If your CBR600RR's velocity...
If your CBR600RR's velocity stacks have this all-short "velocity stack mod," be sure to change them back the way they came from the factory. Unless you want to lose power, that is.
We've all been duped by the thought of something for nothing. So when we saw several threads on the top Honda CBR600RR forums claiming that interchanging two sets of factory velocity stacks for a certain combo would result in another 3 HP at the wheel, we had to see it for ourselves.
Before delving into our experiment let us give you a rough definition of what a velocity stack is. Essentially they are trumpet-shaped devices fitted to the air entry of an intake system, fuel injection or carburetor. The velocity stack is designed to smooth and regulate the incoming air into the engine by directing the incoming air around the walls of the stack. Velocity stacks can vary in length, as the different lengths combined with the airbox design act as resonating pipes that adjust the frequency of the power pulses inside the airbox. In other words, change the length of the velocity stacks and it will alter the powerband.
In general, the shorter the stack the better the top-end power, but usually at the expense of low-end grunt. Conversely, longer stacks generally make more power down low at the expense of high-RPM power.
In the case of our 2008 CBR600RR, it came stock with short velocity stacks on the outer cylinders and two longer stacks on the inside holes. The "velocity stack mod" consists of using the shorter outer velocity stacks on all four cylinders. The claim is that this will offer several more HP at peak without costing anything in the middle.
A quick "Buy it now" on eBay netted us two sets of factory velocity stacks. We then cut apart the two pieces to form two different combos; all short and all long.
The stock baseline (green)...
The stock baseline (green) made more power where it counts-from idle to peak. The long stacks (red) were down everywhere and the short stacks (blue) were also down, save for a few HP on the utterly useless over-rev. Keep your stacks stock.
To keep things fair, we kept the stock exhaust and air filter on our CBR600RR but used a Power Commander to dial-in the fueling in hopes of maximizing each combo. The different dyno runs were also done less than five minutes apart on the same day, the same dyno and at the same engine/coolant temps.
After our baseline runs, we jumped to the quartet of long stacks and lost power and torque everywhere. From idle to redline it consistently cost us several horsepower compared to stock. Next up was the "velocity stack mod," of all short stacks. It too cost us power everywhere except for a jump after the power peak. From 14,250 to the limiter we did see several more horsepower, but at the expense of less power everywhere else. While it did truly make an additional 2-3 HP at the wheel, it did so in a generally unused portion of the powerband-the over-rev zone that you'd never use.
Unless you ride beyond the power peak in the 14,000+ RPM zone and don't mind losing power until then, you're better off leaving the stock stacks because they made significantly more power and torque.
Busted
The stock setup proved...
Busted
The stock setup proved to be the best. Don't you think if there was another 3 HP easily gained by tuning the velocity stacks Honda would have done it already? Myth busted!
members who could have potentially installed the useless "velocity stack" mod.