Ht Performance Porting Success

Discussion in 'Performance Mods' started by 210061741, Nov 10, 2009.

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  1. 210061741

    210061741 Guest

    Ok if you have been following things here you know i recently tore down my 48cc Gubee GT-2B motor.

    Before assembly i decided to try some modifing of the transfer port area in the cranKcase.

    This was my theory ......:detective:

    The area of the transfers in the case is fairly restricted due to the casting and the way it is production machined.

    Alot of material restricting the flow from the crankcase through the transfer.

    Removal of material and modification of shape could greatly reduce the restriction ( allow the intake cycle to breathe better and give it a better chance of sucking more of a charge in the amount of time produced by the duration of the intake timing ).
    Plus side
    Removal of this material will not change the port timing.
    This makes it less succeptiable to error and less of a chance that i would destroy the engine with this experiement.

    The transfers in the Cylinder will ultimatly dictate the maximum amout of flow through the engine. This means even if i went a little nuts with it the transfers will compensate cause the flow restriction would occour there in the cylinder.

    Therefor the least restricted and easier transfer into the ports should proove to be a bonus.
    Down side
    Lots of careful time consuming grinding and shaping would be required.
    This took me about 2 hrs per side.
    Also to do what i pictured certinly would increase the crankcase volume by several CC's.
    Really we don't want to do that but later we can add some filler somewhere in the crankcase to compensate if nessessary.

    At this time i havent figured out what the BMEP of the CrankCase should be nor do i know it's exact volume.
    If someone out there does please chime in.
    So i had to decide if the potential gain was worth the risk of ruining the engine. As this whole idea was bassed of off my theory which seemed to follow along with all of the studying i have done about porting.

    As i had allready wrote the engine off and convinced the wife i needed a new one which she agreed, I figured to give this nutty idea a try.

    Now i didnt do much to the transfers in the jug, and nothing nothing at the top of the transfers in the jug.
    I really reccommend unless you have a cnc to machine the transfers from top to bottom don't touch them.
    You won't gain much anyway there unless the whole transfer is machined and if you change the angles at the top of the transfers you could blow your intake charge right out the exhaust.:eek::confused:

    So what i did was grind out the entire step in the crankcase and round and smoothe and blend it all as nicly as possable to get the transfer area under the transfer ports larger than the transfer ports and promote flow without restriction.

    Then i attached the jug to the crankcase half that i was working on and blended the crancase and transfer ports so ther were nice and smooth.
    No steps anywhere. Just nice even flow.

    I took a huge amount of matterial off each side being careful not to thin out the wall too much at the jug / crankcase seal.


    I had some trouble with my magneto but i got it running yesterday.
    I cannot believe the difference in power.
    Now my low end seems to suffer a little due to the increase in volume of the crankase.
    Only the very low end. And only until the engine warms up.

    Overall this mod rocks.
    I gained alot of rpm's more torque everywhere except the very very low end and it runs soooo smoooothe.

    I't is worth the time.

    But remember i run an expansion chamber and a boost bottle.
    Increasing flow in one area but restricting it somewhere else won't help.
    Everything has to work together in harmony.

    I imagine with a good carb that could be finly tuned you could really get the baby pumping.

    Check out the pictures and if you feel up to it give it a go.
    I don't think you will be dissappointed.

    Attached Files:

  2. craisin

    craisin Member

    If one could imagine the total mass of what one ground off and then attach the equivilent mass in the form of a small plate inside of the crankcase,
  3. 210061741

    210061741 Guest

    The amount of mass removed isn't as important as what the optimum psi in the crankcase should be.

    There is pressure created in the crankcase by the piston and to a large degree the pressure in the crankcase will dictate what the max ouput of the motor can be. this is called BMEP "Brake Mean Effective Pressure".

    So the proper thing to do will be to accuratly measure the total volume of the engine determine what the BMEP is and then adjust the volume of the crankcase to give optimal PSI.

    This i will not be able to do until all of my porting is completed.

    :idea:Now if you have followed my 1500 mile teardown thread you will know that i put the old not so good cylinder and piston back in.
    The jug i have not ported yet.

    So i plan to order a new upper end kit this week.
    I will take the new cylinder and do a port map.
    I posted another thread where you can find out how to do a port map.

    Once that is completed i will know the Timing and Duration of the Exhaust and Intake.

    With all the data that will be gathered through that process i'll be able to calculate the BMEP and have some mathematically calculated adjustments to use as guidelines for porting to get an optimal RPM and Torque from the motor.

    Since i will have new parts on hand i will start with the old ones.
    Cut them slowly a little at a time.
    The piston can be cut to some degree to change the duration and port timing.
    I will keep going little by little and keep track of the results, keep pushing and pushing to the max until i cut it too far and get negative results.
    At this time the old cylinder and piston will be no good. "oh well"

    Now after anaylizing all of the results i'll know exactly the measurements needed to cut the new cylinder and get the very best performance possable.

