On the Diesel V Petrol debate...

Hmmm, lighter cars always seem to stop quicker than heavier cars, surely most heavy cars are designed to be heavy from the start? Is it not something to do with the tyres? I think the coefficient of friction decreases as the vertical load increases??? Would the same reasoning mean that you wouldn't be able to corner as fast?

So turbo a petrol motor.....

A diesel engine, all else being equal, will have a thermal efficiency around

40%, while an otto cycle petrol engine will manage around 30%, these are measures of how much of the energy in the fuel is turned into motive power and how much is dissipated away as heat.

I said "all else being equal", and that's not normally true. Diesel engines are noisy, which doesn't matter too much, and heavier than their petrol equivalents. There are differences in rev range, so a comparison must include the fact that a diesel has to be geared lower so that it doesn't redline at a low speed, and must manage a range from minimum to maximum revs that permits it to function as an engine in the same way as the petrol engine.

To put this in a nutshell, when a diesel engine can be made of similar weight and power output to a petrol engine, it's fairly obvious to anyone with basic physics skills, that it will power a car in much the same way and deliver more or less the same performance. By merit of the shape of the power curve, it will actually put out better figures, on the whole, due to the elevated torque and flatter power band, but that depends on the design of the petrol engine as well as you can make them peaky or torquier.

The main difference, in that situation, is you have of the order of a third better mpg with the diesel, when equivalent, and have a bit less heat to get rid of through the radiator.

Course, this assumes diesels can be made with similar output to the petrol engine while not being heavier. I dunno how feasible that is, at present.

S'what i thought too.... Whats the point in an impressive space frame superlight chassis, filled with a ridiculously heavy 5 litre diesel lump. Summet like a Twin Turbo 3.0 petrol from a Supra would be way better...

And some problems such as the restricted revs.

With a decent alloy V8 being as light as a steel based 4 or 6 cylinder car, I reckon I could make something that handled.

I mean Ferrari have been sticking 5 litre 8 and 12 cylinder engines into car for donkeys.

lol thats in my area, hmmm.

Heavy cars have greater inertia... you need more force in order to change the direction of movement (stop, turn or whatever). Of course, the more weight you have on the wheels, the more your tyres grip to the road but I don't know to what extent or how the greater grip compares to the increased inertia of the car. All that being said, the engine is nowhere near as responsible for making a car heavy as it is every other 'toy' that you get in a car. A massive engine in a spaceframe chassis with very little in the way of bodywork, safety features, electronic gadgets or windscreens would *still* make an incredibly light car. My Mazda RX-7 Turbo II, despite having such a small engine, weighs over

2000kgs - yet my top-of-range Peugeot 305 GTX (yes you can stop laughing now) with a 1.9 litre 4 cylinder weighs 1050kg as standard. .. (I said stop laughing) ... Almost perversely though, lets say you got that *huge* V8 engine and tried to make the lightest car you possibly could with it - you'd probably find yourself in all sorts of trouble trying to actually get the power down on the road.

Chris.

Are you absolutely sure about that 2 tonnes? - neither of my biggish Alfas comes in anywhere near that!

Just checked, it's 1284kgs, which is more like it.

You know what? I have no idea where I got 2 tonnes from. I just checked as well and my model seems to weigh in at 1315 kgs or thereabouts. I did rather think that 2 tonnes was heavy, but I was trying to prove a point ;) .. not the best way to do it, trying to compare a well front-heavy FWD car with a

50/50 weight distributed RWD car, especially if I can't actually get the weight right!

Chris.

That's why you want wings :) Not to look like a tosser, but if you're really looking for track performance, forcing the rubber into the road without having a weighty bodyshell is a good thing.

On the road, you're never going to be legally doing the sort of speeds required to make moderate angles work, and severe wing angles that might do something will really bugger up your mpg.

I quite like the 305GTX. It's got that chunkiness about it that gives it more presence than the jellymold crap made nowadays, IMO.

If it's sheer traction that's required, having the engine in the "Wrong Place (copyright Porsche)" would help, but for good lap times, I would imagine having a good suspension set up and if you really must, TC, would be better. Might not have ultimate grip in terms of getting the power down out of a tight turn, but the balance of the car going into and around the corner should compensate, IMO.

Yup, you're absolutely right on both counts.. my experience on prepping a car for track performance is. ummm. nil.. Didn't even occur to me about using wings, although of course it makes perfect sense.

Ta for that. I've kinda gotten used to making my daily transport into a bit of a joke, but I secretly believe it looks fantastic ;) As I point out to people, it's Pininfarina-styled (do you think they get some work-experience boys to design the Peugeots?) so that automatically qualifies it as beautiful, surely?

What do you mean by TC? You've lost me there...

Chris.

I've been doing some comparisons of my own recently.

My latest Alfa (the 155) is 200kgs heavier than it's predecessor in the range (my 75). And I was wondering why the 155 felt so slow after jumping out of one and into the other.

More shockingly, the later versions of the 155 are even heavier, since they switched to the Fiat-based cast iron block from the all-alloy Alfa twin-cam.

The 75 2lt actually has the same power:weight ratio as a 156 V6, ffs!

Cars have got fat and bloaty these days :-(

I think it's the other way round, GPM.

Oh yes. Nice noise too.

It's all dissipated as heat n*****ts, every last microjoule.

Yup, and they keep getting heavier! It wouldn't seem to be the economical solution, would it? It's all these modern toys.. ABS, traction control, airbags, who needs that eh? ;)

Seriously though, along with the weight, engines have got a *lot* more efficient these days but it does seem to be much harder to find a lightweight, seat-of-your-pants driving machine brand new that doubles as everyday transport. You don't get that same sort of response from driving, say, a modern Golf GTi as opposed to a Mk1 or Mk2. It's like you're disconnected from the road.

Chris.

How about something a bit bigger and inline?

It looks like you get some really tiny people with it to put it all together for you as well ;)

Aero downforce only makes sense if you're going to be driving in the right conditions to make it work though.

Now, ground effect, that's a better idea, but also bloody impracticle to implement to any serious degree as you need to try and seal the sides of the car to prevent air getting pulled in. Doesn't have the drag factor of wings though, which is good. All you need in basic terms is to make the air go quicker as it goes under the car, so reuslting in a drop in pressure, therefore the car is sucked/ pressed (depending on your view of physics I think there's a difference) onto the ground without having nasty great bits of crap sticking into the airflow over the top of the car as you find with wings.

I've never prepped a car for the track in my life, but talked to people who have, and read up on all sorts of things I'll never remember because I'm a sad bastard who likes that kind of thing :) Plus I'm still contemplating the idea of smoothing the underbody of one of my cars as much as possible, and creating a couple of channels to give me a very mild ground effect. More than likely just lowering the front and raising the rea enough to create a definate angle of attack would be quicker, easier, cheaper, and more effective.

And none of it will ever happen anyway!

My daily transport is Michelloti :) Well, a facelifted one anyway. My other daily driver is the original version, but that still needs me to get off my arse and do the brakes.

Sorry - Traction Control. When you've bitched on an F1 group for long enough, you start to get used to the abbreviations and not think!