Safe Speed Forums

Having seen the recent JCB exploits it re-kindled a question I've been wondering for a while...

Bear with me on this one.

Looking at buying a new (used) car. Currently drive a 1.9TD Peugeot. Have only driven small-engined (1.2) petrols or an old (1982) 1.7 Volvo 340 / Tank.

Wondering about the old argument of diesel vs petrol. I like my current Pug's power when overtaking between 40 and 60 and everyone goes on about how diesels are good for this. Are they better than equivalent petrols for doing this quickly, therefore safely? Every diesel fan goes on about torque and how it "wins" in day to day driving but wanted your expert advice. Not driven many different cars enough to see for myself and a test drive doesn't normally give you chance to experience it for a fair indication.

Like I said, bear with this.

Looking at figures only (from Used Car Mag) I found:

Citroen c4s

1.6HDi - BHP = 110, Torque 177 lb/ft, weight = 1270kg, 0-60 = 11.2

1.6 petrol - BHP = 110, Torque = 108 lb/ft, weight = 1200kg, 0-60 = 10.6

Equal BHP, petrol has less torque but "wins" to 60? Surely this would be best for overtaking?

Again: Volvo S40

2.0D sport. BHP = 136, Torque = 320 lb/ft, weight = 1371kg, Top speed = 130, 0-60 = 9.5

T5 2.5 Petrol. BHP = 220, Torque = 320lb/ft, weight = 1392kg, top speed = 149, 0-60 = 6.8.

Petrol has equal torque, is marginally heavier but is quicker than diesel in 0-60 and with a top speed 19mph more.

I know the arguments about the diesel using less fuel for equivalent economy, but what I'm interested in is if I'm driving along behind a car travelling 50-55 in NSL and an overtaking opportunity comes up, can I do this quicker and safer in a petrol or diesel? I know they all vary, so you can take one of the two examples above - either the C4 with identical BHP or the Volvo with identical torque figures. I know that in gear the diesel will be quicker due to low-down torque, but seeing as how I think about when I will overtake I am going to be ready and able to down-shift for power.

So...rapid and safe overtaking vehicles that are travelling at the following speeds:

25 (tractor)
40 (40 everywhere brigade)
55 (playing it "safe")
70 (motorway / dc)

Diesel or petrol?

Thank you.

A diesel will overtake with less apparent effort in these circumstances - to get the equivalent performance in a petrol car you have to rev it very hard which a lot of people don't feel comfortable about doing.

It is also more likely you'll be able to do the overtake in a diesel without changing down.

Having said that, the fact that diesels deliver their best performance in a mid-range power band isn't to everyone's taste.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

I think when comparing petrols and diesels you need to look at the 0-30 & 40-60 acceleration times rather than just 0-60. I also drive a 406 1.9TD and find from 40-60 it will easily beat a lot of petrol cars...but from 0 it is very sluggish.
I don't think there's any clear answer when deciding which is better for overtaking - it all depends on immediate circumstances. For the tractor at 25mph, I'd say petrol is better - don't know about yourself but I find I need to take my car down into 2nd to accelerate quickly past at that speed. However, for the 40/50mph everywhere driver I reckon diesels will win, if only because of the mid-range power that PeterE mentions. And as for motorways, I think at 70mph+ it becomes fairly even and is down purely to bhp/engine size rather than fuel type. I often find that I feel I'm overtaking quite swiftly at these speeds until I spot some 10l BMW/Audi/Merc roaring up behind

Just my experiences; I'm sure someone here who knows a lot more about it than I do can provide much better answers

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Why can't we just use Common Sense?

For overtaking it's acceleration from mid range speeds that matters - things like 40-60, 50-80, and 65-whatever

Don't forget that all the "high-performance" diesels are TURBOS - find me one that isn't. It is also true that smaller diesels (by which I mean not huge rather than small) do indeed tend to deliver their power "in a lump". If the lump is where you want it - fine.

I have a 2 litre petrol turbo SAAB (high output Aero) and it's acceleration from steady speed up is mind-boggling! Any huge engine (regardless of whetehr normal aspirated or not) will of course do the same though

Comparing engine torque in this way is misleading. See: http://www.safespeed.org.uk/forum/viewt ... 2073#12073

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Paul Smith
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1. Have a close look at the road test results in Autocar for 30-50, 30-70 and 50-70. These can be quite revealing.

2. In the case of the two Citroens with apparently the same power but the diesel with the higher torque accelerating less rapidly, have a look at the weight figure. The diesel engine probably weighs more, and you should consider the affect of this heavy nose on the handling.

Diesels will also accelerate slower to 60 because of the gearing meaning an extra change is required; this also affects times for 30-70, for example.
I find that diesels regain the advantage in the 20mph increments despite taller gearing; plus as revs are mid range rather than closing on the redline it's less frenetic.
Sometimes I like frenetic, but most of the time I like not HAVING to revs the nuts off the engine.

There's simply no arguing with a test drive!

It's almost impossible to look at the figures and try to imagine it. I have a 3 litre normally aspirated car with just less than 200 horses amd more torque than you could ever want! My wife's is 140 horses and no torque to speak of. Both petrol. Normally, I drive a diesel MPV (Peugeot 110 HDI) which is very heavy and, of course, a diesel. My car feels the quickest but when you actually look at the speedo, it doesn't actually GO that quick! My wife's is better. Although it has no torque, it has a vey wide power band so you can overtake and let the engine rev to 7000 whereas in my car, it's all over by 6000 and it needs another gear. The MPV - despite being the least powerful AND by far the heaviest, doesn't pose much of a problem with overtaking either. You could try looking at the 50-70 times in top gear quoted in road tests. Up to a point, these are a good indicator of overtaking ability but who in their right mind would stay in top gear in my wife's car to overtake? Whatever you drive, you soon adapt driving style to get the best out of it!

