Chris Gilbert: "No more than 90% of the power band"

[jameslb101]

Thanks for taking the time to answer, and it's interesting to hear how theory was proved wrong in practice in the case of the 627 and Omega. Then again, why haven't I heard this wisdom from racing drivers (of any class, not just F1), who all seem to change up at the redline? Perhaps there's only way to find out...who's got Bruntingthorpe's number!?

[Ancient]

Thanks for taking the time to answer, and it's interesting to hear how theory was proved wrong in practice in the case of the 627 and Omega. Then again, why haven't I heard this wisdom from racing drivers (of any class, not just F1), who all seem to change up at the redline? Perhaps there's only way to find out...who's got Bruntingthorpe's number!?

Perhaps because racing cars tend to be tuned differently?
Chris doesn't say what state of tune the Rovers and Senators were in, but I would have thought most cars on the road would not be tuned to give the same power-delivery characteristics as racers. Would even Police pursuit cars be race tuned?

[Horse]

I think it requires two dedicated AD members with identical cars to try this out

[ChrisGD4]

Police vehicles are all standard except for the suspension and braking system; no engine is tuned. My explanation to you is not opinion it is what we discovered through the process I have explained. At one time we had an MG which had instead of a rev counter, a graph like display with a green line that climbed a hill to a peak 90% before dropping down the otherside to the red line, some of you may remember that. So, together with the advice from the manufacturers, and our experience, all 100 plus instructors at Hendon used the 90% rule when driving flat out. However, I am quite happy to conceed that other vehicles may be different and I'm also very comfortable with other member's opinion on the subject, none of us should get angry when discussing our favourite subject! It would be very interesting, if two of you with identical cars were to repeat the test we did in 1988/90 and let us know what you discovered. I leave it there.
Last year on 22nd November I celebrated 50 years in driver training - now 71 and I still enjoy driving and driving instruction. My fire has recently been lit doing training in the McLaren 12c What an awesome but docile machine! I would buy one if I had the money, it is that good. All the very best - Chris Gilbert

[Ancient]

I for one do not find it surprising that road cars were tuned to give peak power somewhere below the red line. Without putting each individual vehicle through expensive testing, 90% seems a sensible approximation, especially as that has proven to have advantage over 100% in a (single) test.

Race vehicles (and tuned engines that some choose to use on the road) are another matter entirely. Race drivers will not use the 90% rule because (a) their cars will be exactly tuned to give peak power at specific speeds in different gears (which may well be changed to suit the track they are on); (b) they will know (be told) exactly what revs to change up at for each gear to give optimum performance and (c) their engines get regularly rebuilt.

Individual cars may well differ, models will differ even more. Some may perform better when using 95%, some when using 80% of the red line. Modern cars 20+ years on from Chris' test may be more closely tuned with improved engine management technology and commercial pressure (for economy, emmission control as well as performance), but I doubt many manufacturers would put peak power up in the redline (apart perhaps from some specifically marketed as 'performance cars' which will approach race tune) because they want to keep warranty claims down.

[jont]

Actually it needs 1 dedicated AD member who doesn't mind his gear clusters exiting the side of the car... Shift time is going to make a hell of a lot of difference to the difference in distance, particularly at the speeds Chris was mentioning - 115 mph is 51 and a bit metres per second, a difference of 0.1s in shift time is about a car length.

Couple of PDK Porsches should remove that from the equation

[Astraist]

There are of course, three values – peak torque, peak performance and peak revs (limiter). [...] 90% of where the red line starts would be around peak performance.[...] We moved off together and accelerated flat out, I changed up at 90% and Barry waited to the red line before changing.[...]We were neck and neck until I changed up into 3rd gear and began accelerating hard whilst Barry was finishing off 2nd gear and I began to move ahead.

I was under the impression that the 90% rule related to peak performance, not peak engine speed. If it is related to 90% of the motor speed, than obviously some (many, if not most) cars still reach peak performance before the red line...

[Porker]

Would it help the discussion if we used the term "peak power" rather than "peak performance". Using the latter, when I believe the former is what is meant, clouds the issue by confusing the end result with one of its inputs.

P.

[stefan einz]

Many cars generate peak power below the red line. Chris refers to the 12C above, which is a car I am lucky enough to own. Peak power of 620PS is generated at 7,500 rpm, against an 8,500 rpm redline.

However, without question, the fastest acceleration is generated by taking the car to the red line in each gear.

That said, for road driving, I'd tend to agree that making maximum progress rarely means using all the rev range. For example, taking a corner in a low gear may give maximum punch out of the corner, but then shortly after a gearchange is needed. So perhaps it is better to take the corner in a higher gear and ride the torque, and avoid the need for a gear up.