    At this time the engine will be as good as it can get.

    Then i'll have to start with the 66er but hopefully by then i can get a Grubee GT4 High Performance 66cc.

    I am very satisfied even with the porting that i did.
    I was passing cars riding down the road today.
    I run a single speed 36T.

    Wish i had a jackshaft.
  4. 210061741

    210061741 Guest

    geeze i figured you guys would have alot to add to this.

    Guess i'm just not that popular :ack2:
  5. AussieJester

    AussieJester Member

    If i was playing with one of these china motors i would be cutting the skirt on the piston also, we used to "short stroke" 3.5cc motors in the 1/8th scale on road race cars using this method when i was a teenager, they would absolutely screeeeeam touching 40 000 rpm... I think the biggest performance 'upgrade' would be fitting a reed valve setup to these motors have you thought of this 210061741?

  6. 210061741

    210061741 Guest

    Yes that is a major flaw in these engines?

    At tdc the intake port is blocked partially by the piston.
    Also at BDC the port is partially blocked.
    This mease restricted flow.

    To cut the bottom of the piston is a common practice.
    It will change the timing too.

    First i want to do a port map and see if it is even possable to get the proper timing and duration with these HT jugs.

    Figure out the optimal and try to achieve it.
  7. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi 210061741,

    When I raced Yam, Kaw, Bultco, & Carabella [Mexico] we always stuffed the piston to reduce the crankcase volume with great results. On most motors we were able to only cut the rear of the piston skirt approx. 1 MM without major low end losses. I don't know if 1 MM would be way too much on a small motor, but we cut the pistons on motors from 100 CC to 360 CC with the same results.

    Have fun,
  8. 210061741

    210061741 Guest

    So how could you stuff the piston without adding much weight?
    Interesting concept though.
  9. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi 210061741,

    I think we used epoxy, and mostly used Wiesco piston which were often lighter than stock. Chances are the stuffing simply made the Wiesco piston closer to stock weight. I do remember the piston didn't need much to decrease the volume.

    I always worried the stuffing would work loose, but it never did.

    Have fun,
  10. 210061741

    210061741 Guest

    I was thinking epoxy in the rear of the crankcase
    Or even better.
    Remove the steel plate flywheel weights and replace them with larger and lighter aluminum plates.

    It would be interesting to see if the flwwheel would still have enough momentum for the motor to idle good.
  11. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi 210061741,

    Several factory race bikes were given to me to race with lighter flywheels, & several with lightened & balanced crankshafts. It made a very interesting motor, but be sure the bottom end power will suffer greatly.

    On a Ducati Desmo single the flywheel was replaced with a spacer and the throttle simply became an "Off & On" switch. It reved very quickly, but also shut completely down when the throttle was closed. On a Kaw 100 [baby green streak] with a lightened & balanced crankshaft the power came on so quickly it was impossible to ride on a dirt track because the rear end broke loose too quickly in the corners [fell down a lot].

    I guess the best part of stuffing the piston is it doesn't disturb the fuel flow inside the crankcase.

    Why not try "finger ports" on a HT motor. I moved a Kaw 100 from 18.7 HP to 21.8 HP by stuffing the piston, adding "finger ports", and cutting the rotary valve.

    Another neat trick to get a small 2 stroke to perform is by using a "reed" or "pumper" carburetor. Simply drill a hole in one of the transfer ports, install a hose fitting to supply vacumm to the carburetor, and watch it rev higher than you would think possible [if it doesn't blow up].

    Have fun,
    Last edited: Nov 12, 2009
  12. craisin

    craisin Member

    well reeds sounds like the go.
    and as our experienced friend has indicated cutting the bottom 1mm off intake side of the piston, I am wondering if you filed a pattern like a row of saw teeth on that side of the piston
    May help inducing swirl and may help to cool the piston.

    So sounds like where you put the reeds would determine crankcase size to try to reduce recipricating weight.

  13. craisin

    craisin Member

    save stuffing the piston.
  14. AussieJester

    AussieJester Member

    reed valves would make a HUGE improvement, problem is though making these motors rev aint the best thing, they are **** quality and will destroy themselves, you need to upgrade bearings and conrod crank and piston if possible. I would be looking at pocket bike 47cc motors you can get performance parts for these you can buy them off the shelf with 10hp plus and watercooled. You will definitely make the HT motors **** but they wont hold together long can guarantee you that. Like these noddies popping turbos on their stock four packs getting all impressed with their improved performance until their car is on the side of the road with blown motor... If you work the 'top end' you have to beef up the bottom end too :)

    Best of luck...

  15. 210061741

    210061741 Guest

    Yes i do agree on the bottom end.

    That might explain why some people have better luck with the 48cc.

    I believe the 48 and the 60 have the same bottom end.
    Somehow that don't seem right.
    Either one is stronger than need be or the other is too weak.

    We know the answer to that.

    So far i have expected my 48 to blow a gasket !!!!

    But it hasn't and still puts it all out there.

