mrkbbd

CarC55E55S55SLK55SL55CL55Why are the two cars with the best power to weight ratios, the slowest?

Bilal

Simple, and I know I'll get flamed but, power-to weight means essentially nothing, too strong, (very little) in determining whether a car is faster or not. There are a variety of factors, traction, gearing, aerodynamics, weight etc.... To me, the most important are power and torque. It easy saying a car has a better power to weight ratio and is quicker, but thats not always the case...Not to divert too much attention away from the cars you posted, but look at SLR vs CGT, they both have around 600bhp, a huge difference in torque and gear ratios, very similar aerodynamics, but the CGT has a far superior power to weight ratio. Yet the SLR is as quick, and hugely quicker from 0-186mph, 3 seconds difference! Why engine torque characteristics amongst other things.

You can do the torque to weight calculations and power to weight calculations of the 55 cars vs the 55K cars, but the latter cars will always be faster, why? You need power/torque and lots of it! Its all well having 200hp per tonne in a small VW Golf hatchback, that weighs a 1 ton, the SL55 has 250bhp per tonne, but in performance the SL is in a different league. Don't look at power to weight to judge a car but look at gearing/torque/lots of power/traction/aerodynamics and transmission efficiency (SMG vs Auto). EDIT: weight is also a very important consideration, to offset this you need lots of torque, look at the Bentley CGT, 550! hp, but a restricted torque output due to the transmission of 600NM, end result? Not quite as fast as the CL55 or S55, even an SLK55 will level it to 124mph....


The 55K cars are quicker because they have 700NM (minimum) arriving at 2700rpms to offset their weight, and with simple physics, once a heavy object gets rolling, the effort required to move it faster is very small, but because an AMG can produce a lot of that special effort (torque/power), the car is...umm. fast!

mrkbbd

Thanks for claryifying that. So the horsepower really doesn't mean much, it's the torque curve and more importantly the torque output?

smgC32

Power to weight ratio measures the amount of horsepower compared to the vehicles weight. As PTWR decreases, the performance of the vehicle increases, and the opposite holds true. To measure this, you divide the two numbers. Although your math in your example is correct, typically you divide in the opposite order and end up with a whole number represented as 1: x.x.

The PTWR represents the amount of weight each 1 HP on your car need to push. For a true valuation of the PTWR, you need to use the vehicles HP as measured at the rear wheels, as well as including the driver’s weight, since this factor will affect overall performance as the PTWR changes as the vehicles weight changes. This is why you want to maximize the overall reduction in weight in your own vehicle, including unsprung weight (as this type of weight is not supported by the springs of the car) to maximize performance.

If we look at the C55, E55, and S55, you will see that the C55 has a PTWR of 1: 9.7, the E55 is 1: 8.7, and the S55 is also 1: 8.7. For example, it takes 1 HP to move 9.7 lbs of vehicle weight in the C55, while it only takes 1 HP to move 8.7 lbs in the E55 and S55. I don’t have the specs on the S55, and will assume that is close or equal in its 0-60 times for this argument, but this 1 lb difference is why the E55 accelerates faster than the C55.

In HP and PTWR terms, the C55 theoretically could be made equal in performance to the E55 and S55 if its weight was reduced to 3,154 lbs (3,154 ./. 362 = 8.7). In this hypothetical example, all three cars would accelerate from 0-60 in the same time. Now as it has been noted, TQ, gearing, final drive ratio, and a host of other differences will affect the overall outcome of these comparisons, but PTWR is still important. I will take the bet on the C55 tipping the scales at 3,100 lbs! And don’t forget to install the Quaife LSD!

As for the performance differences between the SLR and CGT, what are those vehicle weights and HP? It would be interesting to calculate the PTWR here.

