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  • why do diesels seem faster?
  • lakesrider
    Member

    Been test driving a few cars recently, and just wondering why even for cars with the same 0-60 times between petrol + diesel versions the diesel ones always seem to accelerate faster?. Now i know they probably arent really faster overall but the just seem to suddenly 'go' when the petrol ones just seem slower.

    Premier Icon fadda
    Subscriber

    More torque + more low end grunt and therefore an increased feeling of it "digging in". The petrol engines will generate their power higher up the rev range and so have to "get going a bit" before it kicks in. A bit "layman's terms", sorry…

    PeterPoddy
    Member

    I don't think they do. Even if the numbers say a diesel angine is faster, they are never as crisp and responsive as a petrol, and as soon as you go anywhere near a corner, the extra weight up front has a marked effect on the handling. Try driving the same car with different engines in back to back……

    Premier Icon thisisnotaspoon
    Subscriber

    power = torque x RPM x K

    K = cosntant, depending on your units of measurement

    a diesel wont rev past 4000rpm, a petrol past 6000rpm and a superbike past 14500rpm

    They may well all produce 150bhp, but the diesel produces 1.5 the torque of the car, and the car 2.4x the torque of the motorbike. (note the assumption of peak torque at peak RPM, in fact nearly every engine will be at its peak arround half its max RPM)

    Obviously they are all geared accordingly, so the actual torque at the wheels is aproximately the same. Which is why they all have the same 0-60 time (apart form the bike which weights fek all in comparison to a car) and the feeling is all in your head*.

    *its not quite all in your head, but nearly all.

    Back when petrols ruled the world this was the debate that always raged (In reverse). Why do diesels always feel so sluggish compared to petrols?

    My own take is that people convince themselves of whatever is the popular opinion regardless of what is true. The only other bearing i would say is that if it was a big car then the extra torque of a diesel will make a similar HEAVY car feel faster in the diesel

    Premier Icon nickc
    Subscriber

    more torques innit, pulls harder feels like faster

    coffeeking
    Member

    Torque curve of a D is often flatter than that of a petrol and the max torque occurs lower in the rev range, meaning there feels like more instantaneous "go" when you need it. The same "go" would be there if you drove everywhere in a higher-revving gear in a petrol, but then you'd use more fuel.

    Premier Icon thisisnotaspoon
    Subscriber

    Simple torque analogy,

    petrol – being punched in the face by a featherweight boxer, he jabs a lot but with little force

    diesel – punched in the face by a heavyweight, theres alot fewer punches, but they'r more forcefull (torque) and thus the overall effect (power in car terms) is the same.

    coffeeking
    Member

    The boxer analogy sort of works, but for the fact that gearing is different between petrol and D, meaning torque to the wheel (and hence power to the wheels) is the same after adjustment by the box. It's only the shape of the curve that affects feel.

    glenp
    Member

    Because in the real world you never drive like you would need to to get the best 0-60 time. Magazines should focus on what a car goes like when you drive it gently – in other words the way it is actually going to get driven.

    So, when you shift at 3500 rpm (not 7000+), like normal driving, diesels seem faster cos that's where the torque is.

    Same with handling – what the car is like on or near the limit is virtually irrelevant.

    ahwiles
    Member

    er, the petrol cars i've driven recently all needed the tits wringing off them before they came alive.

    you can cruise along in a diesel, at about 2000 rpm, and all the torque/power is there when you want it, you just need to press the expensive pedal.

    in a petrol car cuising along at the same 2000rpm, i'd need to drop a cog or 2 to get to the power.

    sometimes it's good fun playing with lots of revs, but most of the time i'd rather potter along quietly listening to radio4…

    PeterPoddy
    Member

    Back when petrols ruled the world

    They still do. Diesel technology has always lagged behind petrol: Direct injection, 16 valves etc. All been on petrol engines long before diesels.

    And I'll say one more thing – Compare a 2 litre diesel engine to a similarly specced petrol one – i.e. a petrol engine with a turbo on it.
    Then let's see who wins……
    🙂

    lakesrider
    Member

    edit – hold on hadnt read all the replies 😀

    Ti29er
    Member

    You'll find that engines of the same cc, the diesels have a higher torque output, and because it reaches it's highest bhp at lower revs, say 1500, you'll think they're quicker.

    Clearly that's just the impression, but the oomph! from a diesel engine is in complete contrast to a petrol engine.

    You might want to look at bhp & torque curves of any given engine if you are really asking this question as some engines only come-on-song way, way up the rev range.

