[cmachine]

Hi, I am new to this forum and this is my first post here. I hope you can help me with the following Issue about the transmission.
As I am a potential buyer of a MP4-12C I was doing a test drive with this incredible good car. There is only one concern I have compared with other cars I experienced. It is about the kick-down function in automatic mode.
Usually, the kickdown is to provide maximum acceleration. Therefore the gearbox shifts down one or more gears. That's what I experienced with many cars I drove before (Ferrari, Porsche, BMW M3, etc.).

But in the MP4-12C I experienced the following:

- When driving in automatic with low revs and pressing the pedal down just before the kickdown threshold, it shifts down one or two gears to about 4'000 revs. Thats fine so far.
- When pressing even more over the kickdown “threshold”, the car does not downshift again. But I expected at least one more downshift to get maximum acceleration. Therefore pressing the kickdown threshold had no additional effect. In all oder cars I drove, the kickdown has the effect, that there was at least one more downshift if the revs are at low at 4000. This is somewhat strange, because in the MP-4C user manual is written, that kickdown provides max acceleration. But I only get the max acceleration if I downshift manual one gear more down.

I tested this on two mp4-12c, both with the same behavior of not taking kickdown signals into account. Do you have an explanation if this is normal? How is it in your cars.?
Both cars where updated with the newest software. One car was a new test car at the official mclaren dealer. The mclaren could not answer my question.
Please don’t argue, that a sports car is intended to shift manual. There is an automatic mode that I expect to work properly or at least as described in the user manual.

Beside this issue, this is the best sportscar I ever have driven!!

Best regards and thanks for an answer.
Olaf

 

[AugustWest]

It generally goes for the gear that'll put it at 4K(ish) RPM (this is the fat part of the torque curve). The switch you're calling the "kickdown" is there to let the launch control know you've got the throttle fully depressed, I don't believe it's affecting kickdowns.

Set the powertrain toggle to Track (and leave it in "automatic"); let us know what you think!

 

[cmachine]

Thanks for the fast answer

Of course I tried the different modes.

It's correct that it goes to about 4k, where the torque it fully available. But this is not the max. acceleration that is possible. If I shift down one more gear, there is more force (more wheel torque) at the rear wheels because of the shorter gearing.

Even my 550i Bimmer shifts down to about 5.5k when pressing full throttle. And the mc has a higher rev limit than my Bimmer.

So whats the intention about this behavior? A progessive automatic mode is not possible with this behavior.
Is it a bug that will be fixed in the future or is it intended not to drive "agressive" in automatic?

Thanks in advance for your thoughts.

 

[AugustWest]

My guess (without knowing) would be that they figure if you're being really aggressive you'll put it in manual. I'll have some extensive time in a car on Sunday morning; I'll let you know what at least one other car does!

 

[cmachine]

Ok, I am looking forward to your description from sunday. Enjoy!

 

[jkheit]

Hi cmachine. I hope someone posts the 12C torque curve (I posted an older one, I believe pre-625hp update). What you'll see is it's not really a torque curve, and more of a toque trapezoid.

As such, it doesn't really need to kick down lower if it's got 4k revs, which is pretty much hitting peak torque. Youll notice both the torque curve and turbo boost kick in at rather low RPMs on the car, it starts to get pretty good even just above 2.5k RPM.

So I'd be curious, I'm sure when you downshift and get up to 7kRPMs the car will "feel" like it's giving you more go, but I'm not sure you'll actually be going faster. It's an interesting question, and I'd be curious to see if anyone here has tested/or knows what the behavior might be.

Not sure how the gearing/traction interplays with gear selection.

 

[AugustWest]

That's exactly what I'm going to try to figure out on Sunday, jkheit!. The torque "curve" is so flat, they may use 4K rpm as the sweet spot so you have more acceleration available before a shift becomes necessary.

 

[CA_MP4]

It doesn't matter whether you are in the fat part of the torque curve or not, the lower the gear the higher the acceleration - period. Of course it makes little sense to downshift into 7500 RPM, but 2nd gear at 6000 RPM gives you lot more kick than 3rd at 4000.
If the car does not shift into lower gear, there is actually a little know feature in these cars attached to the steering wheel, I heard of this being referred to as shift paddles or something - honestly, I usually don't fumble around with it for fear of scaring my passengers, but sometimes we just have to experiment with these gadgets a little, after all, we paid for them.........

 

[noone1]

My guess is that auto mode just doesn't drive as aggressive. I imagine that driving in auto is a sign that peak performance is not required and therefor adjusted somewhat based on certain variable.



