Engine Stock Air Box Stroked 351/383


15 Year Member
Jun 10, 2008
New Bedford, MA
My engine is a 351 stroked to a 383 90mm Throttle body 90mm MAF

Right now i have a 90mm intake going to a 4" long K&N filter. I know it picks up alot of heat and unnecessary dirt from the engine compartment.

The stock box has an outlet that is quite large and can be modified to fit. If i opened it up inside do you guys think it would work well?

I don't like the idea of my filter in the fender and most CAI arent 90mm/4". I also dont like the idea of an exposed 1ft filter.

Has anyone tried it?

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I did it a long time ago. I did managed to get a Pro-M 77mm in there with just a little cutting. I'm not sure if that helps you or not.


Did you lose power??? I wish someone out there flow tested a modified or stock airbox to at least get an idea of CFM. They make aftermarket air boxes but for the price i would build my own. But i'm trying to go for the stock look and maintenance.
ok i figured out the 90mm will fit in if i do a bunch of cutting. I also did some measurements and found that the "RU-2590" K&N should fit in place of the stock filter but still allow plenty of air flow around the filter and also has a 4" inlet. I think i can keep 4" diameter flow throughout the whole thing. The rubber inlet on the box is going to get cut back on an angle about 2" to open it up there too.

I'm sure when people see my stock assembly they will instantly notify me that i need to upgrade it.

Ford invests alot of time and money designing these boxes....why not use them if you can right???
I don't think there will be any restriction in the filter. Most if the restriction comes from the factory rubber elbow. I doubt it's enough to make any difference.

I remember doing it with the factory replacement K&N filter without any issues. All I had to do was cut out the outlet side a little where the MAF was too wide and too long. The outlet on the MAF stuck into the rubber pipe a little. It worked fine.