Section : Overdrive

Overdrive or not?

Posted By : Alex Rowell, 06-Jun-2003, 03:25pm

I have 1977 1500 non-overdrive. Spitty is used fairly regularly and would be even more if it wasn't quite as "stressed" driving on the motorways. By that I mean that the poor old thing certainly sounds like it is straining a bit at 70mph.

What difference would putting an overdrive in make? Right now it sits at about 4500 rpm at 70mph.

What are the costs i'd be looking at? Not being terribly mechanically inclined i'd be looking for someone to do it for me.

Followups :

Overdrive or not?

Posted By : Michael Davis, 06-Jun-2003, 09:10pm

70mph in my Spitfire 1500 with J-type overdrive is 3000 revs. Much better on the motorway, and overdrive 3rd is really useful of lesser roads too. I hardly ever use standard 4th these days.

Can't help you with costs since I bought an entire car for &163;500 and nicked the overdrive from that.

Mik
- Overdrive or not?
  
  Posted By : Don Cook, 12-Jun-2003, 11:15am
  
  Nig - are they the revs with overdrive engaged; mine (non overdrive) does that anyway (speedo is sus though)? If not what is the rev drop?
  
  While the rest of you were watching footie last night I was purchasing a J Type Overdrive complete for &163;75 and a four pack of 1664, cheers Stuart!
  
  20 mins earlier I acquired a rear spring and two new front springs, Kevin even threw in a couple of decent discs and new poly steering mount bushes all for &163;50!!
  
  There's some pints I'll need to buy at Stafford.
  
  Still haven't seen the goals yet though.
  
  Don
  - Overdrive or not?
    
    Posted By : Michael Davis, 12-Jun-2003, 11:23am
    
    Nig? or me?
    
    without the overdrive, 70mph is 4000 revs i.e. the overdrive is about 1000 revs drop at that speed.
    
    Assuming my speedo and rev counter are correct of course!
    
    Mik
    - Overdrive or not? who am I?
      
      Posted By : Nigel Gibbins, 12-Jun-2003, 03:24pm
      
      me? or NiG?
      
      My GT6 does 3500rpm in overdrive top doing 70mph on the motorway - or 4300 (ish) in 4th only.
      
      Am I Mik now?
      http://www.mikinthecar.net
      - Overdrive or not? who am I?
        
        Posted By : Michael Davis, 13-Jun-2003, 10:19am
        
        http://www.miknotinthecarcostheyrebothbroken.net
        
        Overdrive or not? who am I?
        
        Posted By : Nigel Gibbins, 13-Jun-2003, 12:40pm
        
        That domain is available for &163;30 - with hosting for a further &194;&163;30.
        
        Just checked -
        
        Overdrive or not? who am I?
        
        Posted By : Michael Davis, 13-Jun-2003, 01:45pm
        
        hehe :-)
        
        I think I'd rather spend the money fixing the cars!
        
        Mik
  - Overdrive or not?
    
    Posted By : John Tabor, 12-Jun-2003, 11:51am
    
    Don,
    
    from memory GT6's with o/d had a different final drive ratio so o/d 4th should be about the same cruising RPM as standard 4th in a non-o/d car. On a spitfire the final drive remained the same with or without o/d.
    
    think that's right, John
    - Overdrive or not?
      
      Posted By : Don Cook, 12-Jun-2003, 12:26pm
      
      So much for showing off then! I'll need to replace the 3.27 diff with a 3.89 won't I (unless others know different). Good job I haven't got round to fitting the rear spring yet. I think I'm gonna leave the spring/diff replacements until autumn assuming it gets through the MOT that has just run out. Oh bugger!
      
      Thanks for the reminder John.
      
      Don
      - Overdrive or not?
        
        Posted By : John Tabor, 12-Jun-2003, 12:39pm
        
        or just enjoy very relaxed long distance cruising!
        
        John
      - Overdrive or not?
        
        Posted By : Graham Reeks, 12-Jun-2003, 01:15pm
        
        I'm possibly getting confused, Don, but why change the diff.? I've got the 3.27:1 on a GT with D-type o/d and it gives a quiet(ish) 75mph at 3,000rpm.
        
        Overdrive or not?
        
