Debunking airbox mods On Yamaha YZF-R125? – part 1

Ok so this seems to be an ongoing urban myth, whether it’s chavs drilling the airbox of their mum’s Fiesta 1.1 popular or little 125s, or even big bikes, people seem set on the idea that airbox designers are actually our mums who are trying to sneakily slow our bikes down. or something.

Why am I even going down this road?

On my bike I’ve shown the exhaust side of the engine quite a lot of love and attention, but despite the nice, big-bore shiny pipesness, she continues to run better with a little baffle in the end, like so:

The best reason I can come up with that the bike runs better with a smaller diameter exit/baffle in the end (effectively restricting the flow again) is that perhaps the exhaust is a bit much pipe for the bike – and by that I mean for the amount of air the engine is moving, the pipe diameter gets too wide at the end, which ends up with the gasses slowing down too much and actually impeding scavenging of the exhaust system. Or maybe there’s some sort of sound-waves stuff going on like with two-strokes.

Either way with the baffle in you get a little bit of torque around that 70mph point where you shift into top gear. With the open pipe it didn’t have the oomph to push through that but with the baffle it does (albeit slowly).

A couple of months ago, under what turned out to be ideal conditions on a private runway, thanks to that baffle we hit 85mph.
That was on a 15t front gear too so would have needed even more torque to make it through to the power band.

And it was absolutely equal parts terrifying and exhilarating..,as you would expect flying along on a glorified moped! 😀

Ok so whatever, I preferred the sound of the open pipe but faster always wins so now we go with a baffle…

but wait…

What If I’ve sorted the exhaust side but haven’t really done much to improve the intake side of things, so perhaps that’s why we can’t make better use of the full pipe.

So improving flow on the intake side – we’ve got a K+N filter but otherwise no changes. So what other changes could we actually do? There’s larger bore throttle-bodies and inlet manifolds intended for hot 125s or 150/180cc conversions but they’re surprisingly expensive, and I’m pretty sure the airbox won’t fit afterwards, so maybe start at the end and work our way upstream to the more expensive stuff.

I’d seen airbox mods mentioned on the forums and it seemed like a reasonable place to start.

If you google “YZF-R125 airbox mod” you get a variety of stuff, much of which is noise:

This forum post seems to be actually talking about the EGR delete rather than anything actually to do with the airbox.

There are some mentions of actual airbox mods further down but bottom line is nobody seems to know for sure.

There’s some mentions of removing the snorkel which comes out of the pipe going to the throttle body but that didn’t seem like a good idea to me since it would bypass the air filter and I’d rather not have grit in my engine.

If you look at the YZF-R125 airbox:

If anything is posing a restriction, then I guess it’s probably the narrowest point where the air comes in (circled in red).

The other main restriction would be the little pipe where the air goes into the throttle-body, but I’m not able to change that at the moment (bigger ones do exist. very expensive though).

One thing I noticed with taking the airbox apart is that those intakes, and actually all the curves on the box, look very *deliberately* and *carefully* thought out, perhaps so that they bring air in with the least turbulence or something.

I know everyone’s keen to break out the drill on their airbox but if the shape wasn’t important do you think they’d spend so much time and effort making such a beautifully shaped piece? Just saying 😉 Anyway, in the name of science…onwards!

I got a spare airbox off ebay so that I could modify it it needed but still be able to go back to stock if it was terrible. Looking at it though, the obvious route to reduce intake restriction would have to be to just remove the top half completely.

Drilling holes or cutting sections out would be very hit and miss – given I can’t see or predict how the different flows from holes/cavities will interact. Removing the top half though would keep the filter (which I wanted), but would let me test completely removing any intake restriction (from before the filter).

Just taking the top of the airbox off obviously isn’t something which would work long-term but it should let me test whether it makes any difference.

Just checking it runs ok and looks like it seals

So how does it ride dammit?

As I tried to remove my petrol cap at the station it slipped out of my hands and made a direct run for my gopro, somehow obliterating the bike mount. Shit.

