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Why not 2 x 2?

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I don't mean to be inflammatory here, and I know it's accepted wisdom to relieve the top of the airbox to a 3 x 3 inch aperture. I also know that this seems to be a successful modification for carburetors which have been re-jetted and exhausts that have been more fully aspirated, so I'm not challenging the validity of this modification in tandem with other modifications.

I have two question; one purely arising from curiosity, the other from a desire to achieve a marginal performance increase with little effort.

Question # 1: What is the cfm increase, on average, in air volume by doing the 3 x 3 modification?

Question #2: Can the airbox be optimized beyond its factory-delivered configuration to boost the performance of an un-jetted, stock 2005 SM carburetor with an equally stock exhaust? I'm not in California, so I have no additional emissions apparatus.

Please recognize that I understand that the 3 x 3 modification with the re-jet is the optimal configuration, so I don't need lectures about how that's the "best practice" and "You're less than a man if you don't do things this way"; I'm just honestly interested in if I can achieve a performance gain with a slightly more open airbox, what size that would be, and any rationale for or against opening up the airbox on an otherwise stock DR-Z400SM.

Thanks

Edited by giotto

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You can make the opening any size you want. Once done, jet it accordingly. You can follow a known, working path, or develop your own. Should not be too hard, Dyno, Wide Band EGR set up, 20-30 jetting combinations later you should have it. Post your results when done.

Best of luck.

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The bike comes lean from the factory.

Adding more air alone makes the machine even more lean.

Too lean is not good.

The equation for flow through an orifice is a simple one to understand. Only the units are somewhat awkward.

Q = AV

Q = The flow in cubic feet per second (ft
3
/sec).

A = The area of the orifice in square feet (ft
2
).

V = The velocity of the liquid in feet per second (ft/sec).

3x3 = 9 sq inches

2x2= 4 sq inches

So you can see that 3x3 is more than double the area and will result in more than double the flow

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Opening the airbox without jetting to suit is not a good idea. Also, opening the airbox bigger than 3x3 on a completely stock SM isn't going to make any difference other than making the task of jetting the bike a nightmare. A stock bike simply isn't going to require the CFM of air that a bigger hole would allow. Jetting a DRZ with a 3x3 is known and understood, therefore making the jetting calculations is relatively easy. When you go bigger, you'll need dyno runs with a/f ratio readouts to determine if the jetting is right or not.

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You can make the opening any size you want. Once done, jet it accordingly. You can follow a known, working path, or develop your own. Should not be too hard, Dyno, Wide Band EGR set up, 20-30 jetting combinations later you should have it. Post your results when done.

Best of luck.

E. Marquez,

I appreciate your response, but I have access to neither a Dyno nor a wide-band EGR system, nor do I have the resources to do twenty to thirty jetting trials. I understand that the 3 x 3 airbox modification achieves the claimed results with the attendant jetting modifications which have been well-documented in dozens of posts. I am respectfully asking if others in this forum have experience or expertise with airbox modifications that might otherwise achieve even very modest gains in performance without necessitating re-jetting. I might add that the bike which I purchased new in Iowa had been re-jetted by the local dealer to run properly at the mean elevation in Iowa which is around 1,000 feet above sea level, so while I don't know the actual size of jets in my carburetor (having never disassembled the unit), my belief is that the dealer took steps to enrich the otherwise lean fuel mixture which is factory shipped to meet emission standards in the U.S. My own, admittedly, less-than-professional experience with my bike suggests that the bike may be running slightly rich since it, at mid-rev range, spools up less quickly than at lower revs; while at higher revs it seems to be breathing more fully and is more responsive to the throttle. It does not pop or burble excessively on deceleration, and does not hesitate to start at either cold or hot starting.

That said, I further recognize that the best course may be to leave the bike in its factory configuration, and I'm content to do just that if I judge that modifying the bike might jeopardize its performance or risk damage to its engine, but I thought it was worth asking if a slight increase in air flow to the carburetor might not nudge the performance upward.

I appreciate the expertise I find on this forum; it is an expertise I lack.

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You should have a little bit of popping on deceleration. Also, you should typically have the choke engaged to start the bike regardless of it being hot or cold outside (at least until the engine is warmed up properly). This could also point to a rich condition. If you suspect your bike is running rich, go for a ride with the airbox door off and see if it peps up a little bit.

When I first got my SM, it would start no problem without the choke, no decel pop. Rejetted it a couple years ago and now I always need to choke it to start after sitting for a while. :smirk:

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The USA/Canadian E model came wirh rubber snorkel in a 3 X 3 hole. Pull the snorkeld out an the hole is about 3 X 3. In some countries the E came without the skorkel so 3 X 3 was the original size of the hole in the air box. I believe that was the model for the 3 X 3 hole in the S/SM air box.

In countries where the E came with the 3 X 3 hole, the E main jet was 165. With snorkel it was 142. Suzuki knew the difference.

Suzuki also knew the difference on exhaust. Some countries got the E with no baffles in the exhaust.

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reading some of the replies to this legitimate question makes me ashamed to be part of this community as not everyone wants to be a mechanic or engineer.

the op has imho a valid querry and had no intention of insulting anyone's intelligence or experience by seeking this information.

