M3 accerlation

[2fast0]

Is it just me or is the car designed this way.

I dont feel any jump in HP after i hit 3500rpms. The car pulls just as hard at 8000 as it does at 4000rpms. If this is the case because of the flat torque curve, what's the point of downshifting when trying to gun it while going at, say, 80mph?

[J08M3]

Originally Posted by 2fast0

If this is the case because of the flat torque curve,

yes... I would imagine so

[ansch]

Originally Posted by 2fast0

Is it just me or is the car designed this way.

I dont feel any jump in HP after i hit 3500rpms. The car pulls just as hard at 8000 as it does at 4000rpms. If this is the case because of the flat torque curve, what's the point of downshifting when trying to gun it while going at, say, 80mph?

if you want to gun it you'll want to down shift to a gear that puts the rev in the 4000+ power band.

[Paddy]

Originally Posted by 2fast0

Is it just me or is the car designed this way.

I dont feel any jump in HP after i hit 3500rpms. The car pulls just as hard at 8000 as it does at 4000rpms. If this is the case because of the flat torque curve, what's the point of downshifting when trying to gun it while going at, say, 80mph?

Just the smile on your face and the beautiful exhaust note from the higher revs.

[spyderco10]

Originally Posted by 2fast0

Is it just me or is the car designed this way.

I dont feel any jump in HP after i hit 3500rpms. The car pulls just as hard at 8000 as it does at 4000rpms. If this is the case because of the flat torque curve, what's the point of downshifting when trying to gun it while going at, say, 80mph?

yes

[kbk_75]

I dunno about you guys, but my car's acceleration really gets serious once I've hit 6,000 rpm, not 4,000. Also to say that there is no jump in HP over 3,500 rpm is just...I dunno...dumb. The car definitely pulls much harder at 8,000 rpm than it does at 4,000.

So, yes, if you want to accelerate hard, shift down to a gear where you're above 5,000 rpm.

[swiss_cornholio]

Originally Posted by 2fast0

Is it just me or is the car designed this way.

I dont feel any jump in HP after i hit 3500rpms. The car pulls just as hard at 8000 as it does at 4000rpms. If this is the case because of the flat torque curve, what's the point of downshifting when trying to gun it while going at, say, 80mph?

Once again I repeat the explanation to this topic:

Yes, the engine torque at 4000 is about the same as at 8000. But the relevant number is the torque at the wheels, i.e. engine torque multiplied by gear ratio. So if at a given speed you can downshift, then you should do so to get maximum acceleration.

[mkoesel]

Originally Posted by swiss_cornholio

Once again I repeat the explanation to this topic:

Yes, the engine torque at 4000 is about the same as at 8000. But the relevant number is the torque at the wheels, i.e. engine torque multiplied by gear ratio. So if at a given speed you can downshift, then you should do so to get maximum acceleration.

+1

Another reason why I love my M-DCT. You can downshift quickly to eke the last bit of power out of a lower gear and then upshift rught away once you run out of RPM. The two shifts cost you virtually nothing, so it is worthwhile.

[gr8000]

Originally Posted by 2fast0

The car pulls just as hard at 8000 as it does at 4000rpms. If this is the case because of the flat torque curve, ...

Yes it is the flat torque curve

Originally Posted by 2fast0

what's the point of downshifting when trying to gun it while going at, say, 80mph?

Because the higher you are in the rpm range, the higher the nominal engine power delivered. Engine power in the S65 increases linearly with engine rpm all the way up to the redline. In order to get the full 420hp at the crank (whatever that is to the wheels), you need to be as close to the redline as possible.


The childish, non-scientific way I understand Torque and Power in my mind (and I know some people will kick me with this):

Torque represents the "force" of the engine to turn the crank. The more the torque, the faster the engine climbes in the rpm range (all other things being equal). In the case of the S65 with the flat torque curve, the engine climbes the engine rpm equaly fast irrespective of where you are in the rpm range.

Power is the actual "strength" of the engine. In order to get the most power out of the S65 you need to be as close to the redline as possible because this is where this engine deliveres the peak horsepower.

[earlyapex]

Torque provides a measure of instantaneous acceleration. Horsepower gives you a way to measure how LONG you can sustain that acceleration with RPMs and gearing.

In any individual gear with a flat torque curve, there will be no difference in acceleration at 4000 vs 8000 rpms if you are making the same torque at each RPM. The car doesn't accelerate "harder" at higher RPMs unless it is making more gross torque.

