Power to Weight Ratios

[Ben]

I know what Power to Weight is. I don't know how it is calculated.

A motorbike has a better power to weight than a Truck for example.

My question.

2.0 TDI in a A3, A4, A6, A8 whether these actually exist is besides the point. also assume all 2wd or 4wd

would all the engines be identically tuned, thus the A3 be the fastest accelerating as it is the lightest car.

or

would the engines be tuned differently to compensate for the extra body weight as the size of the cars increase through the range.

[prolfe]

The power to weight ratios will be different for all of these cars because they could use the same lump and same power but the weights are different.

A3
Weight 1340kg
Power 140bhp
0-60 9.5
Power to Weight 104bhp/tonne

A4
Weight 1490
Power 140bhp
0-60 9
Power to Weight 94bhp/tonne

A6
Weight 1540
Power 140bhp
0-60 10.3
Power to Weight 91bhp/tonne

A8
Weight 1830
Power 230bhp
0-60 7.8
Power to Weight 126bhp/tonne

If the A8 had a 140bhp tdi then:

Power to Weight: 77bhp/tonne

The A4 must have a different ECU as it is faster 0-60 than the A3 with an identical engine.

And although the A8 is a 3.0 TDi with 230bhp the A3 170bhp has a power to weight of 127bhp/tonne !!

[prolfe]

Sorry, Power to Weight is calculated very crudely:

BHP divided by the Weight in Tonnes

So

140bhp / 1.340 tonnes is 104bhp/tonne

1000kg is 1 tonne

[Ben]

Thanks prolfe, this is what i suspected would be the case, Identically tuned engines, with the weight of the car being the negative.

But your comment about the programming of the ECU has thrown it right out the window.

Suppose i'm at the lights in my 2.0 140bhp TDI golf and next to me is an A4 with the same engine, even though i weigh less, i'll lose as his ECU is programed differently giving him more power.

[prolfe]

I was once told that Audi's have a different ECU to VW, Seat and Skoda and that although they might have identical 0-60 and top speed that they were different through the gears and to drive.

I suppose much like a re-map; responsiveness and fuel consumption maybe.

The Audi ECU in the A4 and A3 look different if the 0-60 is quicker by 0.5 in the A4.

hmm, looking at what cars site again - i have quoted 0-60 for the A4 Avant which is quicker than the saloon.

A3 140 - 9.5
A4 140 Saloon - 9.7
A4 140 Avant - 9

Interestingly the peak power on the A3 is different to the peak power on the A4 1750rpm to 2500rpm.

This suggests that the turbo might be different. Same bhp but one spins up faster and dumps its peak power lower down.

Maybe an A4 enthusiast can shed some light.

[Ben]

what about the gearing of the cogs in the box? they could vary in size?

[Col]

what about the gearing of the cogs in the box? they could vary in size?

Yes.

The Audi illustrated above is probably lower geared which enables it to have a better 0-60 despite a lower power to weight ratio.

If you really want to see how much difference this makes, look at the Caterhams for example, they may only have a humble Honda 2.0 pushing a mere 160 (ish) bhp but can kill Ferraris etc.

Astonishingly... I've just looked on Google for a table of power to weight ratios I've seen before.... I didn't find but THIS THREAD is on there (Google that is!!)

[RoRu]

Another contributing factor in 0-60 is weight distribution/wheel base, this plays a major factor in how a car car get it's available power down to the road.

Once it's moving the cars aerodynamics have a huge role to play, I suppose even the rolling resistance of the tyres fitted to the car make a difference.

So it's not necessarily the car with the largest power to weight ratio that will be quickest.

[prolfe]

Astonishingly... I've just looked on Google for a table of power to weight ratios I've seen before.... I didn't find but THIS THREAD is on there (Google that is!!)

Quite a few times i have searched google and found my way back to here.

[cbrpaul]

My old Honda Fireblade 1000cc was 780 bhp per ton


All that power for £6k too ,