Oops, going on a bit now, but just adding the point that in my mind I equate a diesel powered car with a petrol engine that's 20-30% more powerful, but not driven to it's fullest revs, which lets face it, that's the norm for most cars on their daily journeys on congested roads..
i.e. I compare in terms of I think a 180bhp diesel drives like a 220bhp petrol engine, providing the driver of the petrol never exceeds 4000rpm. :)
How many people do you see driving who regularly shove their engines above 4000rpm with a manual box? Even an auto in kickdown doesn't tend to go much more above that until you really push it...
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Well my motor don't even get going till 6000 revs, happily revs right up to 9000 revs
That REALLY is outstanding for overtaking on the motorway.
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I am struggling to express myself here.
Power is Torque x rpm - you can't just ignore the rpm part. That's without the pleasure that using a good petrol engine hard brings to an enthusiastic driver, and which no diesel, for all its accessible shove, can get close to.
Yes, a diesel does more useful work (makes more power) at "everyday" revs, and yes they are often quicker in potter about, A to B, lazy gearchange driving. They excel at providing fuss free thrust in response to a simple prod of the accelerator pedal, which is just what you need in a family car or a motorway cruiser. They also cope with lardy bodyshells and heavy loads far better.
In many ways they are superior for general motoring, but that is not the whole story. The feel of a good petrol engine as it climbs on cam, builds its power delivery as the revs climb, and howls for the redline is a simple pleasure that some of us get a real kick out of.
To take an extreme example, what is more emotive to listen to, a Formula one car, or an Audi R10 TDI Le Mans car? And that's without the engine braking and response to small pedal inputs that diesel engines just don't do. A good chassis, which lets you adjust line on the throttle for example, will always feel more agile and responsive with a petrol engine. They are also lighter engines, which makes them much less challenging to place for optimum handling. There is a reason you can't buy a diesel Lotus Elise or Caterham.
I've posted on here many times defending modern diesels. We have one, and it's superb to drive. I wholeheartedly agree that on today's roads, in today's heavy cars, diesel engines make a lot more sense for general motoring. But impressive as they are, I see them as engines for motoring, not driving. Many see wringing a car's neck on the public road as irrelevant or irresponsible, and that's their look out, but petrols will always do this better than diesels. I am happy to drive a diesel, but frankly not one of my top 10 lottery list is powered by anything other than Super Unleaded.
All IMHO.
Cheers
DP
Edited by DP on 03/03/2009 at 15:08
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DP, I sympathise, I'm also completely failing to get my point across.
I believe that the only sensible measure of revs is a % of maximum revs. How power and torque are delivered in relation to that maximum point, along with the gearing, will determine what the car is like to drive. Anything else is just irrelevant IMO - I'm really really not trying or interested in reigniting the diesel vs. petrol debate.
My point is this - most diesel and petrol engines have useful power between about 40% and 100% of their maximum rev range. That's true pretty much whatever - a massively revvy petrol will probably have not much below 40%, a slow revving diesel likewise. Most cars will be geared to put this range, 40% to 100%, at a useful place relative to road speed.
The *absolute* number of revs is meaningless to this argument; if I removed the numbers and gradations from a rev counter, and made sure you couldn't hear the engine, then I bet you'd not be able to tell the difference between a diesel and petrol turbo in terms of performance relative to % max revs. You'd get about the same speed range in a given gear: for example, both would build up to 100% revs as your reached about 80 mph in 3rd.
Yes, in general the power and torque delivery curves are slightly different shapes across that 40 - 100% range, and you may have a strong preference for one shape of curve or the other (I know I do), but the absolute number of revs is just irrelevant - you run out of gear when you reach 100% of the available revs in either engine, not when you reach some aribitrary number.
I think. I also think I may be on my own on this one.
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Gordon I get your point. Let's say we do away with rev-counters for arguments sake.
Who could tell the difference between 100mph at, say, 3500rpm driving a diesel and 5000rpm driving a petrol car? What about an electric engine? How would you measure that?
They are simply different types of engine.
I think martini23 is right - it is not the rev range that is useful but the range translated into mph.
