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i was reading my latest copy of practical sportsbike (an awesome old skool bike mag in the uk)

and a common mod, that actually worked, for one of the old kawasaki h2 or suzuki slingshot bikes was to leave the lid off the airbox. this was a cheap and effective way to help maximise the airflow.

right enough this was b4 bikes had ram air induction, but its worth bringing up
 

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I had an H2 back in '72 and the hot tip was to take off the airbox top, because the intake horn on the top of it was very restrictive. The horn was there to quiet down the intake noise, which was really noticable when the cover was removed, but the power was much better. On the modern bikes, like the CBR's and RC51's, the only real restrictions in the intake tract are the flapper valves, which are there also, to lower the noise from the intake and the paper element OEM air cleaners, which are somewhat restrictive as well. Removing the flapper valves, upgrading the air cleaner to a higher flow after-market item or upgrading the the air box to a higher volume item will see an improvement in performance for a modern bike.

Leave the cover alone.
 

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I had an H2 back in '72 and the hot tip was to take off the airbox top, because the intake horn on the top of it was very restrictive. The horn was there to quiet down the intake noise, which was really noticable when the cover was removed, but the power was much better. On the modern bikes, like the CBR's and RC51's, the only real restrictions in the intake tract are the flapper valves, which are there also, to lower the noise from the intake and the paper element OEM air cleaners, which are somewhat restrictive as well. Removing the flapper valves, upgrading the air cleaner to a higher flow after-market item or upgrading the the air box to a higher volume item will see an improvement in performance for a modern bike.

Leave the cover alone.
http://www.rc51.org/
One of our members has already dyno tested the aftermarket air filters, they do not increase hp and actually lose some in a RC51 application. ;)
 

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