Velocity Stacks/Filters?

reborn650 wrote:

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I ran Emgo pods last year and didn't like their performance. A couple of times I pulled them off completely (for a quick spin around the block) and it seemed like the bike pulled better. Maybe it was just my wishful thinking.

Thanks in advance gang
Cheers-Colin Firth-Ontario Canada

Maybe the problem with the Emgos was the same as described in this excellent post kzrider.com/component/option,com_joomlab...mit,20/limitstart,0/

I'm happy with my K&N filters.

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Woodstock ON Canada
1980 kz750E1, 4-1, K&N pods

wiredgeorge wrote:

Moved the topic to the carb forum where it seems to be more on topic.

Jay, why would not running filters hurt valve seats? Can you explain your comment? I can understand sucking in dirt/insects/tree branches might not due valve seats any good... is this what hurts the valve seats?

The trash passing thru the intake port get hammered by the valve everytime it closes. It beats the stuff into the seat and valve face. We can tell the heads that come into the shop that have been running no filters.

The seats and valve faces don't last long if you are sucking dirt in the motor.

Back before pro stocks had air boxes, they would keep a towel wrapped around the carbs right up until the starter was plugged in. The rider would put the towel in the front of his leathers, then make the pass, kill the motor, and make the turn off, stop get the towel out and wrap the carbs before being towed back the return road.

Even with the motor not running, there was a risk of the rear tire throwing dirt around and some could get in the carbs, only to be sucked thru the motor next time it was started.

If this was allowed to happen, you could see the performance drop off as the valve job went away.

Jay

Post edited by: APE Jay, at: 2006/01/03 00:38

Personally I would run a filter no matter what conditions you are going to ride in.

I'm amazed at the amount of crap the air filter picks up from street riding. I have more of a dirt riding background where of course filters get dirty and it's understandable since you are riding on dirt, but street miles will accumulate dirt too. Seem to find quite a few granules of sand in those filter pleates too.

Be careful.

Thanks Jay... I was thinking about more convoluted reasons for seat failure than just plain old junk being sucked in... I was never advocating running a bike without proper filtration. I think you could devise a velocity stack where you put some oiled UNI bulk filter material in the stack and put a mesh over the ends to keep it from getting sucked into the carb and from bigger stuff from getting into the intake end. I think I have seen this done.

Carb tuning on race engines is done by controlling the length of the intake runners by using different length stack type intakes. I think the different lengths that can be used vary the airspeed coming into the carbs but I certainly ain't an expert on such things.

Would there be a way to put bigger pods on the ends of the velocity stacks, or would there be room?

there used to be a product i think it was called a "stub stack" that was installed on the top of a carb under the aircleaner that was supposed to smooth the transition into the carb venturi(?) it was an automotive thing,i dont remember ever seeing one on a car i wonder if would be feasable to make something like that for a bike?

The problem with putting anything, including wire mesh to keep the birds out, in the mouth of the velocity stack is it will screw up the airflow, causing turbulence, thus loosing the benifits gained by using the stack. The best, if room is available, is to put a high flow air filter around the velocity stack, this will allow the air in, and allow the velocity stack to work it's magic.
I realize that the stack in a filter is a problem for bikes, just don't have much room. I'm no wizard, most of this comes from a book "Tuning BL's (British Leland) A-series engine" by David Vizard. British cars use SU carbs (well some of them anyway), which are verrry similar to CV carbs. more simple actually than the bike carbs, but all the principals apply.
Wiredgeorge is right, intake tuning is done by adjusting the length of the velocity stack, or intake runners (cars). BUT, you don't necessarily have to have a long stack to get the benefits. You need to break that 90° edge where the air goes into the throat, it is possible to use a short stack (and they do have them, so short in fact they look like rounded rings). My MGB uses a 1/4" plate with a radius to help smooth the air flow.
The shape of the mouth of the velocity stack is verry verrrry important. You want a large smooth flair, so the air can flow smoothly into the carb, if the edge of the mouth is too sharp you don't get a good boundry layer, the air will not flow as smoothly, and you get turbulence. You want something that looks like these (this is not a recomendation for the manufactuer or product, just an example of shape) www.velocity-of-sound.com/velocity_of_sound/velocity_stacks.htm
Not sure if any of this helps with the original question, but thought I'd toss in my pennies (better than tossing my cookies, eh?).

Never run velocity stacks on a street bike. If you have that much money to do rebuilds, please donate to those of us less fortunate. For pure power velocity stacks are the way to go. You can even tune the lengths for the power band/spread you desire. All bikes have velocity stacks, it's just that the factory sticks an air box over the ends of them. You can buy velocity stacks in different lengths for new sport bikes and still keep the air box. On my dragbike they were the absolute cheapest high performance mod I made, WOW do they liven up an engine. Cars are crap. You can drop the exhaust and pitch the air filter without much in the way of negative consequences. Shows you how slow assed and under developed car engines are, try that with a motorcycle and see how long out takes to lean it out so bad literaaly burns itself out.