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Old 31st December 2001, 10:57   #101
Jim Wald
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Re: OK.

Quote:
Originally posted by HBRAMSTEDT
I think Wald and KnightRider are on drugs.

Those numbers aren't that far off??
You need not get nasty, especially when I NEVER defended AMS's numbers, I just accepted them until they are proven wrong, and I further only postulated a few possible reasons for the SL 55's good showing.

I don't have to defend those results, AMS does. Having driven all five of those cars to 100, 200, & 300 KPH, as well as to top speed, I figure THEY are in a better position to answer that than EITHER you or I. No, the numbers don't BOTHER ME (not as they seem to bother you), but they will be proven right or wrong over time. Until that time, they are the best information we have, RIGHT? AND AS I DID SAY:
"Until proven otherwise, I am going to assume that AMS's numbers are correct. But, if AMS or other magazines prove these numbers to be wrong, it won't matter to me. If it were the SLOWEST of this group, instead of the fastest, it would still be in top class company, and it would still be the only one with an automatic (for daily driving) with a glass roof (with the Panorama option) that can turn into a convertible in 16 seconds. THIS is going to be an AMAZING car.".

So rather that I being on drugs, might I suggest that you TAKE SOME DRUGS to calm down.
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Old 31st December 2001, 11:08   #102
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OK.

It is just that those numbers are so bad that even a simple analysis says something is wrong. I was frustrated initially because nobody noticed it, and I thought you were defending it.


I just get a bit upset when I hear stuff which is way off and
no one puts up the B.S. flag. In this case it seems a lot of people now have problems with this data.

My favorite complaints like this are things in the past such as
the Holden HSV phenomenon, the mysterious Pontiac WS-6s that do 12.6 in the 1/4-mile, and SL55 AMGs that rocket from 200 to 300 km./hr. in a couple of seconds.

Fair enough. We'll wait for another test.
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Old 31st December 2001, 11:22   #103
Jim Wald
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Re: OK.

Quote:
Originally posted by HBRAMSTEDT
Fair enough. We'll wait for another test.
Agreed. Like I said, I don't care if it is the SLOWEST of THOSE Five, given the exalted company they are. It is going to be a GREAT car even if it is the slowest. If it is at or near the top, that is even cooler. But I can't imagine AMS would risk it's reputation with a test that was completely flawed, do you? If the test was as AMS's numbers show, I bet even some of those AMS staffers were just as incredulous as you were, AND THEY WATCHED THE TEST. There may be something that aided the SL 55 in it's results, such as temperature, driver, engine break-in, who knows. I don't, I wasn't there. Heck, I have not even seen the article.
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Old 31st December 2001, 12:06   #104
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HBRAMSTEDT

SL isn't on drugs, but SL55 AMG probably is on some kinda doping
I understand your frustration and taking into account how opinions splitted up, we should take a deep breath, step out of the circle and wait for another test, preferably by some other car mag than AMS I guess. I know I did that.
Happy New Year to all of you, both to tuners and manufacturers, both to the gullible and doubting folk
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Old 31st December 2001, 12:21   #105
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I cant see what is so strange about those numbers. The 0-100 numbers conform to what was to be expected from other tests. Apart from the Vanquish(where Ford seems to have failed seriously in getting it to accelerate) the SL 55 is the slowest car. This is probably due to it's Automatic gearbox and it's high weight. Aerodynamics should be a very small factor in 0-100 performance.

From 0-200 the difference between the 911 GT2 and the SL is still 0,6 seconds . The GT 2 power/weight advantage that made it much faster to 100 has started to evaporate due to it's much worse drag coefficient. Both cars have approximatly the same height and width so the frontal area should be almost the same. The GT2 is at this point a little bit slower than earlier test suggest it should be but it's still faster than the SL. The SLs automatic gearbox has had to shift gears three times in the 13,7 seconds so if it has slower gearchanges than a good driver with a manual gearbox. How much could the diffrence be? 0,2 seconds/ gearchange? In that case 0,6 of the 13,7 seconds could have been lost.