If you have 300ft lbs at 4000 rpms and 300ft lbs at 8000 rpms and a 12x effective gear multiplication, you've got 3600ftlbs at the wheels whether you are at 3000 or 8000. The acceleration will feel exactly the same if you are in the same gear making the same torque.

So, what is horsepower and why does it matter? Horsepower takes into account RPM limits and gearing. If you had a car with a redline of 4000rpms and 300ftlbs, you would have to shift into the next gear and get a lower torque multiplication earlier. The M3 is fast because it can take advantage of the torque multiplication of lower gears longer (all the way to 8000rpms rather than having to shift at 4 or 6000).

in any individual gear, the car will accelerate fastest at the torque peak. However, if you had a perfectly efficient continuously variable transmission, the highest acceleration rate of the car would occur at the horsepower peak (you would keep varying the transmission ratio to keep the engine at it's hp peak).

horsepower peak gives you the mathematical point at which the torque curve falls at a faster rate than theoretical torque increase available via high rpms and lower gearing.

Said another way, you are better off with more torque multiplication in a lower gear at the hp peak (even though you are making less torque) than in a higher gear with less torque multiplication at the torque peak (depending on the transmission gear spacing of course). The gear multiplication more than makes up for the drop in engine torque between the two, so generally HP is better than torque because of transmissions.

Thought of another way, the AMG C63 makes a lot more torque than the M3, but the M3 can sustain it's lower level of torque much longer (8300 vs 6800) and the two roughly balance. The M3 is a little guy that can carry 100lbs up the stairs every minute, the AMG is a big guy that can carry 200lbs up the stairs every 2 minutes - same work same amount of time at the end of the day, different styles.

[Alias]

Originally Posted by earlyapex

Torque provides a measure of instantaneous acceleration. Horsepower gives you a way to measure how LONG you can sustain that acceleration with RPMs and gearing.

In any individual gear with a flat torque curve, there will be no difference in acceleration at 4000 vs 8000 rpms if you are making the same torque at each RPM. The car doesn't accelerate "harder" at higher RPMs unless it is making more gross torque.

If you have 300ft lbs at 4000 rpms and 300ft lbs at 8000 rpms and a 12x effective gear multiplication, you've got 3600ftlbs at the wheels whether you are at 3000 or 8000. The acceleration will feel exactly the same if you are in the same gear making the same torque.

So, what is horsepower and why does it matter? Horsepower takes into account RPM limits and gearing. If you had a car with a redline of 4000rpms and 300ftlbs, you would have to shift into the next gear and get a lower torque multiplication earlier. The M3 is fast because it can take advantage of the torque multiplication of lower gears longer (all the way to 8000rpms rather than having to shift at 4 or 6000).

in any individual gear, the car will accelerate fastest at the torque peak. However, if you had a perfectly efficient continuously variable transmission, the highest acceleration rate of the car would occur at the horsepower peak (you would keep varying the transmission ratio to keep the engine at it's hp peak).

horsepower peak gives you the mathematical point at which the torque curve falls at a faster rate than theoretical torque increase available via high rpms and lower gearing.

Said another way, you are better off with more torque multiplication in a lower gear at the hp peak (even though you are making less torque) than in a higher gear with less torque multiplication at the torque peak (depending on the transmission gear spacing of course). The gear multiplication more than makes up for the drop in engine torque between the two, so generally HP is better than torque because of transmissions.

Thought of another way, the AMG C63 makes a lot more torque than the M3, but the M3 can sustain it's lower level of torque much longer (8300 vs 6800) and the two roughly balance. The M3 is a little guy that can carry 100lbs up the stairs every minute, the AMG is a big guy that can carry 200lbs up the stairs every 2 minutes - same work same amount of time at the end of the day, different styles.

excellent post

[2fast0]

wow, what a great explanation. very enlightening. thank u!

but sorry, i missed the part where u explained the "12x effective gear multiplication" or torque multiplication.......... i dont really understand that, but that seems to be a crucial concept because,
given what u said, we can upshift at say, 7000rpm and still have the engine fall within our peak torque after the upshift. so what's the point of taking it to 8300.....besides the advantage of cutting down the number of times needed to shift to reach a given speed compared to a car with a lower redline.

