I was having a debate with a friend about gearing and torque. We became somewhat stumped in our analysis, so I'd like some help in understanding this concept. I did a Search but didn't find much.

It you had two cars that made the exact same power(150hp), torque(110 ft/lb), redline(8400 rpm), gearing(USDM ITR FD & gears)and tire diameter, BUT one produced peak torque at 5500 rpms and the other at 7000 rpms, which would accelerate quicker in a 1/4 mile. Consider driver err and traction constant for both.

Please explain your answer?

 

that would depend on the rest of the torque curve of course, but just to give
an educated guess, I would say it would be the car that peaks at 7000.
Reason being when you shift at redline, the car with the 7000 peak will be right
before its peak whereas the other car will already be past it.

 

Ok...I follow you. That is exactly what I thought.

Now, how would the gearing need to be changed on the car with the lower peak tq to maximize acceleration? Would it need to be longer?

 

It is impossible for two cars to have the same peak power and same peak torque, but with a different peak torque rpm. Power is a function of torque and rpm.

If both cars have the same torque, but one has more power, then the car with more power would be faster. If you look at car stats, as well as quarter mile calculators, you'll notice that torque has little to do with how fast a car is. If you know the horsepower to weight ratio, then you can generally predict how fast the car will be.

If you've ever done work to a car, you'll notice that even you can put out as much torque as any car out there. Just get a longer ratchet or breaker bar and you'll be able to loosen/ break any bolt there is. This doesn't mean that you can accelerate a car fast.


[Modified by Lsos, 10:14 PM 3/6/2002]

 

*Torque.

A Caterpillar Dump Truck with C-12 engine
weighs somewhere between 30,000-40,000 lbs.

Some may think more Horsepower is needed to pull that weight. Nope..
The engine only produces 430 hp But... 1650 lb-ft of torque

Lesson: If you have more weight you need torque.

Its not really a matter of if its N/a or FI output
Gear selection will depend on the torque power band.
Sometimes an engine has undesirable power curves for drag because torque dies out in the highend (depends on engine geometry & airflow).
Horsepower is not necessarily the key, since you have the potential to make more
horsepower if you can produce more torque. None of the top normally aspirated drag Hondas use anything less than 1.8 litre. In fact, most of them use higher than 2.0 litre. Horsepower sells products, Torque wins.

Engines that produce earlier torque can have large gears. If ours produced early torque like a 4G63, our OE gear set would be useless. Ratios are too short ..you would get no traction.
Highend torque is key for a OE gear set. Must have enough torque in the highend to keep on pulling against the forces (friction, weight, and the exponential increase of wind resistance).

someone said "torque has little to do with how fast a car is". That is true, horsepower has a lot to do with (speed) how fast a car can go.

Torque and Horsepower are different things. Dynometers dont measure horsepower, instead they measure torque directly. The horsepower figure is calculated from torque.



[Modified by Quick 200k Mile Motor, 12:36 AM 3/7/2002]

 

I haven't thought this completely through, but I think it is possible

 

Actually, it is entirely possible. Remember that peak torque and power figures tell you nothing about the actual shapes of the torque curves. To get a better understanding of this, consider two torque curves that are basically the same. Both might be quite flat (around 108ft-lb) from, say, 2000-8000RPM. But one could have a small spike at 5500RPM and the other a small spike around 7000RPM. Since the spike is not much higher than the average torque, it would not really affect power output.

 

You're right. I screwed up

 

In that case, I'd say that both cars should be equally quick, as long as you shift to keep it around the power peak, which is when the highest acceleration takes place.


[Modified by Lsos, 4:25 AM 3/7/2002]

 

A lot of constants here, except peak torque rpm, and I think you forgot to add weight as being the same also. Lets make Car#1 as the one that makes peak torque @ 5500rpm, and Car#2 the one that makes peak torque @ 7000rpm

The most important thing to analyze here would be the rpm 'drop' after shifting.
Assuming that both drivers shift at the same rpm pt..
The USDM ITR gearing & tire diameter wont allow rpm to drop near 5500rpm, Thereforeth, it is the one with peak tq @ 7000rpm that will win in 1/4mi ET.

After shift, the rpms will land above 6200rpm. Car#1 wont get as much torque to the ground as Car#2. In other words, Car#1 will miss most of the torque. ITR Gearing is too short for it. Car#2 will benifit from the short gearing, because it produces more highend torque.

-Quick 200k Mile Motor