Torque v. gearing

Old PH post (written by fritz_269):

The best torque curve to have is…..

Dependent entirely on your gearing!
(or vice-versa) J

The power that your engine makes is dependent entirely on the engine’s RPM (as we know from my earlier post). If we could magically run the engine constantly at it’s peak hp RPM, we could deliver that peak power continually, and get the fastest possible car. In fact, the CVT (Continuously Variable Transmission) is designed primarily with this fact in mind.

To accelerate, the wheels have to go through a wide range of RPMs, from zero (stopped) to 1250RPM (100MPH on a 27” tire). So, how do we move the most power from the engine to the wheels at these different speeds? By deciding the best points for the gear ratios, such that the engine is in a RPM range that provides as much hp as possible at any given wheel speed.

Lets talk about the three basic torque trends and how we would gear them:
1) Downward trending torque, like big diesel engines, and the Hummer. If the torque curve trends down, the hp curve will tend to be flat. So these engines make the same hp no matter what the RPM they’re at. So the only reason to have gears is to vary the speed of the wheel. Gearing is selected such that an up shift will bring the engine from near it’s redline to near it’s idle RPM – the hp to the wheels will not change with this shift. This is known as a ‘wide’ ratio. (Note that many big diesels have a very close idle & redline, necessitating the use of many gears to get many speeds – especially the slow speeds necessary for big loads or off-roading.)
2) Upward trending torque, like 4cyl Porsche & high-output Honda engines. If the torque curve trends up, the hp curve skyrockets up, sometimes called a ‘peaky’ engine. Thus, these engines make their maximum hp right at the redline, and we want to keep the engine there as much as possible. So we make first gear ‘deep’ – a large number – to get off-the-line RPMs up to redline as fast as possible. Then we make successive gears ‘close’ ratio – a small difference – such that each upshift brings the RPMs down as little as possible, keeping the hp output near the max.
3) Flat torque, trending softly down on both ends, like a truck V8. This results in a gently upward sloping hp curve. Gearing is not as critical, the ratios are generally smack in the middle.

So, which of these makes the fastest car? The total power transmitted to the ground is the average horsepower within each gear band multiplied by the time spent there. So: any of them (1,2 or 3) can be fast, as long as the ‘powerband’ (where the engine makes its most hp) and the gearing match up well.

More practically, #3 and especially #1 are more insensitive to the driver, if you miss your optimal shift point by 500 RPM or so, the average hp won’t change very much. But, missing a by a few hundred RPM in a #2 type engine will take you way off your powerband & slow you considerably.

In essence, high displacement, low revving engines are forgiving of driver error; and low displacement, high revving engines are not. But both can produce very fast cars.

To make your Honda faster on the strip – concentrate all your time, money and energy on the powerband, don’t worry about the 2kRPM performance, your engine is never there! Often it is difficult to push the peak hp number up, but if you can raise the hp near the edges of the powerband (like porting the heads to keep the torque up a little right at the redline, or tweaking your collector diameter to push the torque up a little right at the bottom of your powerband) you can get a higher overall average & thus more power to the ground.

I’ve found this a difficult concept to explain, please ask questions if you don’t understand, and I’ll try to think of a graphical diagram that might explain it more clearly.


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For a point of reference, my 98 ‘lude the gear ratios are such:
1st gear: 3.285
2nd gear:1.956
3rd gear: 1.344
4th gear: 1.034
5th gear: 0.812
Final drive: 4.266
(The gear ratio is the number of times the engine turns for the driveshaft to turn exactly one revolution. The final drive is the number of time the driveshaft turns for the axle to turn exactly one revolution. i.e. in second gear, the engine turns 4.266*1.956=8.344 times every time the wheel turns once.)

Hey Fritz! Thanks a lot for all this cool info!

But, question:

If a turbo Honda motor (say a B18C1 running moderate boost - 16psi) is still 'peaky' - y does tall-gearing have a positive affect? Since the motor still makes it's power in high RPM?

