Theoretical question.
Discussion
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.
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.
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
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 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.
So far this is the one reply I have read that seems to make the most sense and I understand it 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.
Also I second what gareth.e said, sometimes I have no idea who is right or wrong but thanks all the same
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