4 speeds??

[fortnum1977]

Hello all...

I have a P5B coupe 1971 auto and it's great except there only appears to be threee gears and therefore it revs to much on the motorway at high speed.

Is there an overdrive switch somewhere I'm missing?? and if not, does anyone know of a modification I could perform or different gearbox to fit.

Many thanks,

Chris

[stantondavies]

Do a search on ZF, Chris. Key in 1000days and 1000 results in the options. Sit back and spend the evening reading what is possible. Or stick to 70mph!

[Deleted]

Whats wrong with the odd burst up to 80 + to overtake the odd P5 3 litre?

[Phil Nottingham]

DEPENDS WHICH 3 LITRE YOU ARE CHASING

Anyway a P5b should not be overreving at 70mph - either your rev counter is over recording or the brake bands in autobox is slipping

[fortnum1977]

Yes, Phil from Nottingham... I don't think it's the auto box it's just, you know, you must have thought to yourself it could do with a higher gear for high speed... it sounds a bit stressed I reckon. And why give an overdrive to the P5 and not the P5B which surely warrents the higher gear more??

Anyway, sod it, when I get some cash together I might invest in a 4-speed GM box but I'm sure some modifications will be needed to fit it, so that'll be fun.

Thanks,

Chris

[Phil Nottingham]

The overdrive in the 3 Litre which is a lower reving engine was only in the 4 speed gearbox models not the auto's.

Using the correct Diff final drive with the effect of the torque converter renders an overdrive unnessary.

Modern engines have give their opitimum power output in a much narrower range than the the V8 and straght six engines so they need extra gears in both manual and autos to make the car perform

[rov5b]

I don't think these are the cars to buy if it's real performance you are after in a classic car anyway - I thought it was relaxed comfort and style! - Remember the V8 was only stuck in to stretch out the useful life of what was even then an old design.

[fortnum1977]

'Using the correct Diff final drive with the effect of the torque converter renders an overdrive unnessary.' Thanks for this comment Phil ; how do I determine if my diff is set up ok?? and i didn't know it had a torque converter...

The car just doesn't feel relaxed at 70mph and i think it should given the amount of grunt it's got... 184 bhp?!? I'm not looking for performance at all, just relaxing motorway driving.

I might consider changing the box... she did actually have a new one about 8 yrs ago and it is in great nic. Only a slight slapping on the diff I think when changing from neutral to drive making a bit of a thud and move forward.

The only other thing is that the electrics seem to cut out sometimes, only briefly though.

I thought it must be the battery connections so I checked it out and found the earth wire wasn't properly bolted down and tightened and one connection was a bodge... just sitting on top of the stub. So sorted this and it was fine until this morning when i was starting her and she shut down for about 5 secs... it's def an electrical thing.

[Phil Nottingham]

Its the ratios of the cogs in the diff that determines the road/engine speed. It may have had a none standard diff of another P5,P4 put on but the only way to check this is to dismantle the rear axle - unlikley.

In auto's the torque converter replaces the clutch and also gives up to approx 2:1 ratio between the 3 standard box gears. If the box is worn or need adjusting the brake band and the torque converter vanes can breakup due to cavitation of the flud when iverworked/overheated.

You need to try another V8 P5 to see if yours is not normal!

There re more conections at the starter motor and numerous bullet connectors/relays in the engine compartment and under the dash. These can become loose or corroded or by the sound of it bodged