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Cams and Revs: How and Why

    Ok, so, we figured out how camshaft and flow changes effect the powerband of an engine. Basically they make the engine put out torque higher in the rev range, which, according to the equation stated in earlier sections, gives us more horsepower.  But, why is this so? Well, thats what we are going to answer in this section, along with some other stuff.
    Basically, these changes alter the volumetric efficiency curve of the engine. (say what?). As an engine speeds up through its rev range, its volumetric efficiency changes. At lower speeds, the piston goes down slowly during the intake stroke, and the intake valve is open for a relatively long time. Thus, at the end of the intake stroke, the air pressure inside the cylinder is almost (85%) the same as atmospheric pressure. However, as the engine speeds up, the air pressure in the cylinder drops in relation to that of the atmosphere(60%). This cuases torque to drop off as well. Their is another factor to consider here as well. The column of air rushing into the engine has a certain amount of inertia. The faster the air is moving, the more inertia it has. This is a key point in making horsepower.
    An engine designed for moderate speed operation has an intake and valvetrain optimised for those speeds. So, the intake ports are designed to be relatively small in diameter in order to have high intake velocities, even at lower engine speeds. These velocities make the air pile into the cylinder as they slow down when the piston reaches BDC.  The valvetrain is designed to close the intake valve relatively early though, for if it closes to late the piled up air in the chamber will blow back out. This early closing decreases the overlap of the cam, that is, the amount of time that both the intake and exhaust valves are open.  At high rpm, both the smaller ports and the early intake close have a deterimental effect on engine horsepower, both of them restricting engine breathing.
    On a high speed engine, the ports are much larger, and the cam overlap much longer. This allows larger volumes of air to flow at high velocity into the engine, and the increased inertia of these columns allows the intake valve to close much later, taking advantage of the increased amount of intake piling. But, this has a tradeoff. At low speeds, the intake velocity is too low to have a signifigant ramming effect, so torque drops. The lack of intake ramming makes the intake valve duration excessive, so as the piston comes up, some of the air charge is pushed back into the intake system, further decreasing torque. The crux of all this is an engine that is peaky, and requires high revs in order to make power, assuming of course variable valve timing is not used.
    If you go this route for performance improvement, be sure to make other intake/exhaust changes as well. In order to flow all of that extra air at speed, the intake and exhaust systems will need to be optimised as well, so a good, high flow intake and exhaust are essential. A port and polish job on the head is also a plus. Keep in mind also that many high performance factory engines have pretty long overlap cams and smooth ports to start with, so there may not be much room for improvement here. It's very model specific.

My Sleds:

    1972 Élan 250:

    1972 Élan 250 with a 71' TNT 335 engine, drivetrain, and suspension:

    1973 T'nT Silver Bullet 294:

    1980 Blizzard 5500:


    Bob's Snowmobile Page
        :Big site full of info on T'nT and Blizzard SkiDoo's

    Antique Ski-Doo & Vintage Ski-Doo Restoration Resources

    Tech Report on Power Valve Maintenance
        :Keep those R.A.V.E. valves clean

    Ram Air Article
        :An interesting article on the fable of "ram air"

    Rear Cylinder Siezures
        :A tech article about rear cylinder siezures in PWC twins due to crankshaft torsion (very interesting)

    Polaris Triple FAQ
        :A page that has the skinny on Polaris triples (ie which ones to avoid)

    New Hampshire Snowmobile Association

     Don's Vintage Arctic Cat Site

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