Noble's answer is appropriate and quite enlightening as it explains how the 3x3 size came to be the accepted standard.

Edited by restosud

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So you can see that 3x3 is more than double the area and will result in more than double the flow

incorrect.

its just like in a city when a 6 lane road merges down to two lanes. A motor, road, or anything else only flows as much as the lowest flowing pinchpoint. So the airbox may flow more air, but not when its attached to a motor. In that case, it only flows as much air as the motor can draw in.

p.s. a local suzuki dealer has done a few variants of this, and on their dyno their was no gain from going from 2x2 to 3x3

Edited by the bartender

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I don't mean to be inflammatory here

None taken

E. Marquez,

I appreciate your response, but I have access to neither a Dyno nor a wide-band EGR system, nor do I have the resources to do twenty to thirty jetting trials

Your welcome and thank you for taking it in the manner it was intended.. vice other posters trying to read into my response and inject there own victim mentality. My response was short and too the point as i had a helicopter to go get on, and only a minute to type something out.

My point was and still remains.. You can make the intake hole any size you want, just need to jet it accordingly.

The bike starts lean, opening the hole in the air box up more will not make that better.

Jetting done to an established known is well within the most bike owners capability's

Jetting an unknown set up is not.

The Dyno and wideband does not have to be yours.. make changes, do a seat of the pants feel, once you think you have found a good set up.. Take it to a shop with a dyno and wide band... Get the results, make changes from there.

If your unwilling to make jetting changes , leave the snorkel in the air box and ride.. It wont be great, but it'll run just fine.

All stock except an air box mod and jetting to a easy to follow known set up will make it run better.. And I'd be willing to bet, almost any user here, can be assisted though the process by a number of helpful folks on this very forum.

So a better responding mostly stock DRZ can be yours with a bit of work and a few changes.. But it's your bike.

As to your concern about damaging the engine though jetting changes, worry not. On this bike and most 4strokes.. Lean jetting that is bad enough to hurt the engine, would make it run so bad, you would not even try to ride it that way..

If not changing the jets and needle, leave the air box as delivered from the factory.

.

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incorrect.

p.s. a local suzuki dealer has done a few variants of this, and on their dyno their was no gain from going from 2x2 to 3x3

Find a better dealer, that one does not know what they are doing, at least in regards to what you posted on their behalf.

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Find a better dealer, that one does not know what they are doing, at least in regards to what you posted on their behalf.

so a 3*3 bike with JD jets and their settings (air fuel was in the normal range), was back to backed with a 2*2 bike with almost exact same air fuel settings gave the same results on the dyno. Please explain to me oh great one, what they did wrong.

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Bet they were both completely stock bikes. And almost exact isn't exact at all, so whatever differences in jetting (regardless of how slight they are) between the two may have allowed the 2x2 bike to run properly with that size hole. As Marquez said before, the hole you cut in the airbox can be any size, then jet to suit. He never stated any exact differences in jetting based on the size of the hole. It has also already been stated that the 3x3 comes from the E model with snorkel removed, and so we use this as the template for all other DRZ models for airbox modification and jetting. :smirk:

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Thank you for the informative responses; I appreciate the detailed explanation regarding the origin of the 3 x 3, its development, and the follow up posts which debate (in lively fashion!) the pros and cons of these mods. I'm at a point where I'm likely to try the whole jetting thing; the five-year warranty on the bike (yeah, I bought the extended warranty) ran out twelve months ago, so I don't risk voiding that warranty with modifications, and I know there is an excellent post on how to do the 3 x 3 hole in the airbox, and equally detailed posts on jetting the stock carburetor. As E. Marquez rightly points out, the skills are likely within my ability, and this forum has proven itself to be graciously supportive of five-thumbed goofballs like me. Again, I value this forum and appreciate your patience and responses.

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Ideally the airbox would be much larger than you could package in the frame of the bike, and the box opening would be sized "just" big enough to keep the box refilled. The air would enter the box through a horn shaped apparatus that protruded into the box so air could come in but most the sound wave would get trapped in box. Unfortunately compromise has to be made.

In some applications a slightly restricted airbox is used as sort of a safety net. The "choke" effect at wfo high rpm makes the main jet pull more fuel, keeping the user from overheating the piston on some cold morning pinned wide open from Barstow to Vegas. The extra fuel from the choke can come without using a bigger main jet which could overlap into a different part of the tuning circuit.

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incorrect.

its just like in a city when a 6 lane road merges down to two lanes. A motor, road, or anything else only flows as much as the lowest flowing pinchpoint. So the airbox may flow more air, but not when its attached to a motor. In that case, it only flows as much air as the motor can draw in.

p.s. a local suzuki dealer has done a few variants of this, and on their dyno their was no gain from going from 2x2 to 3x3

Yes I agree with you, I was only talking about the restriction in the airbox intake, not the whole system.

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I think you've got the gist of all the arguments. Eric is extremely knowledgeable and you can count on his answers being based on experience, not from what he's read or heard.

But, maybe there have been too many words...

The foundation of the 3x3 is simply that there IS a wealth of experience with that size opening in the airbox, and you can take that experience and use it to get the correct jetting with minimal experimentation and testing on your part. Any variations on the opening size will take personal experimentation and testing to come up with the correct jetting.

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