Aurora 4.0 Northstar
K&N Filter &Air Box Mods

By Greg Garnes

I drive a 1998 Aurora, with the 4.0L Northstar engine and 4T80e transmission.  This article documents improvements to be gained by use of the K&N panel filter, details air box modifications I have performed, and documents improvement from modifying the stock air box.

Air Box Mods:  The modification to the air box was to remove the plastic insert in the bottom of the air box. I believe it is there for sound reduction. I then used a dremmel tool (wear goggles!) to cut the plastic hole in the bottom of the air box larger to match the existing hole in the metal under the air box. Next, I removed the inner plastic liner from the top of the air box (again this is for sound reduction). Then I just installed the air box with a K&N filter.

How I tested: The tests consisted of one with the stock air box (a spare) with a new paper filter, a second test with the stock air box with a new K&N filter, and a third test with the modified air box with the new K&N. A fourth run confirmed the results of the third run. The tests were on a chassis dyno, so the power is at the wheels. 


  • Stock air box with new paper filter - 189 HP, 204 ft-lbs torque (blue line below)

  • Stock airbox and paper filter vs. K&N drop-in panel filter:
    Stock versus K&N Panel filter

  • K&N panel filter alone:  +7 peak HP, +4 peak torque.  Gains of 3 to 4 HP.  (Red line above) throughout curve past 4,000 rpm.  Gains of up to 7 ft-lbs in the 5,300 to 5,760 rpm range.

  • Modified air box with K&N panel filter: +1 peak HP (at best), +3 ft-lbs peak torque.  Gains of 6 HP from 4,600 to 4,900 rpm, plus gains of 2 to 3 HP throughout much of the HP curve. Gains of up to 8 ft-lbs of torque from about 4,640 to 4870. Past 5200, no additional torque. (Ignore the Max torque reading on the upper right corner of the graph. For some reason, it seems to be using the HP graph against the torque curve.)

  • K&N Panel Filter vs. K&N Panel Filter and Airbox Mods Horsepower:
    K&N panel filter vs K&N panel filter & airbox mods

Total gain: +8 peak HP, +7 peak torque. The surprising bonus was how much the air box mods fattened up both curves and added considerable area under the curve. The total gain in the torque seems to be as much as +10 in that 4,640 to 4,870 range and as a result, also extended the peak torque (or near peak) out through this rpm range.

RPM through the gears -- Stock filter and airbox vs K&N Filter and Airbox mods Torque:

I highly recommend making these modifications to any Northstar engine. I believe the Aurora 4.0 (mine is a 98) and the Caddy 4.6 are virtually the same except that the 4.0 has a smaller bore. I also believe the layout under the hood to be very much the same. My experience is no extra noise/almost no noise for around town driving, and no extra noise on the highway. You get a nice throaty sound when punching it. My Aurora (when stock) pulls air from behind the front fender. Now I am pulling it from there and the area below the box - which turns out to be the same air as behind the fender. The air is separated from the engine compartment and is cool and fresh. In addition, I've run some tests with 2 different paper filters driving in the rain and have not yet found any considerable increase of wetness on the filter. I do carry an extra stock air box just in case for a long highway drive in a down pour, but the modified box will probably do just fine.

RPM through the gears -- Stock filter and airbox vs K&N Filter and Airbox mods.

I really like to think of having transformed the Aurora from a 250 HP, 260 torque car to 260 HP and 270 torque, with much fatter power curves - all with some really simple improvements.

Future Projects: the opening in the bottom of the box could be made much larger by cutting the metal.  Additional improvements would be
cutting and filing some cross bracing on the inside of the outer box (on one side they run opposite to the air flow and probably cause turbulence).  The sharp edge on the round hole exiting the box could be rounded off a little too.