Shorter Vs Longer Velocity Stacks

[koko64]

Did some dyno tests with both stock length rubber velocity stacks and ones that were cut and carved shorter. They are the carbed Monster/900SS type that run from the carb opening into the stock air box.

My bike has FCR 41s, some porting, JE high comps, Termis, open air box/K&N and advanced stock cams.

I cut them down and radiused them with a sharp knife. "Craftsmanship Madam"!  

The stockers are about 50-51mm in total length to where they clamp to the stock carbs or FCRs with the alloy air box adapter. Cut down they become 32-33mm. So they have been reduced about 18mm in length. This 18mm is reduced at the end of the 'trumpet' which protrudes into the air box, the end of the velocity stack. The stacks also were increased in maximum width from 74mm at their ends to 80mm when they were cut down. This is because I cut them down to the lip where they connect to the air box which is a little larger in diameter. This makes them quite flush with the bottom of the air box, which is by the way raised into the air box 20 + mm with a convex radius. This fits quite well with the shape of the shorter velocity stacks.

The stacks are therefore shorter 18mm and about 6mm wider giving a more pronounced trumpet shape with a greater diameter drop over a shorter distance. Taking the FCR air box adapters into account the shortened stacks are within 5mm-ish of the optional plastic blue stacks provided with the FCR kit.

I expected that with the same jetting for there to be a loss of power down low in the rev range and a gain in peak power. You know, the expected short stack Vs long stack tuning characteristic. The stacks were now 18mm further away from from the filter element and I was interested in any resonance effect this may have. I was interested to see if this would simulate the effect of a larger air box.  

Well, there was a range of 1-2 ftlbs more torque and 1-2 more hp from 3500-8200 rpm with the short stacks. There was a little more all the way through the rev range.

There was a difference in the A/F ratio that occurred between 5700 and 8000 rpm that indicated that the short stacks flowed a little more air or had a resonance characteristic that displayed up to 0.5 leaner in the A/F. The long and short stacks were within 0.5 on the EGA through the rev range. Later jetting tests with the long stacks (dropping the main jets from 165 to 160) nearly matched the top end power of the shorties within 0.5 hp but only from 7300rpm. Below that there was still 1-2 hp/ftlbs less. The bike was happier with the bigger main jets on the rich side with the shorter stacks. This gave more power right through the rev range on WOT power runs.

I'm guessing that with the incredibly long inlet manifolds, maybe the shorter stacks help offset the resonance issues with this type of engine. Or maybe the short stacks give the result of a slightly larger air box which thickens up the power curve a little. Or maybe a bit of both. I think it's more about resonance either in the manifold/inlet ports and/or air box than big changes in air flow (although there was some in the upper rev range).
I took some of the dyno charts to Brad Black to discuss and we threw a few ideas around. I've pondered the charts, jetting and velocity stack tests for hours, I am left with more questions. But I learned stuff.

Its not much difference in power, but every pony counts on these little jiggers. I think the biggest difference was in response and acceleration, from road testing. Pinning the throttle and roll ons had more immediate engine response.

At least the short stacks made it easier to remove and replace the air box.

[Speeddog]

In my limited testing with carbs and velocity stacks, and from all I've read on the subject:

Carbs like as short a stack as possible.
Whatever intake tract length is necessary for best tuning is best accomodated between the carb and the intake valve.

[koko64]

I should have just called you!

[Speeddog]

I should have just called you!

My post wasn't backed up by dyno-runs. 

[greenmonster]

Thx f sharing!

The stockers are about 50-51mm in total length to where they clamp to the stock carbs or FCRs with the alloy air box adapter. Cut down they become 32-33mm. So they have been reduced about 18mm in length. This 18mm is reduced at the end of the 'trumpet' which protrudes into the air box, the end of the velocity stack. The stacks also were increased in maximum width from 74mm at their ends to 80mm when they were cut down. This is because I cut them down to the lip where they connect to the air box which is a little larger in diameter. This makes them quite flush with the bottom of the air box, which is by the way raised into the air box 20 + mm with a convex radius. This fits quite well with the shape of the shorter velocity stacks.

The stacks are therefore shorter 18mm and about 6mm wider giving a more pronounced trumpet shape with a greater diameter drop over a shorter distance. Taking the FCR air box adapters into account the shortened stacks are within 5mm-ish of the optional plastic blue stacks provided with the FCR kit.

Pics?

[koko64]

I'll try and get my kids to help me do some pictures. (old fart smiley).

[greenmonster]

 

[junior varsity]

I'm hoping to use the split singles on the jako motorsport short manifolds.  This is not a near-future plan, this is a "I dream a dream of days [not yet] passed by". So maybe this time next year...

[koko64]

Hey  j v
I have probably asked you this before, will those manifolds clear the frame, or do you have cut n' shut?

[junior varsity]

don't know just yet

[suti]

My homemade short stack: http://www.bikepics.com/pictures/2231896/
pic 2 http://www.bikepics.com/pictures/2231900/

[junior varsity]

No more bellmouth? I would think that would substantially hurt air flow. The short stacks discussed refer more to the manifolds on the 'motor-side' of the throttle bodies as well as with the velocity stacks / bellmouths on the 'atmosphere-side'

[koko64]

Guys, that is exactly what I did.

The gain was surprisingly through the rev range which makes me think it was greater distance from the filter element that fattened up the torque and power curves, rather than what I understand to be more traditional vstack tuning. I think it helped offset the small air box volume, tricking the ports a little as if air box volume was increased, rather than just giving more top end with the shorter stack.

To get more top end by moving the torque peak higher in the rev would require shorter manifolds. They are so long as to dominate any vstack tuning.
It wasn't the result I expected. I thought I would lose a couple down low and gain a couple up top.  It was just thicker all the way through, more so the torque which got me thinking less tuned vstack length and more and more to do with available air box volume. Still a bell mouth just deeper in the air box.

Cheers

[koko64]

My homemade short stack: http://www.bikepics.com/pictures/2231896/
pic 2 http://www.bikepics.com/pictures/2231900/

Your pictures pretty much captured what I did. I think I carved a bit more of a gradual radius but its hard to tell. If you are good at whittlin' it aint too hard. You saved posting photos which I gotta bug my kids to do.

For what its worth, I think the gain was made by increasing the available air volume between the stacks and the filter element. Considering modern air box designs like ln my gixxer or my mates 1050 Sprint, you can see the stacks at one end and the filter at the other, with lots of air/space between.
Hey jv, makes me wanna make a carbon air box! Filter at one end end stacks at the other, but still with a big, open filter area. I would then try and utilize the blue FCR stacks which come in different tuned lengths...I shudder at the dyno time$$.

[junior varsity]

Agreed. I would love to do that, have you seen the one-off airbox the Jako Motorsports guy did for his SS w singles? Deeelicious