BHP - Diesel and Petrol

Tony Danza

Inspired by the other thread on BHP, I'm wondering, why is it that diesel engines seem to have a lower BHP. For example, friend of my has a 97 primera 2.0TD. He used to have a 1.6 Avensis. Apparently the Avensis has somewhere around the 108 BHP mark, where as the Primera has something like 80 BHP (correct me if I'm wrong, but I'm almost certain on the Avensis BHP), yet I've driven both cars and the Primera hands down is the quicker of the two.

Also you see Golf/A4's/Passat advertise Red I's with 130BHP or so on and I'd be fairly confident that they'd go quicker than my 2L Petrol 406 coupe, which has 135 BHP? What am I missing here, I assume that it's not a fair comparison comparing the BHP of a petrol to a diesel?

Esel

Torque, not BHP, gives an accurate reflection of the acceleration ability of a car. Diesel engines produce much more torque than equivalently sized petrol engines, and produce it at much lower engine revs.

gyppo

The ingredient you are missing is Torque, the turning force available at the driven wheels. A diesel car has a narrow powerband, and can deliver most of its available power faster than its petrol counterpart.
The diesel engine should start to feel lively to drive from 1800-2000 rpm upwards, whereas a similarly sized petrol engine probably needs to be at 3000rpm.
Theres more factors to be considered, I'm sure other posters will fill you in.

Andrewf20

It seems that some car manufacturers tune the diesels very conservatively, to be more fuel efficient, but low on power. 80bhp seems a bit low for a 2 litre diesel. My brother recently got a 1.9 tdi chipped from 115bhp to circa 150bhp, so more could be got from that 80bhp Primera lump.

Its true that comparing max BHP between 2 cars doesnt tell the whole story. Average power over the rev range would be a better indicator. For example the Corolla GTI thingy has 189bhp but its 0-60 is dismal at around 8.9 seconds. (Autocar figures). Why? - because there is little pull from its NA 1.8 litre engine until, the revs get very big. A 130bhp diesel Mondeo would definetly give it a run for its money.

Diesels pull well from lower RPMs mainly due to turbos.

Im looking here at stats on the 2.3L diesel Opel Rekord from the 1980's. 65bhp, 0-60 in 22 seconds. Argh!!

Andrewf20

esel wrote:

Torque, not BHP, gives an accurate reflection of the acceleration ability of a car.

Thats not necessarily true. Average power over the rev range is the best reflection for how the engine accelerates.

Tony Danza

I was thinking torque would be a factor alright, but there's gotta be a lot more too it that?? How is the best way to measure power output??

Mayshine

The best is high torque produced at high revs. Why? because the gearing takes advantage of this. Gearing muliplies engine torque and gives you the torque that the wheels are putting down on the ground, what you feel. more revs = more torque multiplication

Therefore max torque is more useful at 6000rpm than 2000rpm if you want to maximise acceleration.

Diesel also do not have flat torque curves becuase of the turbos and as such sometimes feel faster than they actually are. Also they have longer gear ratios because of their inherant low revs which is why peak torque figures are quite misleading without context.

Andrewf20

Rolling road will give you a figure of power at the wheels.

Anan1

Tony Danza wrote:

How is the best way to measure power output??

The best way is bhp, or Kw.:) Think of torque as the amount of twist the engine is putting onto its flywheel. This then goes through the gearbox, which can either give you out more torque at lower revs or less torque at higher revs. BHP/Kw is a product of torque and revs, and is therefore the best measure of how much power an engine can put out.

nastysimon

esel wrote:

Torque, not BHP, gives an accurate reflection of the acceleration ability of a car. Diesel engines produce much more torque than equivalently sized petrol engines, and produce it at much lower engine revs.

Power determines acceleration. Plain and simple. The thing is that a diesel engine usually produces more power at low revs, but a petrol ultimately produces more power because it revs higher. Therefore, a diesel will be better at pulling from 2000rpm, but the petrol engine will ultimately give better pulling power if you are willing you use all of the revs.

The thing is that most people do not use the revs, something about not liking to go over about 3000rpm. If you drive the petrol engine in its rev band, and the diesel in its, the more powerful car will be quicker, but you'll be doing 5000rpm all of the time in the petrol. If you are just measuring acceleration from cruising speeds in top, a diesel will usually be quicker as it will already be in its rev band, but you'll be well outside of the petrol engine's one.

JHMEG

nastysimon's explanation is spot on.

A diesel will out accelerate a petrol in top gear. But that's what gears are for.. drop a cog or two in the petrol and bye bye diesel.

maidhc

I have to say this with every one of these arguments, but owning a Petrol car and a diesel of similar bhp, the diesel is superior when it comes to drivability, it you don't need to keep stirring the box, and it doesn't peter out going up hills.

For straight line acceleration to 60mph, the petrol capri is definitely quicker, but at 70mph+ the poor pinto screams its head off when you go looking for more power while the TDCI is only starting to get going.

The differences are more pronounced if you have a loaded car or even a trailer. Torque is vitally important, don't ignore it.

The petrol engine is more "fun", but as a daily driver I would take a diesel any time.

Gerry

Turbocharged petrol car is the way to go Plenty of torque low down, but plenty of top end power too. Can be driven like a diesel, but also keeps going after 3000 rpm

Anan1

Or just get an auto and let the car sort it out.

nastysimon

Gerry wrote:

Turbocharged petrol car is the way to go Plenty of torque low down, but plenty of top end power too. Can be driven like a diesel, but also keeps going after 3000 rpm

Big petrol engines are the way to go They have the same benefits without the poor throttle response, and less than smooth power delivery. They can weigh about the same (all that plumbing, the turbo, and intercooler all weigh a fair bit) and can even give better MPG. Give me a 'Vette Z06 engine over those little 2L turbo 4 cylinder toys