This article was first published in 1998.
One of AutoSpeed's most popular features is our tech story Heavy Breather. In it we describe the excellent gains that can often be had by adding a new intake duct to the standard airbox. But while doing that is all well and good, what if the standard airbox isn't up to much? If the box design is restrictive and the airfilter small, the new intake duct may not improve things!
So how you get around the restrictive box and filter? Airboxes can be swapped from car to car to improve intake airflow, while still retaining the advantage of having control over where the air is picked up. A filter box gives a completely enclosed system, so when it's properly ducted you can be sure that only cool air from outside of the engine bay is being inhaled. But which are the good boxes and which are the bad?
We borrowed no less than 20 different airboxes. But after a bit of thought, we didn't flow-test all twenty. Firstly we've excluded those boxes that use expensive, hard-to-get filters. There's no point in selecting a filter box if you're then forced to pay heaps each time you want a new filter! Secondly, generally the smaller the cross-sectional area of the flat paper filter element, the worse it will flow. So tiny filters and tiny filter boxes are not the way to go on any high powered engine. This excluded a number of boxes, however, we still retained a few small boxes for cars where there simply isn't the room for a large box.
The reason that you see so few tests like this one is that they are an absolute bastard to get together. For this test, Mark Marchesan at Exhaust Technology spent most of a morning making the adaptors that let us fit each box to the flowbench. Bill Hanson of Bill Hanson Engine Developments spent a full afternoon performing more than 30 flow tests, while no less than seven wreckers and workshops made available their airboxes to us. Obviously we use these guys because we think that they are damn good; if you need any of the services that these businesses provide, we can recommend each of them. Flow testing was carried out with the Superflo sucking through the boxes at a test pressure of 1 inch of water and the results are in cubic feet per minute (cfm).
In an ideal world we would have bought a three or four different filter elements for each box and tested them all for flow at the same time. However, our budget didn't extend to the A$2000 or so that this would have cost. This means that we tested most of the boxes with new filters and three of the boxes without filters.
It should be noted that the major determiner of flow isn't the filter element but the box design itself. As you'll see, well-designed airboxes have a flow only 10 per cent or so different with or without a filter anyway. To put it another way, we saw flow variations from box to box of 65 per cent - with exactly the same filter element being used in each of these boxes!
The box dimensions in the following descriptions are given Length x Width x Height. However, if the box is oddly-shaped (and many are!) this doesn't really convey in detail the box shape. Most of the time the dimensions include the exit and intake ducts and any mounting lugs. However, where these could be easily cut off to facilitate mounting, we haven't included them in the measurements. So regard the dimensions as a quick guide as to whether the boxes might fit in your car - for detailed sizing you really need to inspect the box in person. How the inlet and outlet ducts attach to the box will also determine whether they will be able to be used in a different car. For example, some lids can be turned through 180 degrees, but others can't.
The Flow Figures with Filters
The Flow Figures without Filters
Holden VN Commodore
This is an extremely "boxy" design, having near flat sides on each face. Its dimensions are 290 x 180 x 210mm. In early (ie VK-VL) Commodores the airflow meter bolts to the box while in later MAP-sensed models, a section of curved duct is attached to the airbox. Either way, the outlet is 76mm in diameter and a bolt-on internal bellmouth on the exit is used. The inlet to the box uses a rectangular opening 100 x 55mm with a push-in plastic duct leading to the back of the headlight. The filter has an area of 370 square cm and the two halves of the box are clipped together. The lid and body of this box cost only $20 each, both being new prices from Holden.
When the box was tested complete with a filter, the flow bench revealed that the push-in snorkel reduces airflow by 5 per cent! This means that simply pulling off the snorkel can generate better airflow. However, if you do this in a car make sure that the box then can't pick up hot air. Overall this box was about middle of the road for flow.
Box with snorkel and with filter: 82.5 cfm
Box without snorkel and with filter: 87 cfm
Conclusion: Cheap and readily available but only average for flow.
Holden VS V8 Commodore
The later model Holden airbox uses basically the same lower half and airfilter as the box described above. However, the top half of the airbox is revised with the exit duct coming out in a different direction to the earlier boxes. The outlet duct is also much better integrated into the top half of the box and doesn't use the bolted-on bellmouth. The box dimensions are 370 x 180 x 210mm and the outlet duct is 76mm in diameter. The inlet uses a large over-radiator snorkel with the inlet opening at the box itself much larger than the earlier design. Basically, the box has been cut away to accept the larger inlet duct (this can be easily done to an earlier design box too). The filter has an area of 370 square cm and the two halves of the box clip together. The curved lid costs $33 and the bottom section $20. The snorkel assembly costs $42 and comes in two pieces. These are all new prices from Holden.
