Petrol or Diesel for towing.

Jul 15, 2005
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Is there much difference? In a word, yes.

Petrol engines convert about 25% of the energy in the fuel into motive power, whereas a diesel engine converts about 33% of the energy. This is the major reason why a diesel engined car achieves 45+mpg and a similar petrol engined car achieves 33+mpg.

A normally aspirated petrol engine, like the 16 valve 2.0 Mondeo engine, will generate maximum torque above 4000 rpm

A turbocharged diesel engine, like the TDCi Mondeo engine, generates maximum torque below 2000 rpm.

This difference in torque makes a big difference to driving the car with the van on the back, the diesel will happily pull the car and van along at 55+mph in 6th gear. The car will still accelerate and cope with small hills.

It may be possible to get the petrol engined car into 6th (possibly 5th) but any increase in load (slight rise in the road, or you need to accelerate) and you'll be changing down.

So diesel, better mpg and better torque - easier towing.

Robert
 
Feb 3, 2006
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I would like to add, that what rob says is true for the mondeo but not all petrol engines. Subaru make a nice turbo petrol engine in the forester that has 236lb of torque but also a very flat torque curve too + the fact that petrol engines are very tractable at low revs (below say 1500rpm) + petrol engines have more 'raw' power enables the subaru petrol to be an excellent tow car with as much low down grunt yet more power for overtaking. The down side is as rob says fuel consumption. It also depends if you can put up with the diesel clatter, I couldn't so I bought a 3.0 flat six petrol with more than enough low down pull to embarrass any diesel and still manages 30mpg solo and 24 towing, with the low mileage I do I am not concerned if i don't average 40mpg
 
Nov 1, 2005
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The main difference is in the way torque is produced. Petrol engines although producing max torque at relatively high rpm, have wide flat torque curves. It's often possible to see over 90% of a petrol engine's peak torque at any rpm. Diesel engines on the other hand produce massive torque at idle, but the curve quickly falls away above about 2000rpm. That's why large diesel lorries redline at about 1800rpm. As long as your cruising speed equates to this rpm or less torque will never be a problem. I personally prefer a petrol engine, I like the instant power delivery and power through the rev range, but that's just my opinion. Most people who choose diesels do so not because of power or torque, but for fuel economy. I think in the case of a fairly light car like a Mondeo you'd find very little difference in performance, they just invite different driving styles.
 
Sep 7, 2005
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I would like to add, that what rob says is true for the mondeo but not all petrol engines. Subaru make a nice turbo petrol engine in the forester that has 236lb of torque but also a very flat torque curve too + the fact that petrol engines are very tractable at low revs (below say 1500rpm) + petrol engines have more 'raw' power enables the subaru petrol to be an excellent tow car with as much low down grunt yet more power for overtaking. The down side is as rob says fuel consumption. It also depends if you can put up with the diesel clatter, I couldn't so I bought a 3.0 flat six petrol with more than enough low down pull to embarrass any diesel and still manages 30mpg solo and 24 towing, with the low mileage I do I am not concerned if i don't average 40mpg
Baz, sounds like you have the H6 Legacy. How on earth do you get 30 mpg average? Off a cliff?

I can just break 30mpg on a long run at 65 in mine (Outback) but mostly 23 mpg through commute via M25 and West London.

Aside from that, diesels are foul, noisy, gutless excuses for motoring. Even the new ones. But they are beautifully lazy and therefore, ideal for van towing, once you've spun up the blower.

If you can stand the noise. Owners of silent H6 Scoobys will understand

And, PC, next time you review the big Scooby, please get the right picture in the mag.

Nag over, bed, night.
 

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