Airbox vs Pods vs Velocity Stacks

Hey guys,

This is an age old question that plagues the lives of cafe builders, but I think we need to put this discrepancy to rest once and for all. I have searched everywhere and gotten fairly general, but conflicting information on this topic. Here's what I "know" about this:

Airbox: If you aren't racing and don't care about aesthetics, this is the perfect option. Why? It allows your carbs to pull from a common, dead, air source, thus giving your bike a nice smooth flow of air into the engine. This may be the most ideal option for most people.

Pods: Pods can be hit or miss. They are aesthetically pleasing, and they provide just as much filtration as a stock airbox, which is an advantage over velocity stacks. When tuning for pods or K&N filters, it takes an act of God to get the tune perfect where you don't have unsteady air flow. This is as a result of the vacuum pressure being taken away from the vacuum inside the carbs due to the air rushing past and creating lots of turbulence. Even if tuned properly, typically you find the bike still will not have perfect, peak power throughout the throttle range.

Velocity Stacks: Perfect for racing and very aesthetically pleasing for many builders. It's my impression they are hard to tune, but once you tune them properly, there is no performance better. The catch is that you will need an aftermarket exhaust that will not create any kind of back pressure in the system. The biggest draw back to stacks are the lack of filtration.


Having indicated what I know, here if my take on my situation. I currently have K&N's on my bike and I've got the bike working decently, but I know I'm still missing 10-20% power throughout the range. Mechanically speaking, I like the idea of an airbox, but I hate how it looks. It totally ruins my plans for my custom bike. I've seen a few threads showing in detail how to build a custom airbox to your liking, but I don't have the resources or the money to build my own, sadly. As a result, I've been considering putting stacks on my bike and trying to find a fine mesh filter/screen to put over them to protect against dust and dirt. I've seen a lot about pre-filters, but those are typically disposable and I don't have that kind of money. This is an everyday bike, not a racer, that I want to look like a cafe brat. Here's a picture of it so far. It's a 1980 CB650 Custom.

Can you help me get to the bottom of how I can attain the look I want while getting max performance out of my bike without running the risk of damaging it? I am very handing with tuning the carbs and customizing my bike.

Have you changed the jetting since adding the pods?

Yes I have. Stock mains are 95, I'm running 142 and pilots are 35 and I'm running 42.

As someone who has tried stacks on stock carbs with different jetting and stacks on fully adjustable racing carbs let me tell you first hand to just use an airbox. I have free flowing exhaust and the bike only ran well above 4000 RPM. Do yourself a favor and keep the tuning as easy as possible

142 is HUGE step up from a 95 for just K&N's and an exhaust upgrade

I agree with the others, overshot the mark.

Here is an article on jetting procedure that may help. Use OEM jets as they state. http://www.jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm

CV carbs are pigs to get right without an airbox, slide carbs are relatively easy to get right.

If you enjoy wrenching around on your bike why not try to make what you want to work for your specific bike? If you like the looks of pods or stacks and want ultimate performance, have patience and figure it out! I've got a '76 cb550 that I put pods and open exhaust on and I still have not gotten it to run correctly after a year and a half of jetting and messing with the carbs. I recently picked up a nice little '82 xs400 for cheap. I'm leaving the stock airboxes (I actually don't mind the look of the old Yamaha xs airboxes!) and exhaust on it cause it runs perfect and now I always have a reliable bike to ride and I have my 550 to wrench on. There are plenty of people out there running pods or stacks, I just think it comes down to what your specific bike needs and even where you live will effect performance and tuning with the carbs.

Bear in mind, I didn't jump from 95 directly to 142. I went up by 5 incrementally, testing each size. I tried going back down to 125's and it just makes the power worse. I don't like running jets that high because it just way too much fuel being used.

My main problem with the stock air box is it's ugly as sin and I can't work on the bike easily, at all. Not to mention, it's not the look I want. Does anyone know of aftermarket boxes that are a bit more appealing that would fit my bike? I'm not opposed to using an air box and if anything, I'd rather have it on there because it would mean a steadier air flow, but it's too hard to work with. Not to mention, the stock air box is very restrictive. These bikes are typically tuned to meet emissions specs, not performance specs. I'm not looking to make this the ultimate performance bike, but I would like good power with steady throttle response.

