Torque vs Power viz-a-viz gearboxes

Torque vs Power viz-a-viz gearboxes

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Discussion

domV8

Original Poster:

1,375 posts

180 months

Monday 14th April 2014
quotequote all
I wonder if any of the PH massive can help me understand gearboxes...

I have heard 2 statements which appear to me to be contradictory - and I am unable to resolve them..:

1. Engine (without gearbox) accelerates fastest at peak torque
2. Torque at the wheels is what accelerates the car

So firstly, it may well be that one of these is not correct..?

It is my understanding that a gearbox with take BHP (ie.torque at given revs) from the engine, reduce the revs by a factor of X and output the equivalently-factored amount of torque. This is what you would measure as torque at the wheels... (?)

So from that, I would take away that my highest torque-at-the-wheels value would be my highest BHP value..?

That does not agree with the first of the 2 statements above...

So can someone tell me how this works - with regards to torque at the wheels, fastest acceleration point in the range, and torque vs. BHP...?


Thanks!


Dom

EW486

239 posts

165 months

Tuesday 15th April 2014
quotequote all
Hello Dom,

Power = Torque * angular velocity.

So, the highest power output is just the maximum product of these two, it is neither the highest torque nor the highest angular velocity (except a few exceptions maybe).
Just compare a motorcycle engine to a yank big block. The motorcycle engine produces say 200bhp at 13000rpm, the big block say 300bhp at 6000rpm (just numbers, please no complains about accuracy). Just swap them around and you will see the bike engine can't pull the yank car but the yank V8 will just burn the rear wheel of the bike.
You can check each formula about acceleration or driving resistance, it's everytime torque needed for calculations.

Regards

EggsBenedict

1,770 posts

173 months

Tuesday 15th April 2014
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Angular velocity = rpm?

EW486

239 posts

165 months

Tuesday 15th April 2014
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Basically. Agular velocity is 2 * Pi * n (rpm).

NTEL

5,051 posts

239 months

Tuesday 15th April 2014
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All I know is my gearbox keeps stting itself due the 380 torques going through it biggrin

domV8

Original Poster:

1,375 posts

180 months

Tuesday 15th April 2014
quotequote all
Hi Benjamin, thanks for the reply smile
Unfortunately, I am still none-the-wiser frown

So we know that power is a function of torque, as a fraction of 5252 rpms - ie. Torque at given revs.

Now a gearbox takes torque at revs, and then adjusts revs to output an adjusted torque value...

This adjustment of revs & torque sounds to me like BHP, but a RR measures torque at wheels not BHP - so what's actually happening here?

If a gearbox in fact adjusts BHP, then am I best changing at max BHP (hence motorbike engines, high revving 2l engines etc) - or if it adjusts torque, should I be changing at max torque?

Or if it in fact changes BHP into torque (ie. multiple engine resolutions into a single measured output rotation), then what? Aaaaagh! Confused!

What is happening in a gearbox, and how do I make best use of it?

Thanks,

Dom

ChilliWhizz

11,990 posts

160 months

Wednesday 16th April 2014
quotequote all
domV8 said:
What is happening in a gearbox, and how do I make best use of it?

Thanks, Dom
I might be able to help here Dom smile

Many years ago, when I was a young person, I became aware that the opposite sex was different. This awareness occurred over a long period, so it didn't come as an unexpected bombshell. As I grew older, I became intrigued in the 'differences', and as time rolled on I realised that these differences were variable. I stopped thinking about the differences between men and women, and started to give consideration to the variables that women exhibited, and came to the conclusion that whilst these variables (which I later recognised as being moods) were unpredictable, they were often affected, sometimes to quite a dramatic degree, by my own words or actions. With experience, I found that I could generate a required response by using specific words or actions, although the response was still often unpredictable.

So, a gearbox is like a woman, you'll never understand it, it will normally respond in known ways to your input, but can still produce unpredictable and dangerous responses when you least expect it. And in the real world, that's all you need to know smile

Hope this helps,

Chilli smile

900T-R

20,404 posts

256 months

Wednesday 16th April 2014
quotequote all
EW486 said:
say 300bhp at 6000rpm
Snap! It's a wee 4.3 litre, though hehe

sorry for the diversion, move along, nothing to see...

EW486

239 posts

165 months

Wednesday 16th April 2014
quotequote all
domV8 said:
Hi Benjamin, thanks for the reply smile
Unfortunately, I am still none-the-wiser frown

So we know that power is a function of torque, as a fraction of 5252 rpms - ie. Torque at given revs.

Now a gearbox takes torque at revs, and then adjusts revs to output an adjusted torque value...

This adjustment of revs & torque sounds to me like BHP, but a RR measures torque at wheels not BHP - so what's actually happening here?

If a gearbox in fact adjusts BHP, then am I best changing at max BHP (hence motorbike engines, high revving 2l engines etc) - or if it adjusts torque, should I be changing at max torque?

Or if it in fact changes BHP into torque (ie. multiple engine resolutions into a single measured output rotation), then what? Aaaaagh! Confused!

What is happening in a gearbox, and how do I make best use of it?

Thanks,

Dom
Hello Dom,

the gearbox works like a torque converter. 1st gear delivers high torque at low revs (output speed), 5th delivers low torque but high revs. 4th is usually straight through. Maybe you can compare it with lifting an object with a lever. Depending were you set the centre of rotation you can adjust the torque or rotation, but the power output is always the same (lifting the object).

If you are looking for the best rpm for shifting the gears it always depends on the engine output and gearbox as well. There is no 'change from 2nd to 3rd at xrpm' advise possible without knowing the exact gearbox ratio and power/torque-curve of the car. But as a rule of thumb you shouldn't drop too much below the max torque rpm after shifting gears and also do not rev beyond max bhp.

Regards
Ben