What to expect from these port specs?

Discussion in '2-Stroke Engines' started by 2smokebob, Aug 8, 2015.

  1. 2smokebob

    2smokebob New Member

    I put a degree wheel on my PK80, what's it gonna run like??

    Transfer 120' ATDC
    120' Duration
    Exhaust 110' ATDC
    140' Duration
    Intake 124' Duration

    Measurements are with no base gasket. Crank trued with dial indicator. Crank case matched to transfer ports. Cylinder ports cleaned up, but not changed. Piston skirt trimmed to clear intake port. Intake port matched to Manic manifold. SHA clone carb. Piston comes to top of cylinder, so squish will be head casket. Expansion chamber not made yet. Wondering if anything looks wrong here. What RPM/power characteristics to expect so I can make the pipe right for the engine.


  2. jaguar

    jaguar Well-Known Member

    why in the world would you go to the trouble to get incorrect readings due to lack of base gasket?
    my god , man
  3. crassius

    crassius Well-Known Member

    my thought is to run it & see
  4. 2smokebob

    2smokebob New Member

    I found one reference that 120' transfer duration would peak out at 6000 rpm. Another said "120 I find good". So I found it interesting that I hit those numbers with no base gasket. I have a lathe (and I'm not afraid to use it!), so I can trim gasket clearance off the bottom of the cylinder, or just try RTV sealant. The base gasket doesn't concern me at the moment. I'm just wondering what number combinations are healthy.....proper...? I'm sure it has all been figured out by now. I'm not looking for a secret 9HP blueprint, just what numbers work well on a hand built engine.

  5. crassius

    crassius Well-Known Member

    I've got many used cyls here - all different by a few mm in port height - the H2s have one trans port 4mm higher than the other

    all ran acceptably when they were used
  6. butre

    butre Well-Known Member

    my bike has no base gasket, I put a thick layer of paint on the bottom of the cylinder, waited for it to get tacky, and then put it on the bottom end. it seals perfect, boosts compression, and improves intake timing.
  7. jaguar

    jaguar Well-Known Member

    I have considerable experience porting these engines and so would recommend you keep the transfers and the intake the same and then raise the exhaust port little by little till you get the top RPM you want. A duration of 156 worked good on my engines for how I wanted them to be.
    But before going to a higher RPM you need to balance the crank or you will have even more disastrous vibration.
  8. butre

    butre Well-Known Member

    I have to disagree with that. the intake on a stock engine badly needs to be lowered, and the transfers need to be widened and cleaned. the exhaust port should just be widened in my opinion.
  9. jaguar

    jaguar Well-Known Member

    the intake port duration he gave was longer than stock, being now at 120 which from my research is ideal.
    the transfers can be widened toward the rear a bit but due to the natural limitations it can't be done very effectively. I wouldn't discourage it though.
    the exhaust height is only ok if he is ok with the top speed. But consider this, the blow down from exhaust opening to transfer opening is only 10 degrees which is not enough. To correct that he needs to raise the exhaust port.
    ps- all this is assuming he really knows how to correctly determine the durations of the ports.
  10. butre

    butre Well-Known Member

    the problem with changing exhaust height is that you run at a higher RPM when you do. since it just shifts the power band, and doesn't really improve peak power all that much, I find that you're much better off changing the gearing to improve top speed than you are changing port timing.

    of course, changing port timing is free, whereas a good 32 tooth sprocket starts out at about $25
  11. butre

    butre Well-Known Member

    I've actually cut out the transfer walls entirely before. it does really nasty things to your intake velocity but can make for one hell of a top RPM screamer
  12. HeadSmess

    HeadSmess Well-Known Member

    weee, everyones 5 c worth!

    how you mounting em in the lathe? plain old 3 jaw in the bore and then...how to ensure its dead square?

    expanding arbor would be ideal... any consideration of spigotting out the top so you dont need a head gasket?

    then the port timing itself... have you started mapping them? worked out the time areas? done a template?

    opened up the intake and transfer to allow extra flow? this is all before even considering tampering with the durations.

    im with that theory of hacking out the transfer walls... they serve no purpose and make getting into the port itself a breeze.

    allowing for manipulation of flow.
    can always bog em up a bit with poxy or welding...

    then there gets this really tricky bit.

    i figured you could pull it off with spark flash photography, otherwise...

    you want a perspex head, with the same combustion chamber shape as the head you run.

    a source of fog or smoke on the intake...

    a vacuum on the exhaust...

    and to get a true dynamic idea of gas flows...

    a high speed camera so you can spin the engine at near operating speed.

    yep, a flow bench but rather than static, which means nothing...its dynamic. useful.