Here is one great resource I used to educate myself on carb tuning and jetting. I'm not unfamiliar to the process by any means, save for maybe boaring out larger jet holes.
http://www.dotheton.com/forum/index.php?topic=16745.0

Drkblu, I personally love how a set of new filters or stacks look on my bike, but I fear I may go up even higher in jet sizes to make such a free flowing intake work with my system. I'm not looking to make any more performance mods to my system just to get stacks to work. Having said that, if I can find a nice oiled air filter for the stacks, I might be able to get it to work fine on my bike. The problem is I see issues at certain ranges of throttle, just as Finnigan stated. I don't care fro what's the easiest approach, I want the correct approach that with satisfy my requirements.

There may be aftermarket air boxes, but the reason your stock one is ugly as sin is because it was engineered for flow, not for looks. Aftermarket boxes might look better but they won't have the proper flow to ensure a good air/fuel mix at all throttle positions.

How did you "test each size" of jets? Did you do plug chops? Did you adjust the idle and mixture screws with each change? Have you adjusted valves and timing, synched your carbs, verified compression (I assume this was all in check before making the change to K&N's but still have to ask)?

Yes, actually I did all of those, including making sure the valve on the pistons were adjusted properly as well. I've done it enough times now to where I can simply feel the change in performance and adjust accordingly. Of course, with any change I made, it was one step at a time then test it with a warmed up bike. My pilots are perfect. I don't think my bike could be any better off the line.

And what data did you get from the dyno and air:fuel measurements? 142 is such a large increase that it's a sure fire indicator that something isn't right. I am surprised that it runs reasonably well at full throttle. Smooth running is not necessarily a sign of spot on jetting. It usually means that at least one circuit is over rich or over lean - in your case probably the former. It would tend to mask the transition between circuits and soften the powerband.

Getting off the line is only partly about pilot jets and is more about slide cutaway and needle jet/needle and is also influenced by the airscrew of course. Our Butts are really poor for measuring change and it is said that much less than a 10% change cannot be reliably noticed by the so called Butt Dyno. Even on the race track it's hard to feel the difference between different jets and there's a bracket of 4-6 jets that all feel the same and give almost identical lap times.

I have no idea what "valve on the pistons were adjusted properly as well. I've done it enough times now to where I can simply feel the change in performance and adjust accordingly" means.

I would strongly recommend getting it set up on a dyno with full exhaust gas analysis. Those jet numbers may be correct or not but they sound rich and that suggests a lot of unburned fuel which will show up in the unburned HC numbers. I'd also put teh airbox back on and see what it does on the dyno - back to back.

Lots of people run pods on that model of 650, so it's not impossible, but it would be good to know how your pods compare to OEM. Data is really useful.

I use Uniflow pods. My GS850 never came together with K&N's. when i switched, made all the diff

JMO

Here's some data I found on the internet:
http://www.triumphrat.net/air-cooled-twins-technical-talk/120012-dyno-results-velocity-stacks-w-unifilters-vs-k-and-n-pods.html
http://www.triumphrat.net/attachments/air-cooled-twins-technical-talk/17891d1251925476-dyno-results-velocity-stacks-w-unifilters-vs-k-and-n-pods-090902-dyno-runs.pdf
This is just comparative measurements on a dyno.

Last I checked, I had equal compression on each carb, but that was awhile ago and could be much different now.

I've adjusted slide valve clearances and the idle/pilot circuit to get my 1/4 throttle perfect. Every time I've made a change, it's been a noticeable difference. I know our butts aren't as sensitive as a dyno, but I know there have been physical changes in the response when I have made adjustments in the past.

By valve on the pistons I mean piston valves; intake and exhaust. Sadly, I don't have the time or money or connections to put my bike on a dyno. I'd rather spend the cheap change on a bunch of spark plugs and do plug chops for every modification as a method of deciphering my bike than spend a ton just to put it on a dyno.

How many people have been able to run pods smoothly and effectively on these bikes, though? I've heard a few reports of it being fine with no discerning details about it and many reports indicating it's very troublesome and near impossible.

<-----<<< Grabs a large popcorn and sof drink

I gave up with pods on one bike - CB900F and replaced teh stock airbox and finally it ran well. I run stacks on most of our race bikes and have UNI's on a GT750 2 stroke and it runs fine.

The thing about a plug chop is that it is only useful at WOT under load. At part throttle it tells us very little and even less about jetting because the plug tip is not as hot at part load as it is at full load, so the color will be very different even with perfect jetting. I hear you on the cost of dyno runs, but my experience has been that they are singulalry the most cost effective way to get close to final jetting - at least at the low end and top end.

I hate to admit it, but I think I may have to go back to the stock air box then. In the case of a custom box, I'd imagine that as long as the inlet of the box can allow the same flow rate as the stock box, the amount of dead air inside the box is negligible. So couldn't I theoretically get a custom box that would fit and look like the pods I have on and still get the same results?