the pic, it will be a bit more obvious that I was more interested in the noisy outcome rather then the preparation of perhaps even just using a grey lead pencil to mark out a spot that was even.
  17. Hey Muppeteer,

    Pic for you..

    I rode to work this morning (damn cold!!!!!!!) but persisted in riding with my visor up. To hear the bike was worth the stinging cheeks and runny nose.

    To anyone who is not a fan of 250's or who has a bigger bike they may be wondering what all the fuss is about for a slightly different and louder 'note', but Im enjoying it.
  18. I was going to put a can on my Bandit, but the guy who was selling it to me said that thew standard exhaust was a a good thing, and that this is how we fix defective, read quiet cans and he promptly smashed a peice of reo bar down the can, knocking out 3 of the 4 baffles! Sounds great and cost me ZERO!

  19. Until the cops fine you and you have to spring for a new pipe. :p
  20. But it's still well and truly under the DB limits, so in some famous words,STFU! :twisted: