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Why Buy A K&N Filter?
By Bill Watson

Restrictions to airflow cost you power and efficiency. When air is on its way into the engine of a Volvo 700 series, there are a number of items that cause restriction, thereby reducing airflow. Obviously, the first efforts for the tuner should be directed to the highest restrictions, but as we'll see below, many people just guess where the restrictions lie and throw money towards fixing what are in fact the smallest restrictions (the excellent OEM paper filter, or the OEM throttle body).

Many may not care how this testing was conducted, so let's cut to the chase. Here, following the natural flowpath that air takes to get to the intake manifold, are the pressure losses on my 1987 B230FT 740 at 5500 rpm and 10 psi of boost:

  • Airbox snorkel (ie, from the "outside" to the "dirty" side of your filter) 5.0 inches of water
  • Paper air filter (just the loss across the filter element) 1.0 inches of water
  • Air filter lid & AMM elbow (ie, from the "dirty side of the filter to the AMM) 9.0 inches of water
  • AMM (loss across the Air Mass Meter) 5.5 inches of water
  • Pipes to compressor, and from compressor to intercooler 0.0 inches of water
  • Intercooler 38.0 inches of water
  • Pipe from intercooler to throttle body, including 90 degree bend 0.0 inches of water
  • Throttle body 0.5 inches of water

In a search for more airflow (more power), where would you concentrate your efforts? Obviously the intercooler is the first place but it is an expensive proposition. Short of buying an aftermarket intercooler, one option that Turbobricks founding member Philip Bradley has considered is stacking two factory intercooler cores and building custom end-tanks for the pair. The resulting double thickness I/C is a promising, moderate-cost idea. This would also increase the thermal efficiency as airflow would be halved per intercooler. For details of the Volvo factory intercooler thermal and pressure efficiencies and a general technical discussion, click here. If you're accustomed to pressure in psi rather than "inches of water", then click here.

Where is the LAST place I'd recommend you spend your money? Replacing the piping, the paper air filter or the throttle body. And remember, your replacement filter or TB would not have 0.0 inches of water pressure across them, so you're looking at one of the most expensive bang-for-the-buck mods that I can see.

Below, you'll notice I haven't replaced any of these items on my car. In case you think I'm simply afraid of fabricating hardware, you can click here to see some of my homebrew intake and exhaust manifolds, fuel tank, turbo systems, and exhaust systems. This is where my time is presently invested. Someday I'll address these and update this article with results. OK, rant over.

Besides an expensive I/C upgrade, the next place to hunt would be the air filter lid and Air Mass Meter (AMM) elbow, (some fabrication in order here) followed by the AMM. Looking at the AMM, I'd have to estimate that the two screens are 90% of the pressure loss, so I'd consider removing the downstream screen as that should be on the safe side in terms of hurting the delicate hot-wire element. Philip Bradley mentioned that replacement AMMs don't even have a downstream screen if this makes you more comfortable with the thought of removal. I'd estimate its removal would drop the restriction from 5.5 to around 3.0 inches of water.

Even though this is a 700 article, longtime Turbobricks member and Volvo racer Anthony Hyde did some pressure drop measurements on his 200 series car that has the earlier "mechanical flapper" CIS Bosch K-Jetronic mass air meter. These are significantly more restrictive and I believe he found the pressure drop to exceed 30 inches of water!

The next modification in line is the 5.0" loss from the factory inlet snorkel. This loss is commonly alleviated by adding additional inlet holes to the 'dirty' side of the air filter. I've seen a number of sites that show how they made their modifications (try here and here for starters). Keep in mind that cold air is your goal, so obtain your additional inlet paths from the front of the car. In theory, every ten degree F rise in inlet air temp is the same as adding FOUR inches of pressure restriction! As you can tell, cold air is a very high priority.

All that's left is the previously-mentioned air filter and throttle body. I would suggest that modifying these components are very expensive HP gains in themselves.

  1. The advantage to a cone-shaped K&N filter here is that if you don't like fabricating, this would allow you to remove the fist three restrictions listed in the table above, which total 15 inches of water!! If you design a shield so that your new setup does not start breathing underhood air, this should show a nice seat-of-the-pants gain. I'm not a believer that the K&N actually filters out dirt better though than Volvo's OEM filter, (separate rant and my personal opinion) hence I will try to build something that utilizes a factory paper filter.
  2. Why do people who install larger throttle bodies say their car feels faster? Because the larger plate, opened at the same throttle position (angle) is a larger leakpath for air to get in the engine. So it feels faster for the same motion of your right foot. But it's the exact same as just moving your foot more with the factory parts - and wide open, we've shown above that the factory TB is fine. One other reason I don't like the larger TB as a solution on an automatic car; you've now changed the relationship between engine output and throttle plate angle, and your tranny shift points and internal pressures are all programmed off the factory relationship. You increase your opportunity to have too low of main pressures in the transmission during shifts, possibly increasing slip during clutch pack engagement and earlier failure of the clutches. "Adjust your tranny cable accordingly" is all I can say - as your motion ratios will always be wrong now but I'm sure some additional preload will get you by.

These are small pressure drops relative to the pressures being measured. The tool of choice here is a "Delta P" gauge, which has two pressure nipples. You hook up pressure lines from both sides of the element you want to test, and the gauge shows the difference between the two pressures. Here are some photos:

the delta-P gauge

Snorkel-loss pressure tap

Intercooler tap

AMM tap

Air filter loss taps

Hopefully this information will help direct efforts and monies in the most efficient direction.

- Bill Watson


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