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Chloe...Jack...download a schematic of a 6sp 'box to my PDA! ZT CDTi Q&A?

Discussion in 'Motors' started by Oracle, 1 Jun 2006.

  1. Oracle

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    Ok, driving home tonight, I was cruising around 80mph (indicated) and noted the revs were at around 2500rpm (i'm dead on 90mph indicated at 3k rpm), this is a CDTi btw...and I got to thinking

    I'd love a 6sp gear box. I cover a hell of a lot of motorway miles, and having something that bit quieter would be a god send. I have no reason to complain about the 330d only having 5spd as the engine is so much smoother being 6cylinder, etc.

    However, my thought process lies in the following. A six speed box for the diesel would be great if you cruise at 80mph (which I think most of us do). This, if ratios were a trend, would see the engine at around 1600-1800rpm in 6th, pretty much just spooling the turbo. But if you were at 70mph in 6th, this would cause the revs to be too low to smoothly overtake and you'd have to drop a cog to get the power back, but then 90mph would be at 2krpm and would see you fly round to a fair good top speed i'd imagine (private test track permitting!!!).

    I note that Quaife do a six speed FWD box for a very reasonable price (i'd effectively be getting some money back by selling my current box), but what kind of gear ratios would be needed for such an idea, and am i now taking this modification thing too far!????? Instead of dropping 800rpm off with the 6th gear, could it be a closer ratio to 5th and maybe keep it at say 2000rpm at 80 mph!?

    Answers on a postcard!........what are your thoughts!?
     
  2. [TW]Fox

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    My thoughts are that its a totally rubbish idea.

    I will explain why.

    I have had, for a few days, the use of a 2005 BMW 320Cd. It was an excellent car, and it has the slightly more powerful version of the same 4 cylinder engine in your car.

    However, it had a 6 speed manual gearbox. Which drove me mad. Absolutely did NOT suit the car at all. Ratios were too close, the 6th gear just seemed like it really wasn't needed.

    Stick with a 5 speeder - I prefer the 5 speed box in the 530d to the 6 speed box in the 320d.
     
  3. Oracle

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    That was kind of my thinking, although similar to the 530d, i guess the 330d and 530d 5spd boxes are better (or feel better) because the engine is so much smoother.

    I'd purely be doing this from a noise point of view. The CDTi M47 lump, (which is actually vastly different from the BMW M47 lump) has no balancer unit, hardly any acoustic material around the unit, and at 80 or 90 can be easily heard clattering away. Now forgive me, i've not driven a 320d, but the 330d certianly suffers from no such problem even well into triple figures.

    With a Quaife box, I could pick the ratios myself, meaning that when 6th is selected, its bang on the door of the turbo, but at a suitable and comfortable rev range for noise. I'm thinking 80 in 5th at 2600rpm and 80in 6th at 2100 or 2200rpm.

    Fox, can you remember where 6th gear put you in the rev range at similar speeds!?
     
  4. [TW]Fox

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    Nope I can't remember the specific speeds but I did on numerous occasions forget to put it into 6th at all at cruising speed which shows how the noise wasnt really an issue on the 320d.
     
  5. Oracle

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    See, its funny you mention that. I test drove a MK4 GT TDi Golf a year or two back (150bhp), with a 6 spd box and again, it wasnt so much that the car needed the 5th gear...more so, i just forgot it was there, but when selected it just cruised a lot better. The same was true of the MK5 TDi that I took out on the same day. I think i'll investigate anyway, just to see what Quiafe suggest.
     
  6. [TW]Fox

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    You obviously have too much spare money.

    Put it to good use - trade the ZT in for a 330d Sport :)
     
  7. Maz

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    Location: Newcastle.

    6 Speed Diesels rock.

    I've had a couple, and I've driven loads.

    Takes a bit of getting used to, but IMO much more flexability than 5 speed ones.
     
  8. blueboy2001

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    The 6 speed box on my Mondeo ST TDCi works very well. The criticism I had of the 2.0 5 speed I had was that 5th was only any good over 50mph or it laboured a bit, but 4th felt a bit too racy at 45mph. 5th had to be a very long ratio to carry from 50 to about 130.

