How big does the air filter need to be?

[arrabil]

While The Deuce is down for suspension repairs, I want to do a couple other things to it. One of which is a snorkel. I want to use a K&N or equivalent filter because the stock airbox is already gone and the space will be used for dual batteries.

1) I have a design in mind using plumbing PVC and what I'm wondering is if this filter is "big" enough:

K&N RD-0720 3.5" round x 6" long

2) If I stuff the 3.5" filter in a 4"ID pipe would the 0.25" clearance be good enough?

3) Does 2"ID pipe flow enough CFMs for the 2.9L?

Thanks!

 

[MAKG]

The 1988-91 throttle body is almost exactly two inches.

Yes, 2" ID pipe is big enough.

If it worries you, use one slightly larger.

 

[krugford]

I assumed that the filter element would be around 1 inch thick or so. I took a mean filter diameter of 2.5 inches and found the surface area. Comes out to around 47 sq inches. Sounds about comparable to the flat square filter on my 4.0L if the tape measure inside my head is close.

As for the 1/4 inch clearance, you'll have about 3 sq inches of "inlet" area. But that inlet area will have higher losses because of the fact that the opening is only a quarter inch wide. Maybe leave the filter exposed and put a shield in front to protect against debris?

 

[Wicked_Sludge]

or get one of those filter covers, which will allow you to run the exposed element in the rain and water-splashed environments.

 

[arrabil]

The 1988-91 throttle body is almost exactly two inches.

I thought about that. But I also thought maybe the bigger pipe in front allows for the air to gain speed through the smaller throttle body. I can't say I remember my fluid dynamics very well.

If it worries you, use one slightly larger.

There is no 2.5" pvc that I've found and for me 3" is too big for putting outside the vehicle from an appearance standpoint.

As for the 1/4 inch clearance, you'll have about 3 sq inches of "inlet" area. But that inlet area will have higher losses because of the fact that the opening is only a quarter inch wide.

I don't understand what you mean by losses. The only important question is whether that 3sq" inlet area is big enough, no? Thinking about this a little more, my 3.0 air filter is pretty tight in the case but it is a cone. Maybe all I need to do to solve this problem is get a cone filter thats a little longer.

exposed element in the rain and water-splashed environments.

My preferred design has the filter in the engine compartment and 2" or 2.5" tube running out to the roof.