This article is about the airbox modification of the Jimny 5 Door and not the Japan spec 3 Door. The Japan spec K15B Jimny has a different airbox, different in construction but similar principle.
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The airbox of regular mainstream vehicles including the Jimny is a compromise in its design. The compromise not in material, PP-GF30 is very good. The compromise is of performance, fuel efficiency and NVH. High performance variant of a vehicle, when available, always has an airbox which is more conducive to higher flow rather than satisfying low NVH requirements.
Aftermarket air intakes are always an option to gain more performance out of the engine and in some vehicles they do respond very well, especially on vehicles with MAF based fueling. Jimny does not have a MAF sensor to measure the airflow, to estimate the airflow into the engine it uses a speed density model for fueling. In speed density the air flow into the engine is indirectly measured using Manifold Absolute Pressure (MAP) sensor, Intake Air Temperature (IAT) sensor at a given engine speed (RPM) to look up cylinder filling percentage from a volumetric efficiency (VE) table. Because it cannot measure the additional airflow directly like a MAF based system can, the speed density engines does not usually respond very well to air intake changes. There would be some gains, as a result of reducing pumping loss.
Flashback:
I have in the past, a decade ago infact, used the BMC CDA (Carbon Dynamic Airbox) and its higher option the BMC OTA (Oval Trumpet Airbox) in two Suzukis with the K series engine and both running a MAF less MAP based speed density system. Then BMC did not have an India distributor selling it locally and had to be imported. These airboxes were very very expensive and they better perform?! But they did not. The engines gained high RPM performance marginally but lost torque in the low to mid region which is the most used for daily driving. The blame is entirely mine, when the BMC CDA did not work in one Suzuki I should have learnt my lesson but I did not. So I bought the BMC OTA an year later for another Suzuki and found that it was just a glorified CDA with a short velocity stack inlet into the throttle body. For the money paid both were a total waste. What I did next was brush up on Graham Bell and was convinced to try some flow improvements on the stock airbox. There was a snippet about smoothing the airbox and the ribs inside, which is for strengthening the airbox as well as muffling the intake. Which I did and gained a noticeable improvement in the midrange without losing the low end, this was on a manual transmission vehicle. I have mentioned this in an older blog entry. Point to ponder - In performance options of the vehicles the strengthening ribs are external allowing for a smooth surface inside the airbox. Look it up yourself, to give an example the police interceptor vs regular variant of same vehicle with same engine.
BMC OTA (Oval Trumpet Airbox)
The BMC OTA I had on the Suzuki Swift K12/K14B. Photo taken on 31 Jan 2013.
Shipped inside its own cookie can type tin can.
The BMC OTA installed in my Suzuki Swift. Year 2013. The OTA was a nicer kit than the CDA which I had in the Suzuki AStar.
BMC OTA was built better and better supplied feed tube and comes with a bellmouth inlet.
BMC CDA (Carbon Dynamic Airbox)
BMC CDA in my 2010 Suzuki AStar K10B..
Trying various feeds into the airbox, added a bell mouth. Which was sourced from eBay.
There is an opening in the front air dam where the pipes point to away from the radiator fan zone.
And so I have been reading up on the aftermarket intakes available for the Jimny on the web and have interacted over the net with several folks who have put one from name brand companies. Most of them say "its mostly for the sound" and "a little bit but you gotta be putting your foot down feel it. Noticeable change to the intake sound".
I am not saying that aftermarket intakes do not work. They do. They will flow more air than the stock airbox would and to see any proper gains with it you need to have a remap or a piggyback to take care of the fueling. On a stock Jimny an aftermarket airbox will not do much, it will shift the torque curve higher in the rev band, based on what I have seen with the BMC CDA and OTA.
So learning from my past experience, on a stock speed density Suzuki, I decided against, or I should say I resisted the idea of going with an aftermarket airbox. But if I were to go that way it would not be the BMC or the Forge both are just universal filter at the end of throttle body wrapped in a plastic or carbon fiber shell adapted for Jimny. They don't do much so atleast they should look the part, no? A custom made for Jimny airbox looks way lot more cooler than universal options slapped on with adapters. You can get AFE, Gruppem, Armaspeed, Torquit all these are custom made for Jimny K15B they look cool! So decided to modify the stock airbox like I did with the K12B/K14B Swift.
Stock airbox is not very expensive and affordable enough to correct any mistakes. I have gone through five airboxes in total. Changes were made to the top half and bottom half and the type of change made affected where the torque went. The top half of the airbox has a curved funnel shaped inlet. It is a tuned length intake tract and opening into the throttle body with a sort of bell mouthed opening. This length of pipe is very important and if you remove it you will lose torque. This is also why the BMC types and Forge types will not work, but a Greddy type which is an open filter at the end of the tube will work because of that length of the tube, but only till it starts sucking in hot air. So do not cut the curved piece of plastic and do not drill holes in it. Do not drill holes anywhere on the airbox, it is not done. Drilling holes is an old old old "internet airbox mod" and not for the Jimny airbox and where it is placed at.
The bottom half of the airbox is a very compromised design. It is shaped as such to clear the engine valve cover and the fuel rail. A compromise in design to fit the airbox in that particular location on top of the engine resulting in a cramped lower half with an ungainly shape inside. To salvage the poor layout they have included an air flow director (or a plastic wall) placed on the RHS of the air inlet. (Note: RHS/LHS from our point of view facing the Jimny engine bay from the front, and not looking at these photos). The function of the flow director or that plastic wall is to direct the airflow coming in to the rear RHS corner of the airbox. Why? Because the tuned length funnel on the top half of the airbox opens into that corner sucking in air from that corner with the airbox is closed. The designers have tried to create a high pressure area inside the box for the funnel to suck from. When you remove an old filter you will see that the RHS side of the filter is less dirty than the rest of the area, it is due to that plastic wall blocking that side of the lower airbox and due to the way OEM filter sitting angled very close to that wall leaving little space in between.