So alas there’s no footage from today’s test but I did ride with it for around an hour total on a good mix of roads so here’s some initial feedback:

1) It’s LOUD! Not fun loud, more like get’s annoying after about 3 seconds loud. Every time you twist the throttle and a vacuum is created in the inlet manifold there’s a massive induction WOOOOOOOOOOAR. Ugh. It sounds exactly like you’ve drilled all the holes in your airbox. Meh

2) It’s possibly more responsive very low down. By that I mean when you very first twist the throttle it seemed quite a bit more responsive, and there’s a flicker of excitement but that was quickly followed and let down by the now flabby midrange. Normally the midrange is pretty decent so that was a bit disappointing, and I’m not sure why it would happen but hey. Gasses, physics and shit I guess.

The area I was hoping to see the most difference was at high rpm since if the airbox intake ports are a restriction to that side of the air filter, that’s probably where it’s manifesting itself.

It was kinda hard to tell with the high rpm riding. It seemed a little slower than the other day (normal airbox) but did reach 78mph at one point. Whilst technically that’s a little slower than the 79 record from the other day that could just be down to head wind for example,

Interestingly even though it felt slower, later on when I’d put things back the bike *seemed* quick but didn’t do so well in the top speeds, not making it past 75 in the winds, so maybe 78 on the open airbox was not so bad.

On the way there I tried it with the end baffle in the exhaust, on the way back I tried without, just in case if there was now less intake restriction, perhaps it might make use of the bigger pipe exit…but #nope.


Without putting it on a dyno there will always be an element of subjectivity, and even the dyno only tells you what the engine does, it doesn’t take into account wind resistance and how it will actually be to ride.

The only solid/reliable metric I have to test with is the top speed and everything else is down to how it feels to ride. I could possibly do timed runs but there’s variables like traffic and my inconsistency as a rider which would probably make those readings worth very little.

Even the top speed is highly dependent on favourable conditions but with a long enough time sample (i.e. riding with a particular test configuration for a week or longer) means there’s usually enough opportunities for a reasonably fair test and to get an actual top speed.

Today with the airbox mods I only did one run, which although deliberately a nice varied mix of roads and dual carriageway (for topspeed opportunities) it was just one run, and even so it got within 1mph of the previous record (78 vs 79mph) which I’d only hit by chance after a week or so of riding having done the switch back to a 14t front sprocket.

So to just do the mod and then almost hit topspeed on the first run, on a windy day, is actually not a bad result. which reluctantly leads me to think there *may* be some merit to the airbox mods. Further testing needed though I think.

I did not like riding without the top-half of the airbox. I was wondering beforehand if I’d like it and then I’d have to figure out the noise but fortunately that wasn’t a problem 😉 The noise is ridiculous and I’d consider taking a speed hit for that alone (if there even is one).

That said, getting so near the top speed and the improved response right early when you crack the throttle have me intrigued, and I feel somewhat duty bound to test further, but whatever I do will need to make less noise than today’s setup.

But wait…there’s more

No I’m not done yet. Today was just supposed to be a “dipping my toe in the water” exercise to get a feel for the issue and test the theory I’d come up with.

Despite breaking the camera mount so not getting any footage, I still wonder if making the intake ports on the airbox a little larger might be worth testing – I did after all buy a second airbox with the express intention of drilling and/or otherwise modifying it.

So if I were to modify it to allow a bit more air, but without opening myself up to the incessant induction drone, how would one best go about cutting it?

Seeing where the airbox inlet is placed also got me wondering – even if we don’t modify the inlet ports of the airbox, would it be possible to improve the flow of air TO the airbox. It’s pretty tight around there with the battery etc. Perhaps The airbox could also draw from a similar port on each side?

Or maybe that a hole saw/pipe saw and just enlarge the inlet holes a little, trying to maintain the original inlet “flow” as far as possible?

Or maybe try something like a pod filter or basically velocity stacks with a foam filter. Shortest, least restricted intake path possible might be best?

Have you got thoughts on how I should modify the airbox? Let me know in the comments or at, or use the contact form (not you tho Matt. lol 😉 )

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