Filter Size Effect on MPG

[Drifter]

K&N filters are fine. They're neither magical nor demonic - they simply offer a different set of compromises than factory filters.

According to independent tests for a GM Duramax application, the K&N drop-in filter traded 3.13% lower filtration efficiency for 27.13% less resistance at 350cfm compared to the AC Delco filter: https://www.nicoclub.com/archives/kn-vs-oem-filter.html






That AC Delco filter costs $18.30 on Amazon. The equivalent K&N costs $69.99 plus ~$10 for a lifetime supply of air filter oil (you can use dish soap or simple green to clean). The AC Delco has a 45,000 mile change interval so in that application it takes about 200,000 miles for the reusable K&N to pay for itself from filter changes alone.

[steve05ram360]

FWIW I experimented a lot with the air box and found that the flow thru the box can dictate performance in both mpg's and higher end loads. Truck = 2005 Dodge Ram 2500 diesel with a bunch of mods.

Cheap air box V1:

An $8 trailer butterfly vent mounted on the bottom of the air box. Closed got me no vent performance as expected, open got me higher load performance.







Not so Cheap air box V2

Used a 3" electric exhaust cutout with a cab mounted switch. This got me on-demand valve openings, no more crawling under the truck to open the $8 vent.






Not so Cheap air box V3

Used a boost controlled 3" exhaust cutout for automatic control based on boost signal. Valve opens at aprox 5~6 psi of boost and boost is capped at 10 psi with a boost control valve so actuator does not see high boost. Valve performance is regulated with a simple bleed valve shown in the 2nd pic (lower brass valve). The goal was to keep the valve closed under low boost conditions and open under load. What this got me was a mod that jumped up into the top 5 if not top 3 for my truck. I have also been rewarded with 12~15 tanks of 23.0 mpg or better (23.6 peak) last summer and have already recorded a 23.0x mpg tank this year in less then ideal conditions. Previous to this mod mid to upper 22's were the max mpgs over the previous 3 summers.







Not so Cheap air box V4

This will be the same as V3 but with a 3.5" boost controlled exhaust cutout. Will evaluate which one I keep in there.

[XCRN]

That is really fascinating how that worked out. What was your base line MPG? In my situation I cannot find any good airboxes to use so I modified a Wrangler CAI enclosure to reduce the IATs. I previously had the filter right on the radiator and IATs were 145ish. Since moving the filter away into the enclosure it dropped IAT to about 130. I think it can get lower since the upper rad hose is touching the boot and maybe seal the filter a bit better.

I got my cruise control module in yesterday so I cam start logging the effect of the filter a bit better and see if what I saw was a fluke.

[steve05ram360]

Before V3 I was in the 22~22.5 range in the summer and had previously (2018) peaked into the low 23's during a bet with a co-worker. My truck had a makeover in 2017~2018 where I took it back to stock height, lightened up rotating mass on the engine, driveline and wheels with custom tuning (by me using HP tuners) to leverage all the changes. That got me into the 20~22 range consistently and had been averaging 21.0x mpg year round thru 2021. Summer time would bring 22's.

Here is how it looks these days... stock.



I experimented with a lot of aero changes and found 3 things that really stood out, bed seals from the top of the bed down to the body line above the tires, the 2nd air dam and the front wheel liner vents. Never did go back and work on the bottom side of the truck, probably a lot to be gained there.

[Blacktree]

Originally Posted by Drifter

K&N filters are fine. They're neither magical nor demonic - they simply offer a different set of compromises than factory filters.

According to independent tests for a GM Duramax application, the K&N drop-in filter traded 3.13% lower filtration efficiency for 27.13% less resistance at 350cfm compared to the AC Delco filter: https://www.nicoclub.com/archives/kn-vs-oem-filter.html

I'm curious about the "no name" filter in that test. It filters almost as well as OEM, and flows almost as good as the K&N. That looks like a win/win to me!

[Isaac Zachary]

I have always been skeptical of low restriction intake filters helping a gasoline engine with anything other than power at WOT. That's because they don't help "the restriction" from normal driving. "The restriction" is the throttle plate itself. Even going from a 6 month past due dirty stock filter to no filter at all won't change the overall intake restriction one bit.

If you wan't to remove restriction from the intake in a gasoline engine car, remove the throttle first. That's the biggest restriction.

