You might have already heard that the bigger M52 engines (M52B25 and M52B28) were factory restricted. It was mainly because of the German taxation regulations. Basically owners of cars which were more powerful than 193BHP were ripped off on insurance costs at the time. This explains why the newer and bigger 2.8L plant is actually producing 2 BHP less than the smaller, older 2.5L engine it replaced. Due to the intake plenum and runners shape/size design the main difference between the old 2.5L and the new 2.8L unit is felt mostly in lower rpm speeds where the 2.8L plant has a quite substantial torque advantage making a general impression that it is more powerful thafile:///data_disc/krycha/bmc_stuff/website_new/v3_01/e36/m50manifold_conversion_details.shtmln the old 2.5L one. The 2.5L M50B25 engine needs to be revved higher in order to stay within the optimum section of its power band whereas the M52B28 power plant pulls hard, right from low rpm speeds. (good article on runners length formulas)    
           The downside of such low-end torque set up in newer engines is high rpm breathing problems. Above certain RPM's the M52 engines literally struggle to suck the air through small diameter intake runners. I am sure every owner of a standard 328i will confirm that. The car just does not accelerate above 5000 rpm.

Replacing the factory M52 manifold with the M50B25 unit which has around 60% bigger runners results in improved airflow and fuel mixture capabilities. The engine is able to produce more horsepower up the top of the rpm range. Enough for the swap to be worthwhile... This is a list of the main differences you can expect from this modification:

1. Engine revs more freely. The main difference lays in 5000 to 6000rpm band confirmed by various dyno tests but for the average user the car feels quicker right from the 4000rpm mark. When you floor the accelerator on first gear wait until the rev counter reaches 4000rpm... You just won't believe this was achieved by changing the intake manifold alone!!!


2. The engine loses a bit of torque in the low/mid range but this can be compensated by a good ECU remap.


3. The engine idles smoother.


4. Exhaust note is a fraction louder and noticeably deeper and intake roar is... well, more fun at higher rpm's.


5. Improved MPG is achievable(highly dependant on "enjoynment" factor of the driver).


6. The swap combined with a decent remap and installation of a cold air intake and less restricted exhaust should bring your 328i to the region of 230BHP and 323i to over 200BHP(depending on the condition of your engine).

To accomplish the conversion from M52 to M50 intake, a number of differences between the two manifolds and their linkups and mechanics must be accounted for. The main problem with the earlier manifold is that it has different vacuum connections, mainly for the crankcase vent and ICV. Also its mounting points have different locations. To adapt the M50 manifold to the M52 engine systems additional vacuum connections need to be created along with a set of brackets.

If you want to know a bit more on how intake plenum/runner size affects the performance of a combustion engine please follow the links on how the size of intake runners affects a combustion engine or intake length formula

Due to high number of emails requesting further detials on this upgrade, we have prepared an M50 intake conversion information pack. This document is for those who either have bought the bigger manifold or considering purchasing it in the near future. This write-up answers all the basic questions and more. Please follow this link for info.

* suitable for M52B25, M52B28 and S52B32 engines found in the following vehicles: E36 323i, 323is 328i, 328is, usM3 ODBII ('96 onwards) as well as in Z3 E36 2.8, E39 523i, 528i and E38 728i. Not suitable for M52TU engines.

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