HT motor modifications

Discussion in '2-Stroke Engines' started by bigtrucker2007, Aug 18, 2010.

  1. bigtrucker2007

    bigtrucker2007 New Member

    I and looking to modify my 66cc HT motor. I was hoping someone could help me on how to do some port mods and what are good upgrades. thanks

  2. skrufryder

    skrufryder Member

    if your going to port and polish the intake and exast your going to need to buy the cylinder /crank case gasket to replace when you put the top end back toether , its in best intrest to buy the race carb becuse the porting is gona suck alot of fuel and needs alot of air to mix it. if you go with the cn carb get a few bigger jets. as far as the actual porting and polishing goes once you have the cylinder off the crank case and the intake manifold and exaust is off you will notice there is a lip on the inside of the port on the cylinder wall. I use a series of small rattail and flat files to get rid of this lip then a series of emery cloth to start the polising of the port itself. get a fair amount of emery cloth the aluminum is realy "sticky" it gets it the grit of the papper very easly. I then finish up with some brilow pad the trick is to make the ports as smooth as posble and try to make it look like chrome. also get a slant head for it and polish it as close to a chrome finish as possible. I use a wire wheel then work down in emery and brilow when working with the head If you havent done this before try to find a vid on youtube or something to get a good visual. GOOD LUCK!!
    Last edited: Aug 19, 2010
  3. skrufryder

    skrufryder Member

    illl try to get you some pics of the process when time premits. I have that stupid job thing I have to do its a real buzz kill
  4. GearNut

    GearNut Active Member

    Do not make the intake ports mirror smooth, a "brushed" finish will actually run better.
    The little bit of surface irregularity helps create turbulence which aids in atomizing the fuel in the intake charge. The exhaust port and cylinder head both could benefit from a mirror polish.
    When working on the cylinder, be careful to only cut the ports when drawing the file or sand paper away from the bore. If you cut while going towards the bore you risk flaking the Nikasil plating off around the port edges.

    Even after doing all this work, you will not notice a tremendous improvement, but there will be one none the less. Notching the piston about 1/4 inch on the intake side and just cleaning up the ports a little bit seems to do more.
  5. Rconce01

    Rconce01 Guest

    I dont mean to highjack this thread. But i followed another thread on porting the so called "80cc" on this forum. I matched the ports along with the gaskets so that intake and exhaust manifolds flow freely, I took about 2mm of the top of the exhaust port so that its 1.1" form the top of the port to the top of the cylinder and i opened it up laterally from 3/4" to 15/16". I was told the exhaust port can be as wide as up to 70% the diameter of the piston . I lowered the intake port by 1/4". But i think i messed up when i also notched the bottom of the piston skirt 1/4" on the intake side.

    I lost alottt of midrange torque yet the bike revs like crazy. You also see alot of fuel coming out of the carb through the intake when you free rev it.

    Should i order a new piston? or a new motor?

  6. GearNut

    GearNut Active Member

    Porting increases the power in a certain rpm range, but narrows the whole power band.
    You could try a new piston with a stock length skirt. That will open the intake port later and close it sooner. Maby it will help, maby not?.

    Correctly porting a 2-stroke requires a bit of science, geometry knowledge, and careful planning.
    It is not "cut and dry" like making a sandwich.
  7. Rconce01

    Rconce01 Guest

    yes i know its not cut and dry, i didnt mean to undermine the science of two strokes. I've seen entire books dedicated to the optimization of these engines.

    What is cut and dry is that based on the spec i typed earlier people get indredible power gains on these engines.

    I just need to know if you increase an intake port by lowering it 1/4" if your supposed to also cut the skirt off the piston.

    I think its either cutting the port lower or cutting off the skirt because obviusly the combination of the two yiled really bad results.

  8. chainmaker

    chainmaker Member

    If it was me, I would start by getting the thing breathing good first. dump the stock exhaust,air filter,and intake manifold. I went with a SBP EXP exhaust, and one of their "performance" air filters, and a Manic Mechanic machined intake, take time to match the ports clean up the exhaust manifold of welding splatter. You got to go there anyway, might as well be sooner than later,you'll be surprised just how much that little helps without cracking the case. Then go from there.
  9. Rconce01

    Rconce01 Guest

    Cool man thanks,

    The bike ran good before i was just looking for more power, I did match the ports. And last week i ordered the billet machined intake from justin at pirate cycles, but they are on back order. So I am waiting on it.

    The notching of of the piston plus the lowering of the intake port must have prolonged the intake cycle way too far.
  10. Hawaii_Ed

    Hawaii_Ed Member

    expansion pipe, air cleaner, plug and wire, and slant head if you are running a regular head and good enough mods for most...the single best mod is a shift kit, really lets you keep the engine in the range. And the roller clutch bracket is one of the best mods I have on my bikes!
  11. skrufryder

    skrufryder Member

    I have seen some people got ahold of a puch moped top end wich has huge ports and crank out some good power. the one biggest draw backs to these kits is the lack of gears to shift threw. unless of course you have some soert of jack shaft. when working on 2 stroke scooter motors just opening the ports, bigger jets and better air filter realy did it justice, but they have a CVT that realy put this power in it place were you need it. Ive built some of these bike motors with some real tourqe and some hi rpms but it seems even with all those mods your only getting 10 extra mphs sure u get there alot quicker but a shifthing kit of some kinda is realy were the speed is gona come from but at this point a motorcycle seems to be the anwser love my mb's but know have reached the point were they arnt a bicycle any more
  12. HeadSmess

    HeadSmess Well-Known Member

    the induction cycle is symmetrical around TDC, a major drawback of piston ported engines.