    The first 4 ratios on the ST seem more or less the same, 5th has been made a bit shorter and can now be used at 40mph but will take you well past 3 figures. 6th is good for anything above 60mph, 70mph gives about 1900RPM which is close to peak torque and 90mph comes in just under 2500rpm. I can hear the engine over the road noise over 3000RPM but thats about 115mph. It is rather to easy to creep from 70 to 100 without really noticing unless you keep an eye on the speedo.

    If I'm joining the motorway I don't tend to use 5th, just accelerate in 4th and go straight to 6th. Around country lanes I don't use 6th as its really a cruising gear.

    I think Maz has it about right - it adds flexibility and other than maybe cost I can't see any disadvantages. It's not compulsory to use the 6th ratio every time you drive.
     
  9. Smiley Man

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    ive found that 6 speed diesels just have closer ratios to keep it in the narrow powerband

    at 80 in top in a 5 speed car and a 6 speed car are at pretty much the same revs

    6speed on VAG 2.0TDI 140
    5speed on VAG 1.9TDI 130

    ^ the 2 engines and boxes im referring to, i would imagine its the same everywhere though

    it is quite good fun when your mums scared to use it thinking reverse is there though, and when she gets in my car and nearly drives through the house thinking reverse is near 1st
     
    Last edited: 2 Jun 2006
  10. Nathan

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    iv pondered the same thought for my 306 diesel, would make economy awesome :) You could be looking at 80 - 90mph @ 60mpg which would be outstanding. I have a 6 speed box in the gti6 and that is immense just because its so close, the 6th gear though is just like any other 5th because of the close ratios. One of the best boxes ever made imo for a production car.
     
  11. Pug

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    my celica doesnt need a 6th gear at all. Top speed is achieved in 5th where the gearing is low enough to enable the cam profile to change and get me the extra 50BHP upto 8.5k rpm.

    HOWEVER, 6th does enable me to cruise at 90at a fair few rpm lower (still think its somewhere near 4.5k - but thats revvy engines for you).

    So the option may be a suitable one if the aim isnt to go faster, but the same speed at lower rpm.

    HOWEVER (again) the reason your motor probably sounded noiser than the 320 Fox drove is probably because you're in a Rover and he's in a BMW. The acoustic deadening in a BMW is going to be massively better than in a rover (or other car - my celica is noisy as hell tbh - but its a sports coupé so i expect it)

    Another thing to consider is you'd need the rpm to be above the turbo spool (quite a bit above) as dropping in and out of turbo would cause excessive wear on the turbo rotor (possibly)

    Gear ratios are calculateable (word?) by knowing the rpm of the engine, speed, and final drive ratio. I know this is how hypothetcial top speeda re worked out, and i assume the equation could be rearranged to calculate an rpm for a given speed - Wouldnt know how to do it though.
     
    Last edited: 2 Jun 2006
  12. Glaucus

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    I wish more cars where fitted with 6 speeds, thats the one fault on my golf, cruising on the motorway in 5th gear is at 4.5k :eek: it must munch the fuel like that. Now if you could have an overdrive gear not meant for over taking just cruise at 80-90 at around 1.5k that would be awesome.
     
  13. Simon

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    Regardless of engine speed the car will still be doing 90mph and require the power and therefore fuel to do so.

    All you will be doing is reducing internal engine friction slightly. Certainly won't make more than 2-3mpg difference IMO
     
  14. Glaucus

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    I recon you would see more of a saving than that, you can't get away from engine size and revs. yes there are other factors but those are the two main ones.
     
  15. Simon

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    The engine will be going slower thats all, it will need more power to maintain speed and therefore need more throttle opening and therefore more fuel.

    Savings may be greater on a petrol as pumping losses would be reduced however on a diesel this isn't a factor. Also at lower speeds the engine isn't as effiecient in terms of VE.

    The biggest factor on economy is speed once you get about 50mph.
     
  16. Enfield

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    What? Which is it? lol
     
  17. [TW]Fox

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    Read his post and he tells you :confused:

    2500rpm @ 80 and 3000rpm @ 90.
     
  18. Enfield

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    Aha yes, that'll teach me to post first thing in the morning...well when I wake up :D
     
  19. Simon

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    Ah one of these magical gearboxs that the ratio changes depending on the speed :p

    If 90mph is 3000rpm then 75mph would be 2500rpm
     
  20. PhilthyPhil

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    That would assume a linear relationship between engine speed and the speed of the car, which isn't the case since the resistive forces do not increase linearly with speed.