Torque in 1st Gear vs. 2nd Gear

Hey all,

In my continuing question to have a better of understanding of torque, acceleration, horse power and speed, I submit the following question. Please feel free to tell me that it's downright silly or impertinent.

If my ALH is putting out 150 foot-pounds of torque at the wheel at 2000 rpm in 1st gear, what is it producing in 2nd? How does one calculate this? I've got a stock 5 speed, btw.

Knowing this, would it be possible to plot the torque curve in each gear, and simply look at the graph to see where the torque drops below what it would be in a higher gear to know when to shift? Aka, can math figure out shifting points?

Thanks all!
Bill

Torque and horsepower peak will always be in the same rpm range, regardless of gear. Sure there may be some small differences, and you could technically plot it and figure it out mathematically...but I don't think it would really be worthwhile.

Personally I think it is silly and impertinent. Shift by ear and by feel. Drive more - think less.

The engine crankshaft produces a certain amount of torque depending on rpm, go pedal position, tune etc..regardless of what gear you are in. As the power flows through the transmission, the torque is multiplied by the gear ratio (of whatever gear you are in). So the torque available to the wheels is much more in first than second, and so on. That's why you can smoke tires in first, maybe second, but never fifth, all with the same engine.

There are analytical methods to optimizing shift points, but for something that seems simple on the surface, it turns out to be quite complicated.

So just shift when it feels right!!

To find your torque multiplier, multiply your final drive by the ratio of the gear in question. To get wheel torque, multiply your torque multiplier to whatever torque you think the engine is producing.

For example, my 1st gear multiplier: 3.157(final drive) X 3.33(1st gear) = 10.51
So in 1st gear at 100ft-lbs my axles will see 1051ft-lbs. Your numbers will be different, but this should get the idea across.

As for plotting, of course . Check your email.

sonicyellow said:

Hey all,

In my continuing question to have a better of understanding of torque, acceleration, horse power and speed, I submit the following question. Please feel free to tell me that it's downright silly or impertinent.

If my ALH is putting out 150 foot-pounds of torque at the wheel at 2000 rpm in 1st gear, what is it producing in 2nd? How does one calculate this? I've got a stock 5 speed, btw.

Knowing this, would it be possible to plot the torque curve in each gear, and simply look at the graph to see where the torque drops below what it would be in a higher gear to know when to shift? Aka, can math figure out shifting points?

Thanks all!
Bill

Click to expand...

Absolutely yes, the gear torque curves can be calculated and plotted.
But first things first.
The engine's maximum torque output of 150 lb*ft occurs at about 2000 engine rpm. However, that is not the tractive effort the tire is applying to the pavement.
1st gear ratio is what? 3.5 : 1? Final drive is 3.5 : 1? Humor me for simplicity's sake and go with these numbers. 150 lb*ft at 2000 rpm at the flywheel will be 150 * 3.5 * 3.5, or 1,837 lb*ft at the axle, and in my simple world a 24 inch tall tire has exactly a 1 foot radius so 1,837 lb of tractive force, but the rpm will be 2000 / 3.5 / 3.5 or 163 rpm.
2nd gear ratio is 3.0 : 1? Then 150 * 3 * 3.5 is 1,575 lb*ft and 2000 / 3 / 3.5 is 190.5 rpm.

The power doesn't change because "power" is a multiplication of torque and rate. If the units used are "horsepower" and "lb*ft" then the math equasion is:
((lb*ft)*rpm)/5252=HP.
The constant 5252 is only valid when lb*ft and HP are the units. Nm and kW have a different constant but the same relationship.
(150*2000)/5252 is 57 hp
(1,837*163)/5252 is 57 hp
(1,575*190.5)/5252 is 57 hp

Ski in NC said:

The engine crankshaft produces a certain amount of torque depending on rpm, go pedal position, tune etc..regardless of what gear you are in. As the power flows through the transmission, the torque is multiplied by the gear ratio (of whatever gear you are in). So the torque available to the wheels is much more in first than second, and so on. That's why you can smoke tires in first, maybe second, but never fifth, all with the same engine.

