how fast?

[Cabezcoco]

How fast do you think you are going at 6500rpm in 4th in a 92 240 hatch 5 speed?

[black s13]

edit: i actually did the math

4th= 1:11rpm/4.083rpm (final drive)x 6.414ft (circ. of tire)x 60 (min to hour)x 6500 rpm (desired rpms)/ 5280ft (ft to mile)

=116.03mph

i think thats right

[scruffy63]

good job on the math that actually sounds about right. I know that when I shift at 6000 in 4th to 5th i'm going approx. 105mph

[whiterps13]

i think that math is a good ballpark range, but every car is going to be a little differant. there are also other factors such as wind, driveline loss, etc...

[black s13]

i think that math is a good ballpark range, but every car is going to be a little differant. there are also other factors such as wind, driveline loss, etc...

how...? its rpm related, not hp related at all.

engine A spinning at 6500rpm is exactly the same as engine B spinning at 6500rpm, power is irrelevant. 6500rpm is 6500 rpm.

the resasons you cited would expain the rate at which a car reached that final speed, not what the final speed at a given rpm is.

recap: in 4th gear, at 6500rpm you will be going around 116mph, regardless of power, wind, day/night

[bcar240]

for some of the same reasons your car doesn't accelerate to infinity when you pop it in neutral down a hill. The force of gravity continues to act on the car, but there comes a point where other forces interfere too much for the car to continue accelerating even though gravity is still pulling. It is hard to explain, I am not sure I really understand it right. But I wrote a program to find the top mechanical speed of the 240sx on stock components and it comes out around 163 mph when you redline in 5th gear, now we all know that it is impossible to do this speed.

[yelnatsch517]

PSH what are you talking about. I can make anything possible.

[black s13]

you wrote a program for it? its possiblty the simplest math ever...

3000rpm in 5th is almost exactly 70mph...

70mph/3000rpm = 0.02333mph/rpm

0.02333mph/rpm x 7000rpm = 163.333 mph

a stock 240 tansmission is gear limited to about 163mph, however the engine does not have enougyh torque to reach that figure.

not too bad on the math stuff for a highschool dropout, eh?

[bcar240]

oh, lol. I wrote a program where you can input all that stuff, change the variables, and test at different gear/rpm combinations. It is pretty snazzy . I made one specifically for the 240 with presets for the KA24 and the drivetrain and another one where you input all custom info. This is what happens when you have been programming for years and you get put in a computer science 115 level class. I finished an assignment the teacher alotted 2 weeks for in 45 minutes. My next goal is to factor in real-life factors like drag and the available HP/TRQ at speed. But the differential equation-based physics behind these equations eludes me for the moment. I know there are some very close approximations out there and I will probably post a question about it soon if I can't figure it out.

[black s13]

the torque/hp at a given rpm wouldnt be very difficult, just find some stock dyno charts. the difference air resistance and friction make will be very negligable i would assume, seeing as how at a given rpm the tires are spinning at a determined rate, therefore the car must be going at that rate, or really really close to it.

it would defenitly be interesting to see a program that could actually calclate all of that stuff accurately.