25 replies to this topic

[idlethumbs]

My engines coming out for a bit more power next month and I'm going to get the gearbox refurbed at the same time as it's become noisey and occasionally may pop out of 2nd.
I spoke to a company I've been recommended for head work and gearbox rebuild.
They said I should consider changing to a 4.1 final drive and a x pin diff.
I didn't know what any of that meant when they said it but have done some research since.
4.1 sounds too low and I would think will kill my top end.
I have a cooper spi 1993 so I guess I have a 3.2 final drive at the moment. For acceleration would a 3.44 be better without compromising top end?

[nicklouse]

Engine spec? Cam?

[idlethumbs]

I have a 1275spi cooper. Currently it is standard but Im going for work to stage 3 (I know that isn't really a thing but you know what I mean by that!) and I think I'm leaning towards an sw5 cam.

[Cooperman]

A 4.1:1 final drive ratio (FDR) will give just under 13 mph per 1000 rpm in top gear. So 5000 rpm would be something like 62 mph.
That is far too low for a road car and is strictly for a race or rally engine where acceleration is most important and where using over 7500 rpm often will not be a problem.
If you post your engine either I or someone else will recommend a suitable FDR.
I would be a bit nervous about having someone who clearly doesn't under gearing to work on my engine.

[idlethumbs]

Thanks cooperman. I thought I'd lose all my top end. The place I spoke to specialise in minis but more race minis. Maybe that explains there final drive choice.
I'm not sure what more info I can give about the engine for someone to recommend a suitable fd?
I am still trying to work out my choices for the engine if I'm honest. Open up the head, larger inlet valves (stage 3). Possibly sw5 cam kit incl valve springs. I can't seem to get solid info on rockers with the cam, keep stock or go higher lift?!?! Not looking to increase the bore. Sticking with the injection set up.
I just want a nice all round car, not overly quick that it's **** round town. I like the drive just want more poke and a bit more acceleration.

[Spider]

If it were me, I'd run a 3.44. About the best of both worlds, reasonable acceleration and reasonable speed vs RPMs.

[Cooperman]

Yes the 3.44 was fitted to the Mk.1 Cooper 'S' and it worked well.
It will give 16.5 mph per 1000 rpm in top which means about 83 mph at 5000 rpm.

[nicklouse]

If you go 3.44 you don't really need a cross pin diff as the torque forces are less.

If you were going the opposite way (to a taller diff) then it would be a must.

But if your diff needs changing due to wear then is is not a bad idea to change it.

BUT there are done really poor cross pin diffs out there. The Guessworks ones look well sorted.

[idlethumbs]

Thanks for the replies. Confirms what I thought about a 3.44. I'll go down that route.
Now to start saving my pennies!!!

[Midas Mk1]

Having had 2.7, 2.9, 3.1, 3.2, 3.4, 3.7, personally I'd stick with 3.2, one of the best all rounders for fast daily use.

 

Would go 3.44 as a push maximum, any lower and you'll hate the motorway, and I don't find a drop in acceleration between 3.4/3.2 to be honest..

[Mrpeanut]

I'm with Midas. The 3.2 diff from the Cooper spi is the best of all worlds imo, acceleration with an sw5 and stage 3 head won't be noticeably different to a 3.44 and will cruise happily with lowish revs. The 3.2 diffs are sought after and more expensive than the 3.44 and 3.1 for good reason...

Edited by Mrpeanut, 02 September 2016 - 07:03 AM.

[Shifty]

Yup, another vote for the 3.2 here, especially if you're not going too wild with the cam

[Ethel]

3.2's are more expensive because there were loads of 3.4's in Metros - does make your point, considering the Metty's bigger wheels.

The 3.4 would be best for what what Minis do best, but stick with what you have and put the money elsewhere for what difference it'll make.

[Mini Manannán]

I went from a 3.9 to a 3.44, SW5 cam in a 1310, make for a much nicer drive.

Edited by Mini Manannán, 02 September 2016 - 10:36 AM.

[pete l]

What does the standard MPI have ?