Why is first gear on diesel cars always so short?

[pilchardthecat]

I've owned and driven a variety of petrol and diesel cars, and one thing has always puzzled me - why is first gear on diesel cars always so short?
I would even go as far as to say on most diesels it's probably redundant unless you're doing a hill-start whilst towing something

There must be a reason for it - you would think that with a narrower rev band first gear would be longer

[exgtt]

If they had a broad spread of power 1st would be higher geared.

[The Wookie]

Doing a fair amount of work with gear ratios I've often wondered this. From my experience of driving diesels I usually put it down to them being quite easy to stall

[ExPat2B]

Its for towing. Simples !

[kambites]

I have always assumed it was because the complete lack of torque before the turbo kicks in would make them very easy to stall if first gear was longer. First gear isn't actually lower than on a petrol is it? It's just that the engine revs lower.

Edited by kambites on Wednesday 27th May 09:28

[sosidge]

Are you sure it's not just an illusion caused by the smaller rev range of a diesel? If it redlines at 4.5k instead of 6.5k it's going to feel as if the gear is 50% shorter.

Each modern car I have driven seems to have a shorter and shorter first gear. Possibly due to increasing car weights and to acknowledge that a lot of drivers will stall the car in maneouvres with a higher gear.

I can't see any technical reason for a diesel to need a shorter first gear than a petrol. They are harder to stall and have more torque at low revs, if anything they could pull a longer gear.

[ExPat2B]

Its for towing, honestly. You had the answer in your first sentance - Diesels ARE made to able to pull their plated tow capacity on a hill start. This is up to 1800KG, so first gear is very low.

And people do test these things at MIRA......

http://www.galwayindependent.com/motoring/motoring...

[kambites]

I doubt they take towing into account much when designing, say, a Yaris diesel?

[The Wookie]

sosidge said:

Are you sure it's not just an illusion caused by the smaller rev range of a diesel? If it redlines at 4.5k instead of 6.5k it's going to feel as if the gear is 50% shorter.

From the 3 or 4 sets of diesel ratios I've worked with in the past, 1st was in all cases shorter than the equivalent petrol, and that's overall ratio including final drive. It might have been a coincidence, but I think the stalling issue probably covers it

[Maty]

Whatever the reason it's a PITA!! Need to pull out quickly, onto a roundabout say, foot down, bang your at the redline, quick change into 2nd, wait a second for the turb.....and your off!!

[sleep envy]

Maty said:

Whatever the reason it's a PITA!! Need to pull out quickly, onto a roundabout say, foot down, bang your at the redline, quick change into 2nd, wait a second for the turb.....and your off!!

can't say I've noticed that in mine

[blank]

kambites said:

I have always assumed it was because the complete lack of torque before the turbo kicks in would make them very easy to stall if first gear was longer. First gear isn't actually lower than on a petrol is it? It's just that the engine revs lower.

Edited by kambites on Wednesday 27th May 09:28

I'd agree with this.

Before the turbo you have considerably less torque than a same sized petrol engine, so you need a very short gear to compensate.

It seems especially short as the rest of the gears are relatively tall due to the reduced rev range.

Edited by blank on Wednesday 27th May 10:14

[Maty]

sleep envy said:

Maty said:

Whatever the reason it's a PITA!! Need to pull out quickly, onto a roundabout say, foot down, bang your at the redline, quick change into 2nd, wait a second for the turb.....and your off!!

can't say I've noticed that in mine

Maybe the A3 has a better first gear than the Astra's then!

[Parrot of Doom]

This is why some of us have automatics

[F i F]

Somebody demonstrated to me quite a good way to get lightly laden diesels off the line, at least on the flat or slightly downhill.

Basically pull away in second. Feed the clutch in gently which gives a granny start for the first yard or so, but after that clutch fully engaged and floor it. The saved change more than makes up for the granny start. Seemed effective to me, and I know from his fleet records he's never had a clutch changed in the last ten / twelve years.

[Lefty Guns]

Yep one of the reasons big diesels suit auto boxes so well. My benz gets off the line with more vigour than a near-2-tonne taxi with only 200bhp should...

[ExPat2B]

Actually the reason the Automatics do so well off the line is the torque multiplication effect of the torque converter which is around 2:1 to 3:1, this is what gives that "instant" shove of the automatic cars.

[mat205125]

Maty said:

Whatever the reason it's a PITA!! Need to pull out quickly, onto a roundabout say, foot down, bang your at the redline, quick change into 2nd, wait a second for the turb.....and your off!!

Modern tiny power band, high torque Diesel engined cars are crying out for someone to develop a really good CVT transmission.

[Deva Link]

The Wookie said:

From my experience of driving diesels I usually put it down to them being quite easy to stall

Confused by that - diesels always seem less likely to stall to me.

[HellDiver]

All the DERVs I've driven are the same, 1st is only useful for getting rolling. Mondeo, Octavia, Astra, Vectra, even my Lancer are all the same.