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So as i understand it many have tested aftermarket intake systems and it seems oem airbox makes more power most of the time.
what is it about the OEM intake box that makes it work so well? I mean it's just a box, not particularly smooth inside, a typical cone air filter is used, some aftermarket system probably have higher flow intake filter yet make less power.
Blocking out the resonate chamber doesn't seem to affect performance, so resonance tuning is not it probably.
This is a timely thread - having spent several days going through the archives on here as well as other forums to understand the pros and cons of the OE airbox vs other aftermarket systems. I'll summarise my thoughts/findings below, with accompanying sources where possible to help others make their own opinions.
One point that I think is understressed is that the air intake isn't something that should be thought of in isolation, but a component that works in synergy with other accompanying modifications such as larger TBs, different manifolds, headers, test pipes, exhausts and not to mention ECU tuning. These things combined will all contribute to the overall performance and efficiency of the enginer as a whole; each having an effect on resonance, pulse timings and overall air flow. This is undoubtedly why there is so much difference of opinion as different setups will of course produce different results (not to mention being run in different conditions and on dynos with different error margins/correction factors etc.)
Throughout all the threads I read, the most hotly debated topic was whether the OE airbox was sufficiently well designed to outperform other intakes; which mostly boil down to short ram (SRI) and cold air (CAI). The primary area of my focus therefore fell back to the OE airbox and its different modified variants vs the custom CAIs that users have fabricated to different specifications.
When it comes to intakes, most users are looking for either more noise, better looks or more power; sometimes even a combination of all three whilst balancing cost. For the purposes of this post, I'm focussing on those looking primarily to understand which system provides the most overall power gains vs outright looks and noise without spending silly money. I also like something that looks OE/professional as opposed to pure function over form. I don't want a show car, but it should look properly done and as factory-fit as possible. My intent is also to run a setup suited for almost dedicated track days, with very little street driving. This may not be as relevant for those with a fast road setup that don't track/compete and just want the best noise/looks.
Standard airbox
It's clear from various feedback and testing by users that the OE airbox (like many of the standard components on the S2000) is very well designed and engineered by Honda. Caution should be taken before simply hacking it up and/or replacing it with an aftermarket intake without due research and consideration for your desired goals. Sometimes, just because you can doesn't mean you should.
That said, it also seems clear that the overall design of the OE airbox does work to compromise efficiency vs noise, emission control and fuel economy which will always be a trade off.
A particularly useful source of information and research was from MotoIQ's site: .
In summary, his tests suggest that the standard airbox responds well to additional supporting mods (in his case a NACA duct) that produce an increased ram air effect with a direct source of cold air coming from a high pressure zone at the front of the car; albeit fairly nominal given the speeds you need to be travelling to generate an appreciable ram air effect.
He also tested removing the internals of the airbox to see whether this improved air flow, but found it to make no appreciable difference; supporting the widely held belief that Honda did everything with intent on the airbox design to good effect.
What was interesting is that a distinct drop in air pressure can be seen from his measurements in both cases as RPMs increase, supporting the theory that the OE system does present a restriction somewhere. What isn't clear however is where that is, i.e. even upstream of the airbox and intake arm. Removing the airbox lid did (predictably) remove the restriction though this was negated by the increase in IATs and resultant effect on ignition timing; supporting the principle that there are multiple forces at play (flow, resonance & temperature to name a few).
In short, the OE intake system is very good but can still benefit from various modifications to optimise it for outright power gains which when taken to their extremes make the time and money investment questionable. More specifically, the key areas of weakness appear to be: .
A lack of a direct cold air feed into the OE horn that attaches to the airbox to avoid sucking in hot air, as well as providing a ram air effect. The prototype pictures from circa 1999 actually show a longer snorkel before the production version was released, as can be seen at https://www.s2ki.com/forums/s2000-un...riction-921933 .
The placement of the airbox at the front of the engine bay, between the radiator and engine which inevitably lead to heat soak over time when not moving at speed. It presumably also traps ambient air around it that would otherwise move more freely out of the engine bay if it were removed and replaced with a CAI or similar. A vented hood like the J's Racing will also help this point.
