A mate and I were chatting the other day. My Fabia's got the 1.9SDI engine in it, he's got a Mk4 Golf with the 110 TDI in it.

Both of us feel that our cars would have benefitted from an extra gear, as at 55-70mph or thereabouts, the engine seems to sit in the range of revs where you'd normally look to change up. I don't mean for the gear ratios to be altered to get more power through 4 & 5 - simply that a 6th gear would have the car sitting more comfortably at motorway speeds.

 

I know the 115(?), 130 and 150bhp Golfs got 6 gears but is there any reason why VW didn't fit 6 as standard to all their 1.9s? Surely the cost increase wouldn't have been that huge, and they'd have been able to advertise it (legitimately!) as being even more economical than it already is in terms of fuel and emissions. You'd never climb hills in 6th but 5th in the non-turbos is strictly cruising-only anyway so that wouldn't make any difference.

 

Thoughts? I know I've probably missed an obvious factor here, trying to work out what it is!

Edited 23 September, 201510 yr by alessio92

6th would be handy for motorway speeds in my pd100 plus the gear knob looks better lol

For a diesel powered car, I'd say keeping it over 2500rpm would be essential, even when driving on the motorway. How many rpms does your car need to run at 55-60 in 5th gear?

 

Ontopic: according to your gearbox code, this *might* be possible.

There's not a huge difference between 5th on dad's sdi fabia and 6th on my TDi ibiza.

For a diesel powered car, I'd say keeping it over 2500rpm would be essential, even when driving on the motorway. How many rpms does your car need to run at 55-60 in 5th gear?

 

Ontopic: according to your gearbox code, this *might* be possible.

 

Can you explain why you'd think that?

Sure. You need to build up oil pressure in order not to premature damage the engine. Running in high gear at low rpm just to achieve a high mpg can ruin a few things under the bonnet. 

Okaaay....

But surely diesel engines with their naturally lower rev range, and high torque at low revs, would have oil pumps geared to produce enough flow at lower revs?

I'm no longer considering diesels as being low revving engines, not for normal cars at least. A common 2001 VW Golf diesel engine can/has to do >4800 if I'm not wrong (during MOT tests). 0.7 oil pressure at idle, no more than 7 bars, but 2-3 bars should be perfect. Plug in an oil pressure gauge and you will see at which rpm you get that figures.

This page suggests 2 bar at 2k rpm is minimum spec for a PD130, so normal pressures presumably a little better than that.

That's correct. Statements such as 'minimum' and 'should be' determine a 2500rpm as being essential, as I said.    

 

Later edit: IMHO, a modern diesel has to be driven within an rpm range of 2500-3500. You keep the catalysts clean this way, the fuel injectors as well. More important, you have a power reserve that will always allow you to accelerate faster than doing it from 1800 or bellow.

Edited 24 September, 201510 yr by Alexandru

Sure. You need to build up oil pressure in order not to premature damage the engine. Running in high gear at low rpm just to achieve a high mpg can ruin a few things under the bonnet. 

Complete and utter MBF.

 

If you block the outlets from the main gallery you can get 15 bar pressure there, but 0 flow.

 

Also, if you have good flow, through hydrostatic pressure in the bearings, you will have about 100 bar where you actually need it, so a 6 bar pump rather than a 2 bar one will give you just under 4% higher oil pressure where it actually matters!

I have run fabias with 100 bph TDI and 64 bhp SDI engines and I personally accept that the naturally aspirated engine needs to be geared the way it is to be able to deliver the HP needed for higher speeds. Economy may not improve with taller gearing if you end up spending a lot of time at lower rpm with your foot on the floor to get up small inclines. However, I also would prefer a lower cruising rpm and currently run other cars that motorway cruise happily at 1600 to 2000 rpm with zero oil pressure issues

 

When I looked into it I decided you could lift the gearing of the SDI by about 7% by fitting 195/70 tyres but I wasn't confident that the resulting ratios would be too tall for the engine. I would also point out that you don't need 6 speeds to lift the top gear - you can buy higher ratio 5th gear sets for reasonable money on ebay and DIY change them out without removing the gearbox - google it.