    (at least thats what i think, though i never bothered actually doing it...)

    its not just about "port timing"... its about flow...turbulence... and how else can you see that at 5.5k it suddenly stalls at the transfer? anyway...

    then theres that whole subject of the intake, tuning that side of the engine...

    and in the end...using el cheapo engines made from recycled beer cans...

    the realisation that the hours of work involved, the nikhedonistic drive to get it done, was all in vain, and one should have gotten a morini/denardis in the first place.

    hate to say it, but you will spend a LOT of money destroying parts before you get it "right".

    hmmm...say you eat five cylinders...thats $200 at the price i find em at.

    five pistons... another $100... youll always have one drop its keeper pin and destroy everything.

    two engines... 400....

    bearings, parts, time, labour... countless...

    morini 50cc as used in a ktm50... about 5-600 last time i looked... virtually bulletproof.

    i know it takes out the satisfaction of DIY but all ive found with these engines...is NO SATISFACTION!

    PS...when you make a decent pipe, even for a stock standard engine, which you will end up doing because anything else is so much effort after the first three blow up, the gains are extraordinary ;)
    Last edited: Aug 21, 2015
  13. jaguar

    jaguar Well-Known Member

    don't discourage the poor guy. I was very satisfied with my engine after I did all the mods to it and put on a torque pipe. It started easy, ran smooth, had great acceleration, climbed hills like they weren't there. Only thing I hated was the rough ride because it was a bicycle.
  14. HeadSmess

    HeadSmess Well-Known Member

    im just saying its sorta easier to concentrate on the exhaust, intake for stock... only do a little cleaning of ports... the easily swapped bits that actually make it go...but dont spend tooooooo much time making mods that end up making anchors... sure, youll get em to go but are the results worth it? 9hp is sheer dreaming...

    once you make a pipe for a 180 exhaust, 140 intake etc...youre stuck with having to make every new cylinder that way...

    i read lathe.... check out chainsaws instead ;) the results are far more worth it. get a good one... then that 9hp is actually reasonable.
  15. jaguar

    jaguar Well-Known Member

    mine has 157 exhaust duration, 118 transfers duration. not pipey at all.
    here's the big 6:
    ignition system
    balanced flywheel
  16. You really need a base gasket, otherwise, everything changes when you install the base gasket!
  17. HeadSmess

    HeadSmess Well-Known Member

    see, port durations mean nothing.

    just measured up my stihl ms170 quickly... max rpm of 14000, working rpm of 8-9000...

    approx 140 exhaust, 100 transfer. thats just rough, i didnt actually mount the degree wheel, but im pretty sure ive read them as being close to that, if even LESS. 136/96 im sure it was.

    anyways...im sure that if i did a time/area map of them...theyd be pretty close to the "ideals" laid out in them books by jennings blair and the other one... the cross sectional areas are HUGE. couldnt be bothered. just verifying my info before rolling a pipe for it. the time has come!

    so many things to modify, before even touching port durations... angles of port roofs and walls, areas, and flow capabilities always remain the most important.
  18. jaguar

    jaguar Well-Known Member

    "port durations mean nothing". wow, really.
    please get off your drugs (or get back on them) and get back to reality.
    you aren't revving up to no 14 thousand! you can fool others but not me.
    'fess up man
  19. HeadSmess

    HeadSmess Well-Known Member

    MAX rpm 140000

    this is max redline as stated in the workshop manuals. any more and they explode.

    they will do it, unloaded.

    OPERATING rpm...8000 to 9000 rpm... now thats LOADED...

    do you ever read ENTIRE posts or just select the phrases that suit you at the time?

    like the little bit about MS170....STIHL???? CHAINSAW???

    what? you think im saying a HT POS gets to that? :rofl:
    Last edited: Aug 27, 2015
  20. HeadSmess

    HeadSmess Well-Known Member


    transfer of 126

    exhaust of 196

    find a steep hill and it still wont get past 11k. absolutely gutless at 10.5, max band at 10k.

    of course, in this case, its the exhaust to blame. change it and suddenly they wind to 13k quite happily.