Fordboy,
Would you elaborate a little more on:
F1 type power curve. ADVANCE the point of peak flow demand. Longer rod/stroke ratio DELAYS this.
Thanks,
Skip
Skip,
F1 type power curve:Current Cup engines are running rod/stroke ratios in the 1.7/1 to 1.83/1 range. Unrestricted long track engines are producing peak power @ approx. 9500 rpm. They would be running 10,000 rpm, or more, if they could. Raising the rod/stroke ratio to 2.05/1 on an engine 100 cubic inches smaller, and with 4 valve heads, is going to drive the power peak to ~11,500/13,000 rpm, unless the valve motion/flow potential is severely restricted. Is this what you want to accomplish? Not if you are planning to turbo charge it.
ADVANCE the point of peak flow demand.Flow demand/piston demand is a function of build geometry, and is calculated as the displacement in cubic inches (or whatever) * efficiency ratio (Volumetric efficiency) relative to crankshaft motion in degrees.
Piston motion, ie displacement, velocity and acceleration is not uniform, and it is not symmetrical to 90 ATDC. This is due to "the cosine effect" of the geometry. The crankshaft's rotation displaces the con-rod big end laterally during operation,
effectively shortening and lengthening the con-rod, thereby altering uniform piston motion. For V-8s of typical geometry, @ 90 ATDC
the piston will have traveled more than half the stroke. Way more, like 5 to 8% or so. Radical geometry combinations can drastically impact piston motion.
So since the piston travel is not uniform, peak flow demand
usually occurs between 70 to 80 ATDC. The peak demand is closer to TDC with shorter rods.
Longer rod/stroke ratio DELAYS this.A longer rod has the opposite effect, and peak demand is delayed by some amount of crankshaft degrees. The geometric effect can vary based on the difference in length.
So what?Depending on the flow characteristics of the inlet tract and the motion of the valve train, a savvy builder might want to alter/take advantage of the flow/piston demand.
For example: 4 valve/cyl heads are going to have superior low/mid lift flow compared to 2 valve/cyl heads.
1) Fulfilling demand sooner will probably benefit a 4 valve combo by allowing the closing of the intake valve "sooner".
2) Delaying demand will probably benefit a 2 valve combo, by giving the inlet tract time to "catch up".
3) Etc.
Be aware that it is not just flow demand that is altered here. Requisite valve events are altered as well.
BTW: This is a very complex subject, about which not all engineers/engine builders agree. I suggest you obtain some of the collegiate textbooks from the reading list on my tech thread: How do I make my engine better, Racing Engines 101. C. F. Taylor is where I would suggest you start.
Also: Since your engine will be a "blower engine" you will receive some advice that "none of this matters". Think very carefully about that. I suggest that instead of thinking of your engine as a "blower engine" or "turbo engine", think of it instead as a normally aspirated engine operating in a "high air density environment". JMO.
Fordboy