This box flows very well indeed. With the over-radiator snorkel and filter in place it came in at third spot with a flow of 107 cfm. However, to really make it flow we removed the complete snorkel assembly. The flow rose by no less than 21 per cent to 129 cfm! The latter is the highest figure recorded by any of the airboxes with a filter in place. We were intrigued by the change in flow and then tested the box with just the section of duct at the box-end left in place (ie we removed the radiator part of the snorkel). Tested like this, flow was 112.5 cfm, up by only 5 per cent. So the most restrictive part of the intake duct is the section that attaches to the box itself - and when you look at it closely, you can see why!
Box with complete snorkel and with filter: 107 cfm
Box with half snorkel and with filter: 112.5 cfm
Box without snorkel and with filter: 129 cfm
Conclusion: Fit a very large good quality intake to the VS V8 box and you'll have an excellent airbox indeed. At about $15 each (Ryco), the filters are cheap too.
Nissan R31 Skyline
The box uses the same filter design as the Holden boxes above, making filters very cheap. However, inlet and outlet ducts of the box are fairly small - an inlet in the upper box half of 100 x 35mm and an outlet pipe in the lower box half just 63mm in diameter. The airflow meter normally bolts straight to the airbox, with a bolt-on bellmouth situated within the box. The box is 310 x 190 x 230mm and has a lower curved section to match the wheel arch area of the inner guard. The filter has an area of 370 square cm and the two halves of the box clip together. The used price for one of these boxes is about $80.
This box flowed as poorly as we suspected it would. With the filter in place it recorded a flow of 78 cfm - second from bottom in the flow stakes.
Flow of box with filter: 78 cfm
Conclusion: If you have a Skyline with this box, replace it! And if you want a box for a swap, don't get this one....
Nissan Z31 300ZX
The Nissan box can use the same filter element as the boxes discussed above, although the filter is not an exact fit. The box is extremely compact, being only 320 x 200 x 130mm and is the only box tested that is made from steel rather than plastic. It uses an outlet duct that is 78mm in diameter, with the inlet duct 100 x 55mm. A plastic inlet duct containing an external bellmouth pushes into the box, however this reduces the inlet to just a 55mm round hole. Both the outlet and inlet are beautifully contoured into the box shape. Four bolts are used to hold the box together, which costs about $100 in used form.
The flow was a little disappointing with 85 cfm recorded with the short snorkel and filter in place. With the snorkel removed, flow improved by 6 per cent to 90 cfm, putting it amongst the middle runners for flow. While this flow figure is well down over the VS V8 Commodore box, it is still quite good considering the very small box dimensions.
Flow with snorkel and with filter: 85 cfm
Flow without snorkel and with filter: 90 cfm
Conclusion: Uses common and cheap filters in a compact box. Could be useful in some applications.
Ford AU Falcon
This box uses a very large filter having an area of no less than 510 square cm. The inlet to the box is in the lower half, with this duct having a diameter of 80mm. The outlet duct has a moulded-in plastic bellmouth but its outlet ID is only 58mm. However, the outlet assembly unbolts from the airbox revealing that the actual outlet hole can be as much as 80mm in diameter! The lid clips to the base that is heavily curved to match the inner wheel arch area. The box is 380 x 280 x 310mm and costs $58 for the lid and $65 for the tray (new prices from Ford). Clips are used to hold the box together.
The large size of the filter meant that this box flowed quite well, with a filtered cfm rating of 93. However, it's interesting to note that fitting the filter caused a substantial flow drop of 17 per cent.
Flow of box with filter: 93 cfm
Conclusion: Amongst the better airboxes for flow, but see below for how to make a dramatic improvement!
Ford EF Falcon
This box uses the same lower half as the AU Falcon box mentioned above, however the EF lid has a much better integrated exit duct. This duct starts with a well-designed 50mm bellmouth deep inside the lid that grows in diameter to 80mm as it leaves the box. In the photo above, the EF upper half is on the left and the AU on the right. The filter used is the same as the AU, with a 510 square cm area. The box is 380 x 280 x 310mm and the EF lid costs $64 from Ford.
This is a much better flowing design than the AU box. The flow with the filter fitted was 110 cfm, up over the AU box by a very substantial 18 per cent! Even more importantly, the filter caused only a 3 per cent flow decrease, rather than the 17 per cent decrease that occurred when the same filter was fitted to the AU box. This is probably because the outlet duct draws from the centre of the filter rather than one end of the box, in addition to its bigger final diameter. It would therefore appear that swapping an EF lid onto your brand new AU Falcon can improve box airflow by 18 per cent!
