[Insider]

Hi all,

I have a question about gearing and torque. Let's use the 335i and 335d as an example. The 335d has peak torque of 428 at 1750-2250 RPM. The 335i has peak torque of 300 between 1400-5000. Gears 1-6 are the same on both cars, except the final drive ratio is 2.81 for the diesel and 3.46 for the gas engine.

Since the final drive ratio for the 335i is 23% shorter (more aggressive) does this mean that if we were comparing the two cars, we would add 23% to the 300 torque of the 335i to get 369 torque at the rear wheels? Does this make sense? After all, it's torque at the wheels once the power goes through the gears that determines acceleration, not peak torque at the engine.

[bonsai]

I could be wrong (most likely I am), but I don't think TORQUE is affected by gearing in the same way that HP is. Torque is a measure of "twist" force, which is no related to how fast the wheels spin relative to the crank.

[StarrDlux]

torque gets you going; horsepower wins the race


once you get going you use shift extension to maintain peak horsepower for the rest of the (1/4 mile?)

my head is killing me maybe someone else can explain how to shift right at or slightly after peak whp to maintain the fastest speed.

there's a reason why diesels are faster on the highway but suck stoplight to stoplight (or 1/8th 1/4 mile) in general.

bmw and audi are obviously rewriting the rules with high revving diesels with mad boost.

i got my ass handed to me once by a F350 dually with a work chest in the back spraying nitrous and propane. he ran his first run all motor then set me up by pulling a 13.0 which you don't expect in a fricken monster truck.

[noro]

Originally Posted by Insider

Hi all,

I have a question about gearing and torque. Let's use the 335i and 335d as an example. The 335d has peak torque of 428 at 1750-2250 RPM. The 335i has peak torque of 300 between 1400-5000. Gears 1-6 are the same on both cars, except the final drive ratio is 2.81 for the diesel and 3.46 for the gas engine.

Since the final drive ratio for the 335i is 23% shorter (more aggressive) does this mean that if we were comparing the two cars, we would add 23% to the 300 torque of the 335i to get 369 torque at the rear wheels? Does this make sense? After all, it's torque at the wheels once the power goes through the gears that determines acceleration, not peak torque at the engine.

Your train of thought is correct. But it's not as simple as using %. For simplicity let's talk in 5th gear because it's a 1:1 ratio in the tranny. In case of the 335i the 3.46 means 3.46 rotations of the input shaft on the diff equal 1 rotation of the wheel, so the torque is multiplied by 3.46. On the 335d it's multiplied by 2.81.

So the end result in the 5th gear the tq at rear wheels of the 335i is 3.46 X 300 = 1038. For 335d it's 2.81 x 428 = 1202. If you want to evaluate different gears the formula gets complicated because you have to factor the true ratio by applying the multipliers of the transmission as well. Hope this answers your question.

[demosthenes]

Originally Posted by Insider

Hi all,

I have a question about gearing and torque. Let's use the 335i and 335d as an example. The 335d has peak torque of 428 at 1750-2250 RPM. The 335i has peak torque of 300 between 1400-5000. Gears 1-6 are the same on both cars, except the final drive ratio is 2.81 for the diesel and 3.46 for the gas engine.

Since the final drive ratio for the 335i is 23% shorter (more aggressive) does this mean that if we were comparing the two cars, we would add 23% to the 300 torque of the 335i to get 369 torque at the rear wheels? Does this make sense? After all, it's torque at the wheels once the power goes through the gears that determines acceleration, not peak torque at the engine.

You are correct, but there are other factors.

The torque delivered to the rear wheels depends on engine torque, transmission gearing and differential gearing. The 335i's transmission and differential gearing is lower, which makes up for some of the difference in engine torque.

But, the 335i's engine puts out maximum torque from 1400 to 5000 AND it has a higher redline. It can stay in each gear longer, which means it can take advantage of more torque delivered to the rear wheels by the lower gearing.

How does this translate into the driving experience? The 335d would feel more powerful around town, but the 335i would win a drag race because the gas engine performs better at higher revs and it can stay in the lower gears longer. The 335i does 0-100 kph in 5.6; the 335d does it in 6.2.