Debate continues: K&N filter vs. MPG

[nascarnation]

Originally Posted by dremd

Not to go off subject; but synthetic oil does allow you to increase FE; not because of magic; because you can run a lower viscosity oil, which flows better; saving energy.

If you could run a 5W20 syn, there's no reason you couldn't run a 5W20 petro. Viscosity is viscosity I think.
Perhaps a 0W20 syn might be better in cold weather (they don't make 0W20 petros that I'm aware of).

[MetroMPG]

nascarnation: that's my understanding of synthetic oils as well - a small potential benefit to vehicles operated in cold climates (to minimize the warm-up hit). There's a reason the big oil co's don't advertise that synthetics will save fuel... if they could, they would.

[wumpus]

Originally Posted by dremd

I think we can all agree that the SGII will not be capable measuring for this test. THE SGII works off of reported Engine Load which is based on airflow; if the K&N flow was the benefit then the throttle plate would close more, ending up with the same airflow and same reading.

I think this the whole point that everyone has made about "worthless unless WOT". If you let off the throttle for the correct speed, you now have the airflow (air blockage) you need to correctly drive the car. Doesn't matter where in the intake it is, that is what you want. At WOT, you hope to have even more airflow than before, hopefully enlarging a BSFC island. If you aren't in a suddenly found new island, you didn't get any benefit.

On the other hand, has anybody seen real gains from reducing exaust backpressure? There aren't any hard requirements to reduce airflow after combustion (some pressure helps keep it even), so I would expect any MPG gains to be made here.

[nascarnation]

Originally Posted by wumpus

On the other hand, has anybody seen real gains from reducing exaust backpressure?

This is purely anecdotal, because I don't have good records before/after.
However adding a Dynomax "cat back" to my Jeep 4.0 appears to have hurt the mileage.
Since it is considerably louder, I'm assuming it has less backpressure.
If I had a do-over, I'd go with stock or equivalent muffler.

[wagonman76]

Lower backpressure than stock seems to hurt low end torque, and good low end torque is good for mileage. Sure the lower backpressure helps for those who like to race, but not for us.

[some_other_dave]

Depends on a lot of factors, though. A friend of mine put headers on his 65 Mustang, and gained some fuel economy. (It went from "unbelievably hideous" to merely "awful". )

Any potential gains are probably a lot smaller for more modern cars, though. I'm guessing much or most of the FE benefit for him was the stock system having a pretty poor design.

-soD

[worstmechanic]

This is going to be long, but the bottom line is that I may have figured out why some folks are seeing better mpg after installing k&n filters.

I'm going to assume that we are dealing with vehicles that have an electronic engine management system. In addition, I'm assuming that the engine management control program's goal is to hold the air/fuel mixture at a constant target value as measured by the O2 sensors. I'm also assuming that the program is written as a closed loop feedback loop with feed forward. A regular feedback control would wait for the O2 sensors to move away from the target O2, then start changing fuel rate to get the O2 back to target. This control is relatively slow. Feed forward is provided by using a mass air flow sensor. The feed forward speeds up control. In fact, if the feed forward were perfect, there would be no reason for feedback.

So, I think it's supposed to work like this. The manufacturers want a certain target level of richness/leanness in the combustion chamber. They use O2 sensors to measure this, and decide on a target O2 level for the control program. You also have a starting air/fuel ratio target. So, you step on the throttle, the throttle plate opens, and the MAFS sees air flow, and uses the ratio target to set the fuel flow. The O2 sensor reading should be on target, but if it's not, it will change the air/fuel ratio setpoint.

Knowing this, as most of y'all have figured out, a high flow filter like the K&N should not change mpgs. There would be better performance at or near full throttle. But otherwise, all you're doing is taking less pressure drop across the filter, which is being compensated by more pressure drop across the throttle plate.

One of the Chevrolet truck forums is full of claims of improving mpgs by adding a K&N filter, and also, by cleaning the MAFS. Neither one of these SHOULD affect mpgs at STEADY STATE. But while driving, everything is constantly changing. It's possible that the control program just doesn't work as well when a K&N filter is installed, and that the engine may run lean, on average. In the same manner, a dirty MAFS may cause the proram to run rich, on average.

What do y'all think?

[Clev]

Originally Posted by worstmechanic

One of the Chevrolet truck forums is full of claims of improving mpgs by adding a K&N filter, and also, by cleaning the MAFS. Neither one of these SHOULD affect mpgs at STEADY STATE. But while driving, everything is constantly