In my opinion, the arguments for having a petrol engined passenger car these days are getting harder and harder to win. If you do more than (say) 20,000 miles a year, the arguments for a diesel are pretty much overwhelming. If I was choosing a new car tomorrow, it would be a diesel!

(...and you can "chip" diesels to get even more out of them) !

PeterE is exactly right.

The diesel will "appear" to accelerate better if you lazily pull out to overtake and stay in a high gear. And in the real world this probably suits most people.

But if you are prepared to drive the car then the petrol will usually give the better performance.

The real barometer of maximum acceleration performance is not absolute torque figures in foot-pounds, but power to weight ratio. In this case the petrol will generally win hands down as it will typically be both lighter and more powerful than the equivalent diesel model, but this depends absolutely on using the rev range fully.

So the answer is that if you are happy to use the gears and let the engine rev when overtaking the petrol is the one to go for, but if you tend to employ a more relaxed driving style and "waft" past other cars in a high gear then get the diesel.

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Yes, very well put.

Another consideration is that if you are regularly carrying passengers, they may find a "brisk" driving style in a car with a relatively small petrol engine appears a touch brutal.

Of the two Citroëns with equivalent bhp figures, the diesel will certainly subjectively feel better at overtaking in the 30-70 speed range - but you do have to pay a hefty £1745 price premium for it.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

Until last year, I had a Merc E320 CDI.

In terms of speed and accelaration, it outperformed its predecessor - a Scorpio 2.9 12v. - in normal driving; but it could get caught out.

I used to commute on the A34 from Winchester to Newbury and travelling north from the A303 junction is uphill (quite steeply). If I was baulked there by trucks, the Merc took an age to accelerate as it was 'off the turbo' in a high gear and near the top of the power band for the next gear down. No matter what I did - either used the tiptronic or left the 'box to sort itself out - it was slow to accelerate up the hill.

PS. I now have a Jeep Grand Cherokee with LPG and with a 4.7L V8, you hardly notice the hill is there.

Or you could get a petrol engine along with a suitably agressive automatic gearbox (mine is quite happy to kick down from 4th -> 2nd and rev up to 7.8K) and have the best of both worlds

Smaller turbos are better than big turbos for this as well, which helps the diesels, as turbo petrol cars are usually set up for high boost and big power at the top end.

just to throw my two penneth in, my car is a C4 2.0HDi and having tried the 2.0 petrol have found it's 30 to 60 slower than the diesel but it's 0-60 is faster, also the C4 is quite revvy for a diesel, drop a gear and it really acclerates.

Pays your money and takes your picks

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up untill 2002 I had a 1.8 litre 90hp MKII Golf driver, it was swapped for a 1.4 litre 89hp civic. The Golf would do 70 in top at about 2900rpm, the Honda is doing 3500 at 70.

The Golf would accelarate faster in top than the Honda does in 3rd . Suffice to say, my overtaking style has had to change. Overtakes normally are booked a couple of days in advance now...

I do often wonder how accurate the 0-60 times are.

My car

1.6 petrol - BHP = 120, Torque = 146 lb/ft, weight = 1200kg, 0-60= 10.6

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Actually, I'm sure you'll find that the torque on the 1.6 Alfa engine is 146 NM which equates to 108 lb/ft. It's, relatively speaking, a high-revving, sporty engine and so has high power in relation to its torque.

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"Show me someone who says that they have never exceeded a speed limit, and I'll show you a liar, or a menace." (Austin Williams - Director, Transport Research Group)

Any views expressed in this post are personal opinions and may not represent the views of Safe Speed

I used to find the 5.3 litre V12 Jaguar provided a rather nice answer to that. Happy days.

Best wishes all,
Dave.

Ohhhh Yesss

60 in first "Y" in second! and we really shouldnt talk about top!

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"The road to a police state is paved with public safety legislation"

Excuse my ignorance: what does ' "Y" in second' mean?

I've never driven anything that would pull 60 in bottom gear, although I am impressed with my current jalopy that if asked (on the continent of course) would kick down at 115, and where 40 can be achieved in each of the five gears depending on mood.

"Y" means "An impressivly large number"

The rated top speed was a shade under 150 "Units"

What is more the peak torque was at fairly high RPM so as you built up speed you actually accelerated faster!

Thogh one almost certainly wasnt (Even fast cars dont accelerate *that* fast) you got the feeling that you were starting to slide up the seat back!

The shift from 1st to second was a bit like that scene in "Appollo 13" where the first stage booster cuts out and the seccond cuts in!

The momentary pause in acceleration made you lurch forward slightly during the gear change before slamming you back in the seat

I had a lady frend at the time who I have *never* heard swear except on the ocasion of a "Demostration" where the first/second shift caused her to utter "Bloody Hell"

Under "Normal" driving you couldnt hear the engine (Most of the time you never went above fast idle!) Driven "Hard" she would howl like a Spitfire and the small ammount of oil smoke generated would be drawn up into the slipstream coming off the rear wings and look like con-trails!

Awsome! Truely Awsome!

The only downside is that you could almost see the fuel guage move during "Hard" Driving

I must go and get another one

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"The road to a police state is paved with public safety legislation"