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Old 05-30-2022, 08:35 PM   #1 (permalink)
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Filter Size Effect on MPG

I have been experimenting with the intake on my LS Jeep. I had replaced a Spectere cone filter with a smaller K&N one to make it fit my application a little better and I looked at the logs of the 2 different filters, the larger filter required a bit more throttle than the smaller one. Going 55mph the Spectere would hang around 18-20% throttle and the smaller K&N seemed to be happy at 15% throttle.

Does a decrease in filter surface area actually help with economy or is K&N just that much better with airflow? I ordered a larger K&N to see what happens, but it just seems counter intuitive that the smaller filter had better results.

New smaller K&N filter
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Old larger Specter filter
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Old 05-30-2022, 08:47 PM   #2 (permalink)
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Do you have a way to see the airflow into the system? My first thought would be more restrictive meaning more throttle opening for the same speed, but the same airflow as less opening with a less restrictive filter.
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Old 05-30-2022, 08:50 PM   #3 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
Do you have a way to see the airflow into the system? My first thought would be more restrictive meaning more throttle opening for the same speed, but the same airflow as less opening with a less restrictive filter.
There is no MAF with my setup, but I do have MAP readings and the large filter had MAP reading of around 70 but barely dipped below that and the small filter had an MAP reading that would go from 65-70.
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Old 05-30-2022, 08:55 PM   #4 (permalink)
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I can see where a higher flow air filter would make a difference at higher loads, but at light cruise why would efficiency differ throttling with a filter vs a throttle plate?


Edit: unless maybe the ECU were to interpret the higher throttle position as demanding more fuel enrichment & less ignition timing even though actual power demand was equal?
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Old 05-30-2022, 09:01 PM   #5 (permalink)
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Quote:
Originally Posted by Drifter View Post
I can see where a higher flow air filter would make a difference at higher loads, but at light cruise why would efficiency differ throttling with a filter vs a throttle plate?


Edit: unless maybe the ECU were to interpret the higher throttle position as demanding more fuel enrichment & less ignition timing even though actual power demand was equal?
The timing stayed consistent at 36 degrees and target AFR stayed at 15.5.
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Old 05-30-2022, 09:21 PM   #6 (permalink)
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Quote:
Originally Posted by XCRN View Post
There is no MAF with my setup, but I do have MAP readings and the large filter had MAP reading of around 70 but barely dipped below that and the small filter had an MAP reading that would go from 65-70.
That sounds consistent with the throttle decrease. Did you see a fuel economy change with it or just the throttle position?
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Old 05-30-2022, 09:30 PM   #7 (permalink)
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Quote:
Originally Posted by M_a_t_t View Post
That sounds consistent with the throttle decrease. Did you see a fuel economy change with it or just the throttle position?
So far just throttle change and ease of keeping speed up with a few drives so far. Hopefully as I get closer to end of tank ill see what the results were.

Playing with the intake has helped more than I thought so this might lead to something too. I use to have just an elbow with the cone filter sitting on top of the radiator, and for the last year I got a very consistent 13.5mpg regardless of how I drove. I got a CAI enclosure and some aluminum tube to move the filter away from the hot engine bay and on a 200mile cruise through country roads and some small cities it went up to 16mpg.
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Old 05-30-2022, 09:49 PM   #8 (permalink)
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I wouldn't put a k&n air filter on a lawn mower.
With a MAP fuel injection system you probably tricked it into running leaner with a more free flowing intake, at least temporarily.
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Old 05-30-2022, 09:52 PM   #9 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
I wouldn't put a k&n air filter on a lawn mower.
With a MAP fuel injection system you probably tricked it into running leaner with a more free flowing intake, at least temporarily.
I am using a Holley Terminator and the AFR is what I tell it to be, no tricks or work arounds. So do you have other suggestions over the K&N to look at? My constraints are 3.5 flange ID and the largest OD cannot be bigger than 5 and no longer than 7
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Old 05-30-2022, 11:38 PM   #10 (permalink)
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A big paper filter. Unless it's a throw away engine.
If you can run the AFR where ever you want it run it lean at part throttle. I was getting 12 to 13 mpg in my 7.4L, 3-speed, 4.11 rear end aero dynamics worse than a jeep which is hard to do, in the city I never took it on a real road trip but when I did take it on a 50 mile round trip to the next town over I would get between 16 and 18mpg with lean burn.
I was getting around 8mpg in town and about 12 on the highway with a properly tuned carb.

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