Steve Clark

Remember that the HP and torque numbers you're citing are peak numbers.What really matters is the area under the curve on your dyno results.
An engine with a fat plateau of torque will pull hard all the time,but an engine with a higher peak output will pull harder only in that part of the powerband that exceeds the torquier engine's output.At any other point of the rev range,the "slower" engine shoves harder.
BTW- a dynamometer measures engine torque,not HP.The HP is a calculated number generated by torque @ rpm.

With cars having this much power and weight,the bigger issues for acceleration are available traction and driver talent. :p

reggid

Mrkbbd the SC55 and TT65 have better power to weight values than the NA55's, your logic/calulations are incorrect (you need to take the inverse of them since the units are "upside down" and then the smaller the resulting number the better as SMGC32 has already said.

Power and torque are related as most know but when you want to determine approximately which car will perform better you should look at only one(ie either power only or torque only).

Torque:

Torque is the driving force of any wheel driven vehicle but its Torque to the wheels that matters and not peak crank torque values.

Instantaneoues acceleration = (RWTQ/rolling wheel radius - Resistance Forces)/(equivalent mass).....(1)

where

RWTQ = Crank torque X Diff ratio X trans ratio X efficiency.
Equivalent mass = "normal" mass + rotational mass.
Resistance Forces = rolling resistance + drag forces.

So if you know these values you can find the acceleration (Note some of them depend on speed and the gear that you are in)

Power

Instantaneous acceleration = ( wheel power/vehicle velocity - resistance forces)/equivalent mass....(2)

note that if resistance forces are small (at low speeds) and rotional masses are neglected

Accel' = (RWTQ/wheel radius)/mass....(1a)

Accel' = (RWhp/speed)/mass.....(2a)

if you line two cars at the same speed then the relative acceleration is

Relative Accel' = RWhp/mass....(2b) note that the resulting number is not acceleration but it is in relative terms and can be used for a comparison

Note that power/weight is very important and thats why it often used as a reasonable guide to performance but the vehicle must be geared well.

RWhp values need to be used at the speed at which the vehicle is travelling (cars don't always make peak hp all the time, so power delivery is important) but peak values are an ok for a quick hand calculation with a hand calculator/brain.

Note units have not been accounted for in any equations so conversion factors are required.

cntlaw

best "power - weight ratio " what it could mean to me as a car mechanic idiot, is I am not having to waste fuel to lift up the weight of my car to get the corresponding performance. Of course , I am talking about the same engine on different weight cars.

Anyway, a big engine for lifting up a heavy car is a 'luxury'.

Bilal

Of course power is not a measured value. On the engine dyno, the engine is subjected to a load. The load is increased to make the engine work harder and not get past the redline, and torque values are measured. From these values, HP is derived.

As far as the power to weight, in the UK at least, I was under the impression you divide the HP by the amount in tons...So for an SL55, it is 500bhp /1.9 or 2 tons....which gives around 246-250bhp....per tonne.

The SLR has 1768kgs and 617bhp..
The CGT has 1450kgs and 603bhp....

So...

SLR = 349bhp per tonne..
CGT = 415bhp per tonne...

Yet the SLR is faster to 186mph and in top speed and matches the CGt from any speed to any speed despite having one less gear, slightly longer gears, one smaller coefficeint of drag, huge amount of weight over the CGT, whats the deal?

And if its the SLR torque advantage, then what about the Porsche's gearing advantage multiplying the torque output? A linear 500NM/376lb/ft+ is a lot of torque for a 1.4 ton vehicle...

mrkbbd

that would mean a weight to power ratio, not a power to weight ratio.

smgC32

You see it both ways, but the 1:xx format is the easiest to understand. In fraction format, the higher the number the better the PTWR. For the C55, you would generate .10226 hp per lb of vehicle weight. The E55 produces .11475, therefore the E55 has a better PTWR. Now if you can reduce the weight of the C55 to around 3000 lbs, replacing all body panels with CF and gutting the enterior, you could end up with a car with a 1: 8.2 or .12066 PTWR, which would put you close to a 4.4 0-60 time. In general, the PTWR is a good estimator at determining which car will be faster if you have no other information to work with.