[Wads]

Originally Posted by 2fast0

wow, what a great explanation. very enlightening. thank u!

but sorry, i missed the part where u explained the "12x effective gear multiplication" or torque multiplication.......... i dont really understand that, but that seems to be a crucial concept because,
given what u said, we can upshift at say, 7000rpm and still have the engine fall within our peak torque after the upshift. so what's the point of taking it to 8300.....besides the advantage of cutting down the number of times needed to shift to reach a given speed compared to a car with a lower redline.

Don't worry too much about the physics..... the main point is that you will always accelerate faster in a lower gear at the same point in the rev range..... so if you can shift down you should.

eg. 4000rpm in 2nd gear will accelerate faster than 4000rpm in 3rd in the same way that 8000rpm in 3rd will be faster than 8000rpm in 4th

[sparkyg]

Good posts guys.

[jm1234]

Originally Posted by 2fast0

wow, what a great explanation. very enlightening. thank u!

but sorry, i missed the part where u explained the "12x effective gear multiplication" or torque multiplication.......... i dont really understand that, but that seems to be a crucial concept because,
given what u said, we can upshift at say, 7000rpm and still have the engine fall within our peak torque after the upshift. so what's the point of taking it to 8300.....besides the advantage of cutting down the number of times needed to shift to reach a given speed compared to a car with a lower redline.

Engine Torque does not equal wheel torque. The engine might be at peak torque at both 7000rpm and at 8000rpm but if you stay in first gear between these two points instead of shifting to second gear you'll maintain significantly higher rear wheel torque. This pictures shows the relative rear wheel torques for any given car speed.

[2fast0]

ooooo ok!
thanks for the wonderful explanations guys!

[jm1234]

Originally Posted by kbk_75

The car definitely pulls much harder at 8,000 rpm than it does at 4,000.

Pulls harder in the same gear of after you downshift? According to BMW the engine is producing slightly less torque at 8,000 rpms than it is at 4000 or even 3000 rpms.

[mkoesel]

Originally Posted by jm1234

Pulls harder in the same gear of after you downshift? According to BMW the engine is producing slightly less torque at 8,000 rpms than it is at 4000 or even 3000 rpms.

Less torque, yes, but much more power.

[earlyapex]

cool graph - good way to see "area under the curve."

The math isn't that difficult really, say you have a car that has a 4:1 first gear, a 3:1 final drive and a 24 inch tire (1 foot from center of wheel to pavement). Now assume you have two engines with flat, 300ftlb torque curves but one revs to 6000 and one revs to 8000RPMs. At 6k, they each make 4*3*1*300 = 3600lbs of torque at the pavement, so the acceleration would be exactly the same.

But, the 6k engine has to shift to 2nd gear at 6k. Say second is 3:1, so the 6k car drops to 4500rpms and is making only (3*3*1*300 = 2700ftlbs) while the 8k engine continues to put 3600ftlbs to the ground for 2000 more rpms.

About the time you have to shift to second in the 8k, the 6k will have to shift to third. The 8k engine will always be making more torque at the wheels due to its gearing advantage which is a result of it's RPM advantage.

Torque at high RPMs is better than torque at lower RPMs because of gearing.

[earlyapex]

Originally Posted by mkoesel

Less torque, yes, but much more power.

There's no such thing as "power" without considering gearing. Power can be simplified as "torque after taking into account gearing."

The M3 in fact accelerates harder in individual gears at 3k and 4k than 8k. It feels faster at 8k because it's a lot louder.

If you are talking about 1 single gear (3rd for example), torque is the only relavant measure and the M3 makes a little more torque at 4k than 8k, so it accelerates harder in 3rd at 4k (albeit more quietly).

[mkoesel]

Originally Posted by earlyapex

There's no such thing as "power" without considering gearing. Power can be simplified as "torque after taking into account gearing."

Hmm, I'm sure you meant to clarify things, but that doesn't really do so in my opinion. Power is defined as torque x RPM. Gearing is definitely a factor in the acceleration as it multiplies torque. But IMHO your simplified definition of power is off.

The M3 in fact accelerates harder in individual gears at 3k and 4k than 8k. It feels faster at 8k because it's a lot louder.

If you are talking about 1 single gear (3rd for example), torque is the only relavant measure and the M3 makes a little more torque at 4k than 8k, so it accelerates harder in 3rd at 4k (albeit more quietly).

I don't dispute it necessarily, but I feel like there is more to explanation, even if true.

[04S]

Originally Posted by Alias

excellent post

+1