The overall length, ID and smoothness of the intake arm. It is suggested that the length is too short to produce any resonance (though Honda aimed to produce this by manipulating pressures within the OE airbox), as well as the fact it is not entirely smooth and provides no distinct step-change in diameter before the TB that would help improve overall velocity (yes it does get smaller, but it isn't a long straight length that seems to produce best results)
I feel it's also worth making note of the actual size and weight of the stock airbox, which can certainly be reduced for those chasing lap times to help overall power to weight (albeit very slight). This is most relevant to those looking to shed weight in as many areas as possible.
A number of users have done different things to compensate for many of the above factors, primarily: .
Fitting an aftermarket (J's/Spoon) snorkel to the airbox to feed cold air into it and produce a ram air effect
Fitting a cooling plate (and/or some form of insulation) above the radiator to try and prevent some of the hot ambient air reaching the airbox
Fitting heat reflective (DEI) gold wrap to the outside of the airbox (and even intake arm) to combat heat soak
Sometimes gutting and insulating the inside of the airbox to further combat heat soak; also allowing for larger filters to be fitted with velocity stacks
Replacing the intake arm with a larger aluminium intercooler elbow for increased, smoother air flow.
Additional ducting from the fender into the side of the airbox to increase overall volume of airflow (though at the potential cost of increased turbulence hitting the primary intake and negating the benefit of a pressurised ram air unit)
A good example:
On that last point, one member (Dolebludger) actually took this one step further by incorporating a "flapper valve" with an additional duct to try and provide the best of both worlds, as can be seen in https://www.s2ki.com/forums/s2000-ta...-i-like-345540
Whilst I credit the theory behind this, it also makes me wonder at what point these airbox mods become over-engineered vs other simpler, equally/more effective implementations. For me personally, simplicity is often king.
Supporting threads/posts RE the above mods can be found here: .
Whilst I was originally a massive proponent for keeping the OE airbox with some slight modifications to aid overall cooling and airflow, since reading through all these threads I'm now of the opinion that the investment may not be justified. In particular, the only snorkels I can now find are genuine J's/Spoon ones for several hundred dollars, and the same goes for cooling plates. Additionally, if you were to make all the above modifications it doesn't actually leave much of the OE components in place anyway other than the shell of the box, defeating the point somewhat of embracing Honda's engineering.
This moved my attention briefly to the K&N FIPK system which is another popular choice and maintains a relative short ram system with the filter in the engine bay.
K&N FIPK
This was one of the primary aftermarket intakes that caught my attention, not least for its relative simplicity, OE fitment, noise and cost. Upon initial reading, it appeared to be a good option to mate with an AUT cooling plate for added cold air flow as well as wrapping the outside of the heatshield and/or the intake arm in DEI reflective heat wrap to minimise heat soak.
The factors that ended up putting me off this option were it's relatively cheap looks (subjective perhaps) and the fact it was still susceptible to heat soak despite being "sealed" when the bonnet is shut. I don't subscribe to that school of thought given it is clearly not a fully sealed unit and air will always follow the path of least resistance to get where it wants.
The deciding factor however was the intake arm and it's small ID, something that has been raised many times (particularly in the CAI threads I'll link to in the next section). It seemed to me that this wasn't a particularly well optimised system that justified replacing the OE airbox, unless of course you were most interested in more induction noise. Whilst the scale of these issues may not be fully quantified, I didn't like the idea of my filter getting more hot air than the OE box and drawing air through a smaller intake arm than even the factory one.
This led to me to look into CAIs in more detail.
CAIs
When I originlly read about CAIs I dismissed them fairly quickly for two reasons. First and foremost because I just felt they looked too DIY and difficult to make look OE (again just my opinion). This is something I still have to reckon with vs the OE airbox. Second, I had reservations around the common filter location in the inner wheel liner getting the filter excessively dirty from being so close to the road, affecting overall flow over time. It also seems like a difficult/incovenient place to access and inspect the filter condition on a regular basis.
