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Old 06-21-2005, 12:02 PM   #1
Mystikal
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Join Date: Feb 2002
Location: Stance
Posts: 12,760
Inexpensive power upgrades for the 325i/iS/iX motor

Thought I'd compile a list of cheap M20B25 HP upgrades, all in USD for a good frame of reference:

Injectors: Factory injectors are barely adequate to even feed a stock motor when they're new, chances are they aren't feeding enough fuel above 5500 any more. You will want to upgrade your injectors before doing any other upgrades. Any other worthwhile modification will max out your stock pieces and be theoretically useless. Cheap replacements include stock pieces from an M50/S50 motor (E36 2.5 and 3.0, rated 17.5lb/hr) and Ford V8 (Mustang/Crown Vic/etc, rated 19lb/hr). Either set will do the same thing, but using the M50/S50 ones is a little easier because they are all Design II type, meaning the fuel sprays out of 4 pintles instead of 1. Better atomization.
Price: Typically go used for anywhere from $30-80, good idea to get them flow matched and rebuilt afterwards ($25/ea typically).
Installation Difficulty: 3/5 (you need to get dirty for this).

Chip: Because the motor was designed to run on 87 octane, there is a lot of potential in aftermarket software programming that is designed to use 91 and above. Good quality chips not only improve maximum power, they change throttle response at all openings and result in a smoother, more responsive feel. Reputable tuners include Jim Conforti (also labeled under Turner Motorsports) and Mark D'Sylva (MarkD, EAT, Ultrachip). Jim's software requires 91 octane, while Mark has two versions, one that requires 91 and a more aggressive chip that needs 93. Back to back comparison tests have been performed between the JimC and MarkD 93, and the MarkD software had a distinct power advantage. MarkD's website
Price: $249 for JC, $199 for MarkD (91 or 93).
Installation Difficulty: 2/5 (don't bend the pins!).

M30 AFM: The stock AFM is a known bottle-neck of the induction system, to the point where improving flow north of it (read: upgrading to a CAI or running no filter at all) doesn't improve power at all. The M30 AFM is larger and allows better airflow, and is worth about 5hp up top. You will want to use an AFM ending with the part number ending in 027, this one is plug and play with the M20 electronics. You can use the stock M20 airbox if you heat it up a bit to stretch it over the slightly larger M30 port, or you can get a 3" K&N cone filter and slap that on for bling and a cool sound (get a heatshield if you don't want to lose power). Keep in mind, after this mod is done.. the throttle body becomes your next intake bottle-neck. EMPOWERD can bore the stock one out (PM him) or with some modding, you can replace the M20 one with the larger M30 body. Dave Adams did a full write up on the AFM upgrade, here. In case the link goes down, here is the dyno graph hosted on my server.
Price: $30-80 for the "027" M30 AFM, $25-45 for a K&N cone (optional)
Installation Difficulty: 1/5 if you get a K&N cone to mount, 2/5 if you want to use the stock airbox.

Any comments or suggestions for additions, post them up and I'll consider adding them to the list. For now, these are tried and true upgrades that provide solid gains for little dough.
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