Hey guys.
Just thought I'd chime in here on a few weight distribution and handling items that have been brought up.
I want to break it into two seperate items, necause the lines are being blurred a bit (which is reality of the situation, but bear with me).
1) Weight increase (overall) and it's placement.
2) Handling charecteristics of a "nose heavy" car.
When you do the engine swap, you are installing a heavier engine. However, what no one here has mentioned is that the I-6 sits further back in the chassis than the I-4 does. Some weight is already moved backward. Remember our basic machine: the lever. A small change in position changes the dynamics more significantly that you might think.
Also covered in my swap manual that no one mentioned is that the front wheels can be moved forward about an inch. Now we have a longer, more stable wheel base and essentially reduced weight on the front wheels because it is transferred to the rear wheels. (we changed the lever again)
The M3 front suspension includes 'caster tops' which increase caster quite a bit. Pushing the wheels forward using my method also increases caster further. More caster is more stable at speed. My suspension is VERY stable over 100 for extended periods. You don't do this on track, but I'm just mentioning it for thoroughness.
Relocating the battery is not essential to good handling. in a more powerful car, 50-50 static is NOT ideal. You want to be a bit nose heavy because the weight transfers under acceleration. This is why you spin if you lift going into a corner too fast. The weight transfers forward and you loose traction in the rear, so it comes around. Everyone had done this in driving school 101 day one...
The lightweight flywheel (strongly suggested) easily offsets the bulk of the rotational mass increase of the "two extra cylinders" of the six over the four. And I do understand the concept; however the M engines (really all BMW engines) are very well balanced for factory engines that the increase in piston reversal inertia is not so significant at the relitively low RPMS these cars operate at. For every piston at TDC there is one at BDC as well- balance. At 8K+, you certainly have to watch it closely, if for no other reason to keep the pistons from flying apart. It is a gyro, that is true, but the impact of the 6 gyro over the 4 gyro is small. You don't notice it in my car.
Weight of brake upgrades is a concern; however it is UNSPRUNG weight. Which, is bad in general (and I will not go there right now), but does not come into the chassis dynamics because (if your suspension is good) the brakes, spindles etc. stay on the road while the body (sprung weight) is listing side to side. Overall weight of the car will be higher; however dynamic weight does not increase so much when it is in perspective. Bottom line: you need to stop, so you need better brakes. Keep the weight you have/must have LOW in the car. Take out the passenger seat and mount the battery there if you are loosing sleep over it.
In summary, the problem is not as 'bad' as some have made it out to be. If the suspension and braking systems are upgraded as strongly suggested in my manual, you will have a very fun, safe and easy to drive car that will carve corners with the best in the world. Ask anyone who has one that is set up properly- they'll tell you the same story. Just get in and drive. You won't be at all concerned with these trivial arguments!
