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      11-27-2007, 03:36 PM   #1
gadget
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Questions on the M3 based XKR impressions

XKR vs M3

First some background.

Iím 27 and finally able to afford and M3, canít wait until it comes to the sates .

My last 3 cars:
1999-2003 Lexus IS300 3.0L I6 215hp
2003-2006 Infinti M45 4.5L V8 340hp, 330 lb/ft torque
2006- Present Z4 3.0 SI 255hp, 220lb-ft torque

Although I enjoy the smoothness of the in-line 6 engines, I miss the V8 in my M45 and the way the torque pushed you into the back of the seat. Even though the Z4 and M45 are comparable on a 0-60 times, the V8 felt like it pulled harder (lets not forget the car M45 was ~900lbs heavier) Ė Canít wait to get another V8; the M3 is my most likely choice.

Recently a friend lent me his Jaguar XKR Convertible for the weekend (420 bhp @ 6,000 rpm 413 lb.-ft. @ 4,000 rpm 3,924 lb) ( I tried unsuccessfully to persuade him to buy the M6 ) I would not spend $100K on this car. From a performance POV, the M3 is better. He made his choice on looks and comfort.

The XKR, for my standards is a very fast, but heavy car. My two main complaints are the carís poor performance in fast turns and the cars inability to put the power to the road during the 1st second of acceleration (tried with both traction control on and off) On the flip-side, once the car gripped it pulled very hard and very fast. The whine of the supercharger with the top down was awesome. I prefer NA engines but canít help but wonder what the E92 M3 could do with a comparable supercharger- will that be an aftermarket option?

So here is my question to current M3 owners. Although I have no doubts that the M3 is fast, how does the lack of torque affect the way the car accelerates? Do you get the same g-force push against the seat that youíd expect with larger displacement engines with more torque? Will the car feel as powerful as the XKR which boasts over 115 more Lb-ft torque?


Iíve also driven the 335i on several occasions (as a loaner) and was not all that impressed with the way the acceleration felt.
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      11-27-2007, 03:42 PM   #2
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I test drove a new XKR Coupe before ordering the M3 and whilst I thought it was a great car I didnt want Auto and felt ti was too big and heavy for what I wanted. Im 37 and not yet ready to hang up my hooligan nature. Possibly after the M3 the XKR will make more sense.
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      11-27-2007, 03:46 PM   #3
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Quote:
Originally Posted by Leg View Post
I test drove a new XKR Coupe before ordering the M3 and whilst I thought it was a great car I didn't want Auto and felt ti was too big and heavy for what I wanted. I'm 37 and not yet ready to hang up my hooligan nature. Possibly after the M3 the XKR will make more sense.
Thanks for your perspective. My friend who bought the XKR is 47 so it made more sense for him. I am with you- . Could you expand more on how the acceleration felt in each car.

Thanks
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      11-28-2007, 05:05 AM   #4
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Quote:
Originally Posted by gadget View Post
So here is my question to current M3 owners. Although I have no doubts that the M3 is fast, how does the lack of torque affect the way the car accelerates? Do you get the same g-force push against the seat that you’d expect with larger displacement engines with more torque? Will the car feel as powerful as the XKR which boasts over 115 more Lb-ft torque?
I am not a E92 M3 owner but have some good feedback for you on your question. It certainly does not take having driven car X to fully understand and rank the kick you will feel in car X! Yours is a good question and with the info. below you will be able to answer for yourself which car will feel more powerful, not just between these two, but for almost any two cars.

What you feel is acceleration (obviously), more acceleration equals more pressure between you and your seat - more kick. Acceleration comes from engine torque and gear multiplication (transmission ratio and final drive ratio) and is then hindered by a cars weight. The gear multiplication effect can be large and is key; this is what most "car guys" completely miss though. Roughly:

a(rpm) = ( Te(rpm) x (1-Tl) x G x F) / (W x r)

(Note: Technically the relationship is actually "proportional to" not equals since there are other conversion constants involved, but this as it is written covers comparing two relative values.)

Acceleration at a given rpm = engine torque at that rpm x (1-drive train torque loss) x transmission gear ratio x final drive ratio all divided by
(vehicle weight x drive tire radius)

As you can see it really has nothing to do with an engines displacement, number of cylinders or torque output alone.

Acceleration is the largest right around launch (at take off) and decreases as speed increases. If you don't have all of the figures above then you can use a couple of approximations. The best one is simply empirical; the 0-60 time, which is pretty much a direct measurement of acceleration when it is the highest. This is however, peak acceleration. You also certainly can feel a lower acceleration kick in any any gear by just flooring it. The best empirical measure of how strongly you will be put back in your seat at these higher starting speeds is a rolling start acceleration time, say the 50-80 mph time (with the big caution that one car may have its best roll on time in one gear and another car will have its best in a different gear, if a magazine only reports both times in the same gear results will look bad for one of the cars whereas in reality it will actually pull harder!).

