I decided I wanted to see whether it had impacted performance in any way. So I went for a play a couple of days ago, and did a crude comparison of the car accelerating with the airbox, versus without. Basically found a stretch of road, ran it both with and without the airbox, in 4th and 5th gear on separate runs. I recorded rpm over time using my Digidash for each run and plotted in Excel: -
Description
Basically, it seems that to all intents and purposes the airbox makes no perceivable difference to performance - which is exactly what I hoped for!
I've also been having a play with the Road Dyno spreadsheet available here : http://www.iwsti.com/forums/how-install/135450-road-dyno-your-car-complete-noobs-guide-56k-run-away.html
Tweaking around with the numbers for gear ratio's, drag coefficients etc I've got something that looks a reasonable approximation to what I got the last time I was on the rollers: -
Road Dyno Plot
I didn't hold much past 10000rpm so don't see the roll-off.
I don't expect the numbers to be accurate, and there's a lot of "rounding" goes on to smooth out the graph, but this again seems to suggest the airbox isn't impeding performance.
The "with airbox" line does seem a bit more "wiggly" than the without. I'll need to do multiple runs to know whether that's just how the data looked for that run or symptomatic. Probably reading too much into it, but it but I guess it could be resonance and harmonics (though every 500rpm?), or running out of air, refilling and being happy again?
I'm tending to think it's just the data though - certainly didn't "feel" to be jumping around, and as my previous post the rpm over time looks pretty much identical.
I've since run a cold air feed from the nose to the filter (not mounted the filter there because it doesn't fit!), so I'll probably do a back-to-back with that at some point to see if there's a discernable difference.
So overall I'm fairly happy that it quietens things down without impacting performance - which was nice.
