Best Points to Change Gears on Diesels
Discussion
I received an A6 2.0 TDI manual as a courtesy car the other day. I don't have much experience driving diesels but have noticed that the acceleration tends to drop towards the upper section of the rev range, which is certainly different to the effect you get on petrol powered engines where - for max acceleration - you usually take the revs close to the limit point.
So is there a method/formula for working out when is the best rev point to change gear on diesels?
So is there a method/formula for working out when is the best rev point to change gear on diesels?
Welshbeef said:
Peak power just like a petrol is produced below the red line.
Your question needs to e more precise is it for economy driving, town driving, a to b cross country driving M way driving, overtaking, max acceleration.
Max acceleration. Sorry thought it was implicit that was referring to that when I compared it petrols in the 2nd sentence.Your question needs to e more precise is it for economy driving, town driving, a to b cross country driving M way driving, overtaking, max acceleration.
Hoygo said:
At the end of the power band,3K.
Well it all depends at what revs peak power is every car is different but generally max bhp is 4k revs with usually red line 4,750rpm. Maximum acceleration from a standstill will be using max power all the time.
Has to be said surely as you know in the car your driving it tails off why would you carry on revving more? First time you'd not know second and third time you have learnt the characteristics of the engine.
LeoSayer said:
Max bhp is around 4000rpm, so to make best progress, change gear a few hundred rpm higher than that.
Exactly. If you drove a Focus ST red line 7k revs peak power 5,500rpm little point going much over 5,500 rpm change up as power tails off.
For overtaking make sure your in a gear which puts the revs at or just under peak torque so generally 1,750 revs then floor it if your doing it in 4th your going to be near on 100mph if you need to change up to 5th using that strategy so only 3rd would you rev just over 4,200rpm and slam into 4th.
For me if I need the more acceleration whilst moving say 40mph I'd put it into 4th and floor it mine doesn't notably tail off until maybe 4,500rpm but I'm changing into 5th again full throttle good fun.
Skodaku said:
Diesel - so surely you'd want to change up at max torque, not max bhp. (?)
You always want maximum power for maximum acceleration.When you get up towards in a diesel 4k you can feel it's pulling less hard than it was lower down the rev range, but it'll still be pulling harder than it would be a gear up - until you're a little past peak power.
Skodaku said:
Diesel - so surely you'd want to change up at max torque, not max bhp. (?)
No the OP wanted to know maximum acceleration. Best 0-60 &0-100mph times are achieved when changing up just after max bhp not peak torque. Else for me peak torque 1,750rpm... So I'd change up at 2k revs then be dropped into 1.100rpm revs that oat the best point for acceleration. The feeling of the power 'dropping off' after around 3k rpm is actually the rate of change of acceleration reaching a plateau. At peak power, you are still accelerating as fast or faster than you where at peak torque, but as the rate of acceleration hasn't changed it feels different.
Also torque at the wheels is much higher at high revs in a low gear than lower revs in a higher gear, so hanging onto gears for longer is always best.
Also torque at the wheels is much higher at high revs in a low gear than lower revs in a higher gear, so hanging onto gears for longer is always best.
Welshbeef said:
If you drove a Focus ST red line 7k revs peak power 5,500rpm little point going much over 5,500 rpm change up as power tails off.
When you consider gear ratios and wheel torque, it may still be quicker to hold onto a lower gear a fair way past peak power. The sensation as rate of acceleration starts to drop seems at odds with going quickly, but as the rate of acceleration is lower in each successive gear it may still be quicker.You also need to consider the revs drop between gears.
busta said:
When you consider gear ratios and wheel torque, it may still be quicker to hold onto a lower gear a fair way past peak power. The sensation as rate of acceleration starts to drop seems at odds with going quickly, but as the rate of acceleration is lower in each successive gear it may still be quicker.
You also need to consider the revs drop between gears.
Very true. But we are talking very very generally You also need to consider the revs drop between gears.
This is a fairly typical TDI power graph. Consider which area of the power curve gives you the highest average power.
2,700rpm-4,200rpm by my reckoning, assuming a 1,500rpm drop in revs between gears.
My point being, to get the best average you have to go beyond peak power even if the seat of your pants says otherwise.
busta said:
The feeling of the power 'dropping off' after around 3k rpm is actually the rate of change of acceleration reaching a plateau. At peak power, you are still accelerating as fast or faster than you where at peak torque, but as the rate of acceleration hasn't changed it feels different.
Ignoring aerodynamic drag etc., acceleration is directly proportional to the force applied (f = ma). Torque is just force constrained to act in a circle, the torque produced by the engine is multiplied by the transmission and converted to a linear force where the tyre meets the road. In a particular gear the acceleration is directly proportional to engine torque, so acceleration really does drop off as the torque drops.mollymoo said:
so acceleration really does drop off as the torque drops.
But if you changed to a higher gear it would drop off even more because the higher gear would mean that wheel torque was lower, even though engine torque would be higher. For best performance, you change up around peak power.
DaineseMan said:
I received an A6 2.0 TDI manual as a courtesy car the other day. I don't have much experience driving diesels but have noticed that the acceleration tends to drop towards the upper section of the rev range, which is certainly different to the effect you get on petrol powered engines where - for max acceleration - you usually take the revs close to the limit point.
So is there a method/formula for working out when is the best rev point to change gear on diesels?
1500-3000rpm normal driving and you won't go far wrong. Boy-racing, up to 4500rpm but it'll probably run out of puff around 4000 or just after if it's not remapped. It was rare that I went much above 2500 in mine unless I was hooning, but it was remapped and would pull from 1100rpm so I could drive around in a high gear everywhere.So is there a method/formula for working out when is the best rev point to change gear on diesels?
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