What RPM to change up at?
Discussion
According to the dynochart my car makes its maximum power of 265.8 bhp at 4550rpm. Is there any point in revving the engine beyond that point if I want to accelerate as fast as possible?
The test was conducted in 4th gear - would peak power also be at 4550 rpm in 1st, 2nd and 3rd gears?
L.F.
The test was conducted in 4th gear - would peak power also be at 4550 rpm in 1st, 2nd and 3rd gears?
L.F.
The peak engine power will be at the same RPM regardless of which gear you are in.
There are a number of gear change calculators floating about the web. You normally enter the torque curve at various set points, the gear ratios including final drive and rolling diameter of the wheels. They will then calculate a curve for each gear and show the ideal overlap point.
Using a more empirical method I have found that changing up earlier than you would expect gives better acceleration with the 500 engine as the main feature tends to be loads of torque. At the beginning of the Hilclimb season I usually forget this and do a few disappointing ruins until I remember to change gear earlier.
There are a number of gear change calculators floating about the web. You normally enter the torque curve at various set points, the gear ratios including final drive and rolling diameter of the wheels. They will then calculate a curve for each gear and show the ideal overlap point.
Using a more empirical method I have found that changing up earlier than you would expect gives better acceleration with the 500 engine as the main feature tends to be loads of torque. At the beginning of the Hilclimb season I usually forget this and do a few disappointing ruins until I remember to change gear earlier.
My torque peaks at 340 lb/ft 3000 rpm but tails off noticeably after 4000 rpm. There would seem to be no point at all in going beyond the peak power at 4500 rpm when torque is down to 310 lb/ft?
Higher RPM band probably sounds better but in reality would we be better changing up sooner if we want to go quicker?
L.F.
Higher RPM band probably sounds better but in reality would we be better changing up sooner if we want to go quicker?
L.F.
lockhart flawse said:
My torque peaks at 340 lb/ft 3000 rpm but tails off noticeably after 4000 rpm. There would seem to be no point at all in going beyond the peak power at 4500 rpm when torque is down to 310 lb/ft?
Higher RPM band probably sounds better but in reality would we be better changing up sooner if we want to go quicker?
L.F.
Can you post a picture of the torque curve ? Higher RPM band probably sounds better but in reality would we be better changing up sooner if we want to go quicker?
L.F.
For track work the objective is to keep the engine running between max torque and max BHP. So you change up when the revs in the next gear will be as close to max torque as possible. Changing up at the max torque revs will be too soon to make optimum progress as power will drop off in the next gear. Exact revs to change up will depend on gearbox ratios, diff ratio and tyre sizes.
Edited by Englishman on Saturday 16th May 19:00
Nothing to do with changing gear but this thread does highlight what must be a difference in the way various rolling roads are calibrated. There was a time when to get over 300bhp/300ft-lbs from a RV8 was considered very good (and expensive) - indeed I believe Steve Heath used to talk of an informal "300bhp club" - Here we have two different Griff 500's one standard putting out a massive 340 ft-lbs torque (yet only 265 bhp) the other with a specially built V8 Developments engine producing 328bhp/330 ft-lbs. Interesting that's all. 
Edited by RichB on Sunday 17th May 10:14
Perhaps it isn’t rolling road calibration. I remember Joo posting that his rollers and Emeralds posted very similar results.
The engines are quite different. Lockhart has a standard cam, I have an H404. He has loads more torque than mine low down. I have loaded the top end at the expense of the bottom because that’s how I like to drive.
My one and only complaint is that sometimes an overtaking manoeuvre requires a gearchange part way through. This only happens when the manoeuvre begins at low speeds. If I start to overtake in third, there isn’t the immediate take off because the revs are too low and I’m not on cam. If I start in second, I run out of rev range and have to shift up. Most Griffs have a ponderous gearchange. Mine is as quick as I can move my hand.
My car is maintained by Taylor TVR. Dan Taylor has a lot of experience on TVR’s, both road and track. He describes mine one of the fastest 500’s he’s ever driven. It wouldn’t be to everyone’s taste. It has the character and sound of a pre-cat, with more performance than most 500’s.
One of the on-line calculators that I found also asked for the frontal area and drag coefficient. Does anyone know what these are? Presumably the pre-cat and 500 would differ very slightly.
ETA
The benchmark for a ‘Monster’ was 300bhp, but I always thought that was 300bhp at the wheel. I don’t have the wheel figures for my car. What does it need at the flywheel to give 300 at the wheels?
The engines are quite different. Lockhart has a standard cam, I have an H404. He has loads more torque than mine low down. I have loaded the top end at the expense of the bottom because that’s how I like to drive.
