Velocity stacks

[banoffee]

Have picked up a set of shiny Factory Pro ones for my newer bike.

I know there is a reason but can't recall or find the specifics - can someone explain why the inner ones are longer than the outer ones?

[Spike16]

the length of the stack affects the power characteristic's i think, as for 2 different pairs it may be to try and get the best of both worlds

[banoffee]

yeah, I know that much. was more interested about the specific reason for the inner pair being much longer. thanks though

[David W]

It's to do with airflow and harmonics in the airbox. Longer stacks tend to prop up midrange and shorter ones suit revvy, short stroke engines. It depends on airbox design though.

[banoffee]

just fitted them and found that the stock rubber ones are different sizes too. not ridden it yet but they do look good

before...

after...

[sparx55]

two long and two short,

reason is basically that all short will help top end, but create a mid range flat spot, opposite applies to all long, it s a half way house solution, remember, area under the graph gives the fastest bike, not the highest peak.

(para phrasically quoted from Andy Quill)

[lorenzo]

Although it does beg the question: why not fit all 4 stacks at a length somewhere in the middle?

[stretch]

I don't know why not, but I know that going for all the same length is a common cbr600rr modification. I think they go for all 4 of the longer length, needs a PCIII set up tohelp it as well, supposed to beef up the midrange.

[Rainer]

I don't know why not, but I know that going for all the same length is a common cbr600rr modification. I think they go for all 4 of the longer length, needs a PCIII set up tohelp it as well, supposed to beef up the midrange.

It's the short V stacks for the 600RR mod which gives more top end power by sacrificing some mid range. I did it to mine as recommended by Tim Radley - he said it's the cheapest horsepower you can buy and he was right - £11 for a spare set of stacks to hacksaw.

I wonder what it would be like to run three short stacks and one long

[lorenzo]

What about sloped top stacks: front edge tall and then sloping down to the back, or vice versa? Genius, or the worst of both worlds? The mind boggles...

[BenW]

As has been said, the shorter are for top-end, the longer for bottom-end/midrange. The reason for this is to do with pressure waves and the column of air coming from the airbox into the engine.

As the inlet valve is open, and the piston descending, air is drawn in but when the valve snaps shut the air still has a velocity. The valve stops this air and a pressure wave travels back up the inlet to the top of the velocity stack, whereupon it travels back down again. If the valve is opening as the pressure wave comes back down again (speeding up the column of air) this stuffs air into the engine.

This means that the length of the velocity stacks need to be tuned to the engine's power characteristics. The faster the engine spins, the more quickly the pressure waves needs to get to the top of the velocity stack and back (so it arrives as the valve is opening, forcing air into the combustion chamber), so shorter is better for high revs. And vice versa for longer stacks/bottom end.

Having both short and long stacks is a way to give reasonable mid-range and top-end across the firing order. If you're after top-end only, short stacks can help.

Or something like that.

BW

[Stephenrc45]

Or short bell mouths stood off the throttle body. Honda used this on the works RC45 as well as Suzuki doing something along those lines with bell mouths with slots around their base on the K1 and K2 1000's if I recall.

Its also important to remember that anything near the bell mouth will disturb the air. That includes air box lids and second injector sets.

When working on the R7's we found good hp gains by fitting shorter bell mouths. However the same effect was achevied if the second set of injectors were moved further away of the bell mouth and the longer bell mouth was refitted.

The trickest bell mouths have to go to Honda though when they designed them with the same pattern as golf balls use on thier surface to aid their performance.

Edit for poor spelling etc. I'm sure I've still missed a few.

[Bat Fastard]

Bell mouths/basic principles etc are covered in David Vizards engine tuning books. For Minis if i remember??

Its all to do with what has already been quoted. Plus treating the air coming in like a liquid with different angled curves at the top of the bell mouth also having an effect on the flow.

The books are probably a little out of date but the rules still apply.

[banoffee]

thanks for the interesting replies. on a less technical note it sounds and goes really well

[Rainer]

What happens if all four ranged from very short, short, medium and long for a spread of power throughout the rev range?

[Tomfoolery]

Wouldn't you have to tune each individual cylinder for the height of the stack that way?

[Rainer]

Wouldn't you have to tune each individual cylinder for the height of the stack that way?

If we're talking about changing them from the standard two short/two tall on a EFI engine the bike would need a PCIII and dyno set up anyway

[speedzephyr]

2007 up R1's have variable length velocity stacks (YCC-I), seems to have solved the Long/short combination.

[Tomfoolery]

As does an MV of some kind. First production bike to have them, IIRC.

[Stephenrc45]

As did the factory RC45's in '96 if I recall. Only for the middle part of the season though.