New Build (Soon) Mods needed

Discussion in 'Performance Mods' started by DIYMark, Oct 29, 2008.

  1. DIYMark

    DIYMark Member

    My 80cc HT will be here Friday and once its installed the mods will begin!

    First Ill run it stock to wear in the engine but once worn in Ill start modding. The first mod will be to rewire the kill switch with a keyed switch (security) and in a way that disconnects the blue wire instead of shorting the magneto.

    Next I will to open up the exhaust by removing baffles, drilling holes and shortening the final exiting tube. In addition the carb will be modded (bore it out - and eventually buy a proper one) and re jet it.

    Lastly will be to match cases and clean up transfers/ports and then finally mill the head. From what I understand the compression ratio is determined by dividing the cylinder + head volume over the head volume. Now if the engine is at 6 : 1 now and has a displacement of 65.9cc. I calculated the head volume to be 13.18cc.

    This is okay but I was wondering what is a good compression ratio to aim for? Most of the 2T bikes out now use 8.5 : 1 - why this number and not more?

    Also if I want 8.5 : 1 on my HT Id need to mill of ~2.625mm off the head (calculated this value). But this seems like an awful lot! And me thinks the HT engines wont even last at this compression. So what my questions are...

    What other mods can I do?

    Whats a good compression ratio to aim for?
    - Why are 2T motorbikes all at 8.5 : 1 and not more?
    - What compression ratios are other people running here?

    How much more power/revs can I expect form an "derestricted stock" muffler vs say one from a pocket bike with a expansion chamber?

    Any help would be greatly appreciated!

  2. fastboy9

    fastboy9 Member

    I milled my head by 0.480mm. This has made quite a good difference, some people have taken it as far as 0.8mm personally I wouldn't go any more than 0.65mm. Just because I want the engine to last a bit longer! If you cant get to a mill, then you can just shave a little bit off by using course grit sandpaper and a sheet of glass on top, then take off a bit of the metal.

    Not to burst anybodies bubble, but a pocket bike engine with expansion chamber will be wayy faster than a derestricted stock happy time engine.

    Those mods are pretty much the same as mine, mine just wont start at the moment so I haven't had a chance to test it all out. When I do though I'll tell you the performance increases.

    Hope this helps!

  3. BSA

    BSA Guest

    Porting is a good one, and fairly easy but you HAVE TO BE CAREFUL!!!!!!!!!!
    use a hand file, makes it less easy to take away too much metal.

  4. fastboy9

    fastboy9 Member

    Yep, I always mark on how much I want to take off with a scriber, then use a dremel (well just griding stone shoved in a power drill) then finsih it off with a file, and sandpaper to clean it up until they are nice and polished.
  5. DIYMark

    DIYMark Member

    After some thought Ive decided that 8.5:1 is used on most 2 strokes and 12:1 is used on most 4 strokes because...

    2 stokes have their powerstroke twice as often as four stokes and they rev higher than four astokes so this means more heat and thus a higher self ignition chance in 2T motor than in a 4T one. So 8.5:1 is the most Id go for in a HT but after reading that most people take off ~0.5mm Ive changed my mind.

    Milling 0.5mm off will change your compression ratio from 6:1 to 6.33:1 - now this sounds quite poor but if you work things out further that extra 0.33:1 gives you 100 watts more power which is a noticable amount. However, milling the head down 2.5mm to get 8.5:1 will give you 700watts more power! I guess you can say goodbye HT at those compressions - chances are the motor would last no longer than a tank of fuel.

    This brings moe onto another issue - longegivity.
    I guess you could go all out with mods, carbs, ports, exhausts, milling - the whole deal. But then at the end of the day your modding an engine that is of an old design and a design that isnt meant for this. And to top it off the thing was made in china!

    So part of me wants a stock setup (well almost) with a few minor mods such as cut out a baffle, tune carb, fix gasket and mill the head (its like a 10minute task) but then as i said, I guess you can go stupid with mods and have EVERYTHING done to it.

    So for now I guess I will wear in the engine and do the simple and most effective mods. Gasket fix, igniton fix, carb float fix, open up exhaust a bit and mill the head (milling the head seems as the only mod that gives the most noticable boost in power).
  6. duivendyk

    duivendyk Guest

    Engines are rated by their displacement,the volume swept by the piston going through it's stroke,not by total cylinder volume,which is displacement+ compression volume.An 80cc displacement engine with a 6:1 compression ratio has a ratio of displacement to compression volume of 5:1 (6-1),this means a compression volume of 80/5= 16 cc and a total volume of 96 cc. I think going up to 8.5 for these underdesigned engines with reputedly marginal bearings is an invitation to trouble.Even 7.5 would be sticking your neck out,at that ratio your compression volume would be 12.3cc, (80/6.5), a difference of 3.7 cc ,with a guessed at 4.5 cm piston diameter (5cm stroke) and a piston area of 16 cm.squared, the amount that needs to be shaved off the head is 37/16=2.3 mm.Taking off 2mm raises the compression to 7.25.Frequently the HT engines have smaller displacements that are quoted,it pays to verify by measuring piston diameter&stroke of the engine.
    Last edited by a moderator: Oct 29, 2008
  7. mountain80

    mountain80 Member

    I removed my stock head gasket(0.057'') and replaced it with a 0.019'' copper head gasket and removed 0.012'' from the head elimanating the small spigot for a total of 0.050''. The engine runs strong and i had to rejet and lean out the midrange by dropping the needle down as the engine is now more efficient. I also cleaned and matched all ports, gaskets intake and exhaust. The carb was taper bored and polished as well. If you look at the chamber diameter where it meets the cylinder head you may find it is smaller than the cylinder bore so check the piston/head clearance. If it is too close you can machine or carefully hand grind a taper so that there is more clearance, plus this has the added effect of inducing a squishband which forces the fuel/air charge into a nice tight ''packet" thereby making more torque and power even though you will reduce the compression a small degree. The engine has now over 1500 miles on it and running strong. Compression went from a stock 90 lbs to 140 now. I did change out the crank bearings for a better quality type and have a higher quality needle bearing than the chinese ones which is easiliy rated to 13,000 rpm overkill but the needle bearings in the top of the rod are not the best. On a final note I custom machined a boost bottle which definitely enhanced low end torque.