My one and only complaint is that sometimes an overtaking manoeuvre requires a gearchange part way through. This only happens when the manoeuvre begins at low speeds. If I start to overtake in third, there isn’t the immediate take off because the revs are too low and I’m not on cam. If I start in second, I run out of rev range and have to shift up. Most Griffs have a ponderous gearchange. Mine is as quick as I can move my hand.
My car is maintained by Taylor TVR. Dan Taylor has a lot of experience on TVR’s, both road and track. He describes mine one of the fastest 500’s he’s ever driven. It wouldn’t be to everyone’s taste. It has the character and sound of a pre-cat, with more performance than most 500’s.
One of the on-line calculators that I found also asked for the frontal area and drag coefficient. Does anyone know what these are? Presumably the pre-cat and 500 would differ very slightly.
ETA
The benchmark for a ‘Monster’ was 300bhp, but I always thought that was 300bhp at the wheel. I don’t have the wheel figures for my car. What does it need at the flywheel to give 300 at the wheels?
Edited by Uncle Fester on Sunday 17th May 12:57
Englishman said:
So you change up when the revs in the next gear will be as close to max torque as possible.
That's not quite right. The optimum change-up is when the new gear provides more power than the old gear. Quite often with a highly tuned engine this will occur at higher revs than the engine can tolerate, in which case you change up at the red line.In a typical road tuned RV8 the last 1000 RPM produces minimal extra power but it does massively increase the wear and tear on the engine, so it would be sensible to keep it below that in normal use.
Interesting point about rolling roads. I got almost identical at the wheels readings from Surrey rolling road and from the rollers at Austec, but the calculated figures at the flywheel from each of these bear litte resemblence. People put too much reliance on the calculated figure IMHO.
At the wheels
At the flywheel
At the wheels
| Rollers | BHP | Torque |
| Austec | 204 | 220 |
| Surrey | 205 | 215 |
At the flywheel
| Rollers | BHP | Torque |
| Austec | 231 | 250 |
| Surrey | 259 | 275 |
Edited by bigdods on Sunday 17th May 17:29
GreenV8S said:
Englishman said:
So you change up when the revs in the next gear will be as close to max torque as possible.
That's not quite right. The optimum change-up is when the new gear provides more power than the old gear. Quite often with a highly tuned engine this will occur at higher revs than the engine can tolerate, in which case you change up at the red line.In a typical road tuned RV8 the last 1000 RPM produces minimal extra power but it does massively increase the wear and tear on the engine, so it would be sensible to keep it below that in normal use.
What you are trying to achieve overall is as close to maximum power as possible in all gears. Practically that means changing up at or just after max power in any gear. Depending on power curve of the engine, that will typically mean you are at an rpm between max torque and max power in the next gear. Looking at the graph earlier you gain an extra 10% power in the last 1000rpm before peak power. Definitely worth having IMHO.
I do agree there is little point continuing to accelerate after the peak power point is reached though unless you are just about cross the finishing line and would lose the race by the time taken for a gear change. Drive as you wish on the road of course, there is no point extending the engine unless you need to.
Englishman said:
GreenV8S said:
Englishman said:
So you change up when the revs in the next gear will be as close to max torque as possible.
That's not quite right. The optimum change-up is when the new gear provides more power than the old gear. Quite often with a highly tuned engine this will occur at higher revs than the engine can tolerate, in which case you change up at the red line.In a typical road tuned RV8 the last 1000 RPM produces minimal extra power but it does massively increase the wear and tear on the engine, so it would be sensible to keep it below that in normal use.
Englishman said:
What you are trying to achieve overall is as close to maximum power as possible in all gears. Practically that means changing up at or just after max power in any gear.
That is incorrect. The optimum change-up point is when the falling power curve in the current gear crosses the rising power curve available in the next gear. This may occur beyond the red line, in which case the best you can do is change up at the red line. The cross-over point will typically be well past peak power in the lower gear.Edited by GreenV8S on Sunday 17th May 19:24
Chimjunkie said:
Have never seen a graph with the peak power produced so low down on a standard cam. Could the bottom scale be wrong?
It does seem odd because they don't even cross because it doesn't reach 5,250. Often there are different scales each side for the power & torque, I wonder if one of the scales was incorectly set? 
RichB said:
Chimjunkie said:
Have never seen a graph with the peak power produced so low down on a standard cam. Could the bottom scale be wrong?
It does seem odd because they don't even cross because it doesn't reach 5,250. Often there are different scales each side for the power & torque, I wonder if one of the scales was incorectly set? 280bhp at 4500rpm and
283bhp at 5000rpm and then
280bhp at 5500rpm
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