Tuning 850cc Mini Engines

Somebody a while back asked about tuning an 850 and most responses seemed to be to fit a bigger engine. While I agree that if you want a fast car that's the way to go, if you want to tune an 850 you can and you use essentially the same principles as for any motor - increase the air flow and raise the compression ratio. With small bore engines though you can't just bolt on huge inlets and exhausts as you need to keep gas velocity high.

Whatever your budget I think the first place to start is the head. Whether you use the standard 850 head or one from say a later 1000cc or 1100 you need to get the compression ratio up. Standard is 8.3:1 I think for the manual gearbox motors. If you can get hold of "premium" fuel you should be looking at CRs of around 10:1 or 11:1. This'll be achieved by getting the head skimmed by a machine shop which shouldn't cost too much. You have to work out how much needs to come off though which you can do accurately by measuring the capacity of the combustion chambers or by a certain amount of calculations and guesswork. If you're sure your head, block and pistons are standard it's quite simple to work out how much you can take off the head by measuring the area of the chamber part of the head and making a few assumptions. The 850 inlet ports are really small so you can open these up a bit. If you have access to a die grinder (or the cash for a machine shop) you can deshroud the valves. This involves cutting metal away from the pointy beak area but make sure you can still get the CR up to what you want when you're done. If you've got the know-how, equipment or cash you can have the complete head flowed (or buy a 998 Cooper head and get it skimmed)

Next you can flow the carb. The carb has a thick spindle going through the middle of it and if you've got an HS2 it seems even thicker! This can be cut down and flowed. Inside the carb the port goes from round to square to round again and you can smooth off the sudden changes. Porting out an HS2 is good as you increase the flow whilst keeping the gas velocity. The early 850 air filter arrangements are tiny and you can change these to the later type which are larger and have a cool air feed. You can fit a K&N air filter too if you're not alarmed by possible extra engine wear.

If you've got the engine out you can have the block skimmed which'll mean you'll be able to deshroud the valves more.

850s don't have a lot of torque so have to be revved. Again, if you've got the know-how, equipment or cash you can have the conrods and crank lightened which mean the engine can be revved more and more easily. 850s should rev anyway in theory as the pistons are so small!

Next I guess is replacement parts. If the engine is out you can ("should") replace the cam for something better. On a street 850 you want something of 255 or 266 duration. The 850 block only has a single replaceable cam bearing so anything more would get eaten up really quick anyway. You can get the other two bearings fitted but it costs! The timing of the cam has an affect of altering the power band so you should really get the timing of it checked and fit offset cam keys if necessary. If you've got an early 850 with the weak springs you'll need to fit standard 998 springs if you plan on revving more than say 5000rpm. Duplex (double row) cam chain and gears is a good idea too as it'll keep the cam in time for longer and will be less noisy. If you can run to it, fitting offset rocker bushes will give you more valve lift.

Just looking at the standard inlet manifold you can tell it was designed to fit in a small space! These are generally crap and you can fit an aftermarket one but again don't go for anything too big, go for flow.

Next I guess is the exhaust. You *can*, if you really want to, keep the standard single box exhaust system but replace the single box with an RC40 box. Exhaust's flow better without the centre box (a common MG-B trick) but they are a little noisier. You'll get the gas speed along the narrow downpipe but you won't get the same back pressure from the silencer. More than that you can get aftermarket long centre branch manifold and slightly larger bore system.

You *need* to get the car rolling road dyno'ed after any changes as an 850'll need every hp it can get. If you've raised the CR and the ignition timing is out you run the run of blowing big holes in your pistons too.

I think I've covered most things. The above is pitched at someone wanting to improve their street motor on a limited budget. Of course if you want to nick the injection and turbo components from a Daihatsu Charade GTi you can.

Article written by Steve Adderson.