In Reply to: Am I running at 'peak' performance? posted by GREG on January 31, 2001 at 13:03:51:
Things that will degrade power (these apply to all cars):
1) Dirty fuel filter
2) Dirty air filter
3) Vacuum leaks
4) Clogged catalytic converter or muffler
5) Dirty injectors
6) Running too rich
7) Running too lean
8) Bad O2 sensor (causes rich or lean condition)
9) Spark leaking from deteriorated plug wires/cap/rotor
10) Plugs too cold (should use Bosch WR9LS silver plugs for 535i)
11) Heavy carbon deposits on intake valves
12) Pitted exhaust valves/valve seats
And specific to 80's BMWs:
13) Bad solder joint from power transistor to board in ECU
My 535i is down on power. Reasons? 4 (muffler) and 13. Also, I had the valves and seats reground at around 220k miles. The 535 naturally runs lean, so that shoots combustion temps up. The exhasut valves and seats were pitted somewhat, so I got a 1 angle valve job when I rebuilt the engine--didn't know any better. I think this killed the power quite a lot. The spec is a 3 angle job on the seats and one on the valves. It makes a difference!!
Once the car is completely sorted, get a 3.73 diff from an E30 325i or a 3.46 diff from a 1988 (only) 528e. Stock is the rather long geared 3.25, which gives you a theoretical top speed of 170MPH!! I've had great results with the 3.73 diff. Your gas mileage will suffer by about 10-15% (max), and the engine will run 14% higher revs with the 3.73.
BTW--what are you used to driving? Porsche? Twin Turbo Supra? Econobox?