Theoretical question.

[MotorsportTom]

Firstly mods if this is in the wrong place I'm sorry but I wasn't 100% sure of where to post and I thought in here my question may be answered.


So onto the question, me and a friend were talking last night and we go onto the subject of peak power in a vehicle and when is best to change gear.

e.g. (All numbers are made up for the purpose of the question)

A car has 700bhp(peak) @6000rpm.

If you change up a gear you drop to 600bhp @ 5000rpm. Now is it better to stay in the original gear up to 6500rpm where you drop to 650bhp, then change up a gear and you stay at 650bhp but now @5500rpm.

I genuinely am interested but I'm unaware as to whether you'd need more information i.e. torque figures, gear ratios etc so sorry if it doesn't make sense.

Cheers Tom.

[MotorsportTom]

Ah ok well as it was a question made up of conversation and not a real car, so that's going to be difficult to come by!

Thanks for your reply though and I have now started to think more about the torque and the torque curves on a dyno graph etc and I'm convincing myself this information would be needed too

[Alfanatic]

Torque at the wheels is what matters. This is multiplied by the gearing, so that even if you drive past the engine's power peak, you're still sending more torque to the road than you would be at maximum engine power in the next gear up.

Typically, maximum revs in one gear will generate more torque at the wheels than any point in the entire rev range of the next gear up. Unless the engine torque drops off a cliff past the peak, and the gears are really, really close together, you will always get the best acceleration by staying in gear as long as possible before changing up, in other words, change at the redline / limiter and not before.

EDIT: Of, I suppose, possibly in some diesels, the torque starts to drop off a long way before the redline, you might get more torque to the wheels by changing up early.

Edited by Alfanatic on Tuesday 3rd May 16:44

[300bhp/ton]

doogz said:

A power curve would be the best way to figure this out.

If you know the rev drop between 2 gears, you want the section of the curve, where you draw a horizontal line between the curve at your specified engine speeds, and have the least area under this new curve you've made.

I think.

I'd go with that.

What you want to do is make the highest average HP over the duration of the gear rpm range.

e.g.

A Focus ST is known for nosing over at the top end, ie the engine revs higher than the powerband of the turbo/cams. So taking it to the redline might not be the best way to accelerate, sticking in the powerband and changing earlier will keep the engine in its sweet spot.

On the flip side, a Rover VVC engine makes it's PEAK power very near to it's red line and max engine revs. So you want to shift at as high rpms as you can to make the most of the powerband. It'll also help to place the engine at higher rpms after each change.


But knowing your gearing will also make a difference. In my Roadster shifting early in the first 2 or 3 gears produces better acceleration times. However the jump to 4th is bigger, so you need to wind 3rd all the way out to get the most from it.

[alock]

A picture says a thousand words...


This shows that for an RX8, you always want to be in the lowest gear possible. At no point does the torque at the wheels drop enough after peak power to make taking a higher gear worth while.

I've yet to see a power/torque curve for an engine where selecting a higher gear is better. Many people (often diesel fan-boys) believe this is the case as they are under the illusion that it is torque-at-the-crank that provides acceleration.

[MotorsportTom]

Sorry for the delay in the reply I'm at work and got busy for once.

So the general rule of thumb is; without a huge torque drop off and close gearbox stay in a gear for as long as possible regardless of peak power etc?

[300bhp/ton]

alock said:

A picture says a thousand words...


This shows that for an RX8, you always want to be in the lowest gear possible. At no point does the torque at the wheels drop enough after peak power to make taking a higher gear worth while.

I've yet to see a power/torque curve for an engine where selecting a higher gear is better. Many people (often diesel fan-boys) believe this is the case as they are under the illusion that it is torque-at-the-crank that provides acceleration.

Torque at the crank does provide acceleration though and is directly linked with wheel torque.

And with a turbo charged car being off boost will make a huge difference or if the engine has nosed over.

Also what a power graph can't show you is the lost time changing gear. While a vehicle might accelerate quicker in a lower gear, you've still got to change gear, so total time to get to speed 'x' or point 'y' may not always be better.

