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Difference between torque & power based remaps?

The rev limiter would *typicaly* be set to where the red area starts, though not always...

It is important to know what is the quoted rpm at which peak bhp is achieved, because there's no point in revving the engine beyond that point.

So looking at the power curve in my previous post, max power is @ approx. 4200 rpm? In this case there would be no point revving the engine above this as power drops off? I'll check the markings on my rev counter later :thumb:
 
Ok, you got me curious enough to have a look at what a good/correct/expected power and torque graph should look like.

I came across a Mercedes brochure whilst searching the internet, and the following graph is for what i believe to be my engine.

View attachment 87742

Notes on interpreting the MB graph which may help.
Torque curve first.
Note how the torque is rising up to 1600rpm – this is due to the turbo spooling up as it gets increasing energy from the exhaust flow which is rising as the rpm does as more air passes through the engine.
From 1600rpm the torque has peaked and is now declining as the turbocharger has reached its air flow limit (a larger turbocharger would compensate but would be slower to spool up, essentially shifting the torque curve to the right along the rpm axis).
At around 4600 rpm the torque begins to drop away much more suddenly as the airflow limit of the engine (due to valving, port sizes, etc) is reached.
The above I have presented as mechanical restrictions but torque can (and will) be managed by controlling boost level via the ECU. This is where the re-mappers tinker.
Power.
Note how the steepest rise of the curve is up to 1600 rpm. As per earlier, power is torque multiplied by rpm and the rpm is rising linearly, the torque is also rising so the power curve here is steeper than the torque curve.
After 1600 to 4600 rpm the power is rising (due to linearly increasing rpm) but at a slower rate than before as the torque is declining. So long as the torque isn’t falling as fast as the rpm is rising, the power will increase.
At 4600 rpm the torque seriously declines and any increase in rpm cannot compensate thus peak power is reached (which coincides with the rising internal losses in then engine from the various frictions).

So I decided to use the figures from the graph i was supplied by the remap company and create my own.

I used the Economy map figures, when in hindsight I should have used the standard map figures. I didn't bother converting to Nm or kW, as I was just looking more for the shape of the curves for torque and power.

I estimated the RPM as the supplied graph didn't state this, and it appears that I was around 500 rpm too great when I've labelled my axis, but again, the shape of the curves should be reasonable.

Apologies in advance for the very crude graph, but this was drawn on the back of a work document during my coffee break, and was just to get a basic idea of the curve shapes.

View attachment 87743

So my conclusion from this is that the power curve quoted by the remap company is possibly correct for my vehicle, but the torque curve certainly isn't, and in fact just appears to be identical to the power curve?!

Yep, looks like they have presented power curves only.

A customer care representative from the remap company contacted me yesterday regarding their quote, and I have advised them that I am currently seeking help interpreting their claims, as the graph they had supplied wasn't in a format that I was familiar with. I don't expect that I will hear from them again.

Yep, I doubt that you'll hear from them!

The question is now, do I still trust in what I believed to be a highly regarded remapping company and consider one of their remaps?
Or walk away being thankful that I had the help of you guys, and feel glad that I didn't spend my money with them?

There presentation is lamentable. That doesn't in itself imply they aren't good at the actual technical stuff, but I expect from engineers a greater commitment to everything - including presentation of data.
I would resolve this with a good ol' fashioned telephone conversation - but I'm fortunate enough to be able to talk 'turkey' with them and gauge if they know what the think they know. Not everyone is in that position.
 
So looking at the power curve in my previous post, max power is @ approx. 4200 rpm? In this case there would be no point revving the engine above this as power drops off? I'll check the markings on my rev counter later :thumb:

For economy, drive to the torque peak (or thereabouts) and upshift.
For maximum acceleration drive between the torque peak and the power peak. Although the engine torque is declining, the torque multiplication of the lower selected gear ratio compensates.

The one exception where overshooting the power peak is valid is when the gear ratios are so gappy that a later upshift prevents the rpm (on the next gear engagement) dropping too far below the torque peak. This is unlikely to be a concern with today's transmissions with so many (closely spaced) ratios available.
With limited ratios and tuning of the type that narrows the rpm gap between the torque and power peaks this can be a concern though only petrol engines are tuned in this way and even then, there is likely to be enough ratios to make it viable. Wasn't always so, hence the availability of aftermarket 'close ratio gear sets' in the past.
 
So looking at the power curve in my previous post, max power is @ approx. 4200 rpm? In this case there would be no point revving the engine above this as power drops off? I'll check the markings on my rev counter later :thumb:
Correct, no point in revving the engine beyong peak power point.

