You guys are killing me............ ....
The book referenced above, unless it has been updated, shows virtually nothing about roller cams although it goes into rocker geometry a decent amount. Mine doesn't anyway, and great amounts of it committed to memory just like all the other intensive engine tech books I have. Yunick. Jenkins, Atherton, the like. I like reading Vizard but in some ways he's off the mark to me. Like the part about half lift and carb standoff. Standoff happens when the intake rammed-by-inertia mass hits a closed valve at the end of the intake event. Half lift has no effect there, you're NOT at half lift. If half lift is good then the column should be a bigger one and the effect worse. If bad at half lift standoff (reversion) will be less but no matter, engine's not making any power then. Half lift is valuable because you go through it TWICE versus only once for full lift. It really has no effect at idle either since the idle mixture is so under negative pressure. The engine has trouble pulling in with ANY size hole at that time because it is pulling down against the carb butterflies. That restriction pales the lift at cam, almost any lift past barely open that allows engine to draw against carb will work. The size of valve pocket affects reversion but so does all the rest and most importantly the intake plenum size, it is the reversion absorber. Why the chase for IR intakes on big prostock engines in the early '70s failed so miserably. You can't stop reversion with big individual cylinders and intake runners with no connection between them. The control of air mass becomes harder and harder the greater the size of the mass. Why smaller engines make more hp/cu.in. than big ones do. Simple physics.
I have one other big disagreement with Vizard and that is his idea that if intake speed is to be kept high then exhaust must as well all the way down to make power. No. NO no. Missing something very basic there. WHY if that is true then does every engine on the planet with truly wave/pressure tuned exhaust run BETTER WITH NO EXHAUST PIPE AT ALL, the exhaust exits the header pipe end to SLOW DOWN GREATLY??? Take that idea and go BACKWARDS with it, you can increase in exponential amounts the amount of power a two stroke engine makes by dropping exhaust speed slower in incredible amounts and even by choking it to reverse BACKWARDS. The ideas work on four strokes as well, simply harder to implement them.
Exhaust tuning is nowhere near that simple, it is about speeding up or slowing down the flow to get the effect you want.......... ..........I will submit to you that if you have fast velocity in the tailpipe all the way down you are actually restricting the engine, the velocity itself is proof of that. Velocity goes up with push behind it, that push itself is restriction. Sorry Dave.
More. I have never seen any muffler that makes the flow change direction work as well as a simple straight through muffler that does not change direction. Never. The physics say no, the turn is a flow killer. Of course you will not get the noise control on a straight through you can get with a double U-turn muffler, the reduction in noise is evidence the muffler also kills power. We had a saying at Dad's shop, 'noise is horsepower', while not always true, it is often enough. Engines make more power when you use physics to make them wave tuned, that tuning is with SOUND waves.
Dig up the book 'The Scientific Design of Intake and Exhaust Systems' but be prepared for some boring reading, it is written by quants for quants only.