Other than that I agree though

[E21_Ross]

i think it also related to the fact if it's producing 650bhp at 6500rpm it'll still pull harder than 650bhp would in a taller gear...no?

[Alfanatic]

300bhp/ton said:

Torque at the crank does provide acceleration though and is directly linked with wheel torque.

The relationship is as a multiple of the gear ratio.

300bhp/ton said:

Also what a power graph can't show you is the lost time changing gear. While a vehicle might accelerate quicker in a lower gear, you've still got to change gear, so total time to get to speed 'x' or point 'y' may not always be better.

Other than that I agree though

Since the debate is whether it's worth changing up earlier than the redline, or at the redline, both options involve the same gear changes to get to where you want to be. If anything, changing earlier than the redline will involve more changes than staying with the lower gear. Unless you mean changing down in order to accelerate from a constant velocity, but that brings new conditions to the argument which I'm pretty sure the OP wasn't discussing. As it is, I'd say that time lost while changing gear can be discounted as it's a constant that impacts both sides of the debate.

[E21_Ross]

ok, what about G forces due to acceleration? 650bhp at a higher rpm (thus shorter gear) vs 650bhp at lower rpm (thus taller gear), surely it'll be pulling stronger in the shorter gear, no?

[MotorsportTom]

So it seems there are lots of variables I didn't think of haha, I really wished I had thought of this question about 12 months ago when I was still at college on my motorsports course.

Oh and thanks for all the replies people it makes for some good reading and thought.

[300bhp/ton]

Alfanatic said:

300bhp/ton said:

Torque at the crank does provide acceleration though and is directly linked with wheel torque.

The relationship is as a multiple of the gear ratio.



But the more flywheel torque the more wheel torque regardless of the torque multiplication

[blank]

doogz said:

A power curve would be the best way to figure this out.

If you know the rev drop between 2 gears, you want the section of the curve, where you draw a horizontal line between the curve at your specified engine speeds, and have the least area under this new curve you've made.

I think.

This is exactly right.

http://www.parkers.co.uk/advice/forum/topic.asp?TO...

[alock]

blank said:

doogz said:

A power curve would be the best way to figure this out.

If you know the rev drop between 2 gears, you want the section of the curve, where you draw a horizontal line between the curve at your specified engine speeds, and have the least area under this new curve you've made.

I think.

This is exactly right.

http://www.parkers.co.uk/advice/forum/topic.asp?TO...

No it isn't.

In his 'change at 4500rpm' example, he states that the gear change is too late because he has a power drop of ~10% (140bhp to 125bhp). To avoid this power loss, he selects a 30% higher gear.

Extreme example of the flawed logic. Does you car produce a better rate of acceleration at peak power in 6th gear or at 70% of peak power in 1st?

Edited by alock on Tuesday 3rd May 22:22

[SSBB]

All about torque at the wheels:

http://www.welltall.com/ymc/discovery/car/shiftpt....

Torque at the wheels is the physical force which the tyres apply to the ground to accelerate the car. The more the better.

[Alfanatic]

300bhp/ton said:

Alfanatic said:

300bhp/ton said:

Torque at the crank does provide acceleration though and is directly linked with wheel torque.

The relationship is as a multiple of the gear ratio.

But the more flywheel torque the more wheel torque regardless of the torque multiplication

I don't think that's true. Gear multiplication typically has a bigger influence on torque at the wheels than engine efficiency.

In, for example, the case of an engine that produces a torque peak of 100nm at 6000rpm, and the torque then drops off to 80nm on the way to the redline at 7000rpm: If you stay in, say, 2nd, you'll have a faster rate of acceleration at 6000rpm than you would at 7000rpm. The same holds true if you stay in 3rd gear, and all the others. I don't dispute this at all - for a vehicle with only one gear.

But lets say 2nd gear is a ratio of 2:1 and 3rd is 1.4:1. Final drive ratio and wheel size doesn't change so we can assume that's all 1:1.

at peak torque in 2nd, the wheels will be transferring 200nm to the road (100nm*2).
At max revs in 2nd, the wheels will be transferring 160nm to the road (80nm*2).
At peak torque in 3rd, the wheels will be transferring 140nm to the road (100nm*1.4).

