Speaking of torque (i.e. no wonder the E90/92 M3 is so fast...)
 

Inspired by dred's thread on torque, HP and work, a buddy and I got into a discussion about how people frequently misunderstand torque and gearing. For some reason, we found ourselves looking up dyno charts for various cars and came across one for the E90/E92 M3, which looked basically flat at 260ft-lbs from 3,000 RPM to 8,100 RPM. Given how fast the car is, I figured it had to be geared pretty short. And it is. But what's shocking is how much torque the car puts to the wheels, particularly in first gear.

Below are plots of wheel torque for the E90/92 M3 (peak torque 260-ish-something) and C5 Z06 (peak torque 400ish) versus MPH for the first four gears, assuming shifts at redline. The M3 puts almost 50% more torque to the wheels than the Z06 at low speeds... and the cars are pretty evenly matched after that.

The other plot is torque to the wheels versus MPH for the gears over the entire usable RPM range.

Of course this all discounts weight. But if you convert torque to thrust and divide that by the weight of the car, the M3 still wins in first gear (even with a 600lb weight disadvantage). That's the third plot.

The long and short of it is that most people would probably not describe the M3's engine as torquey (at least when compared to a big 2-valve pushrod V8). But engine torque is pretty irrelevant. Torque to the wheels is really what matters, and in this case the M3 is very impressive.

Maybe this isn't interesting to you guys, but I was bored waiting for a teleconference to start late last night...

So you are basically saying that somehow the corvette loses the advantage of 140 ft-lbs, or more than half the M3's total torque, through drive train loss? What am I missing? Is it all from the gear ratios? If it is, what's the benefit of giving up that torque advantage? I suppose gas mileage would improve.

From a practical standpoint, does the M3 outrun the corvette b/c of it's torque advantage? I haven't paid attention to acceleration numbers b/c I have started to just think of cars as "fast" "not so fast" and "slow." I have come to realize that, unlike some of you, shaving a half a second off a one minute autocross course doesn't mean anything to me and there are a lot more important factors that go into car choice than 0 to 60 or quarter mile times.

Good stuff.

It's all gearing. I'm assuming that JV ignored drivetrain losses (which should be about the same in two front engined, RWD cars anyway).

The M3 has shorter gearing and more revs to work with.

The difference is reduced by two things:

1) Different max rpm. M3 is 8100 vs C5's 7000. It allows you to reduce the gearing to to increase torque at the wheels.

2) Torque curve. Max torque is only relevant while you're at that RPM.

John, I assume the Z06's torque curve is less flat than the M3's?

The other thing to consider is that it's one thing to look at how a car is set up under an optimal gear selection for a given speed (which counts for optimal performace) and the range of performance being offered in a given gear. Basically, to borrow from that article, what if both cars had a 3-speed tranny? The corvette would be better.

That is a similar situation to feeling lazy, not wanting to hold revs up due to sound level comfort, or a heavy family car with a so-so slushy. In all those situations, you benefit from broad torque down low.

If I keep the 330i near 3krpm, it is a blast and pretty darn fast.

OT: I've started to dislike revving past 6krpm becasue the revs take to damn long to drop and shifting is not as fun. 3-6krpm is my fun zone - 3.5-5.5krpm is even better.

As always, everything is a compromise - unless you don't have to.

Fixed.

But yes.

I happened to dig up and save this yesterday...

http://forums.carmudgeons.com/attach...1&d=1239219288

FWIW, this didn't stem from an autocross-related discussion. And the point wasn't that the M3 outruns the Z06 or vice-versa (I don't even know which one tests faster in 1/4 mile acceleration runs, but they are probably pretty close). The original discussion was one of area under the curve when it comes to torque... which I still haven't had time to calculate. But I digress.

I neglected drivetrain loss for the reason that Nick mentioned - it's probably the same for both cars and certainly for comparison's sake it's in the noise here.

I just find it interesting that people (Corvette people in particular) like to pick the peak torque value of an engine and declare that it's "torqueless" without taking into account gearing. The M3's torque curve is very flat, and it spans a very wide RPM band. The Corvette's engine is more peaky in terms of torque delivery and it doesn't rev as high. See below.

http://www.britishamericanengines.co...s/ls6_dyno.jpg

The LS6 is over 300ft-lbs from 2000 to 7000 RPM (redline is 6500, but it revs to 6800 before the rev limiter kicks in). Sure, the peak is 400, but the engine overall is limited by its two-valve architecture. Even if a stock LS6 could rev to 8400 RPM without grenading, you probably wouldn't want to - the engine can't breathe up there.

