TheTechGod

How do you mesure how many KW's an engine is? I know you can look it up, but say you put a turbo in, how would you mesure how many KW's it added?

Also, how does KW's relate to maximum speed/torque, besides the saying "more is better".

Thanks!

outback_ute

You can measure engine power on a dyno (full word is dynamometer) which applies a measureable load to the engine. Either an engine dyno where the engine (out of the car) is bolted directly to the dyno or a chassis dyno where the car's wheels are put on rollers and the measurement done there. However the chassis dyno does not directly measure the power of the engine, just what is available "at the tyres".

Kilowatts is a power rating (ie the rate at which work is done or force is applied), torque is a twisting force. So the faster the crankshaft is turning for a given torque output, the more power will be produced.

TheTechGod

Thanks for the info about dyno's, didn't know about them.

Is there a way to work out the maximum speed or torque of an engine by knowing it's KW rating?

I guess what I'm asking is, when looking at engine specifications, what is the item that "tells" you how good the engine is in terms of speed/torque/acceleration. Thanks.

outback_ute

The maximum speed and maximum power are at different rpm points (because as the torque curve drops away from its peak the increase in rpm means the power output keeps rising until the torque really begins to fall - hard to describe/picture I know). Therefore you can't calculate the maximum torque from the maximum power figure.

To get an idea of how "good" an engine is, the specific outputs (ie per litre) are a good guide, also how wide the "powerband" is - one way to get an idea of this is to see how far apart in rpm the peak torque and power figures are. Eg if peak torque is 6500rpm and peak power is 7500rpm the engine is likely to be quite peaky & not have much down low. However if the transmission etc can keep the engine in that peak operating band then it will still be quite quick (eg close ratio gearbox).

With newer engines the spread of torque is usually better than it was (especially compared to carburettor cars), with things like variable valve timing, inlet manifolds not to mention turbo engines. You will often hear people talk about a "flat" torque curve meaning the engine puts out high or close to maximum torque over a wide rpm range. So the bottom line is you can't just judge it on peak outputs (although they are certainly not unimportant), you can get a better idea from a power & torque graph buy looking at how much area is underneath the curves.

Confused yet?

TheTechGod

I did a bit of searching and found this graph.



For this engine, the peak horsepower is at 5500 RPM, peak torque at 4500 RPM. Compared to the horsepower curve, I can see that the torque curve is much more "flatter".

The flatter the torque curve the better, as long as the peak is not too low?

It's a bit confusing but your explanation is pretty good because I feel like I'm starting to "get it".