I was just waisting some time tonight and was playing with some engine calculators and found some intresting things about my engine, head and cam selection.

The calculators show the mach index of my 505 to be a .57 at 6000 rpm.

As I understand it, your flow is concidered choked at mach .55 to .60, And with anything greater than mach .50 you should have a cam that has a late intake closing of somewhere between 60 and 90 ATDC to get maximum cylinder filling.

My cam closes the intake valve at 51 ATDC. So I must be leaving some power on the table and not filling the cylinders as efficiently as possible?

The numbers for a 468 CI engine come out to .50, which is concidered the mach index for maximum volumetric efficientcy. So it appears from this info that a 468 is a completely different beast to cam than a 505.

All this also makes me question head choice for our stroker motors. When I put the 049 ovals on my engine I lost .1 in ET. I figured the added weight of the iron heads and shift in weight bias over the rec aluminum heads I had on it made it an even swap HP wise. But, (and I now think it is a big BUT) I never ran the rec port heads uncapped. My restrictive exhaust system and (now too)high intake port velocities and early intake valve close points were all working against each other. Anyways. I think the 074 rec port heads I have would be quicker on the car than the 049 ovals. The choke point of the ovals and mach calculations indicate the 505 needs the bigger ports to fill the cylinders more efficiently with my cam.

Any thoughts or ideas

Scott

Yes, the oval are too small for a 500ci motor and the air speed will be too high. A good flowing rect. port head will make more power. But flow matter too, say a ported oval flows around 330cfm, while a good rect. will flow 400cfm, so you not only get the right min. cross section with the rect. head for a 6000+rpm 500ci motor, you also get more cfm and good air speed. So the motor picks up a bunch of hp with the rect. heads with the added 70cfm, it also gains tq because in order for the rect head to flow the added 70cfm the air speed has to be good, even tho the cross section went up from around 260cc to 300+cc.

Ok, where are you guys getting this data?