    As for the analogy with superbikes, this quite doesn't hold up since the Ducati 916 (for example) which is a twin cyclinder engine as opposed to an inline-four engine rev'ed to under 10,000, but a single cylinder 125cc bike, that'd scream its nuts off before you were in the very thin bower band at maybe 12,000 revs.

    And on top of all that has been said above – the vast majority of diesels have turbos to improve power output. The vast majority of petrol cars are only fuel injected.

    What you need is a petrol turbo then you can compare them more realistically 8) Or drive a non-turbo diesel. 😛

    Ooops – peterpoddy beat me to it 🙂

    Premier Icon tonyg2003
    Subscriber

    Diesels feel faster because in lower rev "normal conditions" driving – up to say 2500-3000rpm- you will accelerate faster than a normal asiprated petrol of similar BHP but lower torque.

    Diesels have progressed far more than petrol engines in the last 10-15yrs. 15years ago I had a 1.9 ZX TD which was fast for the time. 90bhp and 150lb torque. This is a good performance 1.3l diesel now. Newer diesels also have much better rev ranges – 1500-4000rpm with sequential turbo models. 2.0l, 200bhp, 300lb is a BMW twin turbo diesel now.

    Edit – reading the replies. Turbo petrol is usually faster than tubro diesel of same power – that's why hot hatches are petrol turbo usually. You just have to stop at the pumps more 🙁

    lakesrider
    Member

    hmmm i guess i've found them seeming faster then as i dont really rev cars that high as i'm used to driving diesels, so i'm probably not revving the petrol ones high enough to get to the max power of them.

    coffeeking
    Member

    So, when you shift at 3500 rpm (not 7000+), like normal driving, diesels seem faster cos that's where the torque is.

    Same with handling – what the car is like on or near the limit is virtually irrelevant.

    Sort of pointless though, as most cars feel pretty much the same when driven not-at-the-limit, unless they are spectacularly BAD. You might as well just concentrate on service intervals and costs.

    so i'm probably not revving the petrol ones high enough to get to the max power of them.

    Very definitely the case, most modern petrols run on "economy" cam <3500rpm, i.e. dull and boring. The newest version of the celica is a dog below 5000rpm, and for a very short sprint to 7500 it's nippy, but it makes it feel gutless and horrible for normal driving. If you're used to driving a D you'll be reluctant to push it that high, so will get nowhere near the fun-band!

    glenp
    Member

    Not quite, coffeeking. Hondas and quite a lot of other modern petrol cars are horribly elastic at normal revs – combination of the way the engine is tuned and the set up of the engine management. Really frustrating because it means they are slow to respond and need time to wind-up (or you have to change down). I prefer a normal delivery that is designed for good feel from low revs.

    As for handling – again I don't agree that all cars are similar. I think factors such as ride quality and noise, responsiveness to normal driving input, fun facor etc are vastly more relevant that Top Gear lap time or Fifth Gear drift-ability.

    You just have to stop at the pumps more

    I easily get 44mpg on a run and 35mpg around town out of my 2.0l turbo petrol.

    coffeeking
    Member

    Not quite, coffeeking. Hondas and quite a lot of other modern petrol cars are horribly elastic at normal revs – combination of the way the engine is tuned and the set up of the engine management. Really frustrating because it means they are slow to respond and need time to wind-up (or you have to change down). I prefer a normal delivery that is designed for good feel from low revs.

    Most modern "runaround" cars ARE horribly elastic at normal revs and normal use, that was my point. All reviews would be "all cars in this class feel pretty dull and lifeless". Or at least in my experience with modern cars.

    As for handling – again I don't agree that all cars are similar. I think factors such as ride quality and noise, responsiveness to normal driving input, fun facor etc are vastly more relevant that Top Gear lap time or Fifth Gear drift-ability.

    But relatively similar from one car to the next. Noise difference, I'll give you that, but that's not handling. Responsiveness and ride quality – most tests are between similar classes of vehicle, so most of the tests will be "this will be a niggling tiny bit more harsh over speed bumps", tyre changes make more of a difference than suspension during normal commuting use. All IMO of course. When driving a car for commuting I really couldnt care what it was providing it didn't have fly-by-wire throttle and engine-management controlled acceleration (horrible puntos!), while I may notice a slight difference it's not likely to interest me as it makes very little difference to my daily drive.

