So why is everyone so anti-CAI?

[16v of E30]

what.

x2

[dandingo]

1) square/polygonal tracts flow less than circular ones of equal crossectional area, because the perimeter (component of surface area in 3d) is higher relative to the area, and because the corners have deeper boundary layer, so the reduction of effective flow area by boundary layer is disproportionate to that of a circular tract. The airspeed difference between the center of a high velocity flowstream and the port's surface is huge, as are the pressure differentials.

2) 2 small holes of a given measurable area flow less than one big one, because the boundary layer reduces the effective flow area disproportionately, and the perimeter is longer than the single hole. This is also true re the shaft on a butterfly. The contraction and expansion when single tracts split to double, and merge back to single, robs more cfm on top of the area/perimeter equation. You cannot regain all that you lost.

3) flapper AFMs require the loss of the mass of air used (with velocity, total force then varies with air velocity) used to overcome the bloody spring. That air mass is gone, for your purposes, so it doesn't flow what the same area will if held open by a cable. Push it open with your finger, and think about how low air's mass per unit volume is (pretty light and smooshy) and figure out how much actual air is needed in cubic feet to weigh enough to hold it open, that is what the loss is like. That why they are called parasites, they steal flow without any gain to the program.

the AFM also has wake generating crap in it, it is very far from an idealized tract.

Above applies to tubular style tracts. Poppet valves (your intake and exhaust for instance), are a whole different flow problem, as far as ultimate limits they are all about perimeter and usable lift together, which is why 4 valve heads flow more than 2 valve heads with valves that have more area, that fit into the same chamber area. Perimeter is your friend at the valve.

You can make some guesses, but if you really want to know what your cork is, find a good flow analyst in your area and run a full evaluation, piece by piece, individually and as assembled groups. Be prepared to spend some substantial money. Then go verify the dynamics on a dyno.

making it all work at its best is a matter of controlling contractions and expansions, leading and trailing edges, and so on. Velocity changes cost CFM. the best ports aren't big, either, they are shaped to give the greatest flow for a given area, which yields the highest mean velocity. The whole system needs to provide for the projected needs of the engine. Having everything too big is merely wasted surface area.

Road car airboxes are mufflers alright, and they use energy,measurable as lost CFM, to do their muffling, they are parasites.

what.

[mdok]

A smarter question to ask would be where is the biggest restriction in my intake system and how can I eliminate it? Do a little research and figure it out.

http://www.autospeed.com/cms/A_0663/article.html

On the M42:
Intake muffler = 3.46"^2 (2.1" across)
AFM = 4"^2 (2"x2" square)
Throttle plates at WOT= 4.335"^2 (1.37" circle + 2.12" circle = 5"^2 surface area - .665"^2 for throttle shafts)
BMW didn't call the airbox and intake tube a "muffler" for nothing.
http://www.realoem.com/bmw/showparts...23&hg=13&fg=20

it is not quite so simple as that, because in your case

1) square/polygonal tracts flow less than circular ones of equal crossectional area, because the perimeter (component of surface area in 3d) is higher relative to the area, and because the corners have deeper boundary layer, so the reduction of effective flow area by boundary layer is disproportionate to that of a circular tract. The airspeed difference between the center of a high velocity flowstream and the port's surface is huge, as are the pressure differentials.

2) 2 small holes of a given measurable area flow less than one big one, because the boundary layer reduces the effective flow area disproportionately, and the perimeter is longer than the single hole. This is also true re the shaft on a butterfly. The contraction and expansion when single tracts split to double, and merge back to single, robs more cfm on top of the area/perimeter equation. You cannot regain all that you lost.

3) flapper AFMs require the loss of the mass of air used (with velocity, total force then varies with air velocity) used to overcome the bloody spring. That air mass is gone, for your purposes, so it doesn't flow what the same area will if held open by a cable. Push it open with your finger, and think about how low air's specific weight per unit volume (around .08lbs/cfm range at room temp) is (pretty light and smooshy) and figure out how much actual air is needed in cubic feet to weigh enough to hold it open, that is what the loss is like. That why they are called parasites, they steal flow without any gain to the program.

the AFM also has wake generating crap in it, it is very far from an idealized tract.