I have accidentally pulled away in third in an E-Type and accelerated smoothly all the way to 100mph. (Without 'thrashing' the engine.) Such flexibility seems to be rare these days.
My Triumph Sprint, I have to say, seems to offer the best of both worlds. More torque than many sportsbikes at half the revs but if you feel like giving it some revs - wow! what a noise!
(I have been told that Triumph engineers designed it so that all the induction noise is aimed back at the rider. He (or she, of course) gets the full benefit without being obtrusive.
Listen to a Triumph triple overtaking you and all you here is a sort of whistling noise but the rider will be thinking "Boy, this sounds great!"
Whoops - I did not mean to hi-jack your thread or change the subject :)
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In many ways they are superior for general motoring but that is not the whole story. The feel of a good petrol engine as it climbs on cam builds its power delivery as the revs climb and howls for the redline is a simple pleasure that some of us get a real kick out of.
Now that I agree with 100%. Which is why despite defending diesels to the hilt above, I might still find myself looking at a petrol next time.... :D
OTOH, a Volvo D5 or BMW x35d when pushed hard also sounds quite sublime in a way that puts most 4 pot petrols to shame!
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The way I look at it in the real world all the extra revs a petrol engine provides are meaningless until you get to 50mph+ and you're willing to drive along with the engine screaming.
But now you're just as guilty of stereotyping as the people Gordon was bemoaning at the top of the thread.
Not every petrol engine is a Honda V-TEC. My own petrol car has plenty of torque right from the bottom of the rev range, thank you. I can bumble along in one gear and be as quick as 90% of the cars out there. But, being a petrol, I can if I wish allow the engine to sing as you describe, and be quicker than 90% of the cars out there instead! (It doesn't scream though - it growls ;) )
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I wish engineering recognised something like an 'octave' of power.
An octave is the frequency spread from one frequency to double that frequency.
In engineering terms, we could look at an octave of 1000 RPM to 2000 RPM
or 3500 RPM to 7000 RPM.
Using this concept, a modern petrol engine has a rev range of about 2.5 octaves, a diesel is not far off that, I'd suggest about 2.3 octaves.
But - the diesel does hit its upper limit a bit suddenly !
For the Honda V-Tec to have 3 octaves of power, it would need an 8000 RPM limit and still produce usable power at 1000 RPM - and it's *almost* there. I can't think of an engine that can produce a full 3 octaves of power.
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Trouble is, Vtec engines tend to be rubbish at low (engine) speeds. Suitable for boy-racers only, methinks.
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Trouble is Vtec engines tend to be rubbish at low (engine) speeds.
Exactly! I like the octave way of thinking about it. Revving hard to 9k above 6k does not make it easier to overtake, it means that you've only got a half octave powerband, albeit a brutal and highly effective half octave.
A 'normal' petrol or a diesel will have more like 1.5 octaves of really useable power, geared with overtaking in mind generally, and will surge past things without needing a gearchange.
Once again, I am not arguing for a particular engine; indeed, it's notable that one of the things that the MX5 is famed for is not having much power but having a lovely gearbox - you rev it hard and change a lot, making for an incredibly involved drive.
My problem with a high revving petrol is exactly that - if I need to accellarate from 50 at 6k rpm, I'll be out of steam at 75. That's no better than a diesel, not in purely factual terms anyway.
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But why would you want to rev to 9000 (which is thrashing it, with all the associated wear and tear and fuel consumption etc) when a different engine could achieve the same results by changing up at 5 or 6000?
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Is that necessarily the case Madux? Every engine will have a design specification, and while revving to 9k would certainly kill an engine not designed for it, I don't see that it is necessarily bad for an engine that is designed to do so...
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For the Honda V-Tec to have 3 octaves of power it would need an 8000 RPM limit and still produce usable power at 1000 RPM - and it's *almost* there.
I disagree with that - I was using the VTEC as the archetypal example of an egine which sounds incredibly powerful on paper, but has so little torque that you're eaither scrambling round with your pants on fire, or hardly moving. It's the type of engine which gives petrol engines a bad name in discussions such as this.
My Alfa V6 pulls strongly from little over 1000rpm to 7000 and above.
