Velocity stack length - any experts out there

[Oldrat]

Hi all, I've been a long term lurker in the background watching, and at times drooling over some of the iron on here.

I've decided that I'd like to run some nice polished velocity stacks on my cafe racer build and I've been looking at the bewildering options when it comes to velocity stack length.

I have read all about the advantages and disadvantages of using V stacks as opposed to air filters but my question is about wondering if any of you had tried experimenting with different length stacks.

I came across a really useful article about how to work out the correct length for the desired outcome based upon getting the right pressure wave cycle. This suggests that it's not simply a case of longer = more power etc.

<a href="http://www.ehow.com/info_10074561_motorcycle-velocity-stacks-long-vs-short.html" rel="nofollow">LINKY[/url]

I also recognise that longer stacks might need straight inlet couplings too so if I go that route I might need knee pads

My set up, as advised and prepped by an English MG guru, Nigel B, is intended to give me useable power on the road rather than race track top speed, and I'd like the v-stacks to be compatible with that aim
So.. a T3 base
950 cylinders
Double plugged and silent hektik ignition
Standard T3 valves and cams
PHF 36 carbs
Ram clutch
Short OS bespoke SS exhausts

Your advice will be appreciated.

Cheers guys

[redrider]

 

[Triple Jim]

Velocity stacks don't preclude filters.  If I wanted to experiment with velocity stacks, I'd make the bell ends so they accept Uni foam filters.

[Oldrat]

Ha ha
You just wait until when I ask what oil I should put in it 

[charlie b]

The correct answer to that is used deep fat fryer oil.  Best is from Burger King but Sonic is ok too.  :)

Velocity stack length is based on RPM and swept volume (or total air flow at your speed of interest if you have something to measure it with).  There is a formula somewhere.

[AGRO!]

You could make yourself dual velocity stacks!

[LowRyter]

this is the proper length

[Triple Jim]

No hot linking for you!

[LowRyter]

No hot linking for you!

I know what you were thinking but I beat you to it

[guzzimike]

Not an expert, but FWIW, for the past decade I've been running Malossi velocity stacks on my CX100 with 36mmDellOrtos





 - The Stacks are such as these:


http://guzzino.com/alvest30.html


More Sracks:

http://guzzino.com/velocitystacks1.html


My CX100 exhaust system is a set of Bubs with the high crossover. This crossover location supposedly enhances Mid-Range, so any Velocity Stack influenced gains expected in that portion of the power curve would be somewhat mitigated, I would think.. And in fact, immediately after I installed the Malossis, I noted that the power gain in the erange from 4500 to 6000 RPM was more of a low key, subtle effect rather than that of a Rip-Snorting born-again Torque monster.

Induction sound, OTOH, is definitely enhanced.....   


Oh, Yeah...Especially when the Accelerator Pumps kick in; You can feel it in your nut-sack ...

---

[Oldrat]

Cheers guys. Thanks for the responses... Whoops, I nearly plunged this into a 'what type of *il' thread ravine 

Great info and links Guzzimike, in all of my time browsing the GUZZI sites, I'd not come across that one. 


FWIW This link will take you to the/a formula for working out stack size.

http://www.ehow.com/info_10074561_motorcycle-velocity-stacks-long-vs-short.html

I will get around to working it out later on in the build, but at present I'm happy to see what stacks you guys use

[Rough Edge racing]

 I have spent a lot of cash and time on the chassis dyno and LSR track tuning my record setting 650 vintage Triumph. Now a Triumph ain't a Guzzi but it is a push rod hemi head.. "just the facts "
 Velocity stacks with the screens can disrupt air flow in a negative way...Velocity stacks have an effect on the rather complicated principals affecting the intake/ cam/exhaust relationship ...They might cause a slight reversion around 3500 rpm..or not..They might cause a slight gain or loss of power at any rpm....
 On a stock engine with mufflers they shouldn't have an adverse effect on performance and might give a slight boost...They look racy if nothing else..

[charlie b]

Cheers guys. Thanks for the responses... Whoops, I nearly plunged this into a 'what type of *il' thread ravine 

Great info and links Guzzimike, in all of my time browsing the GUZZI sites, I'd not come across that one. 


FWIW This link will take you to the/a formula for working out stack size.

http://www.ehow.com/info_10074561_motorcycle-velocity-stacks-long-vs-short.html

I will get around to working it out later on in the build, but at present I'm happy to see what stacks you guys use

Note that the length of the stacks is so critical that if you are 'off' by just a few RPM any gain from the length is negated.  It reinforces the real world use of trying to match lengths. 

I remember when 454 Chevy engines were being used in road racing with fuel injection.  The intake stacks were different lengths for the cylinders.  They intentionally chose to make sure not all cylinders were 'peaked' at the same time.

[Rough Edge racing]

 Tuning the intake and velocity stack length will also tend to narrow the power band around a particular rpm range of maybe 1000 rpm..Outside of that there might be power losses. This will vary depending on head design.

[Triple Jim]

I remember when 454 Chevy engines were being used in road racing with fuel injection.  The intake stacks were different lengths for the cylinders.  They intentionally chose to make sure not all cylinders were 'peaked' at the same time.

