Frenchie

I've just found the bit of paper I made my original purchase short list on in July. One of the bits of data I collected was the power to weight ratio for each car (bhp per tonne) ...

Chimaera 4.5 = 273
S2000 = 191
Boxster = 179
Z3 3.0 = 173
Elise = 159
TT 225 = 159
Spider 3.0 = 155

Take the other qualities of the car into account and compare them with the bad points of the others on the list (and I included price in these) and I see why I chose the S2K !

Pete

Nick Graves

An interesting statistic.

Of course, torque/weight ratio is far more relevant in the real world, but we don't talk about that!

It may explain my VTEC addiction, however

AnDy_PaNdY

Would be interesting to compare the prices of those cars loaded to the same specifications also

steve_the_greek

Torque is definitely more important than power. But torque's no good if it gives up at 5000RPM - a car with twice the torque of the S2K but gives up / redlines at 5K is going to be geared down almost twice as much as the 9K S2K, and so it's torque at the wheels will also be halved.

Power and torque figures on their own don't tell the whole picture - you need to see the dyno charts to get a real idea. Le Car (I think) did some excellent graphs showing how much torque was available at the wheels as you accelerated through the gears.

UKjasonm

Not being an engineer the subject of Torque and Power baffles me. I know torque is max turning force the engine applies to the driveshaft but where does power (BHP) fit into the equation?

Im probably showing myself up here but I would really appreciate it if someone could explain In laymans terms terms how BHP and Torque affect the performance of a car and also explain the pros/cons of each.

Frenchie

[QUOTE]Originally posted by UKjasonm
[B]Not being an engineer the subject of Torque and Power baffles me.

steve_the_greek

For the moment, forget about horsepower.

What accelerates you is torque - the twisting force applied at the wheel to the road. So, what you want is bucket loads of torque all the way through the rev range. The VTEC kick you feel at 6K RPM is extra torque, because Honda's don't tend to have 'much' torque low down.

Gearboxes convert between torque and RPM - at the crank you have high RPM, but low torque, and at the driving wheels you have low RPM and high torque. The higher the gear, the more RPM at the wheels (to get the higher speeds), but also the lower the torque, hence the desire to change down a gear to get that extra bit of torque. The relationship is linear - double the RPM, halve the torque (ignoring transmission losses).

To explain why 'low torque', high RPM engines like the VTEC work, let's use an example.
A (VTEC) car has 100Nm of torque at the crank. It's torque curve dies at 9K RPM.
Another has 200Nm of torque, and it dies at 4K RPM.
Both cars have identical torque curve shapes - just different amounts of torque.
If the cars had the same gearbox, then the second would get off the line quicker as it has greater torque. However, it also has to change up at a lower speed, and in the higher gear, its torque will drop - at the same time the high RPM car will still be on its way to 9K.
However, in the real world, cars don't have the same gearboxes. Most are geared to do about 35-40 in 1st, 60-70 in 2nd. So take our 2 cars again and make them so that they are geared so that they redline at the same speed in each gear. 35mph might equate to about 300RPM at the wheels (depending on tyre diameter). So, both must be doing 300RPM at the wheels at their redline regardless of engine RPM.
For the VTEC, we have to reduce the crank speed 30 times to get from 9K to 300, and the torque goes up by this much: 3000Nm at the wheels.
For the 4K RPM car, we have to reduce it by 13.3 times, increasing the torque to only 2667Nm at the wheels.
Hence, the VTEC will be faster.
Which leads us to horsepower - which is simply torque multiplied by RPM. It's a bit more meaningful than raw torque figures because it effectively takes into account the gearing advantage that a high RPM car has. However, it is a measure at a single point (the highest !), so it disregards dips in torque - like on the S2K where the torque below VTEC is significantly lower.

Hope this helps.

Steve

UK Paul

Beautiful write up!

UKjasonm

Thanks Steve,

I'll need to print this out and read it 35 times before it will sink into my thick skull but its a little clearer now. I often wandered if the torque was so low on our car how come the BHP was so high but its all thanks to those 9000 beautiful RPMS.

Cheers

steve_the_greek

I found the thread I was thinking of. Check out Luis' graphs comparing the Boxster S and S2000 just over half way through the thread.

Comparing the S2000 with the Boxster 2.7 and Boxster S

This shows clearly why the S2K isn't a drag racer (i.e 1st gear to 35 or so is 'poor'), but also shows how a lowly 2 litre can play with the big boys in the form of the 3.2 litre Boxster S once it's in its power band (torque band really).

The second of the 2 graphs also shows you why you must cane the car all the way to 9K, otherwise you will be left standing.

Steve