I looked at that style also in post #68 but they are only rated at 450cfm and the inline filters that go into the 4inch pipe are rated at 575cfm.The 5inch were a heavier wall around .110 compared to most pipes which are .07 to .08
Even the 5inch od with the large filters are rated at 600cfm but they are heavier yet and for 25cfm I liked the 4inch pipe setup best.
Something like Carls front filter system for the supercharger would be a nice setup and it had dual outlets
and then would of wanted to just run the regular pipes less filters.
A more interesting setup would be John Kuhn's front snout system tied into what I have now.

Yea but how much CFM does the 928 induction system really need? The paper filter cannot be any more than just one of those hooked up to the system let alone 2 providing 1,125 CFM to the MAF.

Some may want to run a supercharger,some may be running stroker motors and look at Carls think it may be rated around 1300cfm so best to build a system that not only feeds NA engines.

I don't think I'll ever go the supercharger route later on; I'll do mine using the inline airboxes and post later on when finished. I am looking forward to the project.

Will look forward to seeing it also.

Oh and by the way, how does it run after the new intake setup?

FWIW, the Spectre 450cfm that Mongo posted uses .080 thick aluminum. I'm measuring one in my hand right now.

Regarding stock vs cone filter:
The stock air filter surface is roughly 90 square inches (19x5),
the 450cfm cone filter is 37 square inches (X2 = 74),
the 575cfm cone filter is 55 square inches (X2 = 110)

I wouldn't compare surface area "apples to apples" though. The filter and filter box designs are quite a bit different.
Here's my "shade tree" look at it:
The stock filter box functions as a pressurized "plenum" in which vacuum from beyond the MAF draws air from it. The only air velocity into the MAF is created by this vacuum pressure. The benefit has already been stated that it creates a very laminar flow.

The cone filter is more of a "pass thru" design, where air, at whatever velocity it enters the filter, leave at a similar velocity (faster/slower depending on the design). The benefit here is you get more of an induction effect, where the air is "charged" as it hits vacuum from beyond the MAF. More velocity. The trick here is finding the right geometry to control laminar flow.

I wouldn't base much on the CFM ratings of the Spectre filters. Only use it on a relative basis, assuming their test did as such.

I'm not sure if flow test have already been done with the stock filter box. If yes, we could get close if we assumed they used 0 static pressure for the test. And then make the same assumption for the Spectre filters.
That would be "apples to apples", but limited.
The only way to get a "real world" CFM of stock vs other is to flow bench test both and use the same amount of vacuum/static pressure (most benefical to use 928 engine vacuum pressure).

Here's a neat look at flow from a BMC CDA filter:
This is a a differnent way of using a cone as the cone is not the filter, but the idea is similar in that it redirects flow (with efficient losses). Then there's a velocity stack to collect air and smooth flow.

I think this is a hybrid of a pressurized "plenum" with objects which maintain or increase velocity.

This is a good design.

90 square inches is pretty big, but the size and volume of the airbox could possibly affect it's airflow???


EDIT: Just saw your illustration of the BMC. No wonder those things aren't cheap. Lots of thought when into their design.

Somethign else to keep in mind:
Porsche used virtually the same airbox throughout the entire 928 production run. 4.5L - 5.4L. Others are using it on strokers (6.4L).

It's a good airbox with more than enough capacity.

I think the real trick towards a better air filter design is increased velocity while maintaining laminar flow.

Thing with the clear cone filter housing is you might be able to run WOT pull using a shop vac on the engine side to simulate engine suction and flow some smoke into intake setup.

Wonder if the Weissach guys ever made clear intakes and do some smoke flow runs.
Or did they have powerful enough CFD rigs to test the intakes back in the 70's.

I gave one of the reps the rpm (6200-6500)and 302 cubic inch of motor and he told me better off going to the 575cfm in the 4inch pipe than the Spectre 450cfm 4inch od with 3inch inch inlet and outlet so went with that 575cfm into the 4inch pipe setup instead.

The question is - it may have less area than the stock paper filter, but it's less restrictive too. And even with the stock paper filter, most of that area isn't used. If you notice when you change the filter element, (at least on mine) there's a small area that's . . . well, seen the most action, let's say, versus the rest which still looks pretty new.

Taking the Blackbird for example - I don't doubt that the cone filters THEY use have less surface area than the paper filter, but obviously there's something like an 11 hp gain, so they flow better. Even though they're probably K&N's, I wouldn't expect significantly better or worse performance out of any other brand of oiled cotton filter just because the name is different.

Here's an online AFM calculator

http://www.ajdesigner.com/phpengine/..._flow_rate.php

80% is a safe bet for volumetric efficiency, you can use 85% (.85) and be on the safer side. Both cones should be big enough, once they're doubled. According to the calculator, the engine only requires 568 cfm at 100% volumetric efficiency (or '1'), and you'll never hit that.

I posted the formula in post #70 but the system has to also feed some of the supercharged cars also which is alot more and you will go way past that cfm.
If the stock air box is so good then why did the blackbird system put out more horsepower?I still think it is the better ram effect look at the Panamera and other Porsche,Ferrari etc that are all using the short ram design.On the dirty filter area on the stock filter it looks to be more dirt over the MAF area as it may pull air thru harder there.The outter edges seem cleaner because air isnt pulled thru there as much thats only my guess but seems what my old air box looks like.With the short ram it is a direct shot.The stock airbox air is coming in from both sides into each other and the ram pipe is directing the air down to flow together as the air enters.If the stock air box was so good they would still be using the design today I feel the stock air box is 1970's technology.