It's not the CAI vs. FWI that's the issue. The other topic was regarding a 3.5" FWI. If you want a FWI, go 3". It'll have to be custom, of course.

And what TB/Top are you talking about swapping? How can you accomodate more flow with a swapped TB when your car doesn't flow the air to begin with? What I'm getting at is your natural flow numbers. Your TB has a 2.25" throttle plate, and your runners are designed to flow a specific amount based on your bore and stroke (displacement) and the RPM's your motor is expected to turn at, as well as the lift and duration of your valves, head geometry, etc.

A good comparison would be L27-L36. The bore and stroke is the same, but the L36 rev's and shifts at higher RPM's, increasing peak flow at the shift or redline over that of an L27. This is why the LIM, UIM, and TB on the L36 flow more than an L27.

Unlike the Gen2-Gen3 swap from the 91-93 L67's, I'm not familiar with a directly swappable TB for the L27 that would allow you to do this, and then your UIM and LIM are still a restriction, not to mention the stock cam and 1.6 ratio rockers.

Even with 1.8 ratio rockers on an L27, I'd think a 3" CAI or FWI would be more than sufficient to supply you. I'd be thinking about PEM's long before going with a bigger TB.

The problem lies in producing laminar flow. Getting the air moving in a uniform direction and laminar manner increases velocity, which is what you need to feed your engine efficiently, and be able to read the volume of air passing the MAF sensor accurately. A 3.5" intake could actually hurt you, as it can flow far more air than your engine can demand. Some of that air is going to 'mill around' looking for a place to go, changing direction, and creating eddy currents as it necks down from 3.5" to the 2.25" throttle plate. As the air changes direction, your MAF sensor loses it's ability to read the flow accurately, throwing off your air/fuel ratio.

By properly sizing (no, I'm not aware of any magic calculators or flow numbers to nail this for you, but common sense and logic can get you close) your intake, you can actually get more flow through a smaller properly-selected size than you would from a larger one.

Too small=bad
Too large=bad

But there's a little wiggle room for X=good somewhere between them.

I recall a FWI someone here put together last year. He didn't have a 3.5" flange on his filter, so he used a 3" filter, stepped up to a 3.5" tube, then necked back down to the TB.

Would your garden hose (say half inch inside diameter) flow any better if you inserted a section somewhere in between the ends that was 3/4" inside diameter, then necked it back down? No. In fact, it'll probably flow slightly less because of the turbulence created at each diameter change.

I'm not saying this is what you're intending or asking about, but it's a better way to visualize a diameter change in an intake. Your really want no changes in ID (inside diameter) until you get to what you can't control.......the neck down transition inside your TB from the bore to the Throttle Plate.

Hang on a few, I'm going to crunch some ratio's that may help you decide.......

Let's say an L27 flows about 335 cfm @ 5000 rpm's.
For comparison, let’s throw in a 94/95 L67 (chosen because I have the flow data on the Gen3 M62 handy, and not the Gen2). The L67 will flow roughly 40% more at 5000 rpm’s. (Someone please check my math on that 335cfm please, I’m thinking way ahead of my typing…..number accuracy isn’t guaranteed, but these are good for comparison for the time being).

*****Important thought to keep in mind*****
The M90 doesn’t actually flow more than an M62 in our application. Our bore and stroke as well as compression ratio’s amongst the various L67’s are the same, and the M62 simply spins faster to flow the same amount per stroke as the slower-spinning M90 due to the diameter difference in the rotors. The M90 is more efficient in moving the same amount of air the Series 1 moves, making the boost charge a lower temperature. Comparing stock-stock, the Gen3-equipped M62 Series 1 and the Gen3 equipped M90 Series 2 will flow the same amount of air at the same RPM.

Back to our regularly scheduled programming…….

So we have a 40% flow difference between the L27 and the Gen3 M62 equipped L67 at 5000 rpm’s. The throttle plates are also different. The Gen3 M62 TB on the 94/95 L67’s is about 25% larger than the L27. Not 40%, but it doesn’t need to be if we’re forcing the air through instead of ‘naturally aspirating’ it. There is a downside to this (heat) but that’s another topic altogether.

Now let’s step back a bit and discuss ‘what works’ based on the experiences of our members. Back in the day, ADTR calculated the ‘ideal’ intake pipe diameter at 3”. This was based on what MOST of us were flowing. A lot fewer of us were modified very much if at all 4 or 5 years ago. I ran that 3” intake all the way through last season when I discovered I was at the point where I was probably capable of out-flowing it. Knowing I was going to port my heads, change my rocker ratio, and a few other things, and knowing I was already flowing more than most mildly modded Series 2 L67’s, I stepped up to the 3.5” FWI based also partially on INTENSE’s research. If it’s a good intake size for a modded S2 L67, and I flow as much or more than some of them, it should be a good size for me. OldBlueEyes also did some back-to-back comparisons with a 3” CAI (good quality) and a 3.5” FWI while running a 3.4” pulley on a S2. He was actually able to feel the improvement of the 3.5” FWI.

If we can A$$-U-ME based on our collective experience and that of some of our vendors that the L67 market is well-served by a 3.5” intake, and we know the flow differences between the L27 and L67 (stated above) we can draw some rough conclusions and see what comes out.

Using simple algebra to see what the ‘suggested’ pipe diameter at the L27’s 335 CFM flow would be if the 450CFM pipe is 3.5”, I came up with 2.6”. Don’t go to the bank on this just yet, it’s just the first step in comparing them and to prove a point.

Now let’s look at the AREA (determining flow capability) of a 3.5” tube:
9.62 is the answer. Keep in mind the Area is the important calculation here, not the diameter of the tube due to the pi function……as a comparison, a 3.5” tube is only 16% larger than a 3” when comparing diameter, but not when you correctly compare area, which is what we’re interested in).

For a 3” tube, the area is 7.07.

Flow difference? 36%. Pretty close to that 40% number right?

We’ve obviously not taken into account the VE numbers or anything else like that, and I don’t have the time right now to go into it, but the whole point of this exercise was to come up with some kind of guide for determining the properly sized intake for an L27. There’s a lot of different ways to do this, but based on what I just ran through quickly, it looks like our ‘standard’ suggestion of 3” CAI or FWI for basically stock or mildly modded cars is about right. Even stock Series 1 L67’s (particularly 91-93) are probably very well suited for the 3”. There could be some gray area here, and calculating where the L36 falls is another question as well, although we know they flow somewhere between the L27 and L67 at WOT. Go 3.25” on those. (joke there, not going into that discussion today).

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