10 replies to this topic

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Hi guys, I've got an 850cc 63 morris mini minor, and I'm looking to modify it to make it somewhat of a screamer, I currently have the car on a single HS4 carb, inch and a half I believe, I've got a few other things to do also, but I don't have much technical knowledge (bodywork being my forte) I have a 12g295 head, which I've heard is a good head for the 850 as long as it's skimmed to get the compression up? does anyone know to what extent I'd need it skimmed? also have a metro cam, but not sure as of yet if thats going in, but a long time friend and in my eyes, bit of an A series genius recommended both!. so In many aspects I have a few of the bits I need to get this engine to really go, I just don't know how to go about it, I've also read a little about having to strengthen the bottom end? as a huge engine novice, I have absolutely no idea what this entails? 
thanks 
Harry

[Dr s]

You might want to start with a different engine!

But maybe not for a period screamer have a read of Clive Trickeys book.

The challenge with the 850 is the lack of cam bearings meaning extra machine work to put them in to allow a high lift cam to be used.

Starting with a 998 would make this easier as it had cam bearings as stock

[Alex_B]

I have heard there are also issues with the 850 crank being weak so any sustained high rpm can cause it to fail. 

I would also +1 on a 998 base if you're not too worried about a non original engine size, they are plentiful and cheap plus can be made into real screamers if thats your goal. 

 

[Midas Mk1]

As said on the 998 thread, the best answer is to buy an MG ZR.

[RossKnight]

As said on the 998 thread, the best answer is to buy an MG ZR.

That's the best answer to someone asking how to tune their 850?

[AVV IT]

As others have said, dropping in and tuning a 998 would probably make a lot more at sense, particularly on a low budget, as decent 998's are cheap and readily available. Transplanting a 998 will undoubtedly devalue a 63 model though, so it would be advisable to keep the standard 850, so that it can be dropped back in should you come to resell.

[carbon]

Harry,

 

I've never tuned an 850, but would suggest that 12G295 head is probably not best way forward. As standard this has the largest of all standard A-series combustion chamber volumes at about 28cc volume.

 

The standard 850 head is about 21.5cc, and gives a CR of 8.3:1 using the normal 850 pistons. To get more power at higher revs one of the changes you should make is aim for a higher CR, maybe about 9.5:1

 

Getting an 850 up to 9.5:1 with a 12G295 head will be a challenge, I think you would be better starting with a standard 998 head, then porting and skimming this and modifying the chambers. The chamber volume you need will likely be about 17 to 18cc using standard 850 pistons to get 9.5:1 CR.

 

You don't mention which Metro cam you are thinking of using, but one point to watch is the 850 small bore blocks were originally used with pin drive cam cam and if you do change to a Metro cam then you may either need to fit a spacer or find oil pump with correct length shaft.

[dyshipfakta]

850s are becoming rare now too and probably be worth more in the years to come unmolested.

[Cooperman]

The 850 was never ideal to modify too much. The crankshaft won't take high revs and in the '60's race 850's had a special crank which is now impossible to find.

A 295 head, combined with a better cam and an HS4 carb would improve it a bit, but you have to do some accurate engineering measurements and calculations to get this right.

If you have the original split-skirt pistons they are also a limiting factor.

You could spend a lot of cash doing an 850, fitting cam bearings, skimming a 295 head to the calculated figure, finding and fitting better pistons, new inlet & exhaust systems, balanced bottom end, better cam, etc, to find that you then have around 45 bhp, an increase of 11 bhp, which is a 30% increase.

But if you were to fit a 998 engine with similar modifications you could easily have 55+ bhp for a lot less money.

I do recall a friend who, when the 997 Cooper was introduced said that he would make his 850 faster than the then-new Cooper. He did a lot of work on his 850 and it did seem to go well, although it needed a lot of revs. In the end it blew up (crank snapped) and engine parts went through the gearbox. It was a very expensive exercise!

[mk1leg]

I built an 850 screamer.......bored it to max and fitted a 731cam stage 3 head and a webber 40DCOE.......it lasted 2 years then as Cooperman says the crank snapped........but it was fun till then....................

[Cooperman]

I suppose you could get a billet 850 crank made and hardened, but it wouldn't be cheap. Are slipper pistons available in 850 size? I've never seen any and I guess that it because the class size for historic competition is up to 1000 cc, so the 998, which can be made to rev without a special crank, would be the ideal.

Best option might be to get a 998 engine and build it to 998 Cooper specification which will give 55 bhp or slightly more if a very accurate build combined with a gas-flowed bigger-valve 12G295 head with around 10:1 C.R. Fit a 3.44 or 3.76 FDR into a standard ratio gearbox and it'll go really well.

You could then keep the 850 to put back into the car to give it its originality back, if or when you sell is because, as mentioned above, original 850's are going to become increasingly rare and valuable.