The lower half of the Japanese K15B Jimny also has the compromised shape to fit over the engine valve cover and the fuel rail but the plastic wall or the flow director is differently shaped and positioned. The OEM filter of that comes in this version of the airbox is also angled ours when it sits in the airbox. Instead of blocking a part of the lower half it sits right in the path of the incoming air and diverts the air to the RHS corner where the funnel opens to in the top half. Notice how the tuned length is built in the Japanese K15B airbox? More elaborate in construction and it is a separate piece. Image source: Croober.
The only aftermarket airbox for the Jimny with a "dyno graph" to support its claim has been the one from AFE. This has been in my cart's "save for later" list since even before I got the Jimny last July. I came close to pulling the trigger on this one during many sales when the price dropped as much as $50US. I am still tempted to get one even just for looks as I type this, it looks fabulous, but the only thing keeping me grounded is reviews by a few who have installed this kit. It sounds good they say and to feel any difference one has to put the foot down, they say. Which in my informed guess is its all about the top end. This AFE does not have the tuned length pipe, but it has a proper "box" around the filter and is designed as an "airbox". For the Jimny the only aftermarket air intake kit with some form of tuned length intake tract is the one made by Torquit.
The ribs have to go and so does the airflow deflector on the bottom part of the airbox. I have tried the bottom half with totally smoothed I refer to as Type - A, totally smoothed but with the deflector standing Type - B, and the non smoothed deflector cut to the same height as ribs - Type - C.
I made the Type - C lower half to check whether leaving the ribs intact and cutting the deflector would nett any improvement in performance, due to the pockets of air formed by the ribs creating a boundary layer effect. But no, the better performing lower half was the completely smoothed Type - A.
I also made a Type - B, not pictured, where I smoothed the lower half completely but leaving the deflector as is, but again no Type - A performed better overall. The deflector might be there to create a high pressure area but without it seems to help with better filling of the airbox.
I then tried the bottom half with different types of top lids and a stock top lid. I also trialed a stock bottom half with different types top halves. But always and very consistently found that the best performing airbox irrespective of mod/no-mod top half is the one with a completely smoothed Type - A bottom half.
There are similar ribs and also a flow director on the top half of the airbox. The ribs are similar in design to the bottom half but more densely packed and there are three long angled ribs which goes into the bell mouth. I think these serves dual purpose based on some readings I have done. One is to stiffen the box up and the second is to help with streamlining the airflow into the curved throat.
I then tried the bottom half with different types of top lids and a stock top lid. I also trialed a stock bottom half with different types top halves. But always and very consistently found that the best performing airbox irrespective of mod/no-mod top half is the one with a completely smoothed Type - A bottom half.
There are similar ribs and also a flow director on the top half of the airbox. The ribs are similar in design to the bottom half but more densely packed and there are three long angled ribs which goes into the bell mouth. I think these serves dual purpose based on some readings I have done. One is to stiffen the box up and the second is to help with streamlining the airflow into the curved throat.
Similarly I did four types of the top half, but only three remains now. The totally smoothed top half referred to as Type -2, The totally smoothed except the curved rib top half which I would refer to as the Type - 1 and The partially smoothed top half which I would refer to as Type - 3.
The Type - 2 totally smoothed top half as the name suggests is cleared of all ribs inside. It is totally devoid of it even on the insides of the intake tract. This box gave me the best performance at high engine speeds but with a minor loss of torque at the midrange which in an automatic with only three speeds (not counting the overdrive) was noticeable.
The Type - 1 totally smoothed except the curved ribs top half had all the ribs removed but left the curved section intact which goes into the intake tract. This lid performed better at the midrange (around 3000RPM) and also offered a better performance at higher engine speeds compared to stock lid. The midrange was noticeably better with the autobox holding on to the gears without dropping it for want of torque during the same driving conditions.
This leaves me with the Type - 3 partially smoothed top half, that is I left the area behind the intake tract as is and did not smooth it. The area in front of the mouth is smoothed except the three curved ribs. This performed similarly to the box immediately above. They felt nearly close but I think there was a slight advantage to the one mentioned above which was totally clean except the three ribs.
The first top half I did was the Type - 1 and I used that for the longest time. When I was trying to evolve the Type - 1 to Type - 2 design and whilst trying to separate the intake tract from the lid it broke (Note: The lid without intake tract sounds very loud and performs poor). So another stock top half was used to make the Type - 2 and this time the intake tract was carefully separated to smooth the insides. Type - 1 was a good design, so to make another one I used another stock another top half starting from scratch but this time I left the area behind the mouth as is and due to the variation in design call it Type - 3. Type - 3 is a Type - 1 with the area behind left as is but performs nearly as good and the amount of work is reduced. Cutting and scraping the ribs and then sanding it as smooth as possible is hard work and doing it multiple times over is boring and so decided to leave it at that.
All the cutting and smoothing work was done without using power tools using craft blades, flush cutting plier and finally sanded it down using coarser to finer grit sand paper. The first box, Type - 0, was done using a Dremel to cut the ribs but the finishing was less than desirable. Type - 0 was very similar to Type - 1 but it was not very well finished.
One of the best things about this mod is that you can use the OEM paper filter and still get much better performance than you think is possible with the stock air filter and airbox. With a replacement filter the performance is as close as you would get from a "performance" intake kit for the Jimny without all of that expense. So if you are not remapped or running a piggyback you may be better off with a modified stock airbox.
Some photos are attached, what is missing is the Type - 0 lid and the Type - B bottom half which was scrapped and lost in evolution respectively.
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