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Not every petrol engine is a Honda V-TEC.
I wasn't thinking of that. Instead I was thinking of the Zafira 2.2 auto I drive most days. Supposedly a torquey petrol engine, and yet the power delivery is so flat and uninspiring it simply doesn't lend itself to having fun at all.
It doesn't help that it only has 4 gears, so at normal road speeds it can only kickdown to 2nd or 3rd and it still isn't generating anything like maximum power by the time the speed limit is reached.
Perhaps I'd be more impressed if it was attached to a good 5 speed manual box.
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I actually have a Honda 2.2 Vtec. It isn't rubbish at low engine speeds, it is smooth and pulls excellently. As the revs rise it pulls strongly and then at 5200 rpm - wham. It is perfectly possible to drive on suitable roads in nothing but second and third gears (of the four-speed auto/sequential box) with huge acceleration and flexibility at the same time - the 8000 (8300 on the limiter) red line means repeated gearchanges are unnecessary and the maximum speed available is well illegal.
It also ticks over almost imperceptibly, makes a lovely yowl when you wring its neck, and does 35mpg (if you don't Vtec it too much).
Back along I drove a diesel Freelander II with six gears. Yes it was torquey and lively but every time I opened it up it ran out of power in a nastily abrupt fashion at about 4000 rpm, just as it seemed to be getting into its stride and meaning I was up and down the gearbox all the time.
The truncated rev range didn't seem like a myth to me.
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Yes, I got a Honda Civic 1.8 vti Estate. Totally usable and granny mode till 6000 then wham. Will take on much more powerful cars which at 6000 are out of puff. They change down, off the powerband (like 210 becomes 150 till the revs rise again) I go on to 8200 and they are a small dot in the mirror. The look of amazement when the old git in the estate car blows them off is wonderful. Not that I get excited over these childish actions - of course!
Best description is like filling a bath of water. Use a small cup to try and empty it. Use a bigger cup to empty it quicker. Or use the smaller cup more often.
Understeer is when you hit the wall with the front of the car.
Oversteer is when you hit the wall with the rear of the car.
Horsepower is how fast you hit the wall.
Torque is how far you take the wall with you.
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Gordon, I've got your point and I'm with you.
Perhaps most drivers would be better off without a rev counter and then they wouldn't be conscious of the rev range used. And in any case surely it's the length of the gearing (poor words I know, but I'm no engineer) allied to the engine that matters. Other contributors have a good point about the 'feel' of a good petrol engine and throttle response at higher revs, but as a paying customer I have to go for economy and torque is rather luxurious. there are some occasions when I miss a good petrol though.
I wonder if we'd all been brought up on Diesel engines and someone developed this new fangled petrol engined thingy. I bet we'd all be saying how limp and thirsty it was.
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"I bet we'd all be saying how limp and thirsty it was"
Or smooth and quiet... :-)
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A highly unscientific experiment carried out by me using a petrol and a diesel car of similar power has established the following:
In third gear, the diesel (a Citroen) pulls strongly from around 2000rpm. This equates to 35mph. The oomph runs out of steam at 4000rpm/70mph. The engine is limited to 5000rpm, by which time you can hear a very well muffled thrashing noise from the engine.
Doing the same experiment in a petrol Alfa Romeo gives decent acceleration from around 25mph, which is below 2000rpm. The red line is at 7000rpm, but the car will continue accelerating past this to over 100mph, by which time your ears have the pleasure of listening to the magnificent howl from the engine.
Therefore it is not just the rev range that is superior in the petrol engine, but the range of speed available before you have to change gear. That's not to diss the diesel - it's a fabulously relaxed motorway cruiser, but the petrol engine is far more fun for the driver. That's why I have both cars - horses for courses and all that.
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James, you've just proved my point. Sort of.
Doesn't matter which is petrol and which is diesel.
Your diesel works from 40% to 80%, so a 40% useful range.
Your petrol works from 25% through to 100% (110% perhaps given your description), so a 75%+ useful range.
The fact that the diesel runs out at 4000 is immaterial, it's the fact that the two engines have differing useable power bands (in % terms) that makes the difference.
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