Click to expand...

Precisely - and this is what I was hoping to graph out visually. My engine produces X amount of torque, and that torque is manipulated by the transmission so that the wheels have less torque in each successively higher gear.

Once the torque drops off at the upper rev range (rather sharply), there is a point in each gear that will produce less torque at the wheels than the next higher gear, aka, more acceleration.

I've been driving for a good while now with my particular car (nearly four years) and am quite familiar with it. I'm at the point now where it's very difficult to judge the effectiveness of refinements by the seat of my pants.

So excluding other complicating factors, I hope to come up with some 'suggestions' based on something other than just feel so as to eek out a few more tenths of seconds on my runs.

Lug_Nut said:

Absolutely yes, the gear torque curves can be calculated and plotted.
But first things first.
The engine's maximum torque output of 150 lb*ft occurs at about 2000 engine rpm. However, that is not the tractive effort the tire is applying to the pavement.
1st gear ratio is what? 3.5 : 1? Final drive is 3.5 : 1? Humor me for simplicity's sake and go with these numbers. 150 lb*ft at 2000 rpm at the flywheel will be 150 * 3.5 * 3.5, or 1,837 lb*ft at the axle, and in my simple world a 24 inch tall tire has exactly a 1 foot radius so 1,837 lb of tractive force, but the rpm will be 2000 / 3.5 / 3.5 or 163 rpm.
2nd gear ratio is 3.0 : 1? Then 150 * 3 * 3.5 is 1,575 lb*ft and 2000 / 3 / 3.5 is 190.5 rpm.

The power doesn't change because "power" is a multiplication of torque and rate. If the units used are "horsepower" and "lb*ft" then the math equasion is:
((lb*ft)*rpm)/5252=HP.
The constant 5252 is only valid when lb*ft and HP are the units. Nm and kW have a different constant but the same relationship.
(150*2000)/5252 is 57 hp
(1,837*163)/5252 is 57 hp
(1,575*190.5)/5252 is 57 hp

Click to expand...

So would it be reasonable to ignore the everything after the transmission since my chief concern here is the affect shifting has on the application of torque to the rest of the drive train? Presumably the rest of the drive train would have the same affect on the torque value in every gear (I'm relying entirely on High School physics here!)

As such, how would I calculate the torque coming out of the transmission in each gear at several rev speeds?

sonicyellow said:

...As such, how would I calculate the torque coming out of the transmission in each gear at several rev speeds?

Click to expand...

You'll need to know your torque or HP at those RPMs. The rest of the gearing math is easy for Excel. Do you have a dyno sheet handy?

Ok - Google to the rescue!

I *knew* someone had to have done all the heavy lifting for me already! Turns out, I wasn't too far off, either (well, according to the sources I found, anyway).

First, I found this, which explained precisely what I was trying to do, and why it would (theoretically) work: http://www.datsuns.com/Tech/whentoshift.htm

I then found this, which magically takes the appropriate info, and spits out an analysis of where you should shift based on your hp curve, rpm and gear ratios: http://www.bgsoflex.com/shifter.html

And then I plugged in my sorry excuse for data:

As you can see, a slipping clutch makes the drive train of a car travel through time. (And Doc Brown needed a Flux Capacitor - ha!).

So according to my best guess as to my actual hp output, I was able to divine that my optimal shifting points are as follows:

Shift from first at 4150 RPM
Shift from second at 3850 RPM
Shift from third at 3650 RPM

That site does, however, point out that there are two schools of thought about this whole ordeal, and describes them thus:

"There are two schools of thought (probably more) on on determining optimum shifting rpms: one is to find a shift RPM which brings you back to the same point on the horsepower curve (ie. horsepower is the same before and after shift, "straddling" the curve), and shifting so that one maintains the maximum area under the horsepower curve."

This second one, however, results in my shifting at about 4500 in each gear, so, well, I think I'll got with the first and not dance with the red line so much.

So now, armed with MATH, I hit the asphalt once more