Also, can you explain final drive ratios a little more in depth? Like, y does the USDM Integra Type-R have a 'bad' final drive?

Thanks a lot!

Originally posted by VTEC_boi
[B]But, question:
If a turbo Honda motor (say a B18C1 running moderate boost - 16psi) is still 'peaky' - y does tall-gearing have a positive affect? Since the motor still makes it's power in high RPM?
Short gearing = large overall ratio = higher torque multiplication = better acceleration = worse top speed

Tall gearing aka Long gearing = small overall ratio = lower torque multiplication = worse acceleration = better top speed

This is true no matter what type of engine (1,2, or 3) you have.

But it does break down at two extreme points - 1) top speed - where aerodynamic drag is the limiting factor, there is no point in having more gear for a speed you cannot reach, so get a slightly shorter gear. 2) acceleration - where traction is a limiting factor, there is no point in having low gearing for torque that you can't put to the ground, so get a slightly taller gear.

For a drag car, it's generally pretty easy to calculate a good final drive ratio. If you want to cross the line at the top of fourth gear, estimate the speed you'll be going (say 130mph), then calculate what final drive ratio will have your car moving at 130 mph in fourth gear at redline. Then you'll have the tallest gear (most torque multiplication = best acceleration) possible that still won't limit the top speed you need to reach.
Also, can you explain final drive ratios a little more in depth? Like, y does the USDM Integra Type-R have a 'bad' final drive?
Final drive is just what every single transmission gear is multiplied by. You can adjust the entire range of the transmission just by adjusting the final drive. That's usually much faster, cheaper, and simpler than re-doing every single transmission gear. I haven't heard that the ITR has a 'bad' final drive. If so, is it either too short (good acceleration, poor top speed) or too tall (poor acceleration, good top speed)?
:cool:

**edited 2:10p 12/7/01 - I had foolishly swapped 'tall' and 'short' - they are now correct! Thanks to texan**

too short

Fritz, do you have AOL Instant Messenger?

Originally posted by VTEC_boi
too short

Fritz, do you have AOL Instant Messenger?
No. I live behind a thick firewall.

A "too tall" final gear is a very common problem in production built cars. Mainly because of one big factor I left out of the above - Mileage.

short gearing = higher rpm at lower speed = worse MPG
tall gearing = lower rpm at higher speed = better MPG

Auto mfgs must always take this into consideration when determining the final drive.

You'll notice two major trends in USDM drivetrains.
#1 - 60MPH (or 62MPH) always happens at redline in 2nd gear. This is to satisfy the all-important magazine reported 0-60MPH (or 0-100KPH) number. Completely regardless if a different ratio would make the car more competetive on the track or the street or just feel better driving around - You always will hit 60 at redline in 2nd!
#2 - 65MPH in the top gear is very close to the most fuel-efficient (BSFC) RPM that the engine can run at. This is to get those HWY MPG numbers as high as possible.

So you notice that 1st gear, 2nd gear, top gear, and the final drive end up all being strictly dictated by these two mfg. concerns. This leaves the poor engineers only 2 or 3 gears (in a 5 or 6 spd respectively) to actually optimize for your driving pleasure. 3spd automatic gearings are totally dictated by these requirements, and 4spd autos only get one gear (3rd) well designed.

Kinda sucks, doesn't it! And I bet you had no idea... :mad:

fritz- I learned a long time ago that gearing nomenclature was as follows:
shorter = numerically higher gearing
taller/longer = numerically lower gearing

Are my years of using this wrong, or did you get them switched around in your post?

Originally posted by texan
fritz- I learned a long time ago that gearing nomenclature was as follows:
shorter = numerically higher gearing
taller/longer = numerically lower gearing
Are my years of using this wrong, or did you get them switched around in your post? Holy freaking shite! I f-cked up badly. Thank you for catching that texan - I'm an idiot. I must have been drunk or something :( I must have typed it out wrong to start with, and then just been consistent throughout the rest of the post. Arrrgh! :mad:

**It's all fixed now. Thanks again texan. :cool:

Did you get my PM fritz?