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head shaving more compression 2 stroke

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by goer86, Jul 21, 2006.

  1. gday i am about 2 rebuild my nsr 150 has 37,500 km on it n has never been rebuilt i have all th e parts 2 rebuild and was wondering about improvin compression i know that it is 6.8.1 standard and i wanted 2 do a 8.91 thru shaving the head
    i have reeds, big jets, expansion, pod
    already on it

  2. I barely understood if you were asking a question, or just letting us know for our information that you're doing some work.

    try some of these ---> .
  3. yer was a question shoulda been more specific, interested in anyones response and ideas
  4. I shave my head at least once a week :rofl:

    Sorry, I have nothing usefull to add.
    Perhaps one of the mechanics that frequent this site can assist.

    There are a few of them so just be patient, they will respond ;)
  5. Not big on the "two bangers" , but have you thought along the lines of, upgrading the head / bore with machined brass inserts, there is a guy in Frankston that does it and it allows you to up the fuel grade without damaging the beastie :twisted:
    Not sure how it works but it does , I got it done for my bro who used to race and being able to use a premium fuel mix, got him back up to being competative again.

    Its a common mod used by most of the better MX racers to get better performance out of there bikes useing fuel instead of playing with compression ratios etc. :)

  6. Be careful you have an idea on what you are doing...

    Raising the compression by that much might make the damn thing unrideable. And then the only way to reduce the compression is via dodgy head gaskets!

    Have you worked out how much you need to take off the head to make that increase? Is that number something you've read elsewhere, or a wild stab oin the dark?

    Back in the good old days, we used to play with 2 strokes all the time. Get it right and they sing. Make a stuff up and you're left with a bike that has a powerband about 2rpm wide at 15,000rpm. Makes for an interesting if slow ride.

    If you're going down the 2 stroke tuning route, you should also consider changing the inlet and exhaust timing. Not sure on the NSR what you can do to the inlet, but exhaust could be changed. Be aware that you are basically changing the entire characteristic of the engine and most changes are one way only.

    Good luck and we await the results....
  7. thanx for the advice guys i want to keep this bike rideable but 2 scream when i rebuild it yes it was a stab in the dark 8.9.1 i wonder how much 30 thou would bring it up 2 am interested about changing the exhaust timing and inlet but not sure of how it works i have a ful race spec expansion and carbon fibre reeds bigger jets to match i was just thinkin head cause would be cheap 2 do and 6.8.1 seems pretty low i always run 98 octane so shouldnt ping on 8.9.1
  8. You have to be very careful when raising the compression that you don't suffer 'detonation' (seperate uncontrolled ignition), these pockets of ignition can cause eroding/melting of the piston.

    I think there is always some mid range response/power to be had by raising the compression, not having all the factories development expertise is going to make it a bit of trial and error for you.

    Remember, as with all two-strokes, to pay careful attention to plug colour at various throttle settings (1/4, 1/3, 1/2 & full) to set all the carburetor metering components.

    Sparkplug colour (the porcelain) shows combustion temperatures; white is too hot or lean, black is too cool or rich. You want a nice chocolate brown colour at all throttle settings.
  9. Leave it as is and don't worry about it. It'll make one hell of a whipper-snipper!

    ...Although Honda are making four stroke edgers now.
  10. o well watch out when i find ur lawn we cant all have generator/compressor motors lmao
  11. pay attention to your piston to head clearance. 0.65mm is about your minimum but I real wouldn't go under 0.75 for a road bike as you probably wont do as much maintainance on it as you would on a race bike.
    Play in the big end/small end bearing and carbon build up will soon take up this clearance and lead to piston to head failer.
    Increasing the compression will give more midrange but go to far and you will reduce your over rev. Also too much comp will cause detonation. You can prevent the effect of this by installing inserts into the head and top of the barrel but if it's detonating too much, you are not making max power.
    To get max power, you need everything matched, pipe length/volume, port heights, transfer port angles, primary and secondary compression ratios, ignition timing, etc.
    Why not try reducing friction and making the most of what power you have.
    This can be as simple as oiling your chain. Did you know a shagged chain can cost as much as 5hp?
    Or you could go more extreme and take a piston ring out or run single ring forged pistons. Why not titanium nitride the piston ring and run a ceramic piston. Or use low friction bearings, aluminium clutch plates, lighten the fly wheel.
    Or you could just buy a bigger bike - but it wouldn't be as much fun would it. :grin:
  12. thanx i was just thinking shave the head a lil when i am rebuilding it cause it would be easy n cheap. a big bike is what i need but i enjoy stuffing rnd on this lil banger and i already hae the parts to rebuild anyways i will keep at least 0.75 clearance between
    thanx for every 1 's help on this greatly aspreciated
  13. The NSR150 is a cheaply made Thai import. I wouldn'nt be trying to squeeze more power out of it for a number of reasons.
    First of all because the suspension and brakes are to be blunt, awfully inadequate even with the stock engine. Second the build quality is suspect so if you start tuning it to the nth degree it's likely to just seize.
    If you want a 2 stroke to tinker with get yourself an RS250 or RGV250.
  14. yeah...GO FOR IT

    I took 6thou off the head on my 2 stroke...VICTA
    and took out the zorst baffle
    and she now cuts the heads off daises, mulches the green stuff
    and chomps thru tree branches
    ..sounds good too :LOL:

    it runs on PULP on a 22:1 ratio (25:1 is too lean for the Victa to-blow)