For the record, Torque is the actual measure of the engines performance not HP or KW (KW is a metric conversion of HP). HP is simply a mathematical measure of torque produced by the engine at any given RPM. The equation is HP = Torque x RPM divided by 5252. The Diesel and Petrol engines have very different characteristics, think of it this way; Peak torque is produced when the engine is operating at its most efficient RPM range, typically the Diesel has a low flat torque curve while the petrol has a ‘peaky’ style torque curve usually towards the upper RPM range. Turbo chargers work well in both designs by increasing the torque in the Diesel which by design has a very narrow power band. The Petrol is helped by turbo charging by significantly increasing the available torque at low RPM where the petrol engine is normally less efficient.
Now, when measuring the ‘on paper’ specs between Petrol and Diesel engines we notice the RPM difference is usually quite significant. In the case of the Holden V6 the peak torque is the same as the Diesel Mondeo but KW are a lot higher, this is because the engine is making its torque at a higher RPM so there fore the resulting KW’s are higher, when the Holden is making its 195kw’s at a very ‘high’ 6,500 RPM, at that RPM its making 157 nm of torque which is way below its peak of 340 but because the engine is capable of revving so high it can still make good power despite the fact that its past it 'prime' or peak efficiency. The Mondeo will out pull the Holden at low RPM but soon runs out of usable revs while at about the same time the Holden is still building power and will pull away with ease.
What has made the Diesel a lot more common in modern cars is the progress of electronics, engine management systems, turbo charging and direct or common rail extreme high pressure fuel injection. All these advances in technology and engine design have meant that cars can perform very well with a Diesel engine and than benefit by the significant saving in fuel usage (the modern diesel is much more efficient than a petrol), the problem we face now however is that the modern diesel requires a cleaner more refined Diesel and the petrol companies are profiteering on the increased demand for Diesel.
To try and put all that simply you could consider the following;
A modern sports motor bike of around 1000cc makes around 150hp which is 20 or so more than the Diesel Mondeo, but if you were to put the ‘more powerful’ bike engine in the Mondeo it would be an absolute pig to get off the line and would likely stop at the first medium hill you tried to get over. By contrast the Mondeo diesel engine in the sports bike would be a weapon (in a straight line at least) provided you could gear it high enough and fit it in the frame. Another consideration is the Diesel truck, when I did my apprenticeship the interstate trucks were only making around 200hp which is less than most standard family cars of today, but it made that power at only 1300 rpm which meant it was making over 800 nm or torque at 1300 which is what the truck needed to get all that weight moving.
So what’s more important Torque or HP? Well the truth is they are mutually important and also mutually un-important. You cannot compare the ‘spec sheets’ against each other simply because they are not actually comparable in the real world. But if you must compare them, than think of it this way; Torque is how strong you are, HP is how fast you are.
Hope that helps.
Cheers
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