What modifications need to be made to an engine to enable higher safe RPMs?

There are 3 key factors to work on when uprating for higher revs:

• Moving parts

Spinning a rod, cam or flywheel faster than it is rated for will result in it destroying itself, often spectacularly, as the stresses become too much. Moving linkages and pistons back and forth also takes a lot of energy - Kinetic energy = 1/2 m v squared, and it is that squared which causes the problem. What they do in formula 1 to enable higher speeds is reduce the mass of each part, but this requires very strong materials.

• High pressures

Fuel must be pumped faster, air needs to enter and exit the engine faster, the compression of the engine will typically be higher etc - so the pressure in pipes and cylinders will be higher. To cope with this, all these areas need to be made of higher strength materials, gaskets need to be more robust, pipes mustn't split etc.

• High temperature

As you are running everything faster, temperatures will be higher in all areas of the engine and exhaust. Using materials with a higher failure temperature point is essential. As with pressure, this includes pipes, gaskets etc., but also the types of metal used in the cylinder head, pistons themselves and the exhaust.

Probably the most critical part for higher RPM's is the valvetrain. So you want better valve springs and retainers and a custom ground cam that is designed for the RPM you are trying to acheive. This is all to ensure that you don't encounter a situation where the valves cannot close fast enough and the piston head crashes into the valve. The higher the RPM the faster the valves have to close and the greater chance of a valve crash if the valve springs cannot close the valve quicker than the piston travels upwards. So having strong enough valve springs and a cam that will close valves at a safe time for the rpm is the most crucial thing.

Raising the RPM of an engine generally increases the peak HP of the engine so you need to make sure that other components in the engine and drivetrain can handle that added HP and stress. This is where upgraded rotating assemblys rods and pistons come in and also may require transmission and clutch components with better HP handling.

As Cinelli said, you'll have to do extensive work to the header and possibly the rods,pistons and bolts too.

But here's a little secret: manufacturers always leave a wide and comfortable margin of error in their mass-produced engines to make them more durable. In the case of your 320i, it is entirely within safe limits to increase almost any performance-related property by 10% and still see very little extra wear. For instance, the rev limiter can be tuned to only engage at around 7000RPM (assuming it's at 6500 from the factory) or the turbo boost pressure can be increased from 0.3 bar to 0.35 bar (or add about 10 - 20% to whatever that is in PSI). I would recommend you buy a colder running thermostat and install a larger, flow-matched intercooler when you do this because what you DON'T want is a 10% increase in operating temperature. The one thing that ISN'T very lenient is the difference between safe operating temperature and "blown headgasket" temperature. Especially in turbo cars. Turbo cars usually run at 90 to 100 degrees celsius and at 112 degrees celsius the heat will warp your aluminium header. And it's very easy to reach that temperature in hot weather while sitting in traffic.

Warning: you can do exactly ONE of those things and still be safe. If you increase boost pressure AND the RPM limit, your car will break in a spectacular fashion.

Or if you REALLY want to rev to 8500 safely, buy a Mazda RX8.

Allowing the engine to aspirate better; increasing its volumetric efficiency, increases it's rpm. Porting, valve angle considerations after the installation of aa larger duration and larger lift camshaft.A head package with a matching compression ratio and balancing of the rotation assembly.