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This topic contains 4 replies, has 4 voices, and was last updated by  Pete Muller 3 months, 2 weeks ago.

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  • #64162

    Jim Silverheels
    Participant

    Higher compression from my reading makes more lower end hp and less top end, why? 2 cycle anyway

    #64187

    Glenn L Riggs
    Participant

    compression will help with torque and power thru out but what happens on a 2 stroke as the area is decreased near top dead center that allows for less flow of fuel in and out

    #64856

    Pete Muller
    Participant

    I have come to the conclusion over many years that higher compression makes more power through the entire range (within limits, obviously… there comes a point where things start breaking!).

    I believe that the “perception” that raising compression increases low/middle rpm power while hurting (or not helping) top end power (or even limiting peak revs) has to do with typically inadequate fuel metering.

    I don’t think there is any question that peak fuel demand *per*cycle* is at peak torque.  Generally, I feel what happens when people test higher compression is that they have to richen the mixture to keep the engine rich enough through peak torque (aka: peak heat).  The fuel demands at high revs (past peak horsepower) fall off quickly, and the amount of additional fuel being provided at peak torque when compression is raised is simply a bit too much fuel at/near top rpm.

    Of course the exhaust pipe and ignition timing can enter into the picture as well, however I feel the perception discussed (I’d even call it a “myth”) has come about due to poor mixture control.

    PM

    • This reply was modified 4 months ago by  Pete Muller.
    #65531

    Ric mcDade
    Participant

    I had a high compression Vortex, seem to limit top end- bought a PVL with built in timing retard, and the thing would zing to 16,500 on a road course with no problem. I’m not very smart, but was it the retard that let it go, or am I the retard?

    #65584

    Pete Muller
    Participant

    Ric,

    Absolutely correct — here’s what happens:

    Retarding the ignition timing drives the exhaust temp up significantly, which *increases* the speed of sound.  This causes the exhaust pipe to “act” like it’s shorter than it really is (allowing the engine the engine to rev higher).

    Ideally, the exhaust would be adjustable, and the ignition timing would stay the same or even be advanced a touch at very high revs, but since most classes have a fixed exhaust rule, retarding the timing tricks the engine.

    PM

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