        Posted By : Nigel Gibbins, 12-Jun-2003, 03:27pm
        
        I have a 3.89 with 'D' type overdrive and have often thought about what difference a 3.27 would make to the 'feel'. I can work out the math easy enough - but does it feel right?
        
        Or even a halfway house and go for the 3.63 (spitfire 1500) diff - and Tacho.
        
        NiG
        http://www.niginthecar.net
        
        Overdrive or not?
        
        Posted By : Graham Reeks, 12-Jun-2003, 03:54pm
        
        Seems like we're hijacking the spit page!
        
        Well my mate's got a 2500s engine in his Mk3 and a 3.89 (it's for sale, actually), whereas mine is Mk2 engine in a Mk3 body with o/d and 3.27 diff. Laurence seems to think this diff. isn't too strong, although it's outlasted the gearbox i 'popped' last July.
        
        The great thing about the 3.27 is the cruising speed - i tend to drive minimum 70 miles on a work day, and as much as 400 on alternate weekends, and it means I can go fairly quick without the revs. Plus on those 'happy' occasions when the o/d isn't activating, it's still comfortable. Having not driven any other GTs I don't know quite how much difference it makes, although my friend insists it's better with a 3.27. Should be more economical too.
        
        I'm touching wood throughout this as knowing my luck ... no I can't bring myself to type anything else now (especially as my neighbour has got my GT today!)
        
        Overdrive or not?
        
        Posted By : Don Cook, 13-Jun-2003, 11:17am
        
        Sorry Spit owners for interloping but diffs and O/D are pretty much all the same, aren't they? Anyway to continue...
        
        Well that's me thoroughly confused!
        
        My plan: I'm going to fit the gearbox & O/D and leave the 3.27 diff in situ and see what it goes like; Graham's message convinced me on that. If I do need to replace the diff then I'll do that the same time as I replace the rear spring (unless it fails it's MOT). So on the way to Stafford keep a look out for a Signal red GT6 with low revs and Spitfire 'sag'
        
        There you go - forward planning...on the other hand...
        
        Well, Alex (yes are you stil there) are you sure you want to fit overdrive now??
        
        Overdrive or not?
        
        Posted By : Nigel Gibbins, 13-Jun-2003, 12:24pm
        
        "...diffs and O/D are pretty much all the same, aren't they?..."
        
        Well, no. There are two Overdrives - D type and J type and I believe the J type is the stronger of the two, but I don't know why. But Diffs... Visibly there are differences regarding the output flanges and input flanges eg, big/little output and square (small) and Round (large) for the input. There are also internal differences although I'm less familiar with these. Bigger, stronger bearings, bigger output shafts and slight carrier modifications to accommodate all this.
        
        This basically give us 'Strong' diffs and 'Weak' diffs, eg I know the standard 4.55 diff is a weak one and cannot handle the power of a race engine.
        
        I think this is explained on the Canley Website : http://www.canleyclassics.com/infodatabase.asp?top...
        
        NiG
        http://www.niginthecar.net
Overdrive or not?

Posted By : Nigel Gibbins, 06-Jun-2003, 11:56pm

Definately Overdrive, my GT6 does 70mph at about 3500 (3.89 diff) and at 4500 is doing something around 90! (not recomended)

Canley Classics do an overdrive 'kit' for &163;695 (I think), it has everything needed for the conversion. They can do the conversion for you too but I don't know the charge - phone them and ask.

NiG
http://www.niginthecar.net
Overdrive or not?

Posted By : Alex Rowell, 16-Jun-2003, 03:40pm

Thanks for all that guys - I'll go and buy a GT6 then.

Just trying to establish what kind of stuff I can get approved in a post wedding personal budget. ie what I can justify and not.
- Overdrive or not?
  
  Posted By : Graham Reeks, 17-Jun-2003, 07:14am
  
  If the knot hasn't been tied yet, they do make a rather nice car to arrive at a wedding in, which might give you some pre-wedding bargaining (after all their haircuts cost nearly as much as a GT6!)
  - Overdrive or not?
    
    Posted By : Alex Rowell, 17-Jun-2003, 10:41am
    
    I thought so too, but I have been reliably informed that it will not "do" - too small and she says her dad will pay for a DB7 for the weekend! So, sorry Spit but I am being unfaithful for a couple of days.