Above applies to tubular style tracts. Poppet valves (your intake and exhaust for instance), are a whole different flow problem, as far as ultimate limits they are all about perimeter and usable lift together, which is why 4 valve heads flow more than 2 valve heads with valves that have more area, that fit into the same chamber area. Perimeter is your friend at the valve.

You can make some guesses, but if you really want to know what your cork is, find a good flow analyst in your area and run a full evaluation, piece by piece, individually and as assembled groups. Be prepared to spend some substantial money. Then go verify the dynamics on a dyno.

making it all work at its best is a matter of controlling contractions and expansions, leading and trailing edges, and so on. Velocity changes cost CFM. the best ports aren't big, either, they are shaped to give the greatest flow for a given area, which yields the highest mean velocity. The whole system needs to provide for the projected needs of the engine. Having everything too big is merely wasted surface area.

Road car airboxes are mufflers alright, and they use energy,measurable as lost CFM, to do their muffling, they are parasites.

[DCColegrove]

Some threads...

Like this one for example...

Make my head hurt.

I suppose I should just stay out of it though.

[SpecM]

If you want more power just pony up for 93 octane at the pump...

what?

91/93 with a chip, 87 without

[E30 Wagen]

If you want more power just pony up for 93 octane at the pump... As has been said before the BMW engineers likely understand this better than all of us so the original design can hardly be improved upon.

Besides, it's your car, do what you want. Experiment and find out for yourself.

[tjts1]

Just wondering how much air do you think the engine actually needs or could even use without increasing the atmospheric pressure and just about everything else?

A smarter question to ask would be where is the biggest restriction in my intake system and how can I eliminate it? Do a little research and figure it out.

http://www.autospeed.com/cms/A_0663/article.html

On the M42:
Intake muffler = 3.46"^2 (2.1" across)
AFM = 4"^2 (2"x2" square)
Throttle plates at WOT= 4.335"^2 (1.37" circle + 2.12" circle = 5"^2 surface area - .665"^2 for throttle shafts)
BMW didn't call the airbox and intake tube a "muffler" for nothing.
http://www.realoem.com/bmw/showparts...23&hg=13&fg=20

[Need4Speed]

Ya except for the fact that the headlights and headlight grill are in the way... and the 2" wide muffler. Otherwise I guess you're technically right. You guys are really grasping at straws.

Just wondering how much air do you think the engine actually needs or could even use without increasing the atmospheric pressure and just about everything else?

[tjts1]

You do realize all those stock designs are different than the stock e30 design. That is why they work. the stock e30 already has a "forward facing" design.

Ya except for the fact that the headlights and headlight grill are in the way... and the 2" wide muffler. Otherwise I guess you're technically right. You guys are really grasping at straws.

[SpecRaceM5]

or turbo it.

qft!!! :-)

[moatilliatta]

I cant wait to take my AFM and punt it!

[nando]

I have two good friends with the miller kit and I've met a few others. Dissapointing is a good description, especially for the cost. It works pretty well as an AFM replacement (no glitches or anything), but it doesn't make much difference otherwise.

[delatlanta1281]

that's a piggy back controller, I'm talking about "bolt on" MAF kits that use a stock ECU with no mods (other than maybe a chip). ;)

and also, when you start getting close to $1000 there are quite a few very attractive standalone options that give you way more control than any piggyback controller..

What about miller?
Sorry nando. I couldn't help it. I think we need to discuss this on this thread now rather than later.

[nando]

safc2

that's a piggy back controller, I'm talking about "bolt on" MAF kits that use a stock ECU with no mods (other than maybe a chip). ;)

and also, when you start getting close to $1000 there are quite a few very attractive standalone options that give you way more control than any piggyback controller..

[delatlanta1281]

If properly designed and built forward facing CAI works better than stock.

http://autospeed.com/A_110824/cms/article.html

http://www.autospeed.com/cms/A_0663/article.html

http://www.mz3.net/articles/149.html

You do realize all those stock designs are different than the stock e30 design. That is why they work. the stock e30 already has a "forward facing" design.