Quote:
Originally Posted by glamcem
yes Evo and STI has a slight disadvantage on drag coefficient figures (.27 vs .30X something) but that disadvantage becomes negligible especially when the speed increases ..see below formula:
I was referring to the highway pulls  not the transient lag that is inherited in turbos, I should've said "better matched gears" because taller gears only have advantage for that specific power levels ..M3 with it's longer gears will almost always walk away from the SC'd FRS/BRZ with less than ~300whp
compared to other FI choices yes but compared to NA? well we know the answer |
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.
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.
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.
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.
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.