Stock air intake duct

[Leedeth]

My question is, does it act like a (restricted) CAI? Because, it runs from the filter box and out to just under the front of the hood.

And I assume that the duct on both platforms are the same, so I posted here.

[wytbishop]

Well it tries to act as a cold air intake by routing air from the front of the car to the airbox and then into the engine. The problem (a very small problem in the grand scheme of things) is the indirect route it takes. I say it's a small problem because the reality is that no one ever sees more than a horsepower or so gain by installing a CAI. The stock intake already flows as much air as the engine can use. If it didn't, you would see greater gains. There is virtually no gain associated with filter flow, so just putting a K&N filter on the end of your stock tube will not help you beyond better filtration and never replacing your filter agian. That's what I think from an engineer's perspective anyway...people will likely disagree with me.

There are definite gains to be had though with a CAI. For one, you can be more certian that there are no air leaks. Unmetered air entering the intake is a common problem for us and many of the surging idle problems common to the MX3 can be traced to the intake tube. The other is the sound. The stock intake tubing and airbox is designed in part to muffle the sound of the intake plenum. Removing the airbox allows the plenum to echo and resonate in a way that most people like. Your engine will sound a little meaner.

So you get maybe 1HP, no leaks and a cool sound. The downside is, if you do it wrong (by sticking a K&N filter on the end of the stock intake tube) it'll wind up sucking up warm air from the engine compartment and you'll lose 10 - 15HP by heating up the entire intake process.

Hope that answers your question. Anyone may feel free to disagree.

[Aston Wards]

Nope, that about sums it up

I've tried all kinds of variations of intake, the best by far is a simple K&N filter in the stock airbox. It doesn't look fast, but it works.

[nsklzemx3]

Nope, that about sums it up

I've tried all kinds of variations of intake, the best by far is a simple K&N filter in the stock airbox. It doesn't look fast, but it works.

You mean the stock box was actually better than every CIA you tried? So if someone wanted to use a CIA just for the sound and looks, they'd be giving up performance?

[Mooneggs]

Nope, that about sums it up

I've tried all kinds of variations of intake, the best by far is a simple K&N filter in the stock airbox. It doesn't look fast, but it works.

but what if you have a big bore TB? doesn't it require more airflow?

[Aston Wards]

If you have a proper CAI then yes you can expect a gain, but with that you usually lose some peace-of-mind ie; you now have to dodge puddles etc.


but mostly people just stick on a pod filter and tell everyone they're pushing an extra 20 horses!


as for the larger TB, it's not something i'd even pretend to be an expert on, but i can't see how it'd make a difference whether you have a free-flowing airbox filter or a pod filter, just as long as it wasn't restricting the airflow ....., i was more referring to everyday mods anyways

[wytbishop]

Nope, that about sums it up

I've tried all kinds of variations of intake, the best by far is a simple K&N filter in the stock airbox. It doesn't look fast, but it works.

but what if you have a big bore TB? doesn't it require more airflow?

Here's my thoughts on the big bore question...I don't know. But you could easily find out.

If the engine has a displacement of 1800cc, then every 2 revolutions of the crank will draw in that volume of air in the ideal, naturally aspirated case. So without changing internal components, which would alter the volumetric efficiency of the engine, if you give it more air and nothing changes, it's becuase it was already getting all the air it could move.

If you started with 99hp and change the CAI and ended up with 100hp that tells me that the engine only got ~1% more air from the mod and ~1% more fuel was injected as a result, creating ~1 more hp. This only tells us that the CAI does not provide substantially more mass of air than the stock unit. But the engine still may not be getting its full 1800cc if the TB is restricting the mass of air flow.

So now your baseline is 100hp and you change only the TB. If you noted a 10hp increase in that case, that would mean, mathmatically, that the engine inhaled 10% more air, the ECU measured this and injected 10% more fuel, resulting in a 10% increase in power. This is because there is a directly proportionate relationship between the amount of fuel burned and the torque produced. I don't know if anyone has reliably tested the gains provided by changing nothing but the TB and I don't know what gain to expect, but if you enlarged the TB and got 10hp I would do a dyno run with no filter at all and the straightest possible shot to the TB of cold air to see if it increased yet more. That would tell me whether or not the TB was now flowing more than the air filter was capable of.

If you got 2% out if a 5mm overbored TB I would be suprised...but that's a total guess. 1800cc is not very much air to flow. I would expect noticeably better throttle response though.

In the end, if you change something and there is no gain, it's because the mass of air being sucked into the engine didn't change. To get more power you need to burn more fuel. To burn more fuel, you need to ingest more air.

I ride an RC51 and a guy actually went through the process of proving that the stock air filters and air box already flowed more air than the engine could use by this reasoning. In the end all the added power people were feeling by modding the air box and buy $220 filters was just quicker throttle response.

[Mooneggs]

If you have a proper CAI then yes you can expect a gain, but with that you usually lose some peace-of-mind ie; you now have to dodge puddles etc.

not if you equip your CAI with a water bypass valve - I drove my klze in all weather conditions with this setup for over a year... the filter was practically on the ground

and as far as the big bore TB question - it's on my 2500cc klze but I agree I guess I would have to dyno to see a real result of a CAI + BBTB...

