Thread: velocity stacks
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Old October 13th, 2017, 01:27 PM   #18
tgold
ninjette.org sage
 
Name: Timm
Location: West Seneca, NY
Join Date: Oct 2015

Motorcycle(s): 2006 1050 Speed Triple, 2010 250 Ninja racebike, YZF320RR? Racebike

Posts: 527
MOTM - Nov '15
The screens in the stack do absolutely awful things for airflow and really defeat the purpose of having a velocity stack in the first place. The whole point of a good air filter is to increase the surface area to minimize the loss of flow as much as possible and restricting the filtered area to a small screened area is, well, if you're using them to look cool, go for it. If you think you'll get more power, forget it.

There is a reason modern racebikes have big airboxes. It's because the total volume of air in the box is relatively slow moving, and the slower the air, the higher the pressure and that means more efficient cylinder filling.

Also, bikes with open velocity stacks are much more susceptible to carburetion problems due to turbulent air, crosswinds, etc.

Velocity stacks may work well on a dyno indoors with no breeze, but when the bike is moving through the air at 80 mph you will get a different story when it comes to airflow and power. This is because there will be air turbulence that will change as the bike and rider move.

An example of what turbulent airflow will do: I had an SV650 racebike with the filter installed in the airbox but no lid on the airbox (a common SV650 mod) I had also modified the bottom side of the gas tank to increase fuel capacity. Modifying the gas tank also meant that I had to raise the tank on its mounts. I started having issues with the bike stumbling while on the gas in some high speed corners. It was very puzzling until I realized that it was a windy day and the crosswind was playing havoc with the carburetion due to it blowing across the top of my airbox from the side and disturbing the aiflow into the box. So even with an airbox you can get caught out sometimes but with just velocity stacks its much more likely.

I have a good sized K&N air filter with no airbox on my Ninja 250 racebike now and it does pretty good. The only reason I removed the airbox because it is a giant pain in the a** when working on the carbs. If I keep developing the bike for next year, I will make a better airbox out of carbon fiber and I would expect that it will perform better than with just the K&N.

Now, put the right velocity stacks inside an airbox (without those awful screens) and then you may start getting somewhere. But even so, velocity stacks by nature will work better only at certain rpm ranges. That is why bikes like the Yamaha R6 have velocity stacks that move in and out which changes the effective length and makes better power over a wider rpm range.
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