BMW magic

I have read various technical articles about ways to increase the performance of an engine. What they failed to explain is how BMW can get its 325i engine to have more torque at lower r/min and more power than the 323i engine of the same capacity. I assume they have detuned the 323i motor in some way and would appreciate it if you could explain how BMW does that, and how one could re-tune it?

JOHN JEFFERYS – By e-mail

I cannot tell you exactly what the differences are between these two engines. Only a BMW expert can do that, but I can point out how two engines with the same displacement can have vastly different outputs. Once the displacement is fixed, an engine’s maximum output, as measured by the values shown on the power and torque curves, can only be modified by changing the average pressure on the pistons at full throttle, or changing the maximum revs. If the maximum revs are kept the same it means that there is only one variable, the average pressure on the pistons, at play.

This pressure depends on the engine’s breathing characteristics as determined by the manifolding, valve timing and valve lift as well as the compression ratio employed. It follows that the differences between the engines will be found among the latter four variables. There may also be slightly different mapping values for ignition timing and mixture, but responsible manufacturers have very little choice in changing the settings because if they stray too far from theoretically correct settings the catalytic converter will not function.

Tuning a 323i engine to 325i specifications would only be worthwhile if you knew exactly what the changes are, not only with respect to the breathing modifications, but also with respect to stress-bearing parts such as pistons, con-rods and even some transmission components. You cannot assume these parts will be common to both models.

Offering engines with the same displacement in various states of tune makes a lot of sense. This policy is pursued by a number of manufacturers because it enables them to create models that seem more attractive to the customer without making any major changes.

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CAR magazine