To effect a compression ratio increase, and still run on 92 octane pump gas? -Jeffy.

 

Milling the head for a CR increase is not always the best option for increased HP. It's often done 'incidentally' during head rework to assure flatness, but it's not always done solely for that purpose. Switching to domed or higher deck height pistons is usually the better bet. One issue is that any sort of 'extreme' head milling on an OHC engine causes retarded cam timing, this needs to be addressed by offset cam keys or pins to reestablish proper timing. Retarding the cam does sometimes make for higher top-end power, but with a sacrifice of bottom-end torque.

I am not aware of anyone who has done a head cut for increased CR on a MC engine. I am sure someone has, but I missed out. As usual.

 

I've heard of the opposite; adding an extra gasket, etc, on an S to reduce CR and the likelyhood of detonation. I'm w/ Greatbear. If you're sure you want to increase CR, milling the head may seem like the simple way to go, but you might not be happy with the side-effects.

 

For about 20 years. It works great, is cheap and improves power throughout the rpm range-if you have optimal valve lift and timing. Using domed pistons can create hot spots in the combustion chamber(the piston being the bottom of it), and adds weight to the pistons. Sometimes if milling a head makes it too thin or weak, racers have "decked", or milled the block. The heads on these Mini engines look to have somewhere between hemispherical and pent-roof shaped combustion chambers, which are quite detonation resistant. 600cc motorcycle engines have compression ratios as high as 12/1, and run on pump gas, all right out of the box. Longer duration cams help cylinder pressures at lower rpm stay low enough to not damage the engines. I'm starting to think that nobody has a really fast non-S Mini Cooper-yet. Or, they don't want to give up the tip and get factored out of their current racing class... -Jeffy.

 

If you are fixing the octane parameter at 92, then increasing the compression ratio will not give you the performance increase you are looking for as the computer will back off on timing to compensate for the detonation (whether the pistons can handle it or not - not a good idea in any event). Focus on getting the air/fuel mix in and exhaust back out again (filter,porting, polishing, bigger injectors, exhaust) I've built race engines for Bugeye Sprites (14:1 cr), Datsun 510s (11:1 cr) and a 331ci Ford engine (9:1 cr ). If I were building a normally aspirated MINI engine for 92 octane, I'd build a stroker (increase the displacement).

If you decide to mill the head anyway, don't go over .040. Then measure the valve to piston head clearance with putty. You may need to machine some wiskers into the pistons to maintain adequate clearance.

I'm assuming, of course, that you have a non-S. If you have an S, just throw a smaller pulley on the supercharger and don't worry about the head.

 

This is a huge mistake for anyone that wants to make serious power, IMHO.

I didn't deck the head I did on my Cooper. We just did a very, VERY thorough job cleaning up the ports. I'm not certain how much more could be produced by decking the head, but it'd be nice if someone found out...

 

Thanks for giving me an important piece of info, Ihoboy. Now I know that up to .040 can be milled without weakening the head. I'm not going to go straight out and mill off that much, because I don't know how much will cause the knock sensor to have to do it's job and spoil the fun(but save the engine from detonation). I have learned that your ear is a great knock sensor, since I built the really zippy engine in my Datsun 240z. There is no knock sensor in there, or any other condition sensing device that would change the engine's behavior to save it from incorrect tuning. I filled it up with 92 octane and went out and tried to step on the gas- Mean pinging was easily detected by my ear, so I retarded the ignition timing and kept doing this until I could floor it while going up a fairly steep hill without any pinging. In the Z is a happy, powerful, non-supercharged, 2.8 liter engine. The Mini's non-supercharged engine is relatively sluggish. I'm hoping that someone out there will share with us the answer to my original question, since it has been done... Everyone wants to win, so somebody has done this and is sneaking it past the tech inspectors. How would they know? -Jeffy.

 

Actually, fairly easily. As a former SCCA tech inspector, it would not be uncommon for a competitor to file a protest if an identical model was blasting past them on the straight-aways in the showroom stock category. First thing we would check is the compression pressure. Shaved head: BUSTED!