BHP/RPM

Which is more?

Well the figures you have shown are (to my knowledge) peak power figures. This means that you will get 'x' amount of force when the engine is running at 'y' RPM. I guess the two cannot be directly compared since they are quoted at different RPMs. A better way is to actually compare the torque/RPM curves of the two engines to find the 'constant power band'. This will give a better indication of which is more powerful. After that consider the whole car's weight. The power-to-weight ratio then gives which car is actually more powerful (NOT necessarily faster). For example consider two engines of the same power (say 100ps/6000RPM). If put in a car that weighs (in total) 1000kg, the power-to-weight ratio is 0.1. The same engine when put in a 1500kg car gives a power-to-weight ratio of 0.067 which means that the latter has a lower overall power output.

However this above is valid only for engines that have similar specifications - it cannot be applied to any two random engines. So consider your example of 80 and 87hp cars - depending on total weight either would compare less, more or the same.

@Mods: Please correct me if I am wrong.

Quite well reasoned out Rohit-way to go.

On paper the 87@6000RPM is more powerful-as its the peak power developed by the engine. But in reality that not the case-even leaving alone the power to weight ratios(as Rohit has explained) and the gearing(which will point to the driveability of the car to a certain extend) the engines themselves provide power over a power band.

So the 87BHP engine might be producing 75BHP at 5000RPM and so also would the 80BHP engine at 5000RPM. It all depends on the power delivery curve characteristic of each engine.

All said and done the power rating is only one part of the issue-the other more practical aspect being Torque-which also has to be compared.

I refine my question here-

To me the 1298cc engine *appears* to lose. For the 1298cc engine, extrapolate back to 5200 RPM, you will see that the Power and Torque both drop - I am perhaps assuming too much here, but an 800 RPM drop in the engine's RPM means that the power will go down from 87 to 80 or perhaps even lower. Similarly the torque - this will drop even more as the RPM reduces from 4500 to 4000 due to the fact that at the quoted RPMs there is only 1Nm difference between the two engines. In all the 1298cc engine, though of greater capacity appears to lose out - though practically this may not be the case and both might well perform equally. Only the power & torque curves will give a definite answer.

On the contrary, I guess engines with larger displacement will peak at a different (i.e., greater) RPM than smaller ones...so the question still stands. At the quoted RPM, the 1298cc engine is delivering greater overall output.

As for which is practically better, I guess it all depends on the driver - how s/he revvs the engine, gear shifts and so on...

In addition to the above post (I cannot see the edit button - probably a timeout issue), consider a 250cc bike and a 1000cc car. Which will accelerate faster? The bike wins hands down. However which can carry greater load? The car. Similarly which has a greater top speed? In all probability the car again. Now reduce the difference in engine capacity and you will see why it gets difficult to do the comparison now. There are a variety of factors: weight (total load, rather), max engine RPM, power curve, etc.

Explanations nicely put - thanks Rohit.

Just to add... please have a look at Power band.

Thanks Rohit and Binoy for the explaination.

This makes my doubt clear.

Ok, fine. Here we go.

Check this.

Let the technical articles talk.

Pls anybody will brief about difference between power and torque in a car?

Example:

88bhp at 6000rpm &

11.11kgm at 4600 rpm