    as stock standard, it is 120 degrees, from memory. the 1/4 inch notch brings it to around 150 degrees. ie, as piston goes UP 75 degrees drawing air IN, it also DESCENDS 75 degrees, expelling some of that fuel air mix back out the carby... :(

    so doing BOTH 1/4 inch notches results in possibly over 180 degrees. great ONCE the engine is doing hi rpm, and the sheer momentum/inertia of the air prevents it from "reverting" back up the inlet manifold... bad when the engines running slow.

    this is what reed valves are for ;) assymetry of induction stroke means good power through the range... ie, the piston sucks IN on the upstroke, but the reed valves CLOSE, compress the mix all the way from TDC till the transfer ports open.

    the EXport mod you made, towards the TOP of the cylinder...increases open time. so what was 140 may now be 150. the shortened time for the power stroke(increase port open, decrease power stroke...) is where your low end torque has gone. (and 70% exhaust port would be over 2/3 of the piston... maybe big on some, but never that big. still need space for the transfer ports, etc.)

    yes, maybe WIDEN the ex port, careful not to exceed the end of the rings or it shall be messy. but really? its big enough. 250ish square mm.

    the transfer ports also need to be increased to some point to match...

    without a custom designed pipe, the portwork is virtually pointless. it will be a marginal gain in these things.

    also, you dont get both low end and top end from twostrokes. dammit! either always less, over more rpm... or one lil revband...

    a nice pipe on these engines do make all the difference from being a lawnmower engine to something slightly more interesting, btw :) you wont believe its the same engine.

    you have a fine line between piston induction, exhaust timing, and intake/transfer timing. one degree too far one way... oops!

    i suggest a new cylinder, left alone, with a pipe. you will die of shock :)
    Last edited: Sep 7, 2010
  13. Rconce01

    Rconce01 Guest

    Thanks man you really put this into perspective for me. Before the long weekend i flipped the piston so that the notch is on the exhaust port and the bike ran a lot better. A new piston is on its way anyways. I'm going to fill in the intake port on the jug with some jb weld and get the floor to about 2mm lower than stock. I'll check back in, and let you know hows it going. I'm going to mess with this piston and jug until i get this right.

  14. HeadSmess

    HeadSmess Well-Known Member

    another tiny thing to consider IF you play with the transfer ports...

    they are open for 104 degrees. around BDC.

    so, they are closed for 360-104=256 degrees.

    any increase on the open time obviously decreases closed time.


    sub-piston induction is ported at say 120 degrees. 256-120= 136 degrees.

    halve that. (upstroke +downstroke..)

    68 degrees.

    this is the amount of revolution that the piston has on the DOWNstroke, to compress the air/fuel mix, that then forces exhaust gases out the exhaust port/improve scavenging and charging of cylinder with new air/fuel....(the actual angle of the roof of the ports is important want it "squirting" up toward the head)

    too little time means too little CRANKCASE compression means bad running....

    the same applies on the UPstroke, but this is the time it draws air/fuel IN

    once again, not enough "suck" means a bad running engine.

    together, maybe a non runner.

    this is the fine line i mentioned...

    reed valves cure this.

    when i get my new lathe...oh when i get my new lathe :D
    Last edited: Sep 9, 2010
  15. skrufryder

    skrufryder Member

    Hey..... I have gone into the local lawnmower/equipment parts and repair and picked up some WAYYYYYYY better exaust/intake gaskets, cdi/coils and notieced a bunch of reed valves....for chain saws I guess and they did have a small exspansion champer hanging on the wall......they cater to "SPORT LOGGERS" u know the lumber jack comp guys...
    I'll see if any reed valves work there only a couple of bucks....let you guys know...
  16. GearNut

    GearNut Active Member

    I an liking the way this sounds, skrufryder!
  17. Well I can tell you what I did, and how it effected everything.

    Boost Bottle (ran better, not much power increase)

    Expansion chamber (best mod, gave tons of torque)

    Carb (might not do much to a stock motor, but after giving it more power it really needed it)

    Porting/Notched piston (gave it alot of top end snap, and some speed. Low end was about the same)

    Cut up stock airfilter (this was a huge improvement, after adding all the extra power this was the one weakpoint of the engine. I gained 3KPH, and had massive torque increases)

    Ground down some bad casting inside of the intake tube, and other parts of the engine. (minimal increases but was free, and did help a bit)

    I'm waiting on a new caise, and smaller rear sprocket. I went from 51.4KPH to 55.0KPM. Although thats a minimal increas, the power all the way thew the RPM is insane compared to what it was stock. Its not like a motorcycle ofcourse, but alot better then it was.
    Last edited: Sep 12, 2010
  18. azterik

    azterik New Member

    This may be related to a loop scavenged engine common in dirt bikes more, but it is very common in those engines to use power valves which upon higher rpms will expose exhaust ports which are higher up on the cylinder thus expelling gases sooner. These systems also reduce expansion chamber volume slightly on opening of these valves.

    This will create a great 5-10 more hp depending on engine size similar to honda v-tech/toyota vvt engines quick rush of power when they engage their different cam profiles. (It also increases the sound output as the combustion is more incomplete and the flame will propagate farther into the exhaust)

    These valves also had circular plate in the main exhaust port which would have a portion of itself cut into the shape of the port itself and would rotate upon this shape as the valves open to effectively make a larger more free flowing port.

    Now with this knowledge whether you knew it before or not, should entice you to shape the exhaust port to allow opening of it sooner. I myself have not tried this and do not know if the configuration is ideally used only if you have a proper expansion chamber or if the cross flow scavenged engine design does not work well with this design. If anyone does venture into shaping the port upwards some ways i wouldnt do it much as the low end will suffer slightly.

    Just wanted to add another idea to anyone out there currently tinkering with their ports!!