Some discussions erupted on Facebook based on my previous post as well as my friend Nathan Parkes’ post regarding pre filter selection and size.

Let’s start by determining the CFM (Cubic Feet per Minute) rating we need for our 4.0l V6 engines. For the sake of keeping things simple, let’s assume that 4.0l displacement equals 244 Cubic Inches.

Summit Racing has a nifty calculator on their site, and plugging in cubic inches and max RPM. I used 5000, as that’s right around bottom of the red area on the tach, and I feel is maximum normal operating RPM. Sure, some of us wheel with the engine on the rev limiter the whole time, but I think a pre-filter is the least of your concern at that point.

So, we’re looking at 300 to 388 CFM based on street/racing.
Whoa, hold on, you say, that’s carburetor CFM rating! I’m fuel injected, and I have a lead foot! OK, fine. Let’s look at Donaldson’s preferred method of determining CFM.

Fair enough. Let’s use that formula in Excel, and run it with our numbers.

Brings us to 317 CFM. Let’s use that.

Now let’s see at Donaldson’s offerings on filters that fit on our snorkel housings. I have a PRG housing, which appears to be the same as the originals. My housing has a 3″ OD (Outside Diameter) tube on top.

On the list for the Full View filters, which is what we’re looking at, there are only 3 pre-filters that meet the 317 CFM requirement, the H000820, H000821 and H000858.
All the other ones, 7″ and under are by CFM rating too small, and provide insufficient air to our engines, potentially resulting in loss of performance.
All of these have a 10.5″ bowl diameter, the differences are the A dimensions, which are the intake tube dimensions, in our case, matching the Snorkel Tube.

EvilFJ is equipped with the H000858 tube, merely due to me not measuring but guessing the snorkel tube on my truck, which is why I went to HD Overland in town here and picked up a rubber 3″ pipe coupler, which turns out to be a snug fit in the Donaldson 4″ housing.
So I trimmed the excess rubber off, and used the supplied clamp to clamp it to my tube, and voila! A fishbowl snorkel head.

I don’t have any advice on where to get the pre-filters for the best price, I got mine from Amazon shipped Prime, so I probably paid a few dollars more than other vendors, but I like my guaranteed 2nd day shipping and tracking.

Now, I can see the comments coming, “I’ve ran a 7″ for years without problems” and “My snorkel came with a 7” and I invented MOAB with my truck,  and even “If the girl is on top, is gravity effective as birth control?” to which my answer is simple:
“Above is the math and the specs per Donaldson. That’s what I go by for Pre-filter selection, and you probably should too. But as usual, your mileage may vary! Oh, and gravity is a treacherous mistress – beware!