You need very big roads in my experience to really exploit the full performance of a car (and for it to make sense to do so). Last summer I took the 12C to Spain and there one could exploit its performance safely - and explore those wonderful upper reaches of the rev range.

[edpratt]

Hi,

I'm totally new to this forum, having stumbled across it whilst looking for advanced driving courses. However, when I saw this thread I thought I might be able to contribute...

There are two sides to the OPs original question. Chris has pointed out that in an actual road test, the theory held true. However, with other posters questioning the blanket 90% rule, there seems to be a good discussion going on.

The OP showed a graph of in-gear torque vs. rpm.

Taking this a little further, with some more complete data, here are two graphs; The first is plainly torque and power vs engine speed, the second, is tractive effort vs. vehicle velocity.

The data is from a Honda 600cc motorcycle, hence the high max rpm. However, the following points still hold true..



The tractive effort is effectively a torque curve, adjust for gearing, drag, lift, wheel size and a few other parameters. Plus a rudimentary tyre model - nothing fancy like a pacejka model, just a simple curve of grip vs. normal load.

Essentially the maximum available forward thrust of the vehicle will be found by the curve which follows the tops of all the other curves, shown in red on the lower graph.
The optimal gear shift points (green dots) can be seen to get closer and closer together towards the higher velocities.

What you are seeing is Chris's experience, that shifting out at the maximum end of the range is costing you forward momentum.

However, looking back at the power and torque curves you can see that for the first (and almost first two gears in this case),it pays to stay in gear for as long as practically possible - due to the massive torque delivered by these first two ratios. But as the speed increases, the shift point moves further back in the range.

This is down to a couple of factors.
Firstly, at higher speeds (where the shift point moves back towards the peak power), "power", or the rate at which work can be done is the most important factor. However, in the early phases of acceleration the amount of work which can be done, or "torque", is the most important, as you have to get the large mass of the vehicle moving.
Secondly, modern vehicles tend to have split gear ratios with 1-3 being acceleration and 4-5, or 6, being town or fast road cruising gears for fuel economy. In this case, the early gears on the tractive effort graph would move left and up slightly, and the later gears would move right and down, exacerbating the effect Chris is talking about.

So...
Basically, I think what Chris was experiencing is what I've described here but without data for an individual vehicle it's impossible to say where the "optimal" shift point in the rev range would be, especially as it moves around with each new gear!


Cheers,
Ed

[Nicola]

Some of my HPC friends might have been quite surprised to see how he teaches the approach to a roundabout. It was definitely lively.

Groan groan - now I am very confused - have a look at what I've said about that when I went out with him last Friday. It takes up a lot of the thread. CG spent a lot of time trying to get me to slow down very early coming up to roundabouts and I have been practicing doing that. Lively would not be a word I would use to describe the method.
I do not like feeling confused.

[hir]

I do not like feeling confused.

May I suggest that there's no need to be confused about any of this. Just listen to everyone, have a go at what each is suggesting - and then YOU decide what YOU want to do, what feels right for YOU; do what suits YOU; what suits your personality; and then you will develop a style that is YOURS alone.

You don't have to drive like CG, or John Cochrane ; or me ; or [dare I say it] Stressed D or anyone else for that matter!

I'll guarantee that when you drive at Oxford you will experience a different style of driving, and yes, even when approaching roundabouts, with each person you drive with. You might regard one technique as better than another, one that better suits your style of driving, but that's for YOU to decide.

[fungus]

I do not like feeling confused.

May I suggest that there's no need to be confused about any of this. Just listen to everyone, have a go at what each is suggesting - and then YOU decide what YOU want to do, what feels right for YOU; do what suits YOU; what suits your personality; and then you will develop a style that is YOURS alone.

You don't have to drive like CG, or John Cochrane ; or me ; or [dare I say it] Stressed D or anyone else for that matter!

I'll guarantee that when you drive at Oxford you will experience a different style of driving, and yes, even when approaching roundabouts, with each person you drive with. You might regard one technique as better than another, one that better suits your style of driving, but that's for YOU to decide.

I think that just about sums it up. I tend to approach in much the same way as CG. I can do it faster using more braking if I wish, but I find the plan to stop, look to go approach works for me. The most important thing is that the speed of approach, especially in the final few yards, allows the driver to assess the traffic flow as the veiw begins to open up, which will obviously differ from one roundabout to another. Another thing which many drivers fail to consider, is the veiw from the road approaching the roundabout from your left. Too fast, and it doesn't give drivers waiting to emerge a chance to see you.