I wasn't trying to draw any particular conclusion other than the fact that the cars are very similar in terms of measured performance but they go about it in very different ways. WHEEL torque is what matters, and WHEEL torque is engine torque times the reduction factor of the transmission. For the M3 it's around 15:1 in first gear and for the Z06 it's around 10:1. Big difference!

And fuel economy is very closely tied to engine RPM (and weight, and to a much smaller degree throttle position) so yes, the BMW gets worse mileage.

Aboslutely.

In machine design in general it's all about the final delivered torque. My biggest machine, for example, has an electric motor that puts out a max of just 21Nm. But in my case, I had functional requirements that led me to know I needed about 1600Nm "at the wheeels" (with some room to spare given only approximations of drivetrain losses). At that point, it's a matter of finding the most (cost-)efficient way to supply that torque. Given the available gearing I could package/supply (and its trade off wrt the speed requirements) the 21Nm motor was the best option I had. And that guy gives me the 1600Nm I need.

In my case, I have no transmission. I have one gear ratio. I rely on the characteristics of the motor to allow me to work through the speed range.

As an example, decades ago, electric motor performance was poor and required us to actually have 2 speeds. Nowadays with brushless motors you can get away without that. Again, as with cars, since I can't change gears at ALL, I need the torquiest motor I can find that can deliver the most torque as far up in the rev range as possible. And I do run out of torque anyway (that's just physics). Fortunately, the performance envelope of my machine is such that I don't need much torque as speed goes up.

EDIT: This also explains why the little extra hp/tq that the ZHP gets over regular 330i's go a long way. It's not just an extra 8lb-ft, it's 8lb-ft AND the shorter final gear, AND an extra 300rpm to use it all.

I wasn't trying to challenge or criticize. I was asking b/c i didn't understand.

The autocross comment was a defense to the realization that I had no idea how these cars perform from a measurable metric. I know both are fast. I know I would pick the M3 as a daily driver out of the two b/c I have kids.

But according to the graphs you have put up, the M3 should be faster than the Corvette at least for a time off the line. That's surprising to me and I would be interesrted to know if it is true in the real world.

I also don't understand why the 'vette would leave so much potential untapped, if that is what its doing. Is this the choice of the engineers for "fast enough?" Perhaps the compromise point they chose? Or are there other considerations that require or at least make the torque drop desireable? I suppose fuel economy might be enough by itself.

It may be splitting hairs and going in a different direction than you're talking about, but fuel economy is also tightly tied to throttle position. If you are traveling at a specific RPM with a constant throttle position, your fuel efficiency will be much better than if you rapidly alternated between wide open and closed throttle positions. If you jump on and off the throttle, obviously, the RPM won't be static, but if you do this over a set distance and turn the same number of revolutions and have the same average throttle position over that distance as the constant, you'll use a lot more fuel (how much depends on lots of variables).

How big or small the difference is relative the addition or subtraction of 500 pounds of load or 500 RPM... :dunno:

More torque at the wheels, though, does not necessarily equal faster off the line. In addition to the weight issue (which is beyond my ability to quantify) there are other variables like traction--the Corvette has much wider rear wheels than the M3 does, and thus may be able to better take advantage of the wheel torque it does have than the M3 is.

JV, are your calculations torque at the contact patch, or torque at the hub? In other words, do you include the effect of tire diameter in your overall gearing calculations?

Also, WRT fuel economy, another huge variable (in fact, probably the biggest variable) is wind resistance. Since the Corvette has a smaller frontal area than the M3 and probably a better coefficient of drag besides, it simply takes less work to move the Corvette through the wind at a given speed.

Torque at the hub. Once you take into account the diameter of the tire, you're talking thrust (i.e. force).

Fuel economy is tied to throttle position but not nearly so much as it is tied to RPM for gasoline engines. Anyone who has ever delved into the programming for a gas FI system knows this, because injector firing rate is ONLY determined RPM and duty cycle is tied to airflow. The trim functions here are throttle position, intake air temperature, coolant temperature, and some other minor contributors. The other factor here is pumping loss - which is lowest when the throttle is wide open. The most efficient way that we know of to operate a gas engine is to have no throttle and manage the speed with valve lift and duration (aka Valvetronic for BMW)... but it's not necessarily practical for all applications (read: it's expensive to implement and not easy to build).

In diesel engines, fuel economy is tied very closely to "throttle" position because there is no throttle - you're directly controlling the pulsewidth of the injectors.

The bottom line is the BMW is damned fast - especially for how heavy it is - but once first gear is done, the BMW is going to get smoked by the vette.

i guess that's the beauty of it, though. that a 3er is as fast for the first whatever period of time.

that's always the appeal of the M series. that a gran turismo is THAT fast.

one obviously pays the price monetarily, carbon emissions-wise, as well as gearing-wise, for that 1st gear performance. for most people of a certain means, it's certainly worth it.

my own personal journey with a leaden sled that has a rocket booster attached to it is that the curb weight really wears on you after awhile. the whole back end of the thrust proposition is that whole braking thing. no matter how good the brakes work, weight is something that is so visceral and it does literally weigh on you over time to constantly fight that static weight from a standstill as well as coping with that sheer mass on decel.

engine to engine, the BMW is almost peerless for a DOHCer in street guise.

