Wow did this thread get crazy.

 

Q. "correct me if im wrong, but wouldnt that prove that 50 lbs torque at 100,000 would accelerate faster than 50 lbs of torque at 5000 rpm. "

If you are comparing two cars, one maxes out at 5000 rpm and the other maxes out at 100,000 rpm, then these cars would have very different rear gear ratios, and the higher reving car would have 20x more horsepower.

If you look at a given car with a given gear ratio, its acceleration will be constant from 5000 rpm all the way to 100,000 rpm if the engine produces 50lb ft of torque throughout that rpm range (this is ignoring wind resistance, friction etc.)

 

I have to agree with disco here.

What everyone is forgetting is that, when you punch it at 4700rpm with the 4.10s versus 3900 rpm with the 3.42s, your torque is multiplied by 4.10 instead of 3.42 at the differential, offsetting the ENGINES's lower torque output at that rpm.

There will be certain speeds at which the 3.42 would out accelerate the 4.10, but overall, your AVERAGE acceleration throughout your RPM range is constant given a constant hp. In some extremes, even if you have 1.00 gears, there will still be a speed somewhere (120+ range) that you can outrun a 4.10.

Like med_rej mentioned, what matters is torque at the WHEELS. Acceleration is simply force/mass. The more force you generate at your wheels, the faster you go. Think about a car running on a turbine engine. It's spinning at 50,000 rpm making no torque, but when geared down, it'll make more torque at the wheels than a VW TDi.

Simply put, changing from 3.42 to 4.10 is like changing from a wide ratio 6 speed to a close ratio import 6 speed. There will be times when the 3.42 is right on torque peak, but the 4.10 is running out of revs and have to grab a higher gear, and the 3.42 win. But there are also times when the 4.10 is right on the boil, but the 3.42 has just ran out of a LOWER gear and has to grab the same gear the 4.10 is in, then the 4.10 will out pull the 3.42.

Just take the formula:

Engine torque * gear ratio * 3.42 or 4.10

to give you the wheel torque at different speeds. Compare the two graphs and see at which speeds each gear has its advantage.

 

Since you still have the same tq multiplication at high and low RPM's the acceleration is the same in either case.

Dan

 

Most of what you say is correct. In every gear with 4.10's, you will have more TQ across the RPM range of that gear. The problem is that you run out of speed to use that TQ much sooner, causing you to shift to the next gear with a much worse TQ multiplication. If you look at my spreadsheet above, ou will see that after 35 mph, you start to trade off TQ between the different gears like you say. If you take the average TQ from 35-110 mph, for example, you will see that the 3.23 gears have more average TQ in that range than the 3.73 or 4.10 gears. Below about 35 mph, the 4.10's and the 3.73's blow away the 3.23's.

This is all considering that you are making a WOT run shifting at redline.

For just putting around town, the new gears will feel stronger because you will have more TQ in each gear than you had before. The problem is that you need to shift sooner, to a gear with less TQ.

Dan

 

Hey dan, thx for all your info.

This is what ive learned. Torque at the wheels is what accelerates a car.

Acceleration (in G's) = [Engine TQ (ftlbs) * tranny ratio * rear ratio] / [1/2 tire height (ft) * weight (lbs) ]

^--- acceleration is a function of a changing TQ curve in each set gear.

(and you were right about constant acceleration from 1k-8k rpm in a set gear with a constant torque curve)

Now the question of which gear ratios will give you the best acceleration has ENTIERLY to do with horsepower. Your torque curve will determine the acceleration in each gear, but when it comes to gear selection.... you must look to horsepower.

For example if you have an engine with a torque curve that gives you 400 TQ @ 3000 rpm (~230 hp) and 300 TQ @ 5000 (~285 hp) and the idea that the most torque will give you the most acceleration here is wrong. When wondering what gear or rpm will give you the most acceleration you need to look soley at HP.