[fieromx3]

putting a CAI on a kl-ze though really makes it wake up in the high end... its DEFINITLY worth the extra money for the power gain for a ze

[wytbishop]

putting a CAI on a kl-ze though really makes it wake up in the high end... its DEFINITLY worth the extra money for the power gain for a ze

That makes sense. The larger displacement engine has basically a similar intake system as the other k series engines I would imagine. Some of the DE and ZE's probably come with larger TB's because of the larger displacement, but I bet they all have about the same size intake tube and filter arrangement. The intake represents a greater restriction on a larger displacement engine.

[shameem]

my 2 cents - maybe completely wrong ....

Well, opening up the intake/exhaust does a few things power wise
-moves the power band into higher RPMs
-adds a few HP
-reduces torque -drastically in the lower RPMs

A 1800 cc engine may not suck 1800 cc of air every stroke - infact no two cylinders suck air at the same time (to avoid charge robbing) - at any given instant only one cylinder sucks slightly less than 300 cc of air (less because fuel gets added too) - for one revolution of the crankshaft 3 cylinders will have sucked 3 times 300cc sequentially...

Opening up the intake actually reduces airflow speed which means the engine has to do more work to fill up the cylinder (in the given stroke time - determined by RPM) - the common example given in most automobile courses is big straw vs small straw - try sucking coke through a huge diameter straw and small diameter straw and you will see which one requires more effort. This effectively translates into reduced torque for the motor.
A huge straw will fill the mouth faster - same way with the engine - the cylinder fills up faster - which means HP gains at higher RPM when the time available to fill the cylinder gets shorter and shorter......

[wytbishop]

my 2 cents - maybe completely wrong ....

Well, opening up the intake/exhaust does a few things power wise
-moves the power band into higher RPMs
-adds a few HP
-reduces torque -drastically in the lower RPMs

A 1800 cc engine may not suck 1800 cc of air every stroke - infact no two cylinders suck air at the same time (to avoid charge robbing) - at any given instant only one cylinder sucks slightly less than 300 cc of air (less because fuel gets added too) - for one revolution of the crankshaft 3 cylinders will have sucked 3 times 300cc sequentially.........

As I said, "If the engine has a displacement of 1800cc, then every 2 revolutions of the crank will draw in that volume of air in the ideal, naturally aspirated case". After 2 revolutions every cylinder will have drawn in it's ~300cc of air in the perfect world. But you're right, a small bit of air is displaced by the volume of the fuel...I am idealizing.

Opening up the intake actually reduces airflow speed which means the engine has to do more work to fill up the cylinder (in the given stroke time - determined by RPM) - the common example given in most automobile courses is big straw vs small straw - try sucking coke through a huge diameter straw and small diameter straw and you will see which one requires more effort. This effectively translates into reduced torque for the motor.
A huge straw will fill the mouth faster - same way with the engine - the cylinder fills up faster - which means HP gains at higher RPM when the time available to fill the cylinder gets shorter and shorter......

This is a much more complicated issue.

When you're sucking water through a straw, gravity is acting on the fluid so the larger column of water in a big straw, having a greater mass, will require greater suction to elevate. This is not so much an issue with air in a horizontal tube. I would tend to disagree that the air velocity is appreciably slower. A large steel rotating mass sucking air through a straw is not going to be slowed considerably by a change in diameter of .5". Having said that, the engine would have to suck a very tiny bit harder to move a greater mass of air through a larger pipe if all other things were equal...but they're not.

Fluid flowing through a pipe experiences energy losses which have to be considered. First is called "minor losses" and this is the bends and inconsistancies in the path of the pipe. Starting at the front edge of the hood and then traveling to the air box and on through the intake pipe to the engine, the path of air through the stock system encounters many more minor losses than a common CAI. The other is predictably called "major losses". This is the energy lost due to friction between the air and the conduit. Again, the stock system with it's comparatively rough surface creates more major losses than a well made CAI with a smooth aluminum pipe. The CAI is larger in diameter and thus has more surface area, but I think this would be more than compensated for by the near perfectly smooth surface. I think the air speed would be higher.

I agree completely though that the cylinders will fill faster and that you will notice gains in the top end as a result.

these are just my thoughts.

[shameem]

http://auto.howstuffworks.com/question517.htm" onclick="window.open(this.href);return false;

[wytbishop]

I thought we were talking about CAI vs. stock airbox and piping. This page is talking about the IM. The intake runners it refers to are the ones that connect the plenum through the head to the valve opening. A very nice explanation of why the VRIS system exists and what it does.

While these physical concepts are theoretically applicable, they are not used in the airbox and associated piping in the same way as the tuning would go out the window when the throttle butterfly opens or closes.

[shameem]

For instance, it is beneficial to have the intake air moving as fast as possible into the cylinders. This increases the turbulence and mixes the fuel with the air better. One way to increase the air velocity is to use a smaller diameter intake runner. Since roughly the same volume of air enters the cylinder each cycle, if you pump that air through a smaller diameter pipe it will have to go faster.

http://www.team-integra.net/sections/ar ... icleID=471" onclick="window.open(this.href);return false;
http://www.ftlracing.com/tech/engine/intake.html" onclick="window.open(this.href);return false;
http://www.mne.psu.edu/me415/SPRING02/i ... ntake.html" onclick="window.open(this.href);return false;