Re: Choose One
A "fluid" may be either what we understand as a "liquid" or a "gas". Fluid mechanics does not distinguish between the two, although most liquids are so marginally compressible that they can be considered "incompressible", which makes analysis much simpler - closed form formulas with pencil and paper quite often.
Not so with many fluid mechanics problems where the fluid is compressible and non-steady as in high speed engine inlet and exhaust flow. Here, not only inertia effects are important, but also the finite speed of pressure waves, and both have a big influence on flow, particulary pressure wave propagation once the fluid flow rate exceeds about one-third of sonic velocity, and a big influence at one-half sonic velocity.
If the fluid medium is compressible, both inertia and compressiblity effects must be handled in a co-ordinated manner. You can't consider one without the other and have any hope of accurately analysing the flow. That's what modern computational fluid dynamics (CFD) tools are all about.
I would say that the smallest, HIGHEST EFFICIENCY port commensurate with achieving a top end power BANDWIDTH objectives is a valid objective for a racing engine, but since the top end power objective in a racing engine is usually "as much as possible", you end up with the biggest ports you can fit into the available space and then work to maximize flow efficiency. That's why F1 gearboxes have ten percent ratio spacing The 90 or 95 percent power bandwidth extends only from about 17,000 to 19000, and below 15000 they fall way off the power curve.
On a street engine, torque bandwidth is a better high level system engineering objective than peak power, so the ports have to be small enough to meet low end torque objectives, and EFFICIENT enough to make the peak power objective, which is critical for advertising as most consumers can only understand engine performance from this very limited one-dimensional perspective.
If the LS7 heads were scaled down to fit the 4" bore size of a 302 with a mild cam they would make peak power at the same mean piston speed as LS7, which is 4200 FPM with a useable power curve to at least 4700 FPM, and these mean piston speeds correspond to 8100 and 9200 RPM on a 3" stroke engine. Of course, it could also easily meet emission standards with modern emission control technology.
Add a racing exhaust system and valve overlap to exploit it and peak power mean piston speed will increase to the 4500-5000 FPM range, if not more, which places revs in the 9000-10000 range, or more. The problem now becomes how to make pushrod valve gear live, and it's possible because NASCAR can do it - with OEM help.
Duke
A "fluid" may be either what we understand as a "liquid" or a "gas". Fluid mechanics does not distinguish between the two, although most liquids are so marginally compressible that they can be considered "incompressible", which makes analysis much simpler - closed form formulas with pencil and paper quite often.
Not so with many fluid mechanics problems where the fluid is compressible and non-steady as in high speed engine inlet and exhaust flow. Here, not only inertia effects are important, but also the finite speed of pressure waves, and both have a big influence on flow, particulary pressure wave propagation once the fluid flow rate exceeds about one-third of sonic velocity, and a big influence at one-half sonic velocity.
If the fluid medium is compressible, both inertia and compressiblity effects must be handled in a co-ordinated manner. You can't consider one without the other and have any hope of accurately analysing the flow. That's what modern computational fluid dynamics (CFD) tools are all about.
I would say that the smallest, HIGHEST EFFICIENCY port commensurate with achieving a top end power BANDWIDTH objectives is a valid objective for a racing engine, but since the top end power objective in a racing engine is usually "as much as possible", you end up with the biggest ports you can fit into the available space and then work to maximize flow efficiency. That's why F1 gearboxes have ten percent ratio spacing The 90 or 95 percent power bandwidth extends only from about 17,000 to 19000, and below 15000 they fall way off the power curve.
On a street engine, torque bandwidth is a better high level system engineering objective than peak power, so the ports have to be small enough to meet low end torque objectives, and EFFICIENT enough to make the peak power objective, which is critical for advertising as most consumers can only understand engine performance from this very limited one-dimensional perspective.
If the LS7 heads were scaled down to fit the 4" bore size of a 302 with a mild cam they would make peak power at the same mean piston speed as LS7, which is 4200 FPM with a useable power curve to at least 4700 FPM, and these mean piston speeds correspond to 8100 and 9200 RPM on a 3" stroke engine. Of course, it could also easily meet emission standards with modern emission control technology.
Add a racing exhaust system and valve overlap to exploit it and peak power mean piston speed will increase to the 4500-5000 FPM range, if not more, which places revs in the 9000-10000 range, or more. The problem now becomes how to make pushrod valve gear live, and it's possible because NASCAR can do it - with OEM help.
Duke
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