Last if you don't have these measured performance times the next best approximation using only basic figures is simply horsepower to weight ratio. Just compare this for the two cars, and if their gear ratios are chosen any where close to optimal for the best acceleration, and their wheels are fairly close in total outside diameter, the actual gear ratios and wheel/tire sizes won't be a huge factor. Some would argue that torque to weight is a better measure (than hp/W) and the formula above looks like that is the case. The argument in favor of hp is that higher hp motors typically have higher redlines, take advantage of staying in a gear longer and typically have higher gear ratios all of which help acceleration tremendously.

Last edited by swamp2; 11-28-2007 at 06:04 AM.
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      11-28-2007, 05:51 AM   #5
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Just a few things to note. The M3 has 295 ft. lb of torque. Thats only 18 less than the XKR. When you consider that the M3 has a redline of 8,400 rpm and the fact that 80% of the torque is available from 2,500 RPM all the way to redline then any concerns about the torque being what it is should be squashed. *edit* So, ignore the first half of this paragraph.

Another thing is that the XKR is 300+ lbs heavier than the M3 assuming both of the weights were taken in similar conditions. And the M3 is not a drag car, its a twistie car. So the rolling acceleration times will be great for exiting corners.

Lastly, I was never too impressed by the XKR. I could never get over it being such a Aston Martin clone. For $100k you might as well just get a V8 Vantage. Not to mention I think the shift knob is way too posh in that car for the rest of the design cues. Maybe I was just never a big fan of Jaguar's 'style'.

I will say though, since you live in the US the XKR will be a much rarer occurrence to see on the road. So if you value scarcity, then the Jaguar wins by far there, but then again so would a AM V8 Vantage.

Last edited by UltimateBMW; 11-28-2007 at 06:15 AM.
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      11-28-2007, 06:10 AM   #6
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Quote:
Originally Posted by UltimateBMW View Post
Just a few things to note. The M3 has 295 ft. lb of torque. Thats only 18 less than the XKR.
...
Another thing is that the XKR is 300+ lbs heavier than the M3..
The XKR is supercharged and a torque monster at 413 ft lb, not 295+18=313

Furthermore an "apples to apples" comparison the XKR Couple weighs in at 3671 lb, only 24 lbs heavier than the M3.
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      11-28-2007, 06:15 AM   #7
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Ah, your right. I fail at math for tonight. Guess it's time for bed.
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      11-28-2007, 12:20 PM   #8
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Ah, your right. I fail at math for tonight. Guess it's time for bed.
Haha ... FAILED!
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      11-28-2007, 07:33 PM   #9
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Quote:
Originally Posted by swamp2 View Post
I am not a E92 M3 owner but have some good feedback for you on your question.
...
After some follow up work on this question, the XKR vs. M3 "kick" issue becomes more interesting. Both CarTest simulation (quite well validated if you cruise around this board) and the forumula I gave above BOTH agree and both show the XKR having more peak acceleration in most gears than the M3. Yet the M3 is faster in most contests. Why?

Well the XKR is indeed a high torque machine, much higher than the M3. The cars are very similar in transmission gearing, weight, and overall wheel size. The M3 gains about a 15% advantage from gear multiplication (almost enitirely due to the 3.8 vs. 3.3 final drive (rounding off a bit...). It is really like the M3 has more like 1.15*295 = 340 ft lb in an apples to apples comparison with the XKR, again just due to final drive. This only makes up for less than half of the torque advantage had by the XKR. What makes the M3 faster is its 6MT and redline. Even though the XKR produces more accleration "kick" the M3 can get to higher accelerations earlier at launch with the MT and from it's high redline it can keep reving and accelerating while the XKR is shifting. The XKR has a fairly slow transmission shift time as well, about 600 ms. So during these initial and final acceleration periods the M3s acceleration is less than the XKR's peak but more time accelerating is more time getting ahead.

FWIW in first gear the peak acceleration of the XKR is about 10% greater than the M3, but again that level is not maintained for as long as the M3 maintains its lower acceleration.

This case is interesting because the 0-60 times and all other times lead you to believe that the M3 will have a higher peak acceleration but it absolutely is not the case, it just has a higher average acceleration. In the end that is more important and the M3 will pull away from the Jag at most speeds. The Jag will win some roll ons however.

Since the cars are fairly close up to about 60 mph (4.4 or so vs. 4.8 or so) in most circumstances such a race will end up being a drivers race.

If anyone is really keen I can post the simulation results showing the actual acceleration curves and summary comparison between the two cars.

IMHO a very interesting comparison.
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