Hey so I've been looking around online and I just wanted to check here first to see if anybody actually makes velocity stacks. I did rejet at my carbs and threw a whole new exhaust on there so I am ready for them. I just thought I'd check here first

Most people just do the K&N pods or the lunchbox air filter. They're suitable for use with a rejet.

"most people" don't do that. Most people just leave it alone, best performance and engine longevity will be had with the stock airbox and an OEM type filter.

Some people who in my view appear to be more concerned with appearance and sound than performance or tuneability will put pods or the lunchbox filter. A few will swear that they have improved performance with pods or lunchbox but I have yet to see anyone produce a dyno plot to show it.

I have a dyno run with K&N and pipe. What I don't have is the stock, just the pipe and the pipe and K&N on the same bike, in the same conditions pretty much at the same mileage with each jetted to the spot on correct jetting.

K&N and pipe will let you pull an extra 4-600 rpm up top - if your pipe is WFO and the K&N is in place and its jetted spot on. I did a D&D slip on and K&N and the thing was fantastic even as I dialed in the jetting over a year in Canada+California. This jetting runs landed me on 150/40/1-2 washers, 3 turns.

Stock filter and slip on pipe - the perfect pipe for a stock filter would need to be a bit more restrictive than the WFO D&D because you'd never get that last 500 revs and might as well keep some back pressure so through the rev range it doesn't give you a hollow response cos more of the intake charge got out due to the lack of back pressure. The perfect main jet for this is 125 all the rest is the same. Its a 20% larger jet - now that doesn't mean 20% more power, cos you lose more in this setup, not to mention 20% more diameter doesn't flow 20% more. The cross section area is 44% more but flow rate may only be 10% more - who knows I didn't calculate it. My gas mileage dropped maybe 45 to 42 mpg - but I also may have been revving it harder just cos it was responding better.

The other way to think of it is - Likely 10% increase in power for maybe 10% loss in fuel economy. Likely you can keep a little tighter pipe, sacrifice a few 100 rpm on top and not hollow out the midrange, and get most of the benefits of both.

Cool.
Buddha.

Quote from: The Buddha on May 13, 2020, 08:00:44 AM
The perfect main jet for this is 125 all the rest is the same. Its a 20% larger jet - now that doesn't mean 20% more power, cos you lose more in this setup, not to mention 20% more diameter doesn't flow 20% more. The cross section area is 44% more but flow rate may only be 10% more - who knows I didn't calculate it.

It doesn't mean any % more fuel or more intake charge because you didn't change the displacement. The reason you need a bigger jet is because the air flows more slowly and therefore you need to dump more fuel per unit time into the slower air to keep the mixture the same. But the power is limited by the volume of the cylinder.

To actually bring in more air (and fuel) would require lots of work and different carbs entirely, not just different jets.

But anyway, if you rev higher you will make more power, at those higher revs. Likely as you point out, in exchange for usable power in the midrange.

In any case putting a K&N or any other cotton&oil type air filter will let in a lot more dirt and reduce the lifetime of valve seats, rings, you name it.

Anyway... I'm gonna go back to riding my FI bike that I don't have to worry with this stuff. My GS is leaking fuel from the tank petcock. that'll kill your mpg for sure.

Well yes and no. K&N or velocity stack which is even better than a K&N improves volumetric efficiency. You cram more in the same chamber cos the moving air mass reacts slower than the valve closing etc etc.
Now that itself is minimal, so discounting that, you use the same amount of fuel air per stroke. The higher revs mean more strokes per second.
But yes, the removal of restrictions before it gets to the carb causes a drop in vacuum in the venturi and hence less fuel is sucked out and hence the need for the larger main and thinner needle (if you put an extra washer under it). Which is why 44% larger CSA only delivers 8% less gas mileage.

Cool.
Buddha.