The M3 is not THAT bad... 3600lbs is the new 3000. A 997TT is as much. But yes, it is a gt, it is never going to feel like an s2000 or a cayman.

True. And as I've mentioned too many times already, the E93 vs. E90 M3 test drives proved how much of a difference 3700 vs 4200lbs is dynamically.

http://forums.carmudgeons.com/attach...1&d=1239244317

I love this thread. I really have nothing to add. It's a rare occurrence that I'm unable to out-nerd somebody. The fact that there are several posters in this thread alone that out-nerd me is either awesome or totally sad.

:D

The M3 has been a GT for 15 years-- like every other BMW with an M badge, other than the E30 M3.

I'm still thinking about an M3 or 997S. They are both GT's, and right now, after 2.5 years of the more hard core cayman (not that it is faster, but it is light, loud etc.) I'm kinda looking for a gt.

:yes:

age sets in.

So let me see if I'm summarizing this properly:

I should buy a 60K M3 so I can race around in first gear. The rest of the time, despite the engine's awesome whizzing noises, the car is still slower than the 7-year old Z06.

Sweet.




EDIT: Isn't 1st gear where you care the least because you're already near the limits of the tires? I want more wheel torque in 2nd and 3rd, and I don't love having to shift out of 1st gear too soon.

The M3 and Z06 are also both slower than a fifteen year old Maclaren. So what?

I didn't think John was trying to say it was better than a vette. I thought he picked a car that is acknowledged as a beast to compare it to. It also lent weight to the discussion about how much of a difference gearing makes. :dunno:

Right on. I found it interesting that a car most people describe as "torquey" (the Z06) puts less torque to the wheels, on average, than a car people tend to describe as "not torquey" (the M3).

It's a good point, it's all about perception - there's years of legacy behind a 'vette, and a lot of it has to do with its "brutishness" - people would never think of a BMW, any BMW quite this way.

Ed

:+1

Good thread though.

Agree, great thread. :cool:

John, do you have the graphs and calculations set up in Excel (or something similar)? It might be interesting to let people plug in values for other cars. Not that I would want you to do the work, but if we could leverage the work you did from another webpage here at 'mudgeons? Just a thought.

Also because BMW gave the E90 M3 really soggy throttle response as a default.

I did a quick calculation of the area under the torque curve for each engine. From 1,000 RPM to 6800 RPM for the Z06 and from 1000 RPM to 8400 RPM for the M3. The total area is shockingly close. The Z06 has 0.7% more total area under the curve than the M3.

Shouldn't this thread be called "Talkin' Torque"?

Sure. See attached. it isn't pretty, and isn't necessarily intuitive either... but anyone with a basic knowledge of spreadsheets can figure it out.

I expected as much due to the higher redline and flat torque cruve.

Cool, I'll see what I can put together. A web page, or mini-site perhaps. Thanks!

I would love to see an E46 330i ZHP vs non-ZHP comparo, given all the comments regarding how little performance value the pkg offered.

You can start with this

It was when I was comparing the impact of changing the E46 rear diff

You could toss in a modified torque curve pretty easily.

Whoops... looks like the spreadsheet link is down... Oh well, not sure I have it any more.

Check the reviews of the first e36 M3s--even the "real" EU versions were widely acknowledged to be GTs, rather than the rough edged homologation specials that the first ones were.

FWIW, I think the weight gain from e30 to e36 was at least as much as the gain from e36 to e46 or from e46 to e90.

The "real" E36 M3 was never sold here, so it may as well have never existed.

The Real-Real E36 M3 that was sold here had a list of standard "comfort features" that included:

[quote 1995 M3 Window Sticker]
-Air Conditioning with separate left and right temperature controls, recirculating function and anti-dust, bacteria, pollen and smoke microfilter

-BMW audio system: AM/FM stereo tuner with auto-reverse cassette deck; 250 watt amplifier with 10 high-performance speakers

-Electric central locking system with double lock

-Electric front windows with one-touch operation

-Split Fold down rear seats

-LCD digital quartz clock with outside temperature display[/quote]

Sounds GT to me.

250 watts of muddled mid-range.

Sounds GT to me.[/quote]

:thumbup:

on the one hand, that's what makes the M3 so appealing.

my lesson from M in the past is that i'd rather be more low key and frugal on the daily drive and find something a little more focused for the fun days.