Diesel? That's for Teachers and Coppers, you Muppet

But who is constrained like that? Do you know anybody who's under the conditions: "You can buy a car - any car, but 170bhp is you lot. After that, chose whatever you want..."

Or you could look at it the other way around - the peak power on diesels is poor compared with their peak torque output, on account of their limited available revs.

But you're artificially comparing peak power output to suit your prescribed rationale.

There is a turbo'd VAG 2.0 FSI engine that you could compare with the 1.9 TDI engine - that would be a much more realistic comparison, on account of them being similar capacities, and both being blown.

Otherwise, you're skewing the argument in favour of your artificial bias. The diesel has much of it's torque on account of it being turbo'd. Otherwise, the only fair comparison is an NA diesel compared with an NA petrol.

So?

The diesel on achieves that because of it's turbo - compare the turbo'd 2.0 petrol, how does that fair?

Which is artificial.

However, my entry into the discussion was because of a more general "diesels produce more torque" type comment.

But why would you have that limit?

Surely you're more likely to be limited by one of: budget, CO2 emissions, capacity, etc....

If you truly compare diesels and petrols, and I mean _truly_ in a representative fashion, the torque thing isn't quite as advantageous as you're making out - unless you compare TDs with NA petrols.

Quite a few marques now produce similar sized turbo'd petrols and diesels, VAG and Volvo being the most obvious I can think of. When you truly compare like-for-like, then it's more the engine economy that is the advantage of the diesels.

I get that, as previously mentioned.

I just don't consider comparing peak power output when comparing diesels vs petrols to be "like-for-like".

Capacities and induction seem much more representative. I don't know of anybody who's restricted on their car buying, by peak power. I tends to be cost, or engine size, or emissions etc...

If you truly compare the two types of engine on a representative basis ie similar capacities, and similar induction systems, then you truly have a "like-for-like" comparison.

Otherwise, it makes no more sense than to compare engines based on peak torque, and see how max power favours each.

Diesels only make the same power as petrol engines, when you compare TDs with NA petrols - in general.

And on a "like-for-like" basis, diesels do make less power - because they tend to have less usable revs.

It makes no more sense to compare diesels with petrols on the basis of peak power, than it does to compare them on the basis of peak torque. The only sound basis is to compare their displacements and their induction systems.

Otherwise is just a case of lies, damn lies, and statistics.

[...]

Constrained power - "That's for Teachers and Coppers, you Muppet"

:)

We need *much* more than 350bhp - supercharged petrol seems the best path...

A

But it's the _area_ under the torque curve that counts.

But you forget that the torque at the wheels is haved gecause the diesel needed double the gearing to travel at the same speed, net result same performance.

Which is why a 1 litre 180bhp bike engine like the latest suzuki 1000cc for example at 1100rpm will power your truck faster than a 110 bhp diesel or petrol car engine. It delivers more torque to the road at a given road speed. Its the reason that BHP figures are used to measure power not torque figures,

Agreed.

However, I don't see what else you can compare ?

But capacities really ARE meaningless. It's quite possible to get a 3l engine making less power than a 2l. Induction is also meaningless really - it's simply a means to more torque (and hence more power). It's not like there's a huge cost/economy/reliability penalty these days for forced induction, so there's no reason to filter it.

It's not a restriction - it's quite the opposite. The more the better :)

Cost comes foremost for me. I don't give a shit about emissions or engine size, as long as the car makes a sensible amount of power (ie, somewhere above the 200bhp mark).

But what's the point of that ? All it will prove, is that Diesels make less power than Petrols, per litre. But so what. If you want your Diesel to make as much power as your Petrol, then just buy a bigger one. If you want 200bhp, then you need a 3.0 Turbo Diesel - I don't see why a 2.0 Petrol is better (other than the fact that it MAY be lighter) ?

Yep - I didn't say otherwise.

Point taken - that was beyond the realms of my ultra-simplified description :)

Yeah. But seeing as basically all Diesels are Turbocharged these days, then that's fine. There's not really any disadvantage to Turboing, so it's not a negative.

Agreed.

That still makes no sense - you can always choose a Diesel with more displacement and a Turbocharger :)

Well you have to agree it doesn't make sense - most petrol cars have a fairly obvious relationship between peak power and torque output. Clearly on diesels because they don't tend to rev as highly, that relationship isn't in corrolation.

Capacities aren't meaningless, nor is induction.

If you compare across marques, similar capacity engines tend to make similar power outputs.

How can you say it's meaningless, then go on to say the "simply a means..." bit????

There's a difference between filtering it, and comparing apples with oranges.

If you compare a blow engine with an NA engine, you are bound to be comparing different things.

As examples: VAG produce TDI engines of similar sizes to turbo petrol engines - so why would you compare a turbo'd diesel in their range, with an NA petrol? Simply because of peak power similarities? When we already know that the rest of the engines performance is gonna be different on account of the diesel being turbo'd?

Same for engines in Volvo's line-up. Quite possibly for other marques, too.

All I'm saying is the dogma that is "diesel engines produce more torque" can be misleading. With similar induction systems, I suspect there's not such a marked difference - which is more like-for-like than peak power comparisons.

So whats the sense in comparing across the petrol / diesel divide by peak power, then, if nobody is absolutely restricted by peak power?

I was more thinking about company car drivers, or people who use them for business purposes, too.

In order to examine the "diesel engines produce more torque" mantra.

I only entered the thread regarding the "diesel engines produce more torque" thing. There are many people out there that somehow believe that there's something inherently different about diesel engines that makes them produce more torque than petrol.

Now whilst there may be some aspects that make some differences, here, it's not as black and white as often perceived.

