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Discussion Starter #1 (Edited)
Over the years my fox has had the stock airbox with a K&N panel, a Denscharger CAI with a 9” K&N in the fender, and most recently an old school Moroso CAI elbow with the small 5” oval K&N clone Moroso supplies. I found punching numbers into an air filter sizing calculator ( Engine Air Filter Size Calculator - How To Find The Correct Size, Is Your Air Filter Too Small? - Street & Circuit ) to be eye opening. Cliff Notes: The factory panel filter is correctly sized, the small 5” oval filters on every CAI out there are undersized, and you need a 9” long K&N conical to get the surface area needed for a NA 5.0 @ 6000 rpm.

I was looking for something different and ran into Corral user Thundetjet302’s post about using a March Ram Air Kit’s airbox, enlarging the airbox opening to match the fender apron, and then not use the hose and scoop. You then have a 9” K&N with surface area close to the size of the factory panel filter, cold air from the fender apron, and an air box that is cleaner looking than the OEM unit. A little digging finds the March kit using a K&N 0810 filter (3.5” inlet ID, 9.0” long, 6.0” base, and 4 5/8” top) which I had in my spare parts cache from the Densecharger (AKA Home Depot CAI). You can still buy the whole March Ram Air package, but they will sell you all of the parts individually. This means you can source a brand new airbox ( March Performance - 1986-1993 5.0L Mustang Ram Air Box ), pick up one off of eBay, or your buddy that got tired of cleaning leaves and road debris out of it years ago.

The March airbox inlet is an oval of about 5.0” x 2 1/8”. I made a cardboard template of the factory apron opening and enlarged the March airbox to a 7 1/4” x 3 1/2” oval. This is very close to my apron opening which is now 8” x 3 1/2”. I used a Dremel with a Rotozip bit to cut through the 1/8” thick airbox and cleaned up the edges with a orbital sander using 80 grit sandpaper. The fender apron has seen a bit of Dremel cut-off wheel action and edge trim attached.

The 70mm Ford MAS mounts directly to the K&N and I needed a 3” x 4” hose to connect the MAS to my old school Tiny Avenger inlet elbow.

Thanks to Thunderjet302 for his posts about this mod. I hope the post helps somebody doing an old school NA EFI Windsor build.
 

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I bought the March kit years ago and added a screen to the scoop to keep leaves out.Moisture would sometimes make its way up to the filter so I removed the scoop and hose.
 

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Discussion Starter #6
Project Update: I removed my 70mm '93 Cobra MAS and installed an old school 77mm Pro-M meter. I modified the March airbox to accommodate the larger 3 stud air filter base of the Pro-M unit. These images were taken with the 6" Pro-M brand dry filter. I have a similar 6" K&N here and there were no differences in fitment. It looks like there may be ample room to put in something like a Anderson 8" Powerstack filter, but it doesn't seen to be worth the expense with my modest setup. Richard Holdener tested 77mm Pro-M's with an air filter on a Superflow and it was well over 1000 cfm @ 28".

MAS Sidebar:
I've been running running the X3Z/24#/70mm Cobra MAS setup forever. No issues. I noticed some old school aluminum 77mm Pro-M meters for sale and I thought I would be a cool retro part to have. So I picked up a 77mm shorty on eBay and sent it to Pro-M to be setup for a '93 Cobra X3Z/24# configuration. Six day turnaround including shipping both ways and ordered the 30 point transfer function sheet. I also now have a Moates F3V2 chip setup with the GUFB/X3Z up using TunerPro RT. The factory MAF curve and the Pro-M curve almost lay on top of one another. It was so close I've had the car out running without the chip and it runs great. Next step is to install the chip to have the Transfer Function 100% on point. I may get into a Quarterhorse, but we'll see.
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Discussion Starter #8
what does the maf signal voltage look like at idle and low rpm cruise?

you have a log?
I don’t have a Quarterhorse to do logging yet. Here is a screenshot from TunerPro RT with the 70mm Cobra meter in red and the 77mm Pro-M curve in black.
 

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I have a March box like that, also modified like that to get full airflow from the inner fender.

I use a 9" Airaid filter into 4" silicon elbow. The MAF is gone since switching to the Holley HP.

1058118
 

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^^^ Is that just a bit of 4" aluminum pipe between the airbox / filter and the silicone elbow, Cougar5.0? I was thinking I would need something like that whenever I get around to installing the Terminator X setup.
 

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Discussion Starter #13
I have a March box like that, also modified like that to get full airflow from the inner fender.
I use a 9" Airaid filter into 4" silicon elbow. The MAF is gone since switching to the Holley HP.
The March airbox certainly offers options in filter fitment and the opportunity to easily be modified. It also looks way better than the (functional, but ugly) factory air box.
How do you like your Holley HP?
 

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The March airbox certainly offers options in filter fitment and the opportunity to easily be modified. It also looks way better than the (functional, but ugly) factory air box.
How do you like your Holley HP?
HP is good, wideband feedback allowed me to tame the somewhat rowdy cam in my 427w.
 
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