Project E46 M3: Part 2 – Cat-back Exhaust and Pulleys


Having tested other cat-back systems on stock catalyst E46 M3s, it was no surprise we didn’t see a significant power gain.  Until the factory cats are eliminated, not much will happen (and the same goes with the E90/E92 M3).  But we were pleasantly surprised with 2-3whp gains throughout the curve, even though the peak was within 1hp.  The peak gain was 4whp at 120mph (7000rpm).

The primary benefit to a cat-back exhaust on an E46 M3 is the sound improvement, which includes the elimination of the stock “rasp”, and notable weight improvement.  Unfortunately, due to a faulty tach signal, we couldn’t get a good RPM reading and, consequently, torque reading.

Next up were the VAC under drive pulleys.  By putting on larger pulleys they spin each unit less times per engine revolution, thus causing less drag on the motor and potentially freeing up horsepower.

VAC Motorsport pulleys are made from 6061 billet aluminum and are seen on most of the racecars they build or are associated with.  We chose their full street setup, which includes lightweight water pump, power steering and alternator pulleys.  We don’t plan to run a large sound system, so we should be good with the latter.

Here’s our stock, hard plastic power steering pump pulley (left) versus the aluminum VAC Motorsports under drive pulley (right), which weighs in at 11oz.  In this case, the VAC pulley is actually heavier than stock by 5.5 oz.
The stock hard plastic water pump pulley on the left weighs 7oz while the larger aluminum VAC unit weighs in at 8 oz.
Here’s the alternator pulley comparison.  Forget about the 6.5oz we’ve gained above.  All of it is forgiven here, as the stock alternator pulley weighed in at 11.5oz while the VAC unit weighs only 4.5oz.

With all three pulleys together, the VAC setup is 0.5oz lighter, even though they are larger, billet
aluminum pulleys when compared to the smaller, plastic stockers.

While the pulley kit does come with a replacement belt for the pulleys, it understandably does not include a belt for the AC compressor.  Still, it’s an opportune time to check it, and good thing we did because it’s not looking good.  Notice the cracking.

We procured an AC compressor belt from Gates, which specializes in belts.  Not only do we have these belts going on our Project Supra, including for its timing belt (the M3 is blessed with a timing chain), but we even saw Gates belts on the 10,000hp NHRA Nitro cars!

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