So what indeed. ;-)

Personal preferences aside, I haven't made any judgements on "better" per se. I was merely striving to point out that the debates are often skewed because they tend to be comparisons of turbo'd diesels against NA petrols. And given that we've both agreed that being constrained by peak power is unlikely, then why wouldn't you compare with other more meaningful criteria - price, performance, cost-to-run, fuel economy.

I didn't say it was a negative.

What I was pointing out was the difference in induction, refuting some of the "diesel engines produce more torque" mantra, by comparing turbo'd diesel engines with similar / the same capacity as turbo'd petrol engines, by the same marque.

Then when you figure in the torque thing, the area under the graph thing, the whole "inherent" aspect to torque output by the two different fueled engines isn't quite as most would paint.

Why? Why does it make no sense to compare engines of similar capacities and similar induction systems? If the supposed difference is the type of fuel as to the torque output of the engine, surely it's quite relevant.

As you could with a petrol, but what would it prove?

As I said, if the _supposed_ difference is some inherent trait of the fuel type and engine, then surely the capacity and induction systems are relevant.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Douglas Hall" saying something like:

Speed of fuel burn is relevant. Ie, diesel burns relatively slowly and exerts a turning motion on the crankshaft for more of the power stroke than a typical petrol cylinder charge, thus increasing the torque figure compared to the same size petrol engine.

No they don't. The increased torque is due to turbocharging - as far as I can see there's no evidence of what you wrote in the car figures - come to think of it I can't think of a non-turbo diesel car in current production.

It was somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Tim S Kemp" saying something like:

Where did I mention turbocharging? Like for like, you'll find I speaka da troof.

in news:4242b3b5$0$10943$ snipped-for-privacy@reading.news.pipex.net, "Douglas Hall" slurred :

I agree, it's artificial, but the "I accept myself that there is a power output which I would call sufficient, I know that technology allows a diesel and petrol engine to both reach this output..."

i.e., the OP is constrained like that.

You make it sound like I have some sort of pro-diesel agenda. I'm not dervman...

I was just dealing with the hypothetical case that power was the limiting factor - I'm not saying diesel is better...

in news:4242b4c8$0$10939$ snipped-for-privacy@reading.news.pipex.net, "Douglas Hall" slurred :

I think people look at costs, then at power & performance figures. I don't think people care much about the specifics of the induction/fuelling systems, in the same way that they don't care about the type of suspension used. A lot of people _will_ compare cars on either the peak power figure or the MPG figures, so the basic premise of all this is not _that_ atrificial.

But my 3.0 V6 makes 210bhp. The very same engine, but fettled, in the Clio V6 makes 250bhp. Peugeot and Renault's current 2.0 engines make about

180bhp. Their other 2.0 engines make about 135bhp.

Capacity has nothing to do with anything :)

Er, cos that's how it is ! Forced induction provides an increase in torque. There's nothing "special" about it. You can either make that torque with a bigger engine (like my current 3.0) or forced induction (like my previous

2.0 Turbo) - there's no difference between the two. The fact that one is big and one is blown, is neither here nor there.

But ALL engines are apples-to-oranges - that's exactly why you have to choose some sort of constant, in order to compare "similar" engines. I choose peak power output.

Honda 2.0 = 200bhp. Rover 2.0 Turbo = 200bhp. They offer very similar real-world performance (providing you stoke the box in the Honda) and broadly similar economy etc. Why are they different things ? It's not like the Turbocharger costs an extra £10,000 or anything.

Why would you not ? Their 1.9 PD can make 150bhp. Their 1.8t can make 150bhp. Their 2.0 FSI can make 150bhp. Why does it matter one jot, that two of those engines are blown ?

It's different anyway, because A. It's a Diesel and B. It's a different engine. The Turbo offers no gain or loss. Honda's VTec has no torque whatsoever, but it offers very similar performance to a Turbo car (with lots and lots of torque) providing you row the box.

How so ? They do produce more torque ! Let's take your average car - a Ford Mondeo.

Let's give it an average 2.0 Petrol engine. It's making 140bhp.

Now, let's swap it for the average Diesel version (it costs about an extra thousand quid, according to

It's a 2.0 TDCi and it's making 130bhp. It's also making *much* more torque than the Petrol car (nearly 100 lb/ft !). Even the 115bhp TDCi lump (which would save you £300) still makes about

50lb/ft more than the Petrol car.

But you can't compare NA Petrols with NA Diesels, because there ARE no NA Diesels :) Almost all Diesels are Turbocharged - it's part-and-parcel of Derv these days.

Because if you want to move from a Petrol engine to a Diesel engine, then you don't want to sacrifice any power. So you choose a Diesel making the same power as your Petrol car. And said Diesel will be making *much* more torque.

But they do :)

The inherent difference, is that they don't rev as high. So all the torque they do make, is lower down the rev range. So to provide a comparable power-output with the Petrol cars (because nobody would buy Diesels if they were way down on power) then they have to make more torque. Luckily, they're all Turbocharged, so they do make more torque than the equivalent (equivalent means broadly similar cost, power, size, etc.) normally aspirated Petrol engine.

Of course :)

But that's cos average-petrol is NA, and average-diesel is turbocharged.

But by comparing an average 2.0 Turbo Diesel with an average 2.0 NA Petrol, then that's exactly what we're doing ! The Turbo diesel is a similar purchase-price (£1000 more for a Mondeo) which is offset by the higher economy, it offers similar performance (it's 14bhp down), cost-to-run is pretty similar (insurance, servicing etc.). The Turbocharged Diesel doesn't lose out on price, performance, cost-to-run, or fuel economy, simply because it's Turbocharged.

Because petrol and diesel engines don't have similar induction systems ! Average petrol is NA. Average diesel is Turbocharged.