25 replies to this topic

[luigi86]

Hi there,

I have decided i want to tune an 850 engine, the reason for this is because everyone considers them rubbish and boring but im determined to make it go well.

So my plans at the moment are.

Block machined to take second cam bearing
Kent 266 cam
1275 head, block will need machining to take this.


What else can i do?
i will get the crank balanced but is there any other cranks that would give better reliability?
Obviously i will fit things such as a center oil pick up and different timing chain so i can have a tensioner.

My question is how can i squeeze some more power and revs out of her?

[valve bounce]

sounds a great idea, you could lighten the flywheel and pressure plate then have them balanced along with the crank rods pistons bottom pulley etc, be careful with the pistons tho as some of them use a split skirt on the piston and if use at high revs alot the skirt can come away from the pistons crown ruining the engine, if you read david vizzards modify your mini it has a good bit in there about 850 pistons, if you have dont have the book al look it up.

other than that i would try and get a 12g295 heads as the 1275 head will be alittle to much on an 850. central oil pickup and duplex timing gear are a good idea, a 998 cooper exhaust manifold and a hif 38 on a good manifold would also help things along abit.

hope this helps rob

[Scallywag630]

Maybe a turbo?

[cagy]

read in one of the minimags a while back of someone who had a rebuilt 850 with a turbo and my god it packed some power!!

[CHUNKY365]

DO IT!

An 850 can be made to scream and stay in one piece.

Make a good one and you can embarrass many of the 1380cc 'big is good' guys.

They make great little revvy engines and to hear a tuned 850, revving its nads off as it goes round a track is fantastic.

Its proper old skool and more people should try it

[luigi86]

so then, I sadly dont have this book? do you have a title of it please?

I have a 998 cooper head if thats the one you mean? so i could use that.

Was planning to send it all to be balanced, A lightened flywheel was on my list of things to buy. Any idea what weight to get?

The pistons in the engine at the moment are the original ones, in good condition, But the engine has sat for a while and has low oil pressure..
Not bad considering the car hasnt been on the road since 76!

the big question is, just how many revs can we get?

I dont want to go down the turbo route, because it will ruin the engine bay asthetically for me, Shes going to be a bit of a screaming sleeper you see....

[valve bounce]

theres the book.
you say you have a cooper head if this is a 12g206 or 12g295 then if it is skimmed by a suitable amout to bring up the compression it will work brilliantly. ive just read the chapter in my book and it says the 850 pistons are safe up to 6000rpm but the standard 998 pistons are 65 thou bigger this brings the cc to about 893cc and they are safe up to 8000rpm, the only problem with these pistons is that the pin to crown hight on the pistons is 1/8 inch less so the pushrods will need shorting to suit, the other thig i would suggest is to have the center cap machined to fit a center main strap to stop the crank flex.

hope this helps rob

[JakeJakeJake]

Sounds like a good plan should be a nice engine nice and revy, should be good for about 8500 i believe if you used electronics distributors otherwise the points will bounce much before that. However without serious mods such as turbos, 16v heads etc. I just cant see it "embarassing" many 1380's.

Edited by JakeJakeJake, 21 August 2010 - 10:20 PM.

[luigi86]

Dont really want it to embarrass anything,
Just want something a little peaky and more fun than a standard 850 that wouldnt pull the skin off your custard

[CHUNKY365]

Sounds like a good plan should be a nice engine nice and revy, should be good for about 8500 i believe if you used electronics distributors otherwise the points will bounce much before that. However without serious mods such as turbos, 16v heads etc. I just cant see it "embarassing" many 1380's.

No because a lot of people are bough into the believe that you can only get power from a 1275!

Believe me, when we were kids and couldn't get hold of a 1275 for love nor money, we played with what we had (not just our dangly bits!).

This meant 850's or 998's.

Believe me, there was people who were putting the right bits together on 850's and producing engines that would shame many of the big bore boys.

Its not just about having a big engine, its what you do with it that counts!

Edited by CHUNKY365, 21 August 2010 - 11:23 PM.

[JakeJakeJake]

Sounds like a good plan should be a nice engine nice and revy, should be good for about 8500 i believe if you used electronics distributors otherwise the points will bounce much before that. However without serious mods such as turbos, 16v heads etc. I just cant see it "embarassing" many 1380's.

No because a lot of people are bough into the believe that you can only get power from a 1275!

Believe me, when we were kids and couldn't get hold of a 1275 for love nor money, we played with what we had (not just our dangly bits!).

This meant 850's or 998's.

Believe me, there was people who were putting the right bits together on 850's and producing engines that would shame many of the big bore boys.

Its not just about having a big engine, its what you do with it that counts!

Generally speaking larger bores are cheaper and easier to get power from than smaller ones, this is true for almost every engine ever. As for "Its not just about having a big engine, its what you do with it that counts!" If you did the same mods to a larger bore you would get more torque and BHP

I do drive a 1380 in case you could not tell

However this is not the point of the thread, this is about 850 power.

Cam choice is about right as the smaller the bore the more lumpy they appear. Have you considered new carbs? a pair of HS2's would give a little more top end but help alot more lower down the rev range, usefull when the cam is changed to something that apprecicates beign revved.

As it is going to be a high rever having the bottom enf balanced would be usefull.

[surfblue]

To revive an old thread, I wonder if the OP ever built his 850?

My Mk1 850 is very tired and could do with a rebuild, a little over bore might just pep it up a little, anyone recently done 850 tuning?

[Minigman]

Id be interested to know how the OP got on too.

I recently abandoned the 850 rebuild due to limitation of good parts. Hunted for ages for a ST crank that would rev well without breaking. Lack of decent pistons around. Cost of line boring for additional cam bearings etc.

Read all the Clive Trickey books and all interesting info but only if you could find the parts required to make them rev.

Ive setlled for a 1098 instead. Built to a good standard, still running HS2 and original manifold and exhaust. Should get low 50s BHP and bags of torque using the MG1100 cam. And, unless you look very carefully you wouldnt know it wasnt the original 850.

Edited by Minigman, 24 February 2018 - 12:28 PM.

[Cooperman]

The problem with the 850 is the rods have pinch-bolt small end location and the normal pistons are not very strong.

The crank is also a limitation and even fully balanced it is not wise to rev it much over 6500 rpm. Of course, one could get a billet crank machined for a cost (high).

There used to be a competition crank available back in the early 1960's, but I doubt one could be found now.

However, the 850 does respond well to improvement so long as mega-power/revs are not wanted. With a modified 12G295 head, a better cam and a small-bore LCB or Cooper 3-into-1 exhaust manifold, plus either a pair of HS2 carbs or a single HS4 it is easy to get c.45 to 50 bhp.  Don't set the CR too high as those split-skirt pistons will not like anything much over about 9.5:1. A 266 cam should be very good, or maybe a Mini Spares Evo001.

Be a very nice retro build and really nice to drive.

[ACDodd]

Best to just make it bigger!

Ac