Weber air intake

[Undead_Rider]

I have a weber 38/38 and i was wondering where i can get a air intake for a 73 at?

Something like this. Ans also is there any other modifications that you would have to do to install this?

[DavidLMM]

I don't know of anything ready-made for our cars, so you'll probably have to do some fabrication.

At your local parts house, ask to see the paper K&N catalog - they have a low-profile housing that's made for 2bbl Weber downdrafts, and wil accept a side-mounted hose.

Next, throw away that aluminum tube - it's COLD air you want, and aluminum is a heat conductor. You want plastic or some other lower conductive material. Then you'll get a good quality paper cone filter. Unless you _are_ racing and plan to re-ring annually, stay away from K&N - they are effective, but pass a lot of particulates. (google some of the SAE papers on air filters - you probably can't read them unless an SAE member, but there is a lot of other discussion online. And for every naysayer, K&N has a reason why "they" are wrong. YMMV, etc.)

Then, you'll need to isolate the filter from the rest of the engine compartment, again with a non-heat-conductive material. Fab it up out of cardboard, then find some sheet plastic that can tolerate oil and heat, and go to town. Don't forget some welting top and bottom for a good seal - any air leaks will minimize the benefits, and may whistle embarrasingly! Just remember, your goal is to provide cool air, not hot.

The above info about K&N is mine, from reading and seeing test results posted. It's my opinion; I'm not out to start a holy war over air filters, or oil, or brake pads, or anything else...

David

[Mike Self]

Take one stock BMW air cleaner, some tin snips/Dremel tool, closed cell foam and about fifteen minutes:

modify the air cleaner so it'll fit the Weber's opening, then take the foam and create a gasket that will seal between the carb and air cleaner. That will give you the advantage of a large surface paper filter, and cold air pulled in from outside the engine compartment.

The only drawback: doesn't look cool. But it works much better than the K&N style filter setups.

cheers

mike

[nbcbird]

Think about it. The Weber 38/38 is a downdraft carb, and the clearance between it and the hood is about 2 inches. It is almost impossible to try to fit a cold air induction type filter system (that you have pictured) on a 2002. I have never seen one one on a 2002 with a downdraft. You have to use the original assembly (as Mike mentioned) or buy an aftermarket filter kit from K&N. Lynx, ITG, etc.

[robsanab]

I've considered this air plenum some time ago, but haven't had the desire to try and fab whatever's needed... looks like you do though... try it and please let us know how it worked!

http://www.piercemanifolds.com/product_p/99217.090.htm

[DavidLMM]

That's very similar to the K&N airbox, maybe the same without paint and branding.

David

[eurotrash]

K&N has a dual snorkel too right?

[TobyB]

Don't put a long tube on the front of any Weber carb-

it wants free air, or at least a big (2l) plenum.

Unless you're tuning it for ram air effect...

t

[Swiftus]

Don't put a long tube on the front of any Weber carb-

it wants free air, or at least a big (2l) plenum.

Unless you're tuning it for ram air effect...

t

Well you COULD put a really BIG tube in front of a Weber Carb. It wouldn't 'know' that there was a tube delivering its less-hot air, but there would be one.

The bold is the important bit...

Whats the CFM of a 38/38 air at WOT at 6000 rpm in an '02?

The cfm of an engine can be estimated with CFM = rpm x displacement / 3456 x VE (volumetric efficiency of the engine)

In this case : CFM = 6000 x 2000cc / 3456 x 85% (lets be nice)

That makes the CFM of the carb right about 180 CFM

The air in a 3 inch tubular intake at this rate would be flowing at about:

576 x 180(CFM) / (pi x 3^2) = 3667 ft/min or 61 ft/s or 90 MPH!

That is REALLY FAST! (If my math is correct which I'm sure its not).

This actually isn't all that fast. Many racers who race with air restrictors have to work very hard to prevent buffeting to their air intake when the intake speeds go beyond supersonic...

If you were to enlarge the tube to 4 inches, the intake speed would drop to 50.5 mph, which is a lot better, considering that the flow rate is normally half that. (Most people don't drive their cars around at 6000 rpm all of the time)

The faster the air gets going, the more laminar its flow becomes. This can cause havoc on fuel delivery systems that rely on turbulent air to mix the fuel into a mixture.

So yes, an CAI may cause your carb to run less efficiently to some degree, but only at higher RPM. Most on this board would be sfe going with a 3 in pipe on a CAI IMHO.

HTH,

Jay

Also, feel free to tear apart my math all you like. I actually have no idea if its true or not, but it makes sense to me!

[TobyB]

Agreed.

The other thing to consider is that it's not constant airflow,

even in a combined- runner intake,

so the intake air will pulse.

Thus, it can/must be tuned.

Yup, a big pipe is a plenum. A small one is a

trumpet or tube, and will affect fuel delivery.

[khale2011]

Agreed.

The other thing to consider is that it's not constant airflow,

even in a combined- runner intake,

so the intake air will pulse.

Thus, it can/must be tuned.

Yup, a big pipe is a plenum. A small one is a

trumpet or tube, and will affect fuel delivery.

Very well said and explained Toby. Thank you for sharing your thoughts.

Anyway, for my air intake I'm using the AFE air intake system. Works great.

[ought2]

Any chance of posting a picture of your installation? TIA

[dmrice]

Wouldn't the numbers be halved since each cylinder only sucks in air every other revolution?

[TobyB]

In a 4- banger, one cylinder's always intaking, one's always exhausting,

one's compressing and one's expanding.

And 2 are in overlap for a good part of every rotation.

It's the 2 that are in overlap that get POODLED by (improperly) tuned

tubes.

Since one's always intaking, you can say that every revolution,

a 2002 motor inhales one liter. 6000 rpm = 6000 liters/ minute.

...fwiw....

t

[dmrice]

i found the formula used above with an internet search and the displacement used is in cubic inches so 121 should be used for the displacement instead of 2000, I think.