Many people also raise valid concerns around an increased risk of hydrolocking, but I would agree with the general consensus that applying common sense and avoiding massive puddles will avoid any real issues. Interestingly, a few users note that in fact the bigger risk comes from leaving the car exposed to rain which can allow water to pool in the piping, depending upon how it is angled. For me this is also not of concern given that the car is garaged overnight and used almost exclusively for track, largely negating both risks. That just leaves the need to inspect the condition of the filter on a regular basis for excessive dirt build up vs having it located in a more convenient location within the engine bay that is also less susceptible to heavy soiling (though an argument could be made for snorkels creating a similar "issue" which has also been noted in a couple of posts).
These reservations aside, I read a lot of feedback from users both on here and Honda Tech that the high revving Honda VTEC engines benefit from a long intake pipe that creates optimal resonance with the obvious side benefit of drawing cold air into the intake from outside of the engine bay. User b.r.i.a.n seems to have a lot of experience in this field alongside tuning these engines, with further testing done by Gernby over the years, though I can't personally account for them.
When reviewing CAI setups a lot of useful testing and comparisons have been made between both different length pipes and different internal diameters including some pertinent findings with regards to incremental step changes before the TB to maximise velocity. The risk with the very custom/DIY nature of these setups is not accounting for the optimal design characteristics that lead to increased efficiencies vs. slapping something together that leads to losses rather than gains.
In short, feedback and testing would suggest that there is such a thing as too long an intake pipe which will take you beyond the point of diminishing returns; much like can be said for too short as well. The most up-to-date information I could find implies that running a pipe down to the fender well nearest the TB is better than routing the piping all the way along the opposite side of the car. Equally important is the ID and step changes of this piping, which research suggests is probably best configured with 3.5" piping from the filter up until 6" before the TB, at which point it is recommended to step down 0.5" to a 6" long straight section of 3" pipe to maximise air velocity and ensure smooth air flow into the intake.
Pictures help explain more clearly. Filter location:
3.5" to 3" straight section before TB (ignore pipe routing):
These points are discussed at some length in these threads: .
There is mixed feedback as to whether anything larger than 3.5" piping can provide greater results though the general consensus would suggest 3.5" is sufficient for most use cases, especially where the TB is no larger than 70mm is used. Porting the inlet manifold to match any uprated TB is also necessary to see any gains when changing this component and is recommended only with an accompanying tune to ensure AFRs remain on-point.
On the point of CAIs vs short ram intakes, the general consensus appears to be that each can be manipulated to change the torque curve higher or lower in the rev range. Remember however that this works in synergy with all other components that affect overall flow and resonance so depending on your modifications (headers, inlet manifold, test pipes, exhaust etc) you will need to carefully consider your overall setup vs the intake in isolation to ensure things complement each other vs working against one another.
In summary, there certainly seems to be merit in utilising CAI setups such as these but I'd argue they require greater research/attention to detail to offer positive results, else you could end up making things worse. With appropriate complementing modifications and a good tune I am now of the opinion this is a very good way to chase some power gains and will likely choose this option above my standard airbox that I stil currently use today.
Other off-the-shelf intakes
A quick word on other intakes like the J's Racing, Mugen and PW:JDM items. I ended up dismissing these fairly quickly not only because of their very high retail price, but lots of reports of ill fitting components and/or the need to cut bodywork/hood framework to make things fit which I am personally not in favour of. A number of the lesser well known ones also seem discontinued now the S2000 is hitting it's 20th birthday...
Supporting mods
Alongside the intakes themselves, it's important to pay attention to other complementing factors that can help with quantifiable gains; namely - .
