| Rsv1000R |
12-16-2008 01:01 PM |
So, a while back I did a bunch of engine simulations where I'd pick a head, and then vary the stroke, and a bunch where I'd keep the same displacement and vary the bore and stroke to see how the output changed.
What I found was that the amount of air you can get in and out of the head was the critical factor. Torque peak occurs when the port velocity is in the 700'/sec range. Since everyone usually changes exhaust and intake manifolds, that means you can focus on the head and valves to determine performance capability. What I found was the more valve area you have, the more power you can make, if you can fill the cylinder. 4 valves usually have more valve area than 2 valves. So for the same engine a 4 valve head will flow more air. But what I also saw was that you can change the stroke and as long as you can spin the engine high enough, you can consume the air the head can flow. So for a short stroke engine torque peak will be at a higher rpm, and the same engine with a long stroke will just pull the torque peak to a lower rpm.
If you have a long stroke, you don't need DOHC's, the torque peak will be low enough that you don't need high rpm valve stability. A pushrod system will work just fine. However if you shorten up the stroke, you might run into valve float before you finish getting to your torque peak.
Also I think, if you normalize torque output per rpm with gearing between the long and short stroke engine the output will be similar.
And lastly if you're racing in a displacment limited category, you are better off going for Big bore, short stroke, high rpm as that gives you the most valve/port area. And this is the exact kind of engines unlimited class racing has evolved.
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