[glamcem]

Originally Posted by bfrank1972

Hi, unless I'm misunderstanding your post, there are some things that might need clarification:

1) As speed increases, drag coefficient becomes exponentially more important, not less important.

I think you're reading the equation little differently , what I meant to say was ; the power to overcome the drag (Pd) is a lot more dependent to the speed than the drag coefficient (Cd), considering all things being equal..when we start changing things like cars everything start changing (tq, power delivery, gearing, weight, surface area ..etc)

2) We're making broad assumptions based on power/weight ratios, but ignoring engine flexibility/area under the torque curve/gearing when comparing 2 cars.

that what I was trying to say in my initial post that's the exact reason why power/weight ratios and the HP figures alone cannot be the indicators of the real world performance

Assuming we have two cars, exact same shape power curves, exact same power to weight ratio, exact same drag coefficient, same drive layout, we can say the following:

- The heavier car will be at a disadvantage compared to the lighter car because the heavier car will have to overcome greater inertia, which at some power point will create traction issues. High powered heavy car can't put the power down.

- The heavier car will have an advantage at higher speeds because force required to overcome the inertia and the associated traction problems will become less of an issue, and drag will become more of an issue. Given equivalent drag coefficients, the heavier/higher powered car will pull away

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that's actually the complete opposite and the reason why lighter cars (again all else being equal) need more down force to put the power down whereas with the heavier car drive shaft and axle does that for you

That said, the FR-S is ligther AND has a lower Cd than most cars with equivalent power to weight ratios. Where the Jackson SC (and all other gear/bet driven centrifugal compressors) fall down is the power curve. They build boost on a slightly exponential curve relative to engine speed. The power is all on the top end, unless you have some fancy variable ratio drive to spin up the compressor at lower RPM's.

that may become a disadvantage or advantage depends on the application, all things being equal, area under the TQ curve is always desirable and I think no can argue about that .. the problem is "all things are not equal"

This is why you see positive displacement supercharger kits making around 240-250rwhp easily matching & possibly outpacing centrifugal kits making 270-280rwhp. More area under the curve - especially important on street driven cars.

again depends ..see above