HT engine True MAX RPM

Discussion in 'General Questions' started by 210061741, Oct 30, 2009.

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  1. 210061741

    210061741 Guest

    Ok they say that these run only 4500 rpm.
    I have a GT-2B 48cc and gotta be running alot higher than that.
    I know i cruise between 30 to 35 MPH with the SBP Expansion CH and no head gasket + DIY Boost Bottle and 36T rear sprocket.

    With the 36T and calculated with a 26" dia nominal tire.
    Which we know is less.

    I would have to have an engine rpm of about 5800 rpm to hit 30 MPH.
    I am confident i coul push 40 MPH out of her before i run out of Fuel / Air to sustain higher RPM.

    At 40 MPH My motor would be spinnin 7632.83 RPM
    I know i've been there.

    A dealer told me 8000 max rpm.

    Anyone have a tack that can tell us the truth.

  2. DougC

    DougC Guest

    If you have a DVM that does freq. counting, you could make a inductive (coil) sensor out of a bit of wire, and hold that near the spark wire with the engine running.
  3. Fabian

    Fabian Well-Known Member

    Hi Rich

    If you have been to 7000 rpm, you won't have a bottom end bearing for much longer.

    I revved my 1st engine to 5600 and it would not rev any higher - 35 miles later the engine blew the bottom end bearing.

    With my second engine, i accidently revved it to 4800 and around 60 miles later, it's making bottom end bearing rattling sounds.

    7000 rpm, it posssible, anything is possible, but you'll have stuffed the bottom end bearing.
    Expect the big end to c.r.a.p itself fairly soon.

    You're best off disassembling the engine before the bearing fails and metalic particles damage the piston/rings and chrome plating on the cylinder bore.

    Last edited: Oct 31, 2009
  4. iron_monkey

    iron_monkey Guest

    These engines are not designed for high rpm.

    Although my engine has probably seen 6000rpm due to someone wanting to WOT it for 1 second in neutral a couple of times.

    That was 100km ago, hope it doesnt fail.
  5. Skyliner70cc

    Skyliner70cc Active Member

    I rev the carp out of my HT engines with no problem ever with lower end bearing. I've had one issue with wrist pin bearing from overrevving but that was 2 years ago.

    Some folks have successfully used a Stihl wrist pin bearing whch is rated for much higher revs.

    I'm sorry to hear about your engine failure Fabian. I'm sure you know that an engine should never be revved high without a load.
  6. wildemere

    wildemere Member

    I've got an old bushing 70cc motor that sees 6000+ rpm every ride.

    Its 2 years old and sees about 100km or 60 miles per week.

    I've only done one big end in all that time, that was the result of a lean out and overheat.

    Its still on the original barrel though.
  7. Fabian

    Fabian Well-Known Member

    I find it amazing at the number of people who have had no mechanical problems with their 2-stroke engine kits.

    The distributor/retailer from who i bought my engine kit has admitted a big end bearing failure rate of 4 in 10 engines, the others failing due to incorrect oil/fuel ratios.

    I am working with the distributor preparing an extensive PDF to be sent to the engineers of the Chinese manufacturer, photo documenting all the failures and suggesting rectification methods to fix their shocking reliability issues.

  8. 210061741

    210061741 Guest

    Well like i said i don't really know for sure.
    And i do think my engine is getting tired.
    However it still runs great.
    I ride hard and like Fabian I play the terrain.
    Only difference is i don't have gears.
  9. Skyliner70cc

    Skyliner70cc Active Member

    Very noble, but do you thing that the same folks who poison (adulerate) pet and people food so lab tests show higher than actual protein levels actually care about making things better? If you get them to improve their product, I'll buy you a beer!
  10. arceeguy

    arceeguy Active Member

    I was unaware that the same factory that made bicycle engines made pet and people food!
  11. DIYMark

    DIYMark Member

    You're all full of it lol.

    My HT is some what modded but the big/little/crank bearings are STOCK and my engine is just fine even though it revs up to 10,000RPM (and thats with load - who knows how high with out load).

    You see my mates followed the bike and at WOT on a flat we hit 70Km/h with a 44 tooth sprocket and a 26" wheel - that equates to ~10,100RPM.

    In fact my mate held it flat out for 3Km of country roads (hills aplenty) and the bike never fell behind 50Km/h (only once it was a 50, 90% of the time is was at 60 to 70Km/h).

    Problem is...the muffler tip fell off and that changed the fuel air - you guessed it! 100m later bike stopped and upon inspection the piston melted! Oh well new piston and rings and it will be all good (I have split the cases and cleaned out the whole motor from aluminum). I still have to make a new muffler tip - the other one fell out at 70K's and sparked on the road and we lost it.

    I should make a video some day.

    PS - I'm making a phosphor bronze bush for the little end as I fear the stock bearing is about to explode at 10K Revs lol
    Last edited: Nov 1, 2009
  12. wildemere

    wildemere Member

    Piston didn't melt, It scuffed itself to death.

    Also, pics or it didn't happen (10,000rpm)
  13. Fabian

    Fabian Well-Known Member


    10,000 rpm - prove it!
  14. 210061741

    210061741 Guest

    This is exactly why i started this thread.
    Everone has diff ideas of haw fast the motor rpm is.