Flow of box with filter: 110 cfm
Conclusion: The second-best flowing airbox after the de-snorkled VS Commodore. However, at $24.50 (Ryco), the filters are not as cheap as those for the Holden boxes.
Mazda 1992-1997 RX7 Turbo
A large flat box, the Mazda unit is 360 x 330 x 230mm. It uses a filter with an area of 500 square cm. The top-mounted inlet air duct is 110 x 40mm and is divided into two by a vertical separator panel. The outlet duct is 70mm in diameter and draws air from a recess below the main box body. No less than eight bolts are used to hold the box together. The box structure is strengthened with internal and external ribs and is very well made. Second hand, the box will cost you about $100.
Unfortunately we did not have a brand new filter available for this box so we tested it with a slightly soiled filter. Note that it wasonly slightly dirty, though. With this filter in the box, it flowed a disappointing 72 cfm, putting it into last spot of the boxes we tested with filters. Even without a filter the flow rose to only 111 cfm, well down over the 168 cfm recorded by the empty and de-snorkled VS design. At about $60 (Ryco), the filters are expensive, too.
Flow of box with slightly dirty filter: 72 cfm
Flow of box without filter: 111 cfm
Conclusion: Poor flowing with expensive filters. Don't use it.
Daihatsu G102 Charade 1.3
The Charade box is small and flat in shape. Its dimensions are 240 x 200 x 140 and its filter has an area of about 315cm square. An inlet snorkel is fitted that has an odd-shaped opening the equivalent of about a 50mm round duct. The snorkel is riveted to the box but can be removed, creating an opening about 100 x 30mm. The outlet duct is 60mm in diameter. The plastic box clips together and costs about $50 in used form.
This box was tested without a filter but still recorded a low figure of 68.5 cfm - the lowest "no-filter" measurement of all of the boxes.
Flow of box without filter: 68.5
Conclusion: Forget it.
Honda 1.8 Integra 1990
At 360 x 150 x 230 the Honda box is relatively tall and narrow, allowing it to be fitted into squeezier engine bays. Unusually, it uses two intake ducts, one measuring 70 x 60mm and the other a round duct 65mm in diameter. The outlet duct is 75mm in diameter. The filter has an area of about 360cm square. The box uses elegant 'over-centre' clips to hold it together and has a second filter for a crankcase breather return mounted in the lid. This box costs about $100 secondhand.
When we saw the empty-box flow figure we wished we'd bought a filter! At 147 cfm this little box had the second highest "no filter" flow figure of all the boxes. Unless the air filter is very restrictive, we would suggest that this box would flow very well indeed.
Flow of box without filter: 147 cfm
Conclusion: We'd expect it to be very good even with a filter in place: the pick of the little boxes.
Mazda 626 FS
The Mazda box is another fairly tall and narrow at 320 x 200 x 200mm. It uses two round inlet ducts, one 65mm in diameter and the other 35mm. The two halves of the box clip together but the top half is hinged to the bottom half, meaning that clearance is needed on one side for it to be attached. The exit duct is about 70mm in diameter, although this could be easily opened-up to 81mm. No bellmouth is used on the sharp-edged exit and the filter has an area of about 350cm square. We were quoted a startling used price of $250 for this box.
With a no-filter flow figure of 92.5 cfm this box was in the 'just competent' league. However, the Integra's airbox flow figures were just so good that the Mazda could only be an also-ran in the small box stakes.
Flow of box without filter: 92.5
Conclusion: Probably OK but the Integra box is blistering!
WHAT TO BUY!
The VS V8 Commodore box is the best of those tested. Not only is it available at a bargain new price, but with a properly made cold air intake to the box, very good airflows will occur. It is also quite compact. Next is the EF Falcon box. Although much larger than the Holden design, it has widely-available filters and flows very well, even in dead-standard form. And for smaller cars, we would expect the Integra box to perform very well.
WHAT TO LOOK FOR!
If you don't live in Australia, recommending the Holden and Falcon boxes won't be much good to you. However, the extensive flow testing that we performed taught us what to look out for:
- A filter area at least 350 - 400 square cm;
- An outlet duct well integrated into the lid - no steps, flanges or bolted-on bellmouths, just a gentle sweep of the lid contours leading into the exit duct;
- The largest-possible intake area (two intakes, if possible);
- An outlet duct at least 75mm in diameter;
- A large diameter, smooth-interior intake snorkel lacking steps and sudden changes of direction.
If you pick a box with these characteristics, you can be fairly confident that it will flow very well.
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