    0-62 is a fairly good test of acceleration which does matter in out-of-junction moments and onto fast roads from short sliproads. Track tests are good for an indication of the roadholding in dodgy conditions, even if you rarely put it there. If they excell in those conditions they will probably be fine the rest of the time, unless you like your bum carefully wafted up to speed in leather seats and silent luxury. I find that sort of car boring to drive.

    But then you are talking to someone who has no interior in his weekend car, removed the air-con to save weight and better balance the car and changes wheels based on their handling characteristics, not looks, so that might not align with your own viewpoints.

    I easily get 44mpg on a run and 35mpg around town out of my 2.0l turbo petrol.

    I easily get 17mpg from my 2 litre turbo petrol (though I did once scrape 28 lol). But then I've had max 67 and min 35 from my 2 litre TD too lol.

    I easily get 17mpg from my 2 litre turbo petrol (though I did once scrape 28 lol). But then I've had max 67 and min 35 from my 2 litre TD too lol.

    Well yeah it doesn't mean a great deal, but in essence performance petrol cars aren't that much worse than performance diesel cars and you don't drive everywhere with whiffy hands from diesel and feet constantly slipping off pedals.

    😉

    We have a petrol car that is remarkable like a diesel in the lazy way you can drive it with low down grunt (torque) which is down to good engine design. The car is a Renault megane 2.0 16v coupe (the origanl one) which has the same engine as the clio williams. The engine revs nice and high for the speed thing but due to the cars light weight it also feels like the 1.9 dci we used to have.

    One of the car programs (think it might have been the one on channel 4) used to do a exposed to danger test with the cars. This was based on the length of time it took to overtake a lorry doing 56mph. I always thought this was a good test.

    Premier Icon tonyg2003
    Subscriber

    Hi mastiles_fanylion

    I never get less than 45mpg out of my 2.2tdi and I don't hang around. A newer efficient dynamics diesel should be ~60mpg (over70mpg combined figure).

    coffeeking
    Member

    😀 you and your inability to control your hose! No idea about the feet slipping off thing, I fill up at pumps that have both petrol and diesel, spillages come from other people not me so I'm not sure how my fuel type matters 😀

    coffeeking
    Member

    We have a petrol car that is remarkable like a diesel in the lazy way you can drive it with low down grunt (torque) which is down to good engine design. The car is a Renault megane 2.0 16v coupe (the origanl one) which has the same engine as the clio williams.

    You're having a giraffe 😀 I've got more torque on my head bolts 🙂

    <sorry, slipped into petrolhead mode there>

    All you diesel owners are very touchy aren't you? Are you all wishing you had got proper cars? 😉

    Bimbler
    Member

    Don't really know that much about cars but isn't part of it because all modern diesels are Turbo diesels, wouldn't petrol engines begin to share the same characteristics as diesels if they were turbo'ed too?

    coffeeking
    Member

    Don't really know that much about cars but isn't part of it because all modern diesels are Turbo diesels, wouldn't petrol engines begin to share the same characteristics as diesels if they were turbo'ed too?

    A large portion, yes.

    glenp
    Member

    Well I did run a Caterham as every day all year transport for a couple of years, which takes a certain enthusiasm for cars. I very quickly worked out however that it was much more fun, not to mention considerate, to short-shift and drive it lightly. I liked the sensations of driving it at normal speeds – the steering feedback, the communicative (but not uncomfortable) ride and so on. Plenty of hot hatches would ultimately be faster around a bumpy B road, but who cares? Just a few seconds less before you catch the car in front up.

    Some magazines focus almost entirely on the stuff that makes no difference in the real world.

    coffeeking – Member

    We have a petrol car that is remarkable like a diesel in the lazy way you can drive it with low down grunt (torque) which is down to good engine design. The car is a Renault megane 2.0 16v coupe (the origanl one) which has the same engine as the clio williams.

    You're having a giraffe I've got more torque on my head bolts

    <sorry, slipped into petrolhead mode there>

    I take it then you have driven both of my cars and that you can happly driv eit in 5 at 30 from which speed it will pull?

    Premier Icon tonyg2003
    Subscriber

    "All you diesel owners are very touchy aren't you? Are you all wishing you had got proper cars?"

    I also own a 240bhp V6 petrol 😀 I just don't drive it around as much as the tdi and it does about 26-27mpg.

    coffeeking
    Member

    I take it then you have driven both of my cars and that you can happly driv eit in 5 at 30 from which speed it will pull?