In this case the engine with 400 TQ produces more crank torque, but given the higher gear (lower numerically) required for the given speed, the RWTQ would be less. For comparison purposes lets say that the 400 TQ would require a 1:1 final ratio ending up with 400 RWTQ. The 300 TQ shift at the same speed would require a (5000/3000) 1.67 ratio, thus making 500 TQ to the wheels.

So in short, Torque transferred to the wheels determines acceleration and horsepower is a way to determine which way is the fastest. If you ever question which rpm to shift to, ALWAYS shift to maximum POWER, not crank torque.

 

LOL this post is funny

I would explain what i ment by HP and TQ both do, but you wouldn't believe it so i'm not wasting my time........keep driving those 420ft*lbs and 275HP.........I will be passing you about half way down the track........which will be quite embarassing


Med knows what he is talking about Proven time and time again...........so basically they both do......TQ at low speeds and HP at high speeds.............anyone want to prove me wrong???? You guys have seriously never seen this at the track?

 

Remember TQ is not produced by it's self Something makes the TQ and guess what it is ________?

 

gasoline?

 

...mixed with oxygen and ignited with a spark.

I'll answer your comments in a little bit.

Dan

 

if you want to determine the performance of your car, the most accurate way to do it is to plot rwtq (y axis) vs. another variable, say, velocity (x-axis).

Lets say you do this for three cars, and you want to know which one will reach 120mph first. assuming they all weigh the same, you would take the "area under the curve" from 0 to 120mph. This is sort of a way of averaging the rear wheel torque from 0 to 120 mph (wrong terminology, apologies to the engineers out there).

Now if you want to find out which car would reach a given distance first, eg. the quarter mile, you would need to distance (x-axis) vs. rwtq (y-axis). Take the area under the curve for each car.

If you want to know which car would be fastest after a given time, say 15seconds, then plot rwtq vs. time, and again, take the area under the curve at 15seconds. You can even change the gears, and that will change the curve entirely. The gear with greater area underneath will perform better (up to the point that you measure, eg. 120mph).

 

thats what im sayin...its a battlefield out here..lol

 

Hmm, trying to get my arms around what you are saying here.

The information below is based on a M6 LT1. In a perfect world, you would have your engine TQ and HP known, and fixed. (you aren’t going to mod the engine anymore). You would then set your gear ratios of your tranny to take full effect of that TQ curve. The problem that we have is that the gear ratios in our tranny’s are fixed from the factory. We can’t change them, well at least not without spending $$$. So what we have to do is plot the resulting TQ in each gear, using your equation above.

Let’s say that you are in 1st gear and going WOT. You need to shift to 2nd gear eventually, but what is the optimum shift point? It’s either “A” or “B”, which ever comes first:
A) When you run out of RPM capability (Hit Redline)
B) 0r when the TQ to the wheels that you would shift into in 2nd gear would be higher than the TQ to the wheels in the current gear.
C) This holds true for all gears.
Option “B” happens when the TQ falls off very quickly at high RPM’s. Stock, our cars are on the verge of having to shift to the next gear because of TQ reasons right when our redline approaches. We end up just hitting our redline before it would make sense to shift to the next gear to get better TQ. So stock, it makes sense to run all the way to redline. The TQ to the wheels in 2nd gear is NEVER better than it is staying in 1st up to redline.

After a cam install, it’s even more important to run all the way to redline, which is typically higher now, like 6000-6500 RPM. Typically, shifting at max power with our tranny’s will result in less overall TQ that you were able to take advantage of in each gear. This is because we don’t have very “close gearing” If we had closer gearing, we may want to shift before redline, but it’s going to depend on the tranny gear ratios.

Hope this makes sense.
Dan

 

So why don't you run your engine where it's making more torque then at 3grand instead of 6grand Seems to be more than torque in this equation

 

Because the gear ratios that you would have to use if you only ran up to 3000 RPM would make acceleration terrible.

Dan