Large Swept Volume - Real World

[junglie]

This is not a Petrol vs Diesel debate!

I have run a number of cars in the 3.0l bracket (BMW Z4, ST220, E39 etc) and also now have a Mercedes C320 CDI with the 3.0l V6 diesel. They have all been great cars and all have loads going from them in different areas.

In real world terms (being lazy) the MB is easy to punt along with 225 bhp and 370 lb ft and it makes effortless progress. However, if you stoke up the Z4, use the rev range as you can with a petrol, I dare say there would be very little, if anything, in it (I am not taking physics of F=MA into account for this).

My question is this - would a large swept volume petrol, thinking of BMW 545i manual, actually give you the best of both worlds with 330 bhp and a similar torque figure, I would imagine it does. I am not after the 0-60 times but more of an overall driving experience. Fuel consumption not an issue (I run an ST220 estate!!!!)

Thanks

[Chris Hinds]

Anything with plenty of low end torque will give the "waft" experience of a big turbo diesel, whether achieved through a big capacity V8 or even just a turbo on a smaller petrol such as the BMW N53/N55 turbo 3.0. Low rev torque makes a car feel faster because you're delivering more power early... power and torque are inherently related:

Hp = ( lb.ft * RPM ) / 5252

Thus for your ST220
207 lb.ft @ 4900rpm = 193Hp @ 4900rpm

For your C320 CDI
376 lb.ft @ 1600rpm = 115Hp @ 1600rpm

If you _assume_ the power curve is roughly linear between 800rpm idle and 4900rpm in the Mondeo and figure at idle it makes about 40Hp then at 1600rpm the Mondeo is producing...

(((193 - 40)/(4900-800))*800)+40 = 70Hp

Given that, no wonder the Mercedes feels more "waftable" when it makes about 50% more power at the same RPM.

However like you say, as the revs rise the advantage rapidly goes the other way...

Kind regards

Chris

[junglie]

Thank you for that - I have always been fascinated by the 5252 figure of the Torque / Power lines intersecting. I assume the final drive ratios also make a difference as well as numerous other factors.

I love the plus points of both cars but thought about a 'proper sized' petrol with a Spanish box as being fun - large rev range but still some thumping torque. Factor in the fact that many people are frightened of anything that is not a 4 pot diesel and it makes for some keen prices!

[varsas]

Jaguar XJR's have plenty of low down power. Large-ish V8 (or 6) and a supercharger (so no turbo lag) gives very linear delivery, and lots of shove from no revs. And, as you say, they are pretty cheap.

[junglie]

The XJRs are fantastic VFM but not really what I am looking for - the &G box in the merc is great but I want something a bit more involving so a manual box is the way forward, especially while we still have big engines and manual boxes which will no doubt become a thing of the past.

[Chris Hinds]

5252 is derived from the mathematical relationship to what a Horsepower is... according to Wikipedia, James Watt estimated that a Pony could lift 220lb by 100ft per minute over a 4 hour shift. He also considered a Horse could probably lift half as much again thus

1 Horsepower = 33000lb.ft/min

As a crankshaft is rotating, all motion must be rotational too, hence for one turn of the crankshaft the OUTSIDE of the crankshaft moves a distance of 2*pi rads per rev.

Thus

33000 / (2 * Pi) = 5252.113122

Trust that makes less sense than before...

Kind regards


Chris

P.S. Yes people are scared of anything bigger than a 2.0TDI - go buy a V8 now. Audi S4 with Manual box are pretty good value.

[junglie]

Chris,

Thanks for that - takes me back to A Level physics!

Looking at a few potential toys and the B6(?) S4 is one of those. I don't know why but I do like the E60 in M Sport guise. The issue is, would I miss not having the ability to put the roof down? This then opens the equation up to a whole world of hurt with Bike engine kits, V8 kits another TVR and who only know what else! Some, none or all with the V8 option!