19 replies to this topic

[Chal1e]

Hi guys i'm Charlie.

I have recently bought a Classic Mini 850cc engine, my aim is to build from scratch a little performance engine. I know alot of gus will say "buy a bigger lump" 998, 1098, etc, however I do fancy a challenge and really do think I can make a perky little 850cc motor.

So far I have looked into a ported / polished head - from cooper s??
Better cams, lifters, valves, etc
Better Crank??
New pistions / options??
Centre strap - as I'd like to aim for 7000rpm+
New carb set up
Exhaust etc,
Stonger gearbox, with LSD
Could possiby look into turbo / supercharge the little beasty aswell, just a thought.

I start to strip down this weekend and will post pics along the way, oh and did I forget to mension I DON'T have a budget!!!

Cheers guys

[Alex_B]

Crikey, talk about an uphill struggle!

I can see the incentive with going against the grain and tuning the little engine but from my research it will cost the same to get a 998 to the power of a 1275 as it would cost to get a 1275 to 100+bhp reliably, the 850 would need even more money thrown at it and its not the best engine for performance.

The only reason why I would go down the 850 route would be for a specific class of motorsport, on the road I would go for a 1275 bored out to 1293 or 1310, balanced bottom end, ported 12g940 head, and depending on cam you can get 110bhp~ from that, at brands hatch last week swift tune had a 1293 in historic appendix K spec running 120bhp on a very hot cam and that was flying,

I would imagine the same amount of work on an 850 would probably mean you could get 50-65bhp before it needs rebuilding at regular intervals and with forced induction possibly 80bhp?

The 998 is quite popular for being a forced induction candidate and the race mini miglias running 998s have had 100bhp out of them but again thats on race cams and not really suitable for the road

Im not trying to poopoo your ideas on tuning the 850 but it may end up being a lot of money and a disappointing end result, as it is myself I am looking into either b16a2 vtec engine conversion (160bhp) for about £2.5k all in I estimate because the a-series is so expensive to tune reliably compared to a modern lump but that might not happen due to another bigger project I fancy currently

[Cooperman]

The biggest problem with the 850 is the strength of the crankshaft. There used to be a competition 850 crank, but I've not seen one for many years. It really is hard to get the standard 850 crank to rev safely over about 6300 rpm, even when it's balanced and lightened. This limits the cam as the power band will need to be over max by 6000 rpm.
There is little point in fitting a 12G940 head as it will be over-valved for the 850.The best head for the 850 is the 12G295 from the 998 Cooper, gas-flowed and set to no more than around 10:1 C.R. It is possible to fit slightly larger inlet valves.
Optimum cam is a Kent 266.
Twin HS2 carbs are good as is the 3-into-1 freeflow small bore exhaust manifold and a small bore Cooper exhaust.
Lighten the flywheel and use the original pressed steel rockers.
The standard gearbox is fine as the power will not be over about 50 to 55 bhp and an LSD is unnecessary and will simply spoil the steering by inducing torque-steer.
As for pistons you could speak with F.W.Thorntons of Shrewsbury who can tell you what is still available.
Make sure you fit cam bearings as the early 850 had the cam running directly in the cast-iron block.

I have rallied in many 850 Minis including a 'works' prepared one which had about 53 bhp. It had the special crank though and revved, from memory, to about 6600 rpm.

Good luck with this project.

[Chal1e]

The engine's not in a car at the moment, I forgot to mention, lol. Its purely just a hobby and I may fit it, if anything into a RWD buggy or gocart.
So I was thinking the 850cc for lightness really.
Thanks for the response though, very informative cheers.

Edited by Chal1e, 24 April 2013 - 03:20 PM.

[Alex_B]

To be honest Im not actually sure the 850 will be any lighter than the 998 or 1275, they are all about the same size and wont be much in it, plus being a cast iron lump light it isnt ;)

[tomgale]

theres really no weight difference, same block, infact, a 998 or 1275 is probably lighter as the bores are bigger

[Chal1e]

So can I just bore this block out then?? If its the same lump??

[Chal1e]

to be honest, I only paid £20 for the whole thing 60k miles on it (i'm good at haggling, lol), I have access to a full machine shop, so boring, porting, etc wont cost me.

[Alex_B]

Its not the same engine casting no I think the 850 and 997 cooper are very similar but not the same and the 998 and 1275s are very different

[Cooperman]

No. The blocks are slightly different, and the big-bore blocks (Cooper 'S' and all 1275's) have a different cylinder spacing and different crankshaft. The small bores, 850, 997, 998 and 1098 are similar, but parts are not necessarily interchangeable. It's quite complicated.

[Chal1e]

Its a good challenge then, i'm liking it already.

[rally515]

If budget isnt a issue why dont you cast a (lightweight) 850 style block & custom head ?

[Cooperman]

Best small classic engine is the Imp 875 cc all-alloy OHC engine. If only the Mini had been designed with an engine like that. Based on the Coventry Climax engine.

[Chal1e]

Yeah, there's a few options, I think I'm going to stick to my guns on the 850 though, its already on the kitchen table & the Mrs was like what the hell?!!
So if I can match a better head to the 850 block, as i'm going to be stripping and colouring the parts as I go along, nice a shiny bits.
A nice set of carbs, has anyone looked into direct injection on the mini heads at all??

[rally515]

bmw bike head ? or is this overdoing the capacity again ?