Relocation of the IAT sensor on pre-06 cars from the inlet manifold to the intake arm for much lower temp readings which the ECU can take advantage of to run more ignition timing and optimise fuel trims
The common coolant bypass and use of phenolic gaskets for the inlet manifold and TB
Use of DEI or similar heat reflective gold tape on any intake piping (particularly metal) to reflect as much heat as possible, furtehr reducing IATs
Ceramic coating also appears another viable option which may look more presentable;
The use of velocity stacks on the end of the air filter seem almost universally recommended to improving airflow, particularly for shorter ram style intakes. BPi and Blox are commonly used and worth reading up on; https://www.bpi-us.com/
That's about as much information as I've managed to pull together so far from the fairly extensive searching and reading I've done. Hopefully it helps someone else; though keen to get opinions and feedback before I take the plunge myself later this year.
So as i understand it many have tested aftermarket intake systems and it seems oem airbox makes more power most of the time.
Where'd you get that idea? Countless tests show power increases from some aftermarket air intakes. Tiny increases to be sure.
The reason the OE air intake works so well is a phenomenon called engineering. The OE airbox is an excellent compromise between air intake and muffling intake noise.
All of the flahsy things aftermarket intakes provide are not necessarilly better for overall power. Lots of marketing goes into those I have known many cars that aftermarket intakes provide no gain or even loss over stock. Truth is that besides econo cars of the 80's and early 90's, most manufacturers do a pretty damned good job and developing their intakes. They are trying to get the most fuel economy they can out of tiny engines and still make them perform enough that people want to drive them. They have orders of mangitude more funding to develop with. So it makes perfect sense they tend to do as well or better than the aftermarket. I would venture to say most intake tube systems from the aftermarket can barely compete, or do not compete at all, with stock systems any longer other than looks and sound. That has been pretty well known for decades now. Even the "gains" found by some are hard to reproduce or are well within the noise of measurement error. There will be some gains by some, but they are tiny. Pretty much negligible in the grand scheme of things. Certainly not anything you will feel.
I mean, people by strut braces for an S2k as well, giving you a shiny arm rest while working on the car They will tell you all the advantages that do not exist, and that is all marketing to make you buy their thing.
Because most aftermarket "cold air" intakes utilize an open filter exposing to engine heat which increases IAT - which does two things to reduce any performance a potentially better intake elbow might have otherwise increased. 1. Our ecu is very sensitive to IAT and pulls timing as this goes up which a lot of the power loss actually comes from, then 2. general hotter air itself reduces power.
There are some things you can do to either improve on the OEM box or create your own actual cold air intake. The oem box still allows some heat soaking to occur, and it also doesn't flow as good as it can potentially which does go noticed when you start adding bigger TB, hfc/decat and some tuning etc.
There are some good threads over the years you can search for, where people have experimented with intake pipe diameters/lengths, velocity stacks, filter sizes and cold air ducting etc and the theories put to the test with some dyno comparisons.
Last edited by s2000Junky; Dec 31, 2019 at 10:47 AM.
The OEM box isn't perfect but understand the parameters the manufacturer has to work with - noise reduction, water safety, etc. But you cant get 120 hp / litre without a well designed intake, so it had to be near perfect. The air filter has more than twice the capacity of some v8 motor filters. The inside of the intake arm is perfectly smooth and maintains a constant diameter despite being curved. The oem air filter also has a built in velocity stack at the base. The OEM intake box uses pressure differences to aid in flow. The only thing keeping it from being perfect is the short intake snorkel that terminates at the radiator, so it still sucks hot air under some conditions. To make it perfect it should have come with a longer snorkel like what was shown on some prototype models, that was something the Mugen intake got right, still safe from water but sucking cooler air outside of the engine bay. I loved my J's Racing snorkel on the oem box and oem arm and oem filter, I also loved my Mugen intake but I would say the OEM setup with a J's snorkel was almost as good and near perfect.
The OEM box isn't perfect but understand the parameters the manufacturer has to work with - noise reduction, water safety, etc. But you cant get 120 hp / litre without a well designed intake, so it had to be near perfect. The air filter has more than twice the capacity of some v8 motor filters. The inside of the intake arm is perfectly smooth and maintains a constant diameter despite being curved. The oem air filter also has a built in velocity stack at the base. The OEM intake box uses pressure differences to aid in flow. The only thing keeping it from being perfect is the short intake snorkel that terminates at the radiator, so it still sucks hot air under some conditions. To make it perfect it should have come with a longer snorkel like what was shown on some prototype models, that was something the Mugen intake got right, still safe from water but sucking cooler air outside of the engine bay. I loved my J's Racing snorkel on the oem box and oem arm and oem filter, I also loved my Mugen intake but I would say the OEM setup with a J's snorkel was almost as good and near perfect.