The big port BB Chevy heads have intake tracts with two slightly different lengths.  I read that the staggered stacks were to make up for that, so they all tuned the same.  I have no idea which is correct, and different teams probably had different setups anyway.      It's clear though, that small length differences make noticeable changes to the torque curves.

[John A]

I was tuning on a hopped up early square fin years ago. Running at WOT I put my hand to the open stack to feel the intake. To my surprise the air was puffing back and forth and it was warm. It was about 50F ambient so I could tell it was warm air puffing back and forth. As I remember I was only able to detect this at WOT, maybe I was too busy hanging on,and it was not what I expected, I thought  there would be more of a suction ,especially at higher rpms. On multiple cylinders with a shared manifold that's what you would get. A filter on a stack should be 2.5 times the diameter in length or it screws up the pulses. Some Guzzi air boxes are good to keep, sometimes in a modified form.

[Rough Edge racing]

 John, that's reversion....I'm no scholar on this but the exhaust pulses tend to travel back and forth and in the process travel back up though the intake and out the carb or throttle body. Al lower RPM's the reversion can be so severe, the air goes in and out the carb several times severally enriching the mixture causing a flat spot in acceleration... Once again this is caused by the relationship between intake, cam  and exhaust. Sometimes it can be tuned out, sometimes not.
 My race bike creates a standing cloud of intake minute at the mouth of the carburetor...

[charlie b]

The big port BB Chevy heads have intake tracts with two slightly different lengths.  I read that the staggered stacks were to make up for that, so they all tuned the same.  I have no idea which is correct, and different teams probably had different setups anyway.      It's clear though, that small length differences make noticeable changes to the torque curves.

I think these are more than just intake port length difference.

https://www.flickr.com/photos/dsullivan/7525372138/?ytcheck=1

[John A]

Yep, reversion. A good book is " The Scientific Design and Tuning of Intake and Exhaust Systems". Organ Pipe theory and all that. I think I like your Triumph!

[oldbike54]

 Memory says the theory behind varying the length of the velocity stacks was to mask the shortcomings of running the stacks to begin with . 4 cylinders were tuned to run well at mid range , the other 4 were tuned for higher RPM power . Hard to believe engines exceeding 500 cubic inches powering Can Am cars weighing 1800 Lbs needed trickery to work well . Basically these cars were sophisticated American style hot rods , unfortunately , the high dollar 1300 HP Porsche brought in by Penske killed off the big block Chevy powered cars .

  Dusty

[Triple Jim]

I think these are more than just intake port length difference.

Probably so, Charlie.  Sometimes I think that not everything I read is true.       I do have some BBC heads in the shop, and I took a look at them when I posted before.  The length difference is pretty small.

[wrbix]

http://youtu.be/rBXe2FV9WnE

[redrider]

Two cylinders, one inside the other with a gap equal to the wall thickness + a few thou. Slide into the gap another cylinder. Join the pieces and with electronics vary the length while running. Simple matter of which end to flare or attach a mounting boss.

Thanks for the video link, very informative.

[Triple Jim]

http://youtu.be/rBXe2FV9WnE

A classic video presentation!

[charlie b]

Probably so, Charlie.  Sometimes I think that not everything I read is true.       I do have some BBC heads in the shop, and I took a look at them when I posted before.  The length difference is pretty small.

They needed to operate over a wide RPM range, hence different length pipes.  I remember something about having some real diffuclt problems when all the pipes were tuned for the same RPM.  Severe dips in the torque curves.  And, yes, Group 7 Can Am became a victim of it's own success.  Too much money being spent for little payback.  McLaren and Penske made out like bandits.  Ferrari lost a bunch.  Jim Hall made his mark there.  A ton of racing advancements in a short period of time.

That's the problem with some tuning solutions.  They work really well at one RPM, and can hurt you at another.

Chrysler cross ram manifold from the old days of NASCAR.  High speed oval tracks and LSR stuff is suited to that kind of tuning.  Road racing not so much.

[Oldrat]

Thanks for all the comments and advice guys. 
I haven't done the math but instead I've decided to start off with relatively short stacks at 60 mm, and then see how it goes with trial and error, they aren't particularly expensive and I'll also enjoy the tinkering.

I'll let you know how I get on.

Cheers
Brian

[56Pan]

John, that's reversion....I'm no scholar on this but the exhaust pulses tend to travel back and forth and in the process travel back up though the intake and out the carb or throttle body. Al lower RPM's the reversion can be so severe, the air goes in and out the carb several times severally enriching the mixture causing a flat spot in acceleration... Once again this is caused by the relationship between intake, cam  and exhaust. Sometimes it can be tuned out, sometimes not.
 My race bike creates a standing cloud of intake minute at the mouth of the carburetor...

I had the exhaust reversion problem on a cammed, high compression Panhead. Straight pipes.  Figured these things were B.S., but thought I'd give them a try: http://www.ebay.com/bhp/harley-torque-cones
They did a lot to clean up the flat spot.  And no more exhaust soot in the carb. intake. YMMV.

[charlie b]

You could have probably done the same thing with properly sized megaphones.  Unless the exhaust ports as designed are too big for the flow rate of the motor to start with.