Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum - View Single Post

240racer · 01-10-2003, 02:49 AM

think about how many rpms your tires are spinning at any given rpm. Lets say we have 24" tires (we actually have 25", but it makes it easier) If we are going 88ft/sec (60mph) and the circumference of the tire is pi*D then it's 2*pi (in feet) which is 6.28ft. and we are going 88ft/sec so that's 14.01 rotations (or circumferences) per sec. Which equals 840.76 rot/min (rpm). that means in order to match 840 rpm with an engine rpm that's higher then that, you need gears. Gears do two things at the same time, change the speed (rpm) and change the torque. If the gears are lower then the speed is reduced and the torque is increased. If the gears are higher, the speed is increased and the torque is decreased. HP does not changed through gears (neglecting friction) remember HP=(T*RPM)/5252 if you imagine that torque increases and rpm's decrease, then you can see that they cancel each other out and HP stays the same.

Now picture having a power band from 8,000-10,000 at 60 mph you could use overall gears (including trans and rear end) of 10:1
which means that your speed is reduced tenfold and the torque is increased tenfold. So if you are making 200ft-lbs of torque at 8400 rpm with 10:1 gears, then you are making 2000ft-lbs of torque at the wheels and they are spinning at 840rpm.
Now if you have a power band from 4,000-6000 rpm, then you would need a overall gear ratio of about 6:1 which means the wheels spin 6 times slower and they have 6 times the torque of the engine. So that would still be 60mph at 5000 (engine)rpm and if you were making the same 200ft-lbs of torque at the crank, now you multiply that by 6 to get 1200 ft-lbs of torque at the wheels at the same 840rpm.
to convert to thrust, which is really all we care about, since that is what pushes the car down the road. you divide by the radius of the wheel, which I conviently chose as 1ft, so it's the same number. that means if you make 200ft-lbs of torque at the crank at 8400 rpm, then you will make 2000lbs of thrust at 60mph with 10:1 gears. Also, if you make 200ft-lbs of torque at the crank at 5000rpm, then you will make 1200lbs of thrust at 60mph with 6:1 gears.
Hopefully you can see that the higher thrust comes from the engine that is making more power. 200ft-lbs of torque at 8400 rpm is 320hp and 200ft-lbs of torque at 5000rpm is 190hp.
you can do the calculations at the wheels too, it works the same.
I said that the wheels spin at 840 rpms at 60mph. So then you enter in the torque at the wheels to the equation and you see that 2000ft-lbs at 840 rpm equals 320hp and 1200ft-lbs at 840 rpm equals 190hp.

01-10-2003, 02:49 AM	#4
240racer Zilvia Junkie Join Date: Nov 2001 Location: Minneapolis, MN Age: 42 Posts: 574 Trader Rating: (0)	think about how many rpms your tires are spinning at any given rpm. Lets say we have 24" tires (we actually have 25", but it makes it easier) If we are going 88ft/sec (60mph) and the circumference of the tire is piD then it's 2pi (in feet) which is 6.28ft. and we are going 88ft/sec so that's 14.01 rotations (or circumferences) per sec. Which equals 840.76 rot/min (rpm). that means in order to match 840 rpm with an engine rpm that's higher then that, you need gears. Gears do two things at the same time, change the speed (rpm) and change the torque. If the gears are lower then the speed is reduced and the torque is increased. If the gears are higher, the speed is increased and the torque is decreased. HP does not changed through gears (neglecting friction) remember HP=(T*RPM)/5252 if you imagine that torque increases and rpm's decrease, then you can see that they cancel each other out and HP stays the same. Now picture having a power band from 8,000-10,000 at 60 mph you could use overall gears (including trans and rear end) of 10:1 which means that your speed is reduced tenfold and the torque is increased tenfold. So if you are making 200ft-lbs of torque at 8400 rpm with 10:1 gears, then you are making 2000ft-lbs of torque at the wheels and they are spinning at 840rpm. Now if you have a power band from 4,000-6000 rpm, then you would need a overall gear ratio of about 6:1 which means the wheels spin 6 times slower and they have 6 times the torque of the engine. So that would still be 60mph at 5000 (engine)rpm and if you were making the same 200ft-lbs of torque at the crank, now you multiply that by 6 to get 1200 ft-lbs of torque at the wheels at the same 840rpm. to convert to thrust, which is really all we care about, since that is what pushes the car down the road. you divide by the radius of the wheel, which I conviently chose as 1ft, so it's the same number. that means if you make 200ft-lbs of torque at the crank at 8400 rpm, then you will make 2000lbs of thrust at 60mph with 10:1 gears. Also, if you make 200ft-lbs of torque at the crank at 5000rpm, then you will make 1200lbs of thrust at 60mph with 6:1 gears. Hopefully you can see that the higher thrust comes from the engine that is making more power. 200ft-lbs of torque at 8400 rpm is 320hp and 200ft-lbs of torque at 5000rpm is 190hp. you can do the calculations at the wheels too, it works the same. I said that the wheels spin at 840 rpms at 60mph. So then you enter in the torque at the wheels to the equation and you see that 2000ft-lbs at 840 rpm equals 320hp and 1200ft-lbs at 840 rpm equals 190hp. __________________ Adam '89 coupe KA24DE+T 14.1 @ 104 MPH '88 Celica All-Trac turbo stock, but no more ecu codes!!