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[jkheit]

It doesn't matter whether you are in the fat part of the torque curve or not, the lower the gear the higher the acceleration - period. Of course it makes little sense to downshift into 7500 RPM, but 2nd gear at 6000 RPM gives you lot more kick than 3rd at 4000.
If the car does not shift into lower gear, there is actually a little know feature in these cars attached to the steering wheel, I heard of this being referred to as shift paddles or something - honestly, I usually don't fumble around with it for fear of scaring my passengers, but sometimes we just have to experiment with these gadgets a little, after all, we paid for them.........

While I agree with you about paddles (I drive strictly in manual mode), the OP wants to do things in auto.

Also, not so sure about the definitiveness of your gearing comment. Same RPMs in a lower gear, means lower speed. If you have enough power to push through a higher gear, your acceleration will be faster than applying that same power to a lower gear. That's why drag racers are looking for the highest gears, but it is a function of gearing and power, and best balancing it.

Here's a good article on it:

http://www.carcraft.com/techarticles/ccrp_9812_all_about_axle_gears/viewall.html

In the real world, typical street machines with aspirations for good dragstrip performance generally run quickest with 4.10:1 gears. Lower gears are required if the car is very heavy, or if the engine makes its power at the upper end of the rpm scale. Sometimes higher gears are used if the engine has gobs of low-end torque and doesn't like to spin at high rpm-

All that said, I generally agree with you, in most situations jamming down the gear gets you there faster.

 

[cmachine]

Good morning

Yes, of course the torque is present 4k. This is the same answer I got from the local mclaren dealer. But it is just the wrong answer to the current problem.
The torque that makes your car accelerating is NOT the torque at the crank, but the torque at the rear wheel. And therefore we have a gearbox. A very important feature of a gearbox is to transform the torque from one value to an other value. A simple example – let’s assume for this example a flat torque curve:

Assume a torque of X at the WHEELS at 4k in gear 4. If your transmission has a ratio difference from e.g. 1,5 when you downshift from gear four to gear three the result will be the following:

• Rev will be 6k = 4k * 1,5

• Wheel torque will be X * 1,5 !! --> You will accelerate 50% faster, because the force that accelerates you is 50% higher. This is also what CA_MP4 stated: Lower gear, faster acc.

Of course it makes no sense to downshift to 8k. But to 6 with an engine that almost revs to 9 – of course.

And yes, I now there is the option of shifting manual. But this is not the point here.
When I kick down, I want full acceleration – as written in the manual, and as every other car (sports car or limo) is doing. Especially when I have a “threshold” at the pedal as the mp4 has and most other cars have. I also tested the 458 and the f12. The 458 does not have the usual kick down threshold, but it shifts downs to max acceleration. So does the f12. Anyway, I like the mp4 much more than the Italians.


Back to the topic: I still hope the behaviour is a software bug that will be fixed. A premium car like the mp4 should behave premium also in this aspect. If I don’t like to downshift for max acc I still can stop pressing the pedal before the threshold.


Looking forward to your comments.


Best regards

 

[Russell996]

Good morning

Yes, of course the torque is present 4k. This is the same answer I got from the local mclaren dealer. But it is just the wrong answer to the current problem.
The torque that makes your car accelerating is NOT the torque at the crank, but the torque at the rear wheel. And therefore we have a gearbox. A very important feature of a gearbox is to transform the torque from one value to an other value. A simple example – let’s assume for this example a flat torque curve:

Assume a torque of X at the WHEELS at 4k in gear 4. If your transmission has a ratio difference from e.g. 1,5 when you downshift from gear four to gear three the result will be the following:

• Rev will be 6k = 4k * 1,5

• Wheel torque will be X * 1,5 !! --> You will accelerate 50% faster, because the force that accelerates you is 50% higher. This is also what CA_MP4 stated: Lower gear, faster acc.

Of course it makes no sense to downshift to 8k. But to 6 with an engine that almost revs to 9 – of course.

And yes, I now there is the option of shifting manual. But this is not the point here.
When I kick down, I want full acceleration – as written in the manual, and as every other car (sports car or limo) is doing. Especially when I have a “threshold” at the pedal as the mp4 has and most other cars have. I also tested the 458 and the f12. The 458 does not have the usual kick down threshold, but it shifts downs to max acceleration. So does the f12. Anyway, I like the mp4 much more than the Italians.


Back to the topic: I still hope the behaviour is a software bug that will be fixed. A premium car like the mp4 should behave premium also in this aspect. If I don’t like to downshift for max acc I still can stop pressing the pedal before the threshold.


Looking forward to your comments.


Best regards

Using this scenario of auto driving and then asking the car to kick down to the lowest gear possible, aren't you left feeling disappointed by the slow speed of a DCT gearbox to do multiple gear changes? One gear up or down is perfect but two or more with any DCT is best avoided if you are after instant results.

 

[cmachine]

... DCT is best avoided if you are after instant results.

the mp4 also shifts down 2 gears. therefore this is not the reason.
shifting down 2, 3 or more gears results in a still faster acc than only go down one gear (DCT, sequential gearbox, old school converter or manual, does not matter)

Imagine the situation you are cruising in 7th. Now you want overtake with full acc.
Kick down and let the automat choose the best gear (approx @6k) will be faster (when done right) than manual downshift 2,3 or more gears.

