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I fitted a K&N filter to my '93 Mitsi V3000 some years ago and was astonished at the immediate increase in performance. A friend suggested this would have been because the engine management system would have increased the fuel delivery upon detected more oxygen coming through. So Yes, more performance but Yes, guzzling the petrol to do it.

I have a 1990 Honda Civic Si (1.6 injected DOHC) and would like to fit a K&N filter to it. But would the Civic have the electronic smarts to simply adjust the air/fuel mix the way the Mitsi did or could I end up with issues requiring different spark plugs, fiddling about with the fuel injection system etc?

The Honda’s engine management system will adjust the fuel delivery into the engine based on signals received from the airflow sensor much like the Mitsubishi.

You do need to be a little bit careful with aftermarket air filters however.

Because the airflow is increased so are the chances of contaminated air passing over the airflow sensor. If this happens the sensor can over time develop a film over it which provides a false reading back to the engine management system. The end result can be the engine starts to run very lean.

Over a long drive period this condition is certainly not desirable. In a worst case scenario this can result in a meltdown of engine internals such as pistons.

Tampering with the air induction system on some makes/models can render a mechanical warranty insurance policy invalid for this very reason.

I'm not saying filters like K&N are bad, just be careful. Follow any instructions provided with the filter and never fit any aftermarket part which is not specifically designed for a particular make/model of vehicle.

Thanks, really appreaciate your thoughts.