[olddog]

Air flow is what makes Hp. Well actually gasoline, but you can only put in the amount of gasoline that you have air to burn. Air fuel ratio has to be right.

The head is the largest factor limiting air flow. Exhaust and intake restriction can make a good head useless, so they too are very important. So these three in unison, pretty much sets the upper limit on Hp.

Eventually the valve size limits the heads flow regardless of how good the ports are. Ultimately the bore limits the valve size. Therefore in the end the bore diameter defines the ball park the engine will be in.

The stroke (within a practical range) is meaningless, when it comes to peak Hp. However the torque, is a whole different thing. The shorter the stroke the higher the rpm where the peak torque is made, and conversely the longer the stroke the lower the rpm where the peak torque is made. Also the shorter the stroke the higher the rpm where the peak Hp is made.

Since the Peak Hp is the same regardless of stroke, the short stroke engine will make less torque than a long stroke engine. Experiments have been ran on a lab engine, where the only variable that was changed was the stroke (2", 4", 6" strokes were used). The peak Hp was the same in all three strokes and occurred at the rpm which gave the same mean piston speed. In other words, the 4" stroke hit its peak at 1/2 the rpm of the 2" stroke and the 6" stroke occurred at 1/3 the rpm. Likewise the 4" stroke made twice the torque and the 6" stroke three times the torque of the 2" stroke.

Playing with valve timing is a rob Peter to pay Paul game. It can move the torque curve up and down the rpm scale, but you always give up torque on one end to get a bit more on the other. The better the heads, intake, and exhaust allow an engine to breath, the less games are needed by the cam.

So the old engine muscle tended to be big bore and short stroke, to allow bigger valves. Canted valves, and Hemi heads was a way to cheat the bore and get bigger valves.

Bottom line: a 289 or 302 can make as much power as a 347 (even a 427W). You have to spin it faster, and it will make less torque, but it will make the Hp. However you do have to gear it down to run the same speed at the higher rpm, in order to have the same performance from the cars perspective.