 

[Russell996]

the mp4 also shifts down 2 gears. therefore this is not the reason.
shifting down 2, 3 or more gears results in a still faster acc than only go down one gear (DCT, sequential gearbox, old school converter or manual, does not matter)

Imagine the situation you are cruising in 7th. Now you want overtake with full acc.
Kick down and let the automat choose the best gear (approx @6k) will be faster (when done right) than manual downshift 2,3 or more gears.

My comment was about the slow speed of multiple shifts in a DCT, nothing to do with why a certain gear is being achieved in auto mode when max acceleration is requested.
To me, a lower gear is always better for acceleration, to the point when the revs dictate an up change would be needed almost instantly.

 

[noone1]

I don't really follow what you're saying.

7th gear, 2000 rpm, 10% throttle -> 49% throttle -> 5th gear 4000 rpm
7th gear, 2000 rpm, 10% throttle -> 51% or more throttle -> 6th gear 3000 rpm

Those are just made up numbers, but is that what you mean? Giving light throttle drops more gears than heavy throttle?

It seems unlikely, but if it's not a bug, perhaps it assumes that being in auto means you don't want brutal acceleration/performance. If it sees that you want to accelerate under moderate throttle, it goes down more gears because it assumes the acceleration with be controlled. If it sees you smash the pedal, it might think that dropping down to the lowest gear would give more acceleration than desired. The 12C is light and has enough torque that it can probably get to a high gear at pretty low speeds. Dropping suddenly to the lowest gear in some cases could probably provide some pretty serious acceleration.

I could see my mom driving in auto at 35 in 7th gear, then me telling her to quickly get around that car, her slamming on the gas, and hitting 70 mph before she knew it.

Again, it seems unlikely, but auto mode is not for performance, so who knows.



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[Markjayw]

Ok so what powertrain setting were you using in your tests. There are 3 available. Try your test in Track mode next time. I drive my 12C in Auto occassionally, and have been very satisfied with the theottle response when needed. Wondering also if the cars you drove have been upgraded?

 

[texas mongrel]

Silly question, but the difference in response is huge when in active versus non-active mode, i.e. For someone not familiar with the car, they could select sport mode etc but without pushing Active its a slushbox. Regarding auto mode being not for max performance, I've had a couple of instructor-types tell me that best lap times come from Auto mode in track/track. I tried this last weekend at the track and 95% of the time box's selection was spot-on. However, I felt that it would somtimes not pick the best gear when coming out of tight corners (too high a gear and thus not enough revs to benefit from the turbos spooling up), and of course, its loads more fun shifting it yourself!

 

[cmachine]

Ok so what powertrain setting we you using in your tests. There are 3 available. Try your test in Track mode next time. I drive my 12C in Auto occassionally, and have been very satisfied with the theottle response when needed. Wondering also if the cars you drove have been upgraded?

of course I tested all different modes (and of course with active button on)

 

[cmachine]

I've had a couple of instructor-types tell me that best lap times come from Auto mode in track/track. I tried this last weekend at the track and 95% of the time box's selection was spot-on. However, I felt that it would somtimes not pick the best gear when coming out of tight corners (too high a gear and thus not enough revs to benefit from the turbos spooling up), and of course, its loads more fun shifting it yourself!

that brings it to the point: mclaren claims that the auto mode gives best performance. but it not true. and what is claimed in the user manual is also not true.
at least not with the two cars I drove (both updated - as the dealer assured)

if it is not a bug or a design mistake, there must be a very good reson for it.
why should mclaren make it behave different than any other car I know?
It is best practice and well accepted, that full throttle gives max acceleration in the lowest gear that makes sense for a specific engine. and an engine that revs as high as the mp4 engine should downshift to a region about 6k.

Because I still have no good reason for this behavior, I think it's a bug or a design mistake.

May be I should write an email to mclaren ...

 

[CA_MP4]

I have not yet used the car on a race track myself, but I seem to remember that the car has a feature that allows for holding the downshift paddle and the gearbox will select the most suitable gear for max acceleration out of a corner....... Have you tried that?

Silly question, but the difference in response is huge when in active versus non-active mode, i.e. For someone not familiar with the car, they could select sport mode etc but without pushing Active its a slushbox. Regarding auto mode being not for max performance, I've had a couple of instructor-types tell me that best lap times come from Auto mode in track/track. I tried this last weekend at the track and 95% of the time box's selection was spot-on. However, I felt that it would somtimes not pick the best gear when coming out of tight corners (too high a gear and thus not enough revs to benefit from the turbos spooling up), and of course, its loads more fun shifting it yourself!