Scottish Elise Rolling Road Day **Now with Pictures**

[Shug]

BHP is simply a calculation of torque. The two are the same readings, one calculated to show work done (BHP) where torque is force applied (this is the figure measured) - so the calculation always throws up this crossover at 5250. If it doesn't, the figures are wrong, or the operator is trying to pull a fast one.

As far as I was aware, you could only measure torque, as horse power is a calculated figure and not a force? Certainly, everything I've read on dynos says that - of course, they can be set up to calulate BHP and output that as a live figure, but the strain gauge will be measuring the torque and working from there.

[robin]

Ah - well spotted Ali - I was a bit sleepy this morning

Shug - it is possible to measure power directly (well slightly indirectly - you need to measure the increase in rotational speed of a known mass flywheel over time) - but I suspect you are right that almost all dynos measure torque and then compute power from that ... the reason being that you need to have a way of holding engine speed fixed AND you need to know how much power that is dissipating - that's not (easily) possible without measuring torque.

Anyway, torque is interesting to engine designers, power is interesting to people who drive the car on track.

Torque is interesting because it gives us a cycle-by-cycle view of how well the engine is performing at each engine speed; if all your variable valve timing/lift/etc. mechanisms worked perfectly you would hope to get a near flat torque curve. Tuned cams/overlap settings/manifolds tend to produce peak efficiency over a narrow band - you can clearly see the effects of these components when you look at the torque curve; the power curve is harder to interpret because by it's nature is a sloping line already; this slope makes it harder to spot the peaks and troughs in engine performance.

Cheers,
Robin

[hiscot]

power is only torque X rpm its torque that counts all rolling roads measure torque not bhp they are even callibrated by torque
heres a good example of how we get to bhp by d andrews

"Torque is a measure of the turning force or push the engine generates and can be increased by improving volumetric efficiency, the more air/fuel imbibed the bigger the bang, and hence the more the torque. Torque is the true measure of how well the engine is performing. BHP is derived from torque and engine speed, it is a measure of work done.

BHP = torque / 5252 * RPM

So an engine making 120ft/lb of torque at 5000 RPM is producing 114BHP, if it were to make 120ft/lb at 7500 it would be producing 171BHP.

An engine producing a low torque figure can have a high BHP figure if it produces the torque at a high RPM value. Engines which are tuned without increasing their volumetric efficiency can produce high BHP figures but their torque figures are usually meagre, giving breathless performance. Increasing BHP is easy, just turn the engine faster while sustaining your torque.

Increasing torque is not so easy.. proper tuning will entail increasing volumetric efficiency and hence torque everywhere in the rev range rather than just moving the torque from one place in the rev range to another. Head porting, fitting of TBs and improved exhaust manifolds can improve Volumetric efficiency and hecne torque, changing cams generally moves the torque further up the rev band thereby increasing power without a corresponding torque increase.

Maximising the area under the torque curve will produce an engine which will perform extremely well, much better than one which has a high peak power figure and no mid range torque.

If you want to see how one engine compares to another, forget the power curve since its slope is entirely dependent on the scal you choose, instead plot the torque figures, these will give you a much better idea of what the engine is doing."

[mwmackenzie]

I live and learn, some smart cookies on here!!! Great info guys

Mark

[Andy]

Brilliant post! Feel quite stupid buying air filters and exhausts as a yoof now to get more power

[Mikie711]

That explains why my car as it is feels a lot more "powerful" than the figures suggest. The way the 270's were timed was for torque, according to the previous owner, and i shows on the dyno plot. Where it looses out is on the track, not much pull past 5500rpm. Will bear this in mind when mapping they new engine at the start of next year .

[oldminiclub]

Mike, thanks for organising a great day out. Very interesting to see a RR in action, trouble is I found myself wondering if I could afford a Ricky ecu remapping. But I ought to be happy with a flat 200ft/lbs.

[Mikie711]

ye, just a pity it's got the wrong badge on the front