It has been said by many that the head design of the E15ET is flawed and unsatisfactory. Basically the E-series head is a poor design on account of its restrictive flow properties.
So what do you do?
Porting and polishing is a general term describing the process in which the engine’s cylinder head can be machined. The head will be removed from the engine block and the intake and exhaust ports in the head will be machined out and polished smooth to allow maximal airflow.
This process, when skillfully conducted, can provide an increase in flow of up to 25%. Note that this does not translate to a 25% increase in power! Your stock E15ET will not put out over 100kW after a simple port and polish job, unfortunately.
Costs for this kind of head work start around $200