A standard 6 speed vRS does 70mph with 1900-2100 rpms (according to the year the car has been built). Running in 5th at 70mph should require the engine turning ~2500rpm. At less than 2000rpm and in 5th gear, you must have been cruising at under 50mph. Am I correct? 

My car has the early longer ratio 6 speed box and sits at 1800rpm at 70mph in 6th iirc i think the engine makes its max torque at just under 2000rpm so its ok 

I think that it is due to cost and lower torque figures that they dont fit the 6 speed box in other 1.9ds

Edited 24 September, 201510 yr by DC2990

I GOT THE 6 SPEED GEAR CABLES PM ME

My car has the early longer ratio 6 speed box and sits at 1800rpm at 70mph in 6th iirc i think the engine makes its max torque at just under 2000rpm so its ok 

I think that it is due to cost and lower torque figures that they dont fit the 6 speed box in other 1.9ds

That makes sense, so in 5th it should be well over 2000rpm.

Do certain pd100 models have wide ratio boxes

I ask as my estate seems to have long gears mine sits around 1900rpm at 60mph

Where as my Mrs mk4 golf pd100 golf is over 2k rpm @ 60mph, but the golf has a noticeably shorter Gearbox as it feel like you are screaming it at higher speeds where as my fabia feels comfortable at higher speed on the motorway

This could be why i notice the 1.9 models get better MPG figures on long high speed drives than i do in my 1.4TDI.70 in 5th sees the revs very high.

Add to last post, i found a bit of road today and at 70, mine is at 2500 RPM. But on a decent hurried drive up the M6 heading north, I notice the revs can get a lot higher

Consider yourself being lucky. A 1.4 8V mpi with (theoretical) 50kW does 60 with ~2700rpm and it's over 3000 at 70 (and it does 38mpg at that speed, fully loaded and with A/C on, but that's another story).

Edited 28 September, 201510 yr by Alexandru

I swapped the 5th gear in my 1.4TDI for a longer one (0.658 ratio) to drop the revs at higher speeds. It is now a lot more relaxing to drive on motorways etc. The conversion kit is available from Darkside. 5th gear at 70mph is around 2.1-2.2k which is perfect for me (not remapped). With the original 5th gear you had to be a bit more careful at higher speeds to get good MPG - anything over 60-65mph would see the MPG plummet.

 

Read more:

 

http://forums.clublupo.co.uk/index.php?/topic/99507-14tdi-5th-gear-mod/

Edited 28 September, 201510 yr by Dazza95

Hi there,

 

it's my first post so please forgive me if the question appeared before anywhere - I looked but couldn't find an answer.

 

I have Octavia 1.9TDI from 2005 and I was wondering what RPMs it should have on different speeds and gears, for example what should I expect it to have on 6th gear around 70mph?

Hi there,

 

it's my first post so please forgive me if the question appeared before anywhere - I looked but couldn't find an answer.

 

I have Octavia 1.9TDI from 2005 and I was wondering what RPMs it should have on different speeds and gears, for example what should I expect it to have on 6th gear around 70mph?

This is the Fabia 1 forum.

 

Oh and we'd really need to know which tune your engine is in. With the 6-speed in an Octy it's probably a 130, in which case it's about 35mph/1000rpm in top.

Most modern 5 and 6 speed diesel gearboxes give an RPM of around 2100 at 70mph approx.

 

The old 6 speed (in the early Fabia vRS and Ibiza PD130s) sat at around 1800rpm at 70mph.

 

I know an Octavia with 1.9 PD105 and 5 speed manual sits at 2100rpm at 70 mph and so does the same car with 6 speed DSG.

 

If you have the time/knowledge/ability then it is probably possible to swap the 5th gear ratio or the whole gearbox. Otherwise it's probably easier/cheaper to just buy a new car!

 

Phil