Diesels reach peak power at relatively low rpm (3000-5000). Petrol higher (4000-7000). Race engines and sport motorcycles peak even higher (10000+......).
 
There are several reasons why power drops sharply when revving beyond the peak point, mostly to do with volumetric efficiency.

But there's also another one... modern engines use hydraulic valve lifters, which, unlike the mechanically-operated lifters of old, do not need regular tappet adjustments.

This is a good thing, usually. Howeve, at very high rpm, the hydraulic lifters starts to 'float' - caused by a minute delay in transferring force to the valve due to the oil compressing.

This means that valve timing goes off, leading to loss of power, and also the valves are never fully closed, which in extreme cases, can also lead to burnt exhaust valve.
 
With forced induction engines that have peak torque at very low revs the approach needed for maximum economy may not be as simple as peak torque revs. Ideally you need a map of specific fuel consumption i.e. fuel consumed for each HP produced. It varies a lot with revs and also with load but specific fuel consumption tends to be roughly a mirror image of the torque curve, maximum efficiency being approximately at peak torque revs and full load. I wonder how accurate MB's "change up now" indicators are on manual cars ? They do seem to take into account load as well as revs. Here is the simplistic graph that doesn't take load into account

Curve.jpg
 
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With forced induction engines that have peak torque at very low revs the approach needed for maximum economy may not be as simple as peak torque revs. Ideally you need a map of specific fuel consumption i.e. fuel consumed for each HP produced. It varies a lot with revs and also with load but specific fuel consumption tends to be roughly a mirror image of the torque curve, maximum efficiency being approximately at peak torque revs and full load. I wonder how accurate MB's "change up now" indicators are on manual cars ? They do seem to take into account load as well as revs. Here is the simplistic graph that doesn't take load into account

View attachment 87751

True, and why I said 'or thereabouts'. It gets more complicated (for SI) at part load acceleration as the torque curve isn't the same as the full load torque curve. Initially it is, but as the revs rise, the throttling (at part throttle) increases impacting negatively on the SFC. Easily dealt with by upshifting the moment the acceleration tails off, but there are so many drivers out there who go beyond that point and have the motor droning gasping for air that they make the case for automatic transmission right there without realising it.
 
I wonder how accurate MB's "change up now" indicators are on manual cars ? They do seem to take into account load as well as revs.

I was surprised how soon I was prompted to change up gears when I first got my car, but I'm used to it now, and pretty much change gear during normal driving at the point the car suggests. I'm still not quite used to changing down at the correct point though, and when going up hills I seem to be prompted to change down sooner than I would without the prompt. :dk:
 
I was surprised how soon I was prompted to change up gears when I first got my car, but I'm used to it now, and pretty much change gear during normal driving at the point the car suggests. I'm still not quite used to changing down at the correct point though, and when going up hills I seem to be prompted to change down sooner than I would without the prompt. :dk:
I am assuming that the car prompts you to change gears at the earliest possible point (low revs) in the interest of economy and reduced emissions. However this is the opposite of sporty driving. Re-programming the TCU as well will deliver more performance-based driving at the expense of fuel economy and emissions (beyond what is available via E/S/M). If it was a race car, you'd want to be promoted to change gears at the highest point in the rev range, not the lowest (in a straight line anyway - there are additional considerations when cornering).
 
I am assuming that the car prompts you to change gears at the earliest possible point (low revs) in the interest of economy and reduced emissions.

It does but it's a little smarter than just the lowest revs. When the engine is under load, for example when accelerating hard or climbing a hill then the change up indication occurs at higher revs. It seems to be intelligent enough to be of use for more than just the maximum possible MPG.

I hired a Transit sized van recently and driving it reminded me how much I've got used to forced induction engines with low rev torque. This puny 1.6L turbo diesel had little torque below 2000 RPM and if I allowed the revs to fall below that figure it was forever telling me to change down a gear. It wasn't just lack of toque, the judder at low revs made me suspect the van didn't have a dual mass flywheel. Part of me dislikes the potential fragility of a dual mass flywheel but modern 4 cylinder engines making peak torque at only a little above idle revs would be impossible without them.
 
I have monitored the gear change indicator the best I could over my last couple of work commutes. As expected, it doesn't seem to instruct to gear change at the same rpm every time for each gear. My journey consists of both flat and hill driving, and the gear change indicator appeared to recommend changing up between 1600 - 2000 rpm. I was unable to monitor close enough to get exact figures for each gear, but it never seemed to be outside of this range.
 

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