The 0-100 and 0-200 times of the SL shouldn't be controversial. Are they?

Where the SL 55 seems to have a large comparative advantage seems to be between 200 and 300. I am not technically educated so please correct me if i say something stupid. At these speeds the weight of the car becomes much less important than aerodynamics so the weight of the SL shouldn't be such a big factor anymore? correct? And the large aerodynamic advantage it has should start to play a bigger role. The drag coefficient of the GT2 is 18% higher than the SLs(and it has approx. the same frontal area) and the Murcielagos(that has a 0,33 when standing still but 0,36 when pushed and the rear nostrils and rear wing are extended) is 24% higher(the frontal area of the 23cm wider and 16cm lower Murcielago should be the same or marginally lower than that of the SL). The automatic gearbox of the SL only needs to change gears once or twice between 200 and 300 so the loss of time using the numbers above should be 0,2 to 0,4 second out of the 18,8 seconds. That the more power full SL and much more aerodynamic SL should be much faster than the GT2 between 200 and 300 doesn't seem strange does it? The large aerodynamic advantage should mean that it's comparative advantage should be at higher speeds and according to the test data it is so. If it had a very large power advantage over the GT2 shouldn't it have pulled away before 200 and not sometimes after that speed?

Sl 55 200-300 in 18,8 seconds
Murcielago 200-300 in 22,8 seconds

These are the only numbers that makes me think "Can this really be correct? Is the SL really faster than the Lamborghini?" Is it possible that the Lamborghini(4th,5th and 6th seems very close) driver needs to shift gears three times between 200 and 300 and that the Mercedes gearbox only once and that this could explain part of the diffrence? Could this in combination with a 24% higher wind resistance and only a 22% hp advantage and lower torque be the explanation?

Regards/Robert
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Old 31st December 2001, 13:59   #106
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Luxobarge,
Thanks for explaining it logically bit by bit. After some quick rough calculations I discovered that SL experiences on itself around 17% less drag force than GT2 and 24% less drag force than Murcielago. Don't you think it is quite a difference???
I think that weight doesn't play such a big role at speeds that high and given SL's huge torque and slightly higher power than GT2 I can see why it is faster at high speeds. Automatic gearbox again doesn't hinder it at those speeds for the reasons Luxobarge mentioned.
OK, I agree that Murcielago has a significant power advantage over SL, but it experiences 24% percent more drag force too!
What do you think Johan?
This is an equation I used Fd = Cd x (1/2 rho x V^2 x A), Fd being the drag force, Cd the drag coefficient, rho viscosity/density, V the speed of the car relative to the speed of the air and A the projected area.
V and rho is the same for the three cars, what is different is A and Cd. We do know exact Cds, however not the Areas. Simply multiplying width by height, we'd ignore an empty clearance space and the fact that the cars sides are not perpendicular to its' roof, i.e. it's not square. Also, different tyre makes and sizes. Would we agree that taking into account SL's narrower tyres and larger projected area and Murcielago's and GT2's wider tyres, but smaller projected areas, these values would even out (tyres vs. projected areas) and be more or less equal for these cars?
If so, then it is only necessary to divide Cds to find out drag forces. I would be glad if someone corrects me if I am wrong in my calculations.
Honestly, the test figures seem quite realistic to me now.
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Old 31st December 2001, 14:21   #107
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so that SL55 is quite a cooker! The question I want to have answered- and I will have answered at some point- is how do these cars drive during "normal" driving? For example, here in the USA, I wouldnt be driving 300km/h So what do they feel like, what are they like to live with. I suspect the SL55 is probably a big winner- but its just a guess. I wonder what the Z8 would do w/o a speed limiter.... btw- I thought all the German manufacturers had a handshake agreement to limit the cars to 155mph......
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Old 31st December 2001, 15:03   #108
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The problems I see...

