Velocity stacks vs pod filters

[PaulF87]

Hey guys,

So I did some searching, seems like a lot of mixed reviews on velocity stacks. My bike currently has BS38s and high flow pod filters on it and it's been jetted accordingly. I've had to do some tweaking to get it running right but it seems to be really happy right now. I got a little anxious one night and dropped the hammer on some custom stacks with fine brass screens. I feel like they will flow better than the pods but how much more messing around with these carbs will I have to do or is this going to be a marginal difference in flow, thus negating any need for further adjustments?

Thanks in advance for any input!

 

[figure8]

I'm no expert but I would think every velocity stack is gonna be different, just as there is a difference in pods.

The first thing you should do when you get the stacks is put a flashlight to the screen and see if light gets through. See how well they are gonna keep crap out. If they seem sufficient then give them a try, see how the bike rides and read the plugs. You may or may not have to jet up but it shouldn't be anything really drastic.

 

[fredintoon]

Hi Paul and welcome,
the problem with stacks is that they don't keep the dirt out which is most likely one of the reasons why those old Britbikes wore out so quick.
By the time you put enough wire mesh across them to keep the dirt out it ain't a velocity stack no more and you may as well have put pods on it.

 

[PaulF87]

Hmmm, Thanks for the input guys... I might just throw them on for shows or something. I love the look and I realize that they do not have the same filtration qualities of an actual filter but the screen should keep big stuff out at least for short trips and such.

 

[5twins]

Velocity stacks are for racing. Racers typically rebuild their motors every off season. If you don't mind doing that then by all means, run the velocity stacks. Looking cool ain't cheap, huh.

 

[pamcopete]

Well, leave it to Honda to give you the cake and allow you to eat it too. On my '74 CB750 SOHC four, the stock velocity stacks are inside the air cleaner box.

The engine will not run properly without the stock velocity stacks and air box no matter what combination of jets, pods or other velocity stacks you attempt to use, which tells us that the carbs, stacks and air box are a complete tuned intake system not to be toyed with, so I would not attempt to install velocity stacks on a set of carbs that came without from the factory without the ability to do a flow test and dyno test. Looks can be deceiving!

 

[PaulF87]

Kinda feel like people are pissed off that I stated that they look cool....?

 

[pekka]

Why not put large "foam sock" filter over the pod and zip-tie it. You might lose some looks but keep the performance and keep the dirt out.

Pekka

 

[pamcopete]

Kinda feel like people are pissed off that I stated that they look cool....?

Well, I don't think anybody is pissed off, it's just that a lot of us are function first before form, like some people think wearing a baseball cap backwards is cool, but then they use a one size fits all and the little adjustment holes leave dots across their forehead which really looks stupid, so you can buy the right size baseball cap that doesn't leave those dots across your forehead and might even have a cool logo, like MLB on the back, er, front, but those hats cost ten times what the one size fits all caps do and most people who can afford to buy a quality custom fit baseball cap actually play golf and they need the visor thing where it belongs, which is in the front, so that leaves the el cheapos with their cheap one size fits all caps with the John Deere logo on the front, er, back that makes them look stupid with the dots on their forehead.

So, as 5twins said, coolness has it's price. If you can afford the trouble and expense to dyno test a set of velocity stacks, then by all means do that and then at least you will be the equivalent to someone with a custom sized baseball cap worn backwards and no dots on your forehead.

 

[dps650rider]

Back in the '70s when I was into snowmobiling I played around with velocity stacks (called ram tubes at the time) on my 433 Yamaha. I had 2 sets, long and short. What I found through experimentation was they could be used to tune the powerband to different RPM ranges. With the longer ones the low end was tremendous, so much that in typical snow conditions the takeoff wasn't as quick due to spinning the track excessively. I also found they killed the top end.

The shorter ones gave less of a low end boost but restored some of the top end. Using open carbs gave the best top end at the expense of a substantial loss in low end. I would expect that changing the length on a 4 stroke would have somewhat less of an effect than it did on a 2 stroke but there should be some.

However, I would never run my bike without filters. There is just too much dust kicked up in summer and I would expect excessive engine wear would result.

 

[lakeview]

Ages ago, (1969?)I put open megaphone slip ons and velocity stacks on my Bonneville and took it for a spin. I do not now remember at what RPM, but it was as if a horsepower switch had been thrown!
That week I rode to Mosport to watch a race and was disturbed to come back to see the whole bike covered with fine beige dust.
The next day it went back to the stock gravel strainers and pea shooters.

