Increasing rpm

Discussion in 'Introduce Yourself' started by CedarHillbiker, Aug 3, 2015.

  1. CedarHillbiker

    CedarHillbiker New Member

    I'm topping out at 7800 rpm. I have a high compression head and expansion chamber exhaust and high performance CDI. I have also cut the piston. How much can I increase the rpm if:

    1. I replace my NT carb. If so, with what CARB?
    2. Raise the exhaust. If so, by how much?

    Any other suggestions? I don't want to lose low end tongue. I've got great uphill speed and power right now.

  2. The_Aleman

    The_Aleman Active Member

    If I were you, I'd be happy with that. Don't get greedy. Many people aren't as lucky with their engines. Many of them start degrading when taken over 6K.

    Raising your max RPM will almost always cause you to lose some low-end torque, because you'll almost always have to sacrifice it to get there.

    If you're dead-set on more RPM, you'll likely need a fully balanced lower end and exhaust tuned for high RPM.
  3. jaguar

    jaguar Well-Known Member

    a Mikuni carb properly jetted will give better power all thru the rev range.
    I wouldn't change the engine porting if it has good power now.
  4. butre

    butre Well-Known Member

    gaining RPM will lose torque, it's just the nature of these engines.
  5. CedarHillbiker

    CedarHillbiker New Member

    Tuned exhaust

    in order to gain higher rpm by tuning the exhaust, should I lengthen the header pipe? By how much?
  6. butre

    butre Well-Known Member

    shorten, and only if you have an expansion chamber
  7. jaguar

    jaguar Well-Known Member

    shorten it an inch at a time
  8. i too am interested in raising my rpms but would like to know how one would go about it
  9. jaguar

    jaguar Well-Known Member

    If you have a stock pipe then shortening its length won't increase RPM. You have to get a rotary tool and take off metal to raise the exhaust port 1mm.
    that shortening length trick only works with expansion chambers. but even then it may not be greatly effective as the stock exhaust port height is not high at all. It is intentionally low to limit RPM/speed.
    But also know that if you have engine vibration then raising the RPM will also make the vibration worse (maybe so bad that the fuel in the carb is too frothy to work right and so that limits RPM). I tell on my site how to balance the flywheels for little vibration.
  10. sbest

    sbest Active Member

    When I look at these engines I see some problems with raising RPM.
    Balance, vibrations of course, the cranks are a bit crooked, not much port area for flow at speed, but the largest problem is the piston ring thickness.
    those slabbing thick rings are not meant to turn up past 9,000 rpm according to Gordon Jennings' formulas. At 38mph with a 44t sprocket I must be pretty close to that.

    Those rpm work on a high performance engine because the cylinder is ringed with ports letting maximum fuel/air and exhaust move in the fraction of a second. Our Happy Time motors have limited port area, so power efficiency will really drop off with RPM.

    Same view (toward intake) of KTM 125 and Grubee GT5A note the difference in number and area of the ports.

    With other engines I discovered that raising the exhaust port alone would generally create a "peaky" engine. Better was to leave the port height where it was, but flatten it in nearly a straight line and make it much wider at the top of the port, leaving the bottom of the exhaust port stock width where flow there is minimal. This gives the effect of raising the port, but keeps the midrange torque. I have not tried this on my Happy Time Grubee GT5A yet.

    Just some thoughts,

  11. jaguar

    jaguar Well-Known Member

    good comments.
    raising the port 1mm generally will let it rev to around 7000 to 7500 rpm.
    I had mine raised even more and it rev'd to 9000 but with thinner rings, Mikuni carb, balanced crank, torque pipe.
    torque at low rpm was still good thanks to the pipe.