6 replies to this topic

[Dope]

I have an 80's mini city that someone has put a 1275 motor in that came from an MG Metro. Chances are he took the path of least resistance and swapped out the gearbox with it - but how do I tell what gearbox it is for sure?

 

I have a 1380 waiting to go in and want to know if I can use the gearbox that's already in the car.

 

Spec for the 1380 is below, thought I'd canvas opinions before proceeding:

 

The engine is a 1380cc 73.5mm bore with a wedged balanced & polished crankshaft with new standard bearings and a centre main strap. It has new Powermax piston & rings.
The camshaft is a 530 spec with slot drive to a turbo high capacity oil pump. It has a lightened duplex chain drive with cam followers, pushrods and solid rockers all lightened.
The head has been fully ported and polished and fitted with 37mm inlet and 32mm exhaust rimflow valves with dual springs. The compression ratio is 11.5:1 & it has ARP studs and nuts.

 

Planning to re-use the existing HIF44 (suitably setup) on a Maniflow inlet and LCB exhaust.

[Cooperman]

You need to check the gear ratios of the existing gearbox to see if it is standard and also check what the final drive ratio (FDR).

Then look at the power & torque curves for the cam you are using.

From this you can determine, possibly with some help from on here, what gearing will match your engine output in order to keep the engine 'on the cam' when demanding absolute maximum performance.

There is no other way of doing this.

[Dope]

Thanks Cooperman. I know that the ratios will be standard for the stock 4 speed 1275 MG Metro gearbox as it was a straight swap out job. If indeed they used that gearbox.

 

I'm yet to check what numbers are stamped on the casing, but assuming its the box that came with the MG Metro we're looking at 3.520 2.210 1.430 1.000 according to this post - http://www.theminifo...ratios-and-fds/

 

Are you saying that we should build up the engine as intended, get it on the dyno, then choose the ratios? I was hoping for a 'best guess' solution! I know that's not the purest approach - sorry! I'm not after a world-beating car, just something for the weekend...

[Cooperman]

I don't have the power and torque graphs to hand for the 530 cam.

I am assuming that it is a fairly 'aggressive' cam so it is important to 'close up' the ratios by selecting a lower FDR.

I can only suggest a 3.44:1 FDR as that is a good compromise with standard ratios.

Cruise in top will be c.16.5 mph/1000 rpm and should give good acceleration whilst not allowing it to get too far from the optimum power band.

I hope this helps, but engine specs and gearing is quite an involved topic for maximum performance.

[Dope]

Thanks again - wish I had even half the knowledge that you do!

 

Looks like these guys will be asked to set it up after the install. http://www.spamspeed.com/

 

I'll pass on your suggestions pre-build though as it would be good to get it right (or thereabouts) first time.

[Cooperman]

It does sound a good specification. I would certainly try a 3.44:1 FDR and certainly fit a cross-pin-differential. The torque of a 1380 will quickly wear a standard diff pin, which was originally designed that size for a 34 bhp 850 Mini in 1959. 

Make sure all the gears, bearings and, especially, the lay shaft in the 'box are all 'as new'.

The other thing is to ensure that the CV joints are good. A high-torque engine will break CV joints if power is applied when there is steering lock on.

 

I hope this helps.

[Dope]

The chap that's selling the engine and will be doing the install has had a lifetime of building very competitive cars that he hillclimbs at Prescott etc - Imps and Escorts judging by the pictures on his office wall, and I'm very much in love with his latest mk1 Mexico. I watched it win best in class at Prescott a few weeks ago.

 

Ian Moss, in an Imp -

 

He didn't build the 1380 himself but I spotted it in his office and he tells me he very much trusts the chap who did build it. It's not been run since being built.

 

Very excited. And it's not even my Mini that this is happening to - I'm helping out the lady it does belong to - but I will get to drive it a bit!