The oem arm isn't 100% smooth, it has accordion sections through it. HPS makes a good upgrade to this which I ran for some time when NA. That intake elbow also gives you an opportunity to build a better 6" velocity sack with the appropriate size washable filter. If you gut the box you can get a bigger area to build this velocity stack and filter large enough over oem to maintain proper flow without bottle necking there, which does happen when you start playing with oversize TB with proper port matching to it, exhaust and tuning. Then top it off with some form of snorkel to the box for proper cold air flow. That's the method I chose to make a true perfect oem set up. Some chose to take to the next level with a few more compromises and actually create a long tube stepped intake changing the stock pulse flow and further improving velocity at the manifold, but requires locating the filter in the airflow path where its more compromising to get to and potential to the elements are greater.
The oem arm isn't 100% smooth, it has accordion sections through it. HPS makes a good upgrade to this which I ran for some time when NA. That intake elbow also gives you an opportunity to build a better 6" velocity sack with the appropriate size washable filter. If you gut the box you can get a bigger area to build this velocity stack and filter large enough over oem to maintain proper flow without bottle necking there, which does happen when you start playing with oversize TB with proper port matching to it, exhaust and tuning. Then top it off with some form of snorkel to the box for proper cold air flow. That's the method I chose to make a true perfect oem set up. Some chose to take to the next level with a few more compromises and actually create a long tube stepped intake changing the stock pulse flow and further improving velocity at the manifold, but requires locating the filter in the airflow path where its more compromising to get to and potential to the elements are greater.
The S2000 oem intake arm is perfectly smooth on the interior throughout the entire length, what you describe is indicative of many Honda intake arms but not the S2000 one, Honda designed that differently. One reason why I never bought a silicon aftermarket replacement arm as the oem piece was good enough. I owned an ap1 but I assume the ap2 would be the same. The oem intake arm is susceptible to splitting when it ages because it does not have the accordion sections.
The S2000 oem intake arm is perfectly smooth on the interior throughout the entire length, what you describe is indicative of many Honda intake arms but not the S2000 one, Honda designed that differently. One reason why I never bought a silicon aftermarket replacement arm as the oem piece was good enough. I owned an ap1 but I assume the ap2 would be the same. The oem intake arm is susceptible to splitting when it ages because it does not have the accordion sections.
I've got one on my shelf I verified before posting. Sorry but its not entirely smooth, only parts of it. The HPS replacement is entirely smooth.
But ive got either for sale if anyone interested.
Last edited by s2000Junky; Dec 31, 2019 at 11:58 PM.
For Honda to have extracted the power it did from these motors necessitated efficient intake and exhaust designs. F1 tech in a street car. BMW did this with their S65 and S85 motors (just as beautiful and incredible as the F20C/F22C). The intake and exhaust manifold/system designs were already maxed out for power within the parameters of their respective governmental/environmental regulations.
why does the oem air box work so well
I have known many cars that aftermarket intakes provide no gain or even loss over stock. Truth is that besides econo cars of the 80's and early 90's, most manufacturers do a pretty damned good job and developing their intakes. They are trying to get the most fuel economy they can out of tiny engines and still make them perform enough that people want to drive them. They have orders of mangitude more funding to develop with. So it makes perfect sense they tend to do as well or better than the aftermarket. I would venture to say most intake tube systems from the aftermarket can barely compete, or do not compete at all, with stock systems any longer other than looks and sound. That has been pretty well known for decades now. Even the "gains" found by some are hard to reproduce or are well within the noise of measurement error. There will be some gains by some, but they are tiny. Pretty much negligible in the grand scheme of things. Certainly not anything you will feel.
The HPS replacement is entirely smooth.
