the dc 4-1 header on the z1 has too much highend/wot bias. remember, the section of the rpm band controls the frequency the gas is pushed ,which indirectly effects the amount of gas. while throttle position decides the amount of gas pushed. the z1 has super small ports and the header is just too large for while in the primary cam. do you follow?
GENERALLY the shorter the primaries and secondaries the lower the intended use in the rpm band. which is why your super expensive headers have adjustable primaries, so that you can tune it for the power band your aiming for and harmonize the lengths of all the primaries so the exhaust flows through the collector sequentially. also the thinner the pipes, the less gas is intended to flow.
google 'anti-reversion' for a little help in understanding some of the tricks with stepped primaries and things of that nature.
don't waste your time or time will waste you
With FI the smoother the better as it minimizes the surface area and keeps the air cooler - my xB has plastic intake manifolds that are extreemly smooth and thermally insulating from the engine block temps all adding to more power and MPG. Raising the compression is going to help the most.
Not with this engine - seems a lot of people are adding cold air intakes and the engine doesn't run right. I get pretty poor mileage when very cold but too hot is bad too - this engine has high compression ratio. Better not to second guess the people that created the motor.
OK then I guess I should play with a warm air intake - as it is, it takes the air from the left fender under the hood next to the battery to the side of the radiator. I can monitor the intake air temp with the ScanGauge so adding a little ductwork to get some air from the rear of the engine near the header should give some different results. Might be cool to increase the MPG even more ha ha ha!