    My guess is at least 6000 rpm.

    Now at 4500 RPM there saying max HP and Torque.
    But if you look at most torque curves the RPM keeps going up but you loose torque. But at such higher RPM you don't need as much torque.

    So how about everyone posts there gear setups and engine types.
    And thosse who have an accurate way of knowing MPH post there speeds.

    Then i can do some math and come up with the average RPM we get from these motors.
  15. 210061741

    210061741 Guest

    specs from Flying Horse HT Engines
    Seem to be closer to what i guess

    Attached Files:

    • rpms.jpg
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  16. 210061741

    210061741 Guest

  17. 210061741

    210061741 Guest

    Key Factor for MAX Engine RPM

    Exhaust Porting and Expansion chambers = RPM

    The exhaust open period determines to some extent
    what the maximum hp will be
    and at what engine speed it will be produced.

    The expansion chamber, on the other hand, 'adjusts' the power characteristics of the engine
    at speeds above and below maximum hp revs.

    expansion chamber design is far more
    critical than exhaust port duration.
  18. DIYMark

    DIYMark Member

    Scuffed? No way it was in mint condition (the sides anyway - I use Castrol Power TTS) However there was a 10cent coin sized hole blown into the top of it lol.

    Anyone can get 10,000RPM - just raise the exhaust port/lower intake - problem is that you have like no torque hence no power. But my bike made it over hills at over 50Km/h so it has revs + torque = power.

    Ill try and get a video once its put back together (may be a while because Im moving house and my engine supplier doesn't want to talk to me now that I mentioned a spare part - amazing how he wouldn't shut up when I asked to buy the engine lol)

    Until then you have to trust me on the 10,000RPM.

    My wheel has a circumference of 2.11meters.

    The speed It reached was a touch over 70Km/h (actually 73ish) lets call it 70 then.

    70,000 meters per hour (70Km/h) is 1167 meters per minute (70,000/60).

    1167 meters per minute is 556 wheel revolutions per minute (1167/2.11)

    Gearing is stock - so internally its 4.1:1 and externally its 4.4:1.

    Therefore my engine is doing 4.1 * 4.4 * 556 RPM = 10,030RPM

    As for the engine its is modded as follows...

    CNC made cylinder head with 50/50 squish band
    NGK B8HS plug
    Transfers/Intake/Exhaust matched
    Gaskets matched
    Intake Lowered + porting
    Exhaust Raised + porting
    Transfers ported
    Piston Machined on intake side
    Rings honed
    Crank Balanced
    Crank Trued
    Timing optimized (make an offset wood ruff key to tune timing)
    Carburetor Bored/Honed
    Custom straight Intake
    Polished head
    Polished Exhaust port
    Honed Intake Tract
    Output sprocket profile corrected
    Custom built Expansion Chamber exhaust ("tapered" header)

    Don't forget all the simple things like correct fuel/air, good oil and fuel (bike needs 98 octane), correct chain alignment and tension, correct sprocket alignment, correct engine alignment, trued wheels, etc.

    I just realized then lol that this is on a STOCK carburetor! All along I thought to my self man I need a new carb - but if the stock one will get you to 10,000RPM...why buy a new one? If anything a larger one will give me less bottom end but more top end - however, I like my power curve the way it is.

    Finally I don't see whats to "unbelievable" - go get a ~70cc moped and go flat out on it. How fast did you get 70/80km/h ish? Now get a HT and do the mods that I did to it (so that it is kind of the same quality you'd get as if the engine came off a moped) then go flat out on that. How fast you get? Again 70/80ish.

    My point is any half decent 70cc engine will get you to 80Km/h (50MPH), its just that a HT engine is not half decent - its ****. Once you bring it into the ball park of a small motorbike/moped engine in terms of "engineering quality" (ie correct dimensions/design) then it will perform the same - only difference is that the HT engine is not a high mileage/durable engine (once brought up to the same power level that is).

    So sure my engine can do 10,000RPM so what? How long will it last - I give it 500Km (the way I ride it) before I need to get new bearings for it (Chinese bearings are rubbish).

    PS - Id like to try a 30T sprocket on my bike - I'd reach like 100Km/h haha! Even better would be a shift kit with a wide range cassette (11-34) then my MB would literally be a 70cc motorbike.
    Last edited: Nov 2, 2009
  19. Fabian

    Fabian Well-Known Member

    h these two factors set correctly - Crank Balanced and Crank Trued, 10,000 rpm is possible.

    What manufacturer and specification of big end bearing are you using.

  20. DIYMark

    DIYMark Member

    All bearings are stock.

    I will be making my own bushed little end bearing (from phosphor bronze) because I trust that more than the stock needle roller. However the stock crankshaft and big end bearings I fear aren't even rated for 10k revs so I'm seriously thinking of upgrading them.

    And yes the crank is one of the main mods that lets my rev that high. However being a single cylinder engine it will always vibrate. But the compromise Ive chosen for is that at around 10,000 rpm the engine vibrate a fair bit but then at cruising rpms (~7K) the engine is smooth.