The best ways to reduce intake restriction for fuel mileage reasons are:

• Install a less powerful engine. A less powerful engine will need you to keep the throttle open more, reducing intake restricion.
• Taller gearing. Taller gears means you get less power from RPM and therefore need more toque meaing you'll need to keep the throttle more open.
• Adding EGR or lean burn. The less fuel there is to everything else in the intake charge means it produces less power. Less power means you have to make up for it by opening up the throttle even more.
• Pulse and glide. This gets you better fuel mileage because every time you pulse you are opening up the throttle a whole lot more than you would to keep a steady speed.
• Load your vehicle as much as possible with people or stuff that needs to be moved. This of course helps anyway since putting 5 people in one car is more efficient than having all 5 people drive each his or her own car. The car will need a bit more power to haul more weight, but that helps open up the throttle a little more and reduce intake restriction even more.
• Get a diesel. Diesel engines don't have a throttle. On them you next step would be things like the air filter.

Unless you're driving around at WOT, putting on a lower restriction air filter only means you'll be driving around with your throttle plate closed a bit more.

[Drifter]

Originally Posted by Drifter

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?

Originally Posted by Isaac Zachary

I have always been skeptical of low restriction intake filters helping a gasoline engine with anything other than power at WOT. That's because they don't help "the restriction" from normal driving. "The restriction" is the throttle plate itself. Even going from a 6 month past due dirty stock filter to no filter at all won't change the overall intake restriction one bit.

After posting the above I was able to find a study on clean vs dirty air filters from the Oak Ridge National Laboratory ( https://www.fueleconomy.gov/feg/pdfs...02_26_2009.pdf ) They concluded:

Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR regardless of intake restrictions, and therefore fuel consumption was not increased. The carbureted engine did show a decrease in fuel economy with increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease in fuel economy (such as that cited in the literature3,4) was so severe that the vehicle was almost undrivable. Acceleration performance on all vehicles was improved with a clean air filter.

Modern Fuel Injection:



Dodge Charger with multi-displacement system:



Carbureted vehicle:


Modern fuel injected car with a severely clogged filter (almost undriveable - the car was barely able to meet the acceleration requirements of the EPA city mpg test):

In the United States, city fuel economy is measured using the Urban Dynamometer Driving Schedule, also known as the Federal Test Procedure (FTP).

Highway fuel economy is measured using the Highway Fuel Economy Test (HFET).

Another relevant test is the US06, an aggressive (high speed, high load) test used to confirm emissions compliance during aggressive driving

[Isaac Zachary]

Originally Posted by Drifter

After posting the above I was able to find a study on clean vs dirty air filters from the Oak Ridge National Laboratory ( https://www.fueleconomy.gov/feg/pdfs...02_26_2009.pdf ) They concluded:

That is what I expected and have seen.

A carbureted car? Sure, it might get a little richer if you drive around with a clogged air filter.

But reducing intake restriction for for fuel efficiency on an EFI car? Pointless. Any increase or decrease in fuel mileage is because of something else that was changed or because the measurements weren't precise.

I'm not saying that changes to the intake won't make changes in fuel mileage. But those changes affect things like the AFR or fuel atomization (or de-atomization) and such.

If anyone wants to remove restriction from the intake, remove the throttle plate or drive around with it wide open. It's like trying to get a highflow water pump to work with your thermostat stuck shut. First remove the valve that's constantly restricting the flow before trying to increase the flow elsewhere.

[cRiPpLe_rOoStEr]

Originally Posted by Isaac Zachary

Install a less powerful engine. A less powerful engine will need you to keep the throttle open more, reducing intake restricion.

Thi might explain why the last variant of the Ford Flathead V8 to be made in France had some quite narrow ducts, which were bad for power yet better for low-end torque. They were fitted to some military trucks, which ultimately had a Renault Diesel engine as a replacement for the Flathead...

Adding EGR or lean burn. The less fuel there is to everything else in the intake charge means it produces less power. Less power means you have to make up for it by opening up the throttle even more.

Occasionally the heat recovered along the EGR flow may lead to a more accurate vaporizing of the fuel too. But it seems to only be good for gassers. Most often on Diesels EGR is quite a PITA...

Get a diesel. Diesel engines don't have a throttle. On them you next step would be things like the air filter.

Choking too much a Diesel engine with an undersized air filter would be a matter of concern, at least for the mechanically-governed ones. But I would still take it with a grain of salt even with an electronically-governed one.

[XCRN]

Just thought I should follow up with this. I took 2 logs of my morning commute to work. There was a difference in ambient temp and looking at the IATs there is a difference, but I would not think enough have a noticeable increase. I want to try some back to backs one of these days.

So with the larger filter, I had an average speed of 35.87mph and an average fuel flow of 12.75lb/hour so my calculations came to 17.44mpg with the average intake air temp of 107degrees F.

The smaller filter, I had an average speed of 37.08mph and an average fuel flow of 11.98lb/hr so my calculation came out to 19.19mpg with the average IAT being 94degrees.

The 2 filters were the same brand (K&N) and constructed the same way, just different size. Traffic was roughly the same and hit the same stop lights. No idea why, but thats the numbers I got.