Airbox Experts?

[chancho196]

Hey folks, I'm trying to wang together a GSX750 cafe racer, and I have a question for all the streetfghter builders out there. It's obviously easier to remove the airbox and fit K&N style filters, rejet the carbs and go from there, but... is there a definite power increase using this method, or is it better to retain/build a new airbox?

I know pod filters have been used from the year dot on pretty much every old GS, Z etc., but I want to hear from people who've actually done it (dynoed it). And can you just upjet the carbs/shim the needles yourself or do you need to buy a dynojet kit?

Interested in your thoughts.

[blur1]

Nothing wrong with a well designed airbox!

[lorenzo]

But there may be something wrong with a badly designed one from the early 80's...

An airbox, if correctly designed, will be able to not only handle enough air for your carbs, but crucially, keep the air from swirling about, which makes for better fuelling. I've found, in my limited experience, that there's not a huge amount to be gained by binning the airbox and replacing with pods, but it does make it hard to set it up, and I've been told by dyno guys that this is especially the case with CV carbs over flatslides etc. I'd say before doing this, you want to make sure that your current airbox is hindering the bike before you bin it, unless you're doing it for looks purposes, then the criteria's different I suppose.

With regards needles & jets, put whatever jets you like in there, so long as the dyno says it's right on the A/F curve, it will be. So long as the mains flow enough fuel then they're cool, irrespective of brand, dynojet kits are just a nice way of getting you in the ballpark from the off, but it'll still want tweaking. Set it up for the right mains at high RPMs then work backwards from there, on to your needles (which can be beneficial using aftermarket like dynojet or factory etc), shimming them, then down to your pilots. Last of all sort your pilot screws. Having said all of that, if you're getting it dyno'd, your dyno man should know all of this anyway!

[chancho196]

Cheers guys, this will be done on the cheap (I doubt very much whether it'll ever see a dyno). I'll take her up a size at a time and go from there. I'm considering making an aluminium airbox with far more capacity. As you say Lorenzo, the stock box is awful - a two-piece contraption that almost needs to be fitted in the frame before anything else. Pods would be simple, but without a steady supply of filtered air I couldn't see how the engine could make more power as the average claim from the filter manufacturers.

[vfiend]

Run em open with big bellmouths and mesh filters, it'll look the tits

On a helpful note, everyone racing the old gsx's seem to be running pod filters,

the gains will be marginal I expect,

the main bonus is getting rid of that hideous airbox and making it easier to work around the carbs.

[ronenige]

When you put pod filters on a 1200 Bandit you have to go up on the main jet to a 150 - 160 ish from a 102 ish , otherwise you wont get more than 30 mph out of 'em , you get a marginal ( 2 HP ) increase in power for knocking 30 miles off your tank range , I soon put the airbox back on . I know its a different bike etc but you get the idea

[chancho196]

Cheers folks, this is the sort of stuff I need to know.

[2moto]

There is no question that a properly designed airbox will easily outperform an "open" pod filter approach. In the early days, before airbox design was well understood, airboxes were mainly there to house the filter and inhibited performance. Hence, the reason why many people put on pod filter and got better performance. Nowadays, with decent airboxes, pod filters will always be a step backwards (as ronenige found out).

What works best for the GSX750 will depend on what's there to start with. If the airbox is poor, and pod filters give better performance, then it's likely that putting on a proper airbox (as opposed to a filter housing) will required a good deal more space, meaning "frame surgery" would likely be needed to make sufficient room.

Are we talking about this sort of thing?

[chancho196]

Yeah that's the general shape. Mine's the single shock version and the inlet is directly under the seat. Didn't want to alter the frame, but thought I could increase the capacity of the box. Carbs 1 and 4 will still be stifled because of the frame tubes but, with more air ready to be taken in, I thought there might be an improvement. Or maybe not.

[2moto]

Also, remember that with most filter designs, it's only the volume on the carb side of the filter that matters. Going by your picture, that looks a typical airbox of that period, and looks like it'll be on the small size. There is a good test you can do to check. Connect a vacuum/pressure gauge to the airbox and go for a ride. When going flat out, I'd be surprised if the gauge doesn't show a negative pressure. That's the reason pod filters give a performance improvement in these older airbox designs.

You'd need a gauge with a small pressure/vacuum range.