I've got a loan of a 130 hp diesel Passat in place of my 100 hp one. The difference in performance is astonishing with, to me, a violent shove in the back from 2 - 3000 revs.
Not wanting to upgrade my car, but what makes such a difference to an outwardly similar engine?
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This is probably a silly answer but I'll post it anyway to try and get a technical discussion going on this interesting question.
Supposing we take a particular instant in time during vehicle acceleration at which point the various transmission, rolling and air resistances absorb about 25% of the engine power (this figure purely 'out of the air'). This leaves us 75 hp from our original 100 hp for accelerating the vehicle. Boost the engine power by 30 hp and we now have 105 hp available for acceleration. Thus we have a 'usable' power increase of close to 50% although the actual engine power increase is only 30%.
I have an interest in this question myself as my 131 ps Rover 75 is so much more lively than the 115 ps demonstrators that I drove.
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This leaves us 75 hp from our original 100 hp for accelerating the vehicle. Boost the engine power by 30 hp and we now have 105 hp available for acceleration. Thus we have a 'usable' power increase of close to 50% although the actual engine power increase is only 30%.
A very interesting point and one I had never thought of! Actually 40% though if your estimates are correct(pedant).
I also suspect that there is an element of perception of speed and power. One of the reasons what modern TD's feel so fast is that they have so little torque low down, then a great bug sudden whack of it.
This issue was well illustrated by my mate's Imprezza turbo. Though well up on peak power from the standard engine tune, it didn't feel particularly fast, because he had adjusted the engine management so that the turbo boost increased gradually through the rev range rather than all-of-a sudden.
Whilst the Passat 130 and Rover 131 have more mid-range torque they probably have no more low-end torque, the rate of increase is probably much more rapid.
Valid?
Andy
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Interesting point about the contrast between on-boost and off-boost. When I first got my Tdi the new engine was very laggy. You would put your foot down and then wait up to half a second before the boost would kick in. Although this was no good for smooth driving it was quite good fun. Travelling down a straight road I found myself putting my foot down just to feel the turbo kick in. The lag disapeared after the first thousand miles and the car is arguably faster and more drivable but not as much fun.
I'm sure I remember a motoring magazine coming to the same conclusions when comparing a first generation 911 turbo to a contempory one.
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Andy,
I agree that the gain is not 50%; it's exactly 40% as you say. I must sharpen a point on my pencil.
However your comment about the element of perception with modern TDs certainly doesn't apply to the Rover (BMW) engine. This engine pulls smoothly and relatively strongly from under 800 rpm right the way through to the red line with no detectable step in torque production. Further, the 131 ps version produces its max torque at a lower point than the unchipped version, at 1900 rpm against 2000 rpm.
I don't have any torque curves to show what happens below those engine speeds but, having driven both versions, I'd be very surprised if the chipped version loses out lower down. The fact that it pulls without hesitation from 800 rpm in fifth (with a gearing of 30 mph/1000 rpm) shows that little is missing.
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I believe the thump in the back you get with the VW diesels is a result of the Pumpe Duise injection system. Common rail systems seem to offer a more linear delivery because of something to do with pre-injection before main injection per stroke which is not possible with the PD system. (Or something like that - hopefully someone can add to this or correct it!).
What I do know is that my Mondeo TDCi 130 does not have this sudden "thump" when compared to the Passat PD130 I had previously although it has more torque.
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I think it is basically down to the way in which the car is set up (ECU Fuelling maps etc etc)
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Both CR and PD engines use electronically controlled staged injection in order to reduce engine noise and minimise emissions. The difference is in the way the fuel is pressurised. A CR engine uses a common pipe that feeds all the injectors (the common rail!), and is pressurised to extremely high pressure (like 1600 bar) by a pump driven by the cam belt. On a PD engine the high pressure (not sure of the figure, but I'd expect similar to CR) is generated at the injector by extra cams on the cam shaft (hence the the need for such high-spec oils in these engines, as this cam generates phenomenal shear forces).
RichardW
Is it illogical? It must be Citroen....
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