 

[chizler62]

There are a bunch of benefits to having a velocity stack. They produce a cool look and nasty induction sound. If the mouth is properly shaped you'll get increased improvement over the entire throttle range not just either end. Texture will dictate the increased speed and captured air. I've seen 10-15%. Throttle response helped by improvement of atomization of fuel. Regardless of velocity stack length.

But, the air flow around the stacks are difficult to keep the same without an air box. If I used a velocity stack on an xs650 it would be a single carb setup. Hand machined not CNC'd. It wouldn't be a daily driver.

Posted via Mobile

 

[fredintoon]

There used to be another drawback to having a velocity stack instead of an air filter.
This was back in the day when they had guys to pump gas for you and they still sold oil by the jugful from a station right next to the gas pump.
A Gold Star pulled up to the pump and the rider told the attendant:-
"Please fill her up with 5-star and top up the oil with a half-pint of Castrol XXL while I buy a packet of cigarettes from the kiosk."
The rider came back with his smokes just in time to see the attendant pouring the last of the oil into the bike's velocity stack.

 

[pamcopete]

But, the air flow around the stacks are difficult to keep the same without an air box. If I used a velocity stack on an xs650 it would be a single carb setup. Hand machined not CNC'd. It wouldn't be a daily driver.

Posted via Mobile

That's what I found with my CB750 SOHC. When I bought it, the PO had already installed pods, I guess for that "cool" look, but the engine stumbled and mumbled and had several flat spots. I tried a different, more expensive set of pods with similar results.

Fortunately, the PO gave me the factory stock air box complete with the factory stock rubber velocity stacks. When I installed those, it was night and day difference. No more stumbling or flat spots and the throttle response was dramatic. The stacks were located inside the air box, so the inlet pressure was the same for all four and of course they got filtered air.

 

[FLEA]

not sure how to link it but on ebay usa theres a double velocity stack by 707 racing that he says solves a lot of the problems of stacks, made for xs650 with vm36s personally I will only use air filters on my bikes

 

[TwoManyXS1Bs]

not sure how to link it but on ebay usa theres a double velocity stack by 707 racing that he says solves a lot of the problems of stacks, made for xs650 with vm36s personally I will only use air filters on my bikes

Found it:

Item: DOUBLE STACK VELOCITY STACKS FOR MIKUNI VM 36 CARBURETORS VM36 XS650 Triumph BSA

URL: http://pages.ebay.com/motors/link/?nav=item.view&id=121500598391&alt=web

Strange design, never seen that before.

We sold a lot of velocity stacks.


We also sold a lot of top end overhauls...

 

[hoffman900]

Velocity stack + properly sized airbox > pod filters and / or velocity stacks alone.

One of the worst things you can do for airflow is put socks over a velocity stack. Pod filters also need to be as large as will fit to not be a restriction. Take a look at how big they are on a XR750 Harley.

 

[pamcopete]

I installed foam / oiled pods on my '81 CB750 DOHC because removing the factory air box to work on the carbs is about a 2 hour job and very frustrating. (The '81 does not have Velocity stacks.) Initially, the pods did not work very well, with flat spots etc. so I reasoned that they were more restrictive than the stock filter and air box, so I connected the inlet to the pods together with some PVC pipe and that fixed the problem because all four carbs got air from all four pods. I also increased the jet size up one number, including the idle jet because 1981 was when the EPA started designing carburetors and these were super lean, just like the stock carbs on an '81 XS650

Something similar may work with the XS650 by connecting the two pods together. You can buy black PVC for a better look. This mod is for Uniflow foam / oiled filters as they are more restrictive than other, cheaper filters.

 

[Jack]

Hey guys,

So I did some searching, seems like a lot of mixed reviews on velocity stacks. My bike currently has BS38s and high flow pod filters on it and it's been jetted accordingly. I've had to do some tweaking to get it running right but it seems to be really happy right now. I got a little anxious one night and dropped the hammer on some custom stacks with fine brass screens. I feel like they will flow better than the pods but how much more messing around with these carbs will I have to do or is this going to be a marginal difference in flow, thus negating any need for further adjustments?

Thanks in advance for any input!

Forget the dirt inviting gimmicks and install Flat Slide carbs,that produce less air drag and will broaden the entire powerband over stacks and as mentioned install the biggest K&N filter possible.