So, at max revs in 2nd, you are still getting more torque at the wheels than at max torque in third. Thus, while acceleration will tail off from 6000rpm to 7000rpm in second, it will tail off even more as soon as you change to 3rd, no matter what revs you make the change at. Hence, for best acceleration, stay in the lowest gear as long as you can.

The gear ratios and torque numbers I have used and the torque figures are made up, but this is what the RX8 chart above is showing, I have seen similar results for a Honda Civic, and I have heard that there are few, if any, road cars that buck this trend. You'd need a very large drop off in torque between peak and redline, or very close gear ratios, to buck the trend.

[paranoid airbag]

E21_Ross said:

ok, what about G forces due to acceleration? 650bhp at a higher rpm (thus shorter gear) vs 650bhp at lower rpm (thus taller gear), surely it'll be pulling stronger in the shorter gear, no?

yes and no (mostly no). Strictly speaking, acceleration=Power/(mass times speed) - note there's nothing in that equation that depends on a gearbox. So 650hp is 650hp regardless of gear, unless first gear is more efficient (wastes less power making noise and heating oil up) than second for some reason (AFAIK in most cars the opposite is true, but it's not that significant). But to get that 650hp in a taller gear, you'll have to be going faster, so by the above equation you'll accelerate slower. If an engine had a constant power output over a part of the rev range, you'd feel its acceleration dropping off in that range even in a single gear - constant torque will generate constant shove in a single gear.

[300bhp/ton]

Alfanatic said:

I don't think that's true. Gear multiplication typically has a bigger influence on torque at the wheels than engine efficiency.

In, for example, the case of an engine that produces a torque peak of 100nm at 6000rpm, and the torque then drops off to 80nm on the way to the redline at 7000rpm: If you stay in, say, 2nd, you'll have a faster rate of acceleration at 6000rpm than you would at 7000rpm. The same holds true if you stay in 3rd gear, and all the others. I don't dispute this at all - for a vehicle with only one gear.

But lets say 2nd gear is a ratio of 2:1 and 3rd is 1.4:1. Final drive ratio and wheel size doesn't change so we can assume that's all 1:1.

at peak torque in 2nd, the wheels will be transferring 200nm to the road (100nm*2).
At max revs in 2nd, the wheels will be transferring 160nm to the road (80nm*2).
At peak torque in 3rd, the wheels will be transferring 140nm to the road (100nm*1.4).

So, at max revs in 2nd, you are still getting more torque at the wheels than at max torque in third. Thus, while acceleration will tail off from 6000rpm to 7000rpm in second, it will tail off even more as soon as you change to 3rd, no matter what revs you make the change at. Hence, for best acceleration, stay in the lowest gear as long as you can.

The gear ratios and torque numbers I have used and the torque figures are made up, but this is what the RX8 chart above is showing, I have seen similar results for a Honda Civic, and I have heard that there are few, if any, road cars that buck this trend. You'd need a very large drop off in torque between peak and redline, or very close gear ratios, to buck the trend.

I was meaning a much simpler situation.

300lb ft @ 2000rpm with 1:1 gear ratio and 3.42:1 final drive

vs

380lb ft @ 2000rpm with 1:1 gear ratio and 3.42:1 final drive


In this scenario the car with the higher flywheel torque will still be making more wheel torque.


I fully agree with what you are saying though. Torque multiplication is highly affective, hence why off roaders have a low ratio gear transfer box.

The only down side to torque multiplication is the trade off in wheel rpms. You can make more torque, but less rpms, so HP remains fairly constant gear on gear.

Edited by 300bhp/ton on Wednesday 4th May 11:57

[gareth.e]

I hate these threads... I never know whos right

[Gizmo!]

SSBB said:

All about torque at the wheels:

http://www.welltall.com/ymc/discovery/car/shiftpt....

Torque at the wheels is the physical force which the tyres apply to the ground to accelerate the car. The more the better.

Which, children, is why when you're practising burnouts you should always drop the clutch at the torque peak, not the power peak.