Top Speeds in Different Gears (6MT)

G37Sam

Not sure I agree aerodynamics has anything to do with that..

If a car make 155 mph @ 7.5k in 6th gear for instance, it will make that much speed at that gear whether on the moon, on earth, or on a dyno. Air resistance will only affect the time for it to get there, assuming it does.

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kannibul

Wrong. Moon has no air to resist. Dyno has no wind resistance. On the moon, the car would go as fast as it can barring friction losses with the brakes, drivetrain, front wheels, bearings, etc (provided it could breathe), and rolling losses (ie, tires). On a dyno, it'd go nearly to the theoretical limit, the difference being the same as on the moon, except without the front brakes/wheels being factors. Of course, I'm discounting gravity effect and lack of air for the moon.


So, let my try again:

You're driving along at 75MPH, 0MPH headwind. Your car is using (example) 65HP to perform this work.

You're driving along at 75MPH, 20MPH headwind. Your car is using 65+N HP now, because there is more resistance.

(You can change these numbers to driving at 75, with a 75 mph tailwind vs 75 with a 0mph wind)


Same effect applies when attempting to determine the maximum speed fo the vehicle. The faster you go, the more resistance is applied to the front of the vehicle.

As you increase speed, you increase air resistance. Lower drag coefficient means the car slips/cuts through the air more instead of creating turbulence which distorts the air, making it more resistant.

Imagine a biplane wing on a F18. It wouldn't got as fast...ever, given all things being equal.

G37Sam

Wrong again, you're arguing a different point than I am

What I'm trying to say

If you're @ 4k rpm on 4th doing say 100mph, you will do that irrespective of any other forces, for the rear wheels to do 100mph, your engine has to be at 4k rpm if in 4th, it's how the car's gearing has been setup. Air resistance aka drag force only affects how much TQ you'll need to get there i.e how much time I'll take you to get there, but once you're there, you will be doing 100 @ 4k rpm.

Take it easy guys I only said on the moon because I wanted to use the 0 air resistance factor, of course the cars won't move there lol

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Lug

http://www.box.net/shared/static/6svnpj7so7.wav

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cjof2003

kannibul

Ha, I knew the 7AT was faster!

(ducks)


Also, it looks like we should be dyno'ing the car in 5th gear?

kannibul

OK, that makes sense...I guess I was going from the point of real-world, not theoreticals...then again, at what point does the Z run out of juice...

GTRFAN

My limited Physics knowledge says that at a certain point, the car will not be able to accellerate any further as the wind resistance will be too great, so it will never see the 7500 redline in 6th. It will just sit on a lower rpm and be unable to push through the wind given it's known HP output.

More power and torque will change this.


I've been saying this all along on other threads. The only problem with using 5th is running into the speed limit of the rollers on the dyno. Some dyno's only support 200km/h, so using 4th is the only option to keep the speed down. (shoud only vary by a few kW anyway.)

my 2c

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My Zed

kannibul

What's interesting is that using 4th, unless compensated for in some way, the 7AT will show higher torque numbers (and therefore higher HP) due to the reduced ratio in the rear end and 4th gear.

Hmm...interesting, but, not that it matters much. Comparative gains is what I'm interested in, which before and after dyno runs is good enough.