    I think and soon will find out that there is a series of things within these motors that must balance each other out.
    Not the crank either LOL

    If one peformance upgrade is done without the other you start getting things out of balance.

    If you work the engine as a whole and Balance the modification out i believe things will go smoothly.

    Obviously if you run your motor hard it won't last as long.......

    But if you have 2 motors.
    1 in operation and one in standby.
    If you notice a funny noise spend the 30 min swap motors and tear that baby down.

    Parts are cheap if you don't destroy the motor completly.
    Fix it before it breaks.

    Then like changing the oil in a car change motors after certin # of miles when you know things are getting rough and repair before there is a major problem.

    Thus far i've only had the 48cc but i have run 6000 / 8000 rpm for a long time and it's holding up good.

    I have a 66cc Jet coming next week.
    I plan to tear it apart and build it up before it ever sees the bike.

    Maybe some $20.00 a piece 6202-2RS hybred ceramic stainless mainbearings.
    Easily available.

    The needle bearings are a little more difficult and i got to get the motor apart before i can work on thoose.

    It seems though that the lack of thrust washers and the incredable amount of space for them to float around are the worst problem.

    So hopefully i can get a wider set of needle bearings with the same dimensions and better quality to take the extra abuse.

    The limiting factor of the motor should be the Max Velocity of the piston.

    With the 66 im gonna try a differant approach.
    I'm gonna mod it for the best low end torque at a useful rpm and run a much smaller gear.

    The 48 take advantage of the rpm.
    The 66 take advantage of the power.

    If i can get the speed i need without the rpm that is wonderful.
    If not i'll push the envelope.

    Worst case i'm out $100 bucks.

    I have been known to spend that on dinner and a few drinks.

    So i guess to make a long story short if it cost me $$ as long as the fun i had was worth it i'm happy.

    Hopefully this 66 dosen't change my tune.....
    Iv'e realized there a different animal.
  16. AussieJester

    AussieJester Member

    At the end of the day its all good fun aint it, sounds like your going about it the right way too. I would definately get onto the reed valve setup for the carby though honestly the difference it will make is light and day...Could eeeeasily adapt a carby manifold and reed valve from a pocket bike motor to the HT motors too.

    Best of luck

  17. 210061741

    210061741 Guest

    Well i guess it's time to learn about Reeds now.

    I haven't done so yet cause i read somewhere they wouldn't work right with our HT motors. I guess it is time to find out.
  18. Quenton Guenther

    Quenton Guenther Motored Bikes Sponsor

    Hi 210061741,

    Reed valves will increase low end torque, but will decrease the top end speed. Reed valves restrict the flow at high RPMs because it is an obstruction in the intake manifold/port.

    Motorcross riders use reeds whereas high speed dirt track bikes don't. As a rule reed valves remove approx 10% off the top end, however the gearing can be adjusted to make use of the additional torque at lower RPMs.

    Have fun,
  19. 210061741

    210061741 Guest

    If iv'e learned anything here at all Ive got to say when you really get into it you find out the smallest changes really do make a big diff.

    Now the bottom end porting wasn't small.
    Very effective too.
    Ran it for a week screaming down the road.

    When i first put it back together i used no Bottom Jug Gasket.
    The motor has been running smoothe at high rpm but tougher at idle.
    I prolly still had some air leak somewhere too.
    It either didn't idle or wouldn't stop ideling at high Rpm.
    But ran like a champ going down the road.

    So in my infinite wisdom i decided to double the base gasket pushing the ports up higher than normal.

    Normally the base gasket is about .045 inch thick.
    I put .090 under the jug.
    Raised ports .090 from where they were and .045 from stock.

    With that extra height from the base gasket this thing won't run right.
    Just can't seem to get the carb adjusted correctly.
    Usually it hits a spot where it just kicks in and smoothes out.
    But now i have 4 stroking symptoms across the board.

    I guess i'll take it apart again ant try with just the .045 thick gasket.

    Just goes to show ya..... .045 inch isnt very much at all.
    So whichever port dosen't like this.
    If you ground .045 off the top of it.
    You wouldn't be very happy.

    The only problem i have right now is my muffeler broke loose from my SBP expansion chamber.

    So thats the question is my missing muffeler causing my poor running or is it the extra base gasket.

    The bike sounds sweet with no muffeler. But Loud as Heck.
    I got a Lawn Mower Muffeler hoocked to it Now.

    Still loud as all ..... and running like poop.

    Gotta give it to the guys at SBP.

    There muffeler Rocks....

    Anyway i'll let you guys know about the base gasket deal when i figure it out.
  20. 210061741

    210061741 Guest

    I have now tried a couple of different mufflers.
    Running a little better but it was running soo much better before.

    I'm gonna pull 1 of the base gaskets out tonight.

    This configuration dosen't seem to be working for me.

    My low end did improve alot.

    But my top end is all but gone.

    Glad i'm playing with gaskets before cutting ports.

    If i had cut them it would be over now.