    Not your car specifically, but at least 4 of the same model, yes, when test-driving them for my other half. I'm not sure what happened to the latter half of your sentence but it makes no sense to me, sorry.

    coffeeking – Member

    I take it then you have driven both of my cars and that you can happly driv eit in 5 at 30 from which speed it will pull?

    Not your car specifically, but at least 4 of the same model, yes, when test-driving them for my other half. I'm not sure what happened to the latter half of your sentence but it makes no sense to me, sorry.

    What the other part means is that you can dirve along in a high gear ie 5 a@ 30mph and the car will still happily accelerate.
    Are you sure that they were the F7R engine ones?

    Premier Icon thisisnotaspoon
    Subscriber

    lost the quote but, someone claimed that diesels produce their highest bhp down the rev range, no they peak their torque lower down.

    Torque curves are invariably almost flat. Even a 2stroke motorbike engine has a flat torque curve. The 'peak' is due to a mirriad of factors like the relationship between flame speed and piston speed (in theory peak torque occours when the two are aproximately even).

    power = rpm x torque x K

    which gives a stright line of power Vs RPM, the tailing off at the end is die to things like th eexhaust and inlet not being physicaly able to deal with any more flow. And rev limiters kicking in to protect the engine, theres not much difference in design between an F1 engine and a motorbike engine, except one has a 19k rev limiter (~33% higher = 33% more power) so it only 'needs' to be tuned for 33% more torque (ok thats a lot, but not hard to do if your service intervals are 8 hours and budget is huge)

    coffeeking
    Member

    What the other part means is that you can dirve along in a high gear ie 5 a@ 30mph and the car will still happily accelerate.
    Are you sure that they were the F7R engine ones?

    Ah, ok, yes, well they will accelerate, just not at a rate I'd consider more rapid than my 90hp diesel. Yup, they were the F7R, as we were comparing them against the 1.6s to see if they were worth the extra cost/insurance etc, we concluded not for her usage though it's obviously more torquey/powerful than the 1.6. But don't worry, it was a humorous willy-waving, not meant to be taken too seriously!

    This is a reasonable explanation, if you like graphs:

    You can see the D on the right produces more torque but at a lower RPM to generate slightly less power, but with a fairly flat torque curve. The petrol has a much more variable torque curve (this is especially true of non-performance petrols trying to achieve economy and low emissions). The extra torque from the D is scaled in the gearbox to produce similar levels of torque at the wheels to get similar performance, but crucially that max torque is delivered slightly earlier in the rev range compared to the petrol (when you scale it WRT revs). SO its a small difference, but enough to make a difference to the feel of the car.

    Premier Icon stumpy01
    Subscriber

    Diesel rules. 😉

    Enough oomph to make motorway driving a relaxed affair with a minimum of gear-stick stirring, 55mpg normally, 60+ if I drive carefully & the worst I've had it 46mpg or so with 2 bikes on the roof and a boot literally full of camping gear driving down to Cornwall on busy roads.
    Nice and quiet at 70mph while revving away at ~1800rpm in 6th.
    It's long geared so the lack of revs isn't an issue, either.

    I think the 20-50/30-70mph type acceleration figures for diesels are quicker in 'normal' driving conditions when you aren't purposefully ragging the nuts off the car.

    Petrol & diesel both have their good points – mates Corolla T-Sport is undoubtedly quicker off the line than my diesel Ibiza, but in real world driving – it'd be a close call & I'd just waft past him as he pulls into the petrol station while I've still got 200 miles in the tank still….but his does feel like more fun.
    Horses for courses…..

    Fair enough. I find it good if a little high reving when on th emotorway but dam good fun on the back roads and find that its pefectly happy around town, even though the ride is a little harsh for that, well compared to the xc90 it is.

    coffeeking – Member Ah, ok, yes, well they will accelerate, just not at a rate I'd consider more rapid than my 90hp diesel. Yup, they were the F7R, as we were comparing them against the 1.6s to see if they were worth the extra cost/insurance etc, we concluded not for her usage though it's obviously more torquey/powerful than the 1.6. But don't worry, it was a humorous willy-waving, not meant to be taken too seriously!

    i din't say it would do it quickly 😉 you need to drop it to second for that. We looked at both and decied that the difference in running costs wasn't large enough to concern us and went for the best example we found.

    coffeeking
    Member

    I think in a nutshell diesel=lazy speed, no need to change gears to get it to shift a bit, providing you're above the turbo spool point (which is usually bearly above idle). Petrols tend to require a shift, unless they're very impressive and large displacement, which is like comparing apples and oranges to be fair.

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