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I’ve done that too and saw no difference. In my opinion based on what I’ve tried the only thing that will definitely require a new tune is when you start cutting out large areas of the air box and put in a high flow (race only) type filter.
 

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You can take the whole airbox off and won't make any more power. She may run somewhat lean compared to before. Unless you are cutting off large chunks of the airbox, might as well not do anything. Even a basic tune flash with opening up the airbox and foam filter will make a big difference.

If you don't want to splurge on velocity stacks, you can get another airbox off ebay and swap to both short stacks.
 

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You can take the whole airbox off and won't make any more power. She may run somewhat lean compared to before. Unless you are cutting off large chunks of the airbox, might as well not do anything. Even a basic tune flash with opening up the airbox and foam filter will make a big difference.

If you don't want to splurge on velocity stacks, you can get another airbox off ebay and swap to both short stacks.
I haven't tried taking off the airbox and running it without, but I have heard it can make a bit more power. I know the airbox I designed and 3D printed did increase power pretty much everywhere but especially around the top end. Only about 1-1.5 hp on my bike because I have other stuff done to it, but on our supersport bike that has stock TB's, stock cams, stock head, it made over 2 hp more.
 

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I haven't tried taking off the airbox and running it without, but I have heard it can make a bit more power. I know the airbox I designed and 3D printed did increase power pretty much everywhere but especially around the top end. Only about 1-1.5 hp on my bike because I have other stuff done to it, but on our supersport bike that has stock TB's, stock cams, stock head, it made over 2 hp more.
You got 2 hp more with just a custom airbox? That's impressive - That's about a 2/44= 4.5% gain. Did it affect intake NOISE level much? Do you intend to sell any?

Jim G
 

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Guys,
Attached are photos of my air box modification that I did on my 2021 Ninja 400 "street bike".
I kept the longer velocity stacks. If you are not racing the bike, the longer stack will work better on a street bike where you are in the 4,000 / 7,500 rpm range.
If you open up the air box, you do need to reflash the bike. I also have a full titanium exhaust system on the bike.
On the flow bench, there is no air flow gain by changing to any other air filter, the stock air filter is so large, compared to the fuel injection internal ID that it's a no gain.
Yes, it will be louder!
BMHS
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You got 2 hp more with just a custom airbox? That's impressive - That's about a 2/44= 4.5% gain. Did it affect intake NOISE level much? Do you intend to sell any?

Jim G
There may be a slight noise change, but not very noticeable. It doesn't seem to be any louder than running a stock airbox with aftermarket shorter stacks.

Yes...eventually...not 3D printed ones like the prototype on my bike, but once we figure out a more efficient way of making it, and more robust (like carbon fiber).
 

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There may be a slight noise change, but not very noticeable. It doesn't seem to be any louder than running a stock airbox with aftermarket shorter stacks.

Yes...eventually...not 3D printed ones like the prototype on my bike, but once we figure out a more efficient way of making it, and more robust (like carbon fiber).
Sounds good. If I still have the Z400 by then (The 2008 ZX-6R has been charming me with its ultra high-rpm nature, performance, and appearance :) )

Jim G
 

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On the flow bench, there is no air flow gain by changing to any other air filter, the stock air filter is so large, compared to the fuel injection internal ID that it's a no gain.
I always wondered how much difference the filter actually makes, I suspected it's very negligible, but I don't have a flow bench and I never did any dyno testing with filters on this bike. I did do that for the new Honda Grom and noticed no difference at all in performance between the stock filter and an aftermarket one that allows more air. The bottle neck is not the filter like you said.
 

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Guys,
Attached are photos of my air box modification that I did on my 2021 Ninja 400 "street bike".
I kept the longer velocity stacks. If you are not racing the bike, the longer stack will work better on a street bike where you are in the 4,000 / 7,500 rpm range.
If you open up the air box, you do need to reflash the bike. I also have a full titanium exhaust system on the bike.
On the flow bench, there is no air flow gain by changing to any other air filter, the stock air filter is so large, compared to the fuel injection internal ID that it's a no gain.
Yes, it will be louder!
BMHS
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Why did you block off the holes for the original snorkels?
 

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Guys,
I originally did a few Ninja 400 air box's for some Midwest racers, where we wanted to set up a mild ram air system. I carried the idea over to my street bike. The aluminum panel on the bottom is a slight deflector for rain. I removed my ABS brake system & the PAIR valve system, so the whole area in front of the air box opening is completely open. I have only done it a couple of times, but when you get over about 75/80 mph at WOT in whatever gear, it really comes on strong. Most performance engines prefer to have the incoming air entering the air box from one opening, to control turbulence. The velocity stack side would ideally want mildly pressurized air that has little motion, or swirl.
BMHS
 

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Spears sells something similar. But while you may be getting cooler air by deflecting some hot air off the radiator away from the air intake I doubt there is any ram effect up there behind the fairing. Norton actually built a big snorkel that extended up to the front in an effort to try and get some ram effect but I don’t think it was successful.
 

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Actually, it's not behind the fairing, it below the front , and I have mounted one of my electronic manometers with a pitot tube and measured the air pressure at the inlet opening. Not great like a MotoGP bike, but it is slightly positive. My opening is 2.5 times the area of the smallest section of the throttle body. If I recall correctly the snorkels are about 87% of the area. So, if your interested in going faster, pull the snorkels.
 

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Not that it really matters but I think it's pretty safe to say the air box is behind the fairing and not below it.

But that's great that you are seeing a positive pressure there. I'm a little surprised.
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If you truly want to get a ram air effect it requires a way of taking air from the high pressure area at the front of the fairing and piping it to a sealed air box. One that can actually be pressurized. But just trying to get cool air to the intake area is a worthy goal too.

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Guys,
This is a photo shot right under the lower triple clamp. I think, but I'm really guessing that the shape of the front fender tends to move the air up there. This Spring, when it's warmer, I make some freeway runs with the pitot tube mounted in various spots. By that time all the bodywork will be back on the bike. This would not work nearly as well if all the brake & PAIR stuff was in the way. It will be interesting to see where all the air is going.
BMHS
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