Originally Posted By: Jim Allen
One thing I'll add is that the K&N gauge is calibrated (as you can see in gaijunnv's pic) to go "red" at 10 inches of water. That is the spec K&N uses for their filters because they are low restriction and high DP tends to start pulling dirt thru. If you have an ordinary cellulose filter, you want to use 15" as a the "red" spec because they have inherently higher DP (differential pressure) and don't flow quite as well. If you used a 10" filter with paper, it might trip too early. A 10" gauge might be a good idea for the performance oriented driver who wants his WOT power at 100% at all times, but you can buy gauges that will trip red at 15" but will indicate actual restriction and then all you have to do is service at 10".
General things to keep in mind RE restriction gauges:
-They come with different settings for different applications and you want the right one for your engine.
-Some engines will have higher initial readings with a new filter than others. That's why you get your baseline with a new filter and go out to "set" the gauge with a couple of WOT runs to rated max revs. Some cars will barely move the gauge, others will show a reading and some will be higher than others due to inherent intake restriction (which could be the plumbing as much as the filter). That is n't necessarily anything to sweat, especially if you are not performance oriented, and rest assured the engine designers know about it and it's factored into their design.
-For fuel injected gas engines, you generally want one that goes red at 15" of water (about 3.7 kPa).
-For light duty diesel engines, you want a gauge that goes red at about 25" of water (6.2 kPa). Some HD engines go to 30 " (7.4 kPa).
-Get on that has a scale so you can follow the restriction as it increases. Try to get one that has as many graduations below your max number as possible and with as low a lower number as possible. This is more for informational purposes than anything... it isn't strictly necessary, it just is geeky fun to track it.
-Restriction under 15" doesn't decrease fuel economy in an EFI engine, but I have been told by experts that if you are an economy minded diesel owner, air filter service at 20" can result in slight to moderate fuel economy gains. Diesels are more sensitive to restriction, moreso older mechanically injected diesels.
-I have been told that restriction gauges on carbureted gas engines do not provide reliable readings but I have some cause to doubt that... but I won't go into it now. trying to get it lined out. I have a gauge on a carbureted engine that came with one from the factory (a gasoline powered John Deere combine).
All good points, Jim.
I originally had the Wix gauge rrounds cited in his post. It has a maximum reading of 25" H2O. When I did my WOT runs to establish a baseline, it still indicated no restriction.
When I substituted the K&N gauge which has a maximum reading of only 10" H2O, however, after my WOT runs it indicated a restriction of a little less than 50% (5" H2O).
Chalk me up firmly on the "geeky fun" side of this one. I am looking forward to noticing the first indication of restrriction on my gauge, and I know I'll see it a lot sooner with the K&N gauge than the Wix gauge.
Since I still have the Wix gauge, I could always plug in the Wix gauge after the K&N gauge indicates 10" H20 so I could watch it climb to 15" H20.
Or ... I could install BOTH gauges on my filter box so I could watch their indications increase sequentially
Now that would be geeky fun