Large Displacement Airboxes

The question of performance gains from aftermarket airboxes has been a thorn in my side for many many years. All the data I have gathered in more than a decade of tuning on the RC51 and other bikes for that matter points to the idea that it is very difficult to get more horsepower out of a stock engine. Many of us have done mod after mod after mod to the stock engines and have seen virtually no gains outside of an exhaust and a Powercommander. I have tested several closed type aftermarket airboxes and witnessed a test performed with pod filters with no airbox at all and found no truly beneficial gains at all with the airboxes. It moved the dyno curves around a tiny little bit, but it was totally nominal so small of a change that it could be simply environmental factors or maybe just the removal of the thermostat which was required to fit the Moriwaki airbox. Without the thermostat the bike runs cooler which puts it into a different A/F ratio window. The pod filters attached directly to the throttle Bodies actually lost horsepower by a fair amount.

Then reports started coming into the forums about these new open air and large displacement airboxes being instant bolt-on horsepower goodness offering gains of 6-9 horsepower, but every owner that installed one and made those claims had also done porting work in the heads or installed high compression pistons, cams or full exhausts etc at the same time they installed the airbox so there was no proof that the airbox by itself was responsible for those gains let alone if those gains would be prevalent on an otherwise stock engine. Having said that it would certainly appear that a larger aftermarket airbox does indeed offer some decent gains on an RC51 engine that has been modified with a quality porting job and various race type parts (pistons, cams etc), but that is not what the majority of RC51 owners are doing to their bikes. Most owners want the longevity of the OEM engine internals without having to worry about reliability issues that typically occur when race parts are installed.

What we needed was for someone to take a stock RC51 engine with just the common mods of an exhaust & Powercommander that was optimally mapped and then install an airbox and do the complete mapping process over again so we could compare the gains. The issue is nobody wanted to pay to map their bike completely without the airbox just so they could pay to remap the bike completely again with the new airbox... If the baseline dyno run is not a fully mapped bike then comparing it to the bike that is fully mapped with the aftermarket airbox does not let us discern if the gains/losses came from the airbox mod or if it was merely a result of proper mapping, but I digress.

As it turns out the best we could come up with is a very generous RC51 owner in Texas named Raleigh Parks that did a baseline run on his stock engined RC51 then another run after a Turn One airbox install and then had it mapped for the Turn One airbox:

You can clearly see the Turn One Airbox by itself put a really big dip in the mid-range which would definitely make the bike feel like it was hitting harder as it came out of the dip. This would falsely lead riders to think the bike was pulling harder because the jerk of the power when it came out of the dip is more perceivable than a more linear and preferable power delivery. Most of that dip was ironed out with mapping changes, but the end result is barely arguable as better than the stock airbox...

The airbox is not making the power gains some of the forum guys have been claiming at least not with an otherwise stock engine.  It is an excellent mod for saving weight, especially weight carried very high in the chassis, but overall it basically just moved the power curves around a little bit with some give & take of where the bike performs versus stock and it costs you the price of an airbox and a custom map to get there.   Excellent data gleaned here, but not so good in the search for performance with a stock engine RC51.



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