Which engine is it?
Most of the BMW engines have Alusil coated bores. It's not expensive to have recoated, but for the 1-2mm that you MIGHT be able to enlarge the bores, it's questionable if it's worth it. Adding iron bore sleeves, and using a turbo is the best option for most of the BMW engines.
If you DO enlarge the bores, the only realistic option for pistons is to buy new ones in a custom size. ($$$) High performance pistons are made on a special Piston Lathe that the tool occilates with rotation to make the piston an oval shape (about .003" out of round), which aides ring sealing. Most BMW pistons have asymetric dishes as well.
A friend of mine had a BMW V12 sleeved by a "BMW Race shop" and when he got it, the block was so warped it wasn't even usable, much less race worthy. He took on doing it himself, and after scraping a couple, has it down now. It's still not a cheap option though.
Cylinder head porting is generally not as much an issue in BMW heads. An average port flows about 220-230 cfm in stock form. I've only been able to increase this to 250 cfm with LOTS of time and tweaking.
For a fuel injected engine, it's kind of a side point, and only really worth it for all out race or forced induction.
The best options if you want a shop project, are to make custom titanium valves, and add a turbo.
Regards,
Eric  |