After trying out the Racing Beat headers
and exhaust, my car finally got its high-end improved. However, I
still felt some kind of resonance, vibration, or lack of
smoothness, when engine speed was over 6000rpm. I suspected the
K&N drop-in was the problem because Terence's '90 Red doesn't
vibrate over 6000rpm. There came a chance when Terence was out of
town for a week. I took the cone filter with custom-made adaptor
out of his car and tried it on my car.
The cone filter is 8" long and has a 5" hose diameter when the funnel is removed. The 5" circle is just big enough to cover the entire rectangular surface of the air-flow meter. So an aluminum adaptor is fabricated with Alfred's lathe. The custom-made adaptor is actually a 3/4" thick, 5" diameter aluminum disc with the rectangular air-intake hole and 4 bolt holes to the air-flow meter. But most importantly, we fabricated the rectangular air-intake hole into a funnel shape to make sure intake air does not need to turn 90 degrees into the air-flow meter. Unfortunately, there wasn't enough time for me to show some close look at the custom-made adaptor before I need to put the cone filter back onto Terence's car. The top view pic shows here the cone filter sticks very close to the air-flow meter.
With the heat shield
customization already in place, it doesn't take much effort to
transfer the cone filter assembly from Terence's car to mine.
Similar to Terence's car, the entire filter assembly has to be
moved towards the front of the car by about an inch to make
enough clearance for the brake master cylinder.
Contradictory to what most people think, the cone filter gives
softer intake noise than the K&N drop-in. This matches my
hypothesis that the smoother the intake air path is, the softer
the intake noise gets. From my dyno-simulation, I found out
interestingly that the cone filter actually increased the power
from 3k to 6k rpm by 2 to 3 hp. This is contradictory to my
believe that the cone filter mod should improve high-end only.
After
Terence put in a fully-programmable ECU and removed the factory
flapper-door type AFM (air-flow meter) in his car in spring 98,
he custom-made his own intake plumbing and no longer need his
home-made adaptor for the AFM. So I took over the adaptor and
bought a smaller Akimoto-style cone filter for my car. Before I
put on the adaptor, I spent a whole afternoon
filing/shaving/sanding/porting/polishing the adaptor to make the
transition between the AFM-side rectangular opening and the
filter-side circular opening as smooth as possible. Later on, I
put on Alfred's custom-made 3" intake plumbing to replace
the factory crimped/wrinkled/collapsable plumbing.
It's hard to say whether my port-and-polishing has any improvement in air flow, but I think the custom-made intake plumbing does make more improvement. The improvement is more noticeable at mid-rpm around 4000 where the intake sound is amplified the most. The noticeable improvement from the new intake plumbing is probably not due to it's big 3-inch diameter. Instead, the non-collapsable structure of the pipe ensures smooth flow of intake air, unlike the factory collapsable plumbing which can create pulses at certain engine speed.
Next: 
Autopower 4-point
roll bar