I'm assuming the current V-12 Vanquish was the one referred to in this post. If so, its 0-100 km. time is 1.2 seconds greater than other tests of the car, including the test in the November issue of Car and Driver magazine. A variance of 1.2 seconds to 60 m.p.h. is a bit too much.

In the portion to 200 km./hr. , aerodynamics still don't impose a real significant effect on the times. In this case the SL only has only a 6/10s deficit to the GT2, despite having roughly the same horsepower, 1000 pounds more weight, and an automatic transmission. I don't believe gear change times are a major factor in this analysis, but the aforementioned factors are. Therefore, I do think the 0-200 time is doubtful to say the least.
Think of this in terms of a drag race a bit longer than a 1/4-mile.


It appears that we agree that the 0-300 or 200-300 interval is questionable. One point is that surface area is not really well estimated by using only height and width dimensions, and your estimations of total drag hinge on this. Though I believe your relative estimations of total drag are correct, you can't therefore say that one specifically has 24% or 18% more total drag. What I believe is more important and missing in your rough estimate is the consideration of gear ratios. Gear ratios can compensate for aerodynamic inefficiencies, but we don't have that data. I believe gear change events are not really significant over the long test interval to 300.

Besides your valid point about questioning the 200-300 interval for the SL and the Murcielago, it is hardly believable that the SL took 1/2 the time to reach 300 than the Ferrari, a car with less weight, a bit more horsepower, and a manual transmission. You are neglecting to account for the large losses an automatic trans. imposes on available power.

The next test will come soon, I'm sure.




P.S. - I appreciate your understanding KnightRider and the clarification, Jim Wald
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Old 31st December 2001, 15:18   #109
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Robert,

The number I found most suprising is the 0-200 (or more correct 100-200). It is as fast as the 996 GT2. Yes the Cw is higher on the GT2 but the frontal area is bigger on the SL55 AMG (if we assume that it's the same or bigger than SL400). KnightRider assumed that the frontala area was the same (which it likely isn't).

Using KnightRiders formula, Fd = Cd x (1/2 rho x V^2 x A)

We see that Fd=CdA(1/2rho*V^2).
Don't really know the difference between Cd and Cw but I'll continue to use Cw since they are known from tests.

So we may find that the difference in air resistance isn't that big, in fact Cw*A only is 5% worse on the GT2 (0.58 vs 0.61).
Also in speeds up 200 it is still weight/power ration that is most importan and second power vs air resistance. The SL55 AMG's weight/power ratio is 33% worse than the GT2's!

Calculating with 1/4 mile ET and speed also indicates that something are off by far and those calculators are fairly correct.
They also assume perfect traction and perfect gearing for 1/4 mile races somthing a street car never has. In other words the should be even worse in reality.

It has also been a lot of discussion about torque and when doing acceleration tests like this you are likely to stay above 5000 rpms or at least close in all gears accept 1st. This means that you in most cars will have passed your top torque and on top. On top of that it is the area under the power curve that is interesting. Saying that engine torque is more important than power when accelrating is just a simplification since IT IS power that accelerates your car.

So let's get back to 200-300. It may seem strange that the SL555 AMG could be faster than the Murceilago. On the other hand this is where low air resistance is really important. The Murcielago is likely to have a clearly worse Cw*A value. It may be so much worse that the power advantage is not enough to make it faster than the SL55 AMG.

KnightRider, I think you're pretty much on track accept that the GT2 has clearly lower frontal area (A) than the SL55 AMG, again at least if the SL 55 AMG has same or larger front area than the SL500.

Cheers,
/Johan
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Old 31st December 2001, 15:45   #110
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Another issue...

I believe from what I have read that the Cd of the SL500 is .29.
This is not necessarily true of the AMG variant with wider tires and different trim details. In the past, AMG variations have had higher coefficients of drag, and it is true in this case, I believe.

If it is true, that would make it even harder to explain the stunning performance in this test.

Is it true?
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