I have a Safari Snorkel on my Montero. I like the added measure of fording safety it gives me, but it takes in more dust than I thought it would. After only a day or two on the trail, I’m already blowing dust out of the air filter. To remedy that, I’ve decided to get a pre-cleaner.

A pre-cleaner replaces the air scoop that sits on top of the snorkel inlet. Once installed, it draws incoming air into a vortex, the centrifugal force of which causes the heavier-than-air particulates to fly out towards the sides of the bowl, where they’re trapped. According to my research this will keep 80–85% 75% of the particulate matter from ever reaching the engine air filter.

Pre-cleaners are rated at a maximum cfm (cubic feet per minute) of air flow. Make sure you get one that matches the needs of your engine. Too large a pre-cleaner won’t spin the air fast enough to remove particulates. Too small and it will restrict your engine’s air flow. And, of course, your engine requires more air at higher rpms.

For most trucks, a 7″ pre-cleaner makes sense. To check if that’s too restrictive at the high rpms typical of highway speeds, I’ll add a formula for calculating pre-cleaner size after the jump.

Donaldson looks like the company that manufacturers the quality pre-cleaners that other companies sometimes rebrand. I’ll include a link below that gives the specs on their line of pre-cleaners.

Update 3/3/2014:
The part number for the 7″  Donaldson pre-cleaner that fits a 3″ opening (the size of my Safari Snorkel inlet) is H001249.

Here are some local, Bay-Area sources to check for availability. I ordered mine from Opperman & Sons and they quoted me just over $30, less than half of the listed prices I found online.

Opperman & Sons, 707-433-4421
Healdsburg, CA

TEC Equipment Inc, 510-715-4358
Oakland, CA

Golden Gate Trucks, 510-632-3535
Oakland, CA

Links:
Donaldson Pre-cleaner PDF, see page 12

Donaldson Pre-cleaners from Metro Fuel Injection

ARB Pre-cleaner at 4WD Hardware
Safari Snorkel Pre-Cleaner Thread on Expo

Here’s a clip from the Expo thread I referenced in the Links which explains how to figure out your engine’s cfm requirements. Compare this to the cfm ratings on the Donaldson Pre-cleaner page, and select the appropriate pre-cleaner.

This is from Expo user TeriAnn. In the formula CID is, of course, cubic inches of displacement.

Prefilters have a minimum air flow needed to work properly and a maximum air flow beyond which the prefilter starts to restrict the flow. The trick is to look at the RPM range you actually use then pick a prefilter that can handle that air flow.

How do you do that?

simple if you use this handy dandy formula:

cfmFormula

Now, my 302 engine is at 2650 RPM at 65 MPH and almost never sees more than 3000 RPM at shift points.

So running the numbers, the 302 pumps 131 CFM @ 2000 RPM and 197 CFM @ 3000 RPM. Looking at the specs for Donaldson top spin prefilters, the H002425 has an operating range of 90 CFM to 200 CFM, and fits a 3″ dia snorkel tube & has a 7 inch dia body. That would basically cover the 302 V8 from low RPM rock crawling through 75 MPH without restricting air flow.

Those larger 10″ diameter prefilters some people mentioned don’t work properly below 200 CFM (At least the Donaldson ones). I’d have the keep my V8’s revs above 3200 PM just to get the prefilter to start working properly.