When an engine is rated as 3L, it should, under perfect conditions, and 100% Volumetric Efficiency, displace 3L of air every revolution. With NA engines, that almost never happens, untill they are highly modified, and then usually only at WOT with some sort of efficient ram air induction.

Turbo motors can provide 100% VE numbers, and much more so when the air is cooled (more O2 per CI) with an efficient intercooler. That is the only reason I stated the engine expells 3L of exhaust.

A 2.5" exhaust will be more than an enough for that tiny motor. The gains to be had are in replacing the bends in the stock system (not madrell bent) and removing the restrictions (cats, resonators, mufflers).

Once you get those fixed, you can still add an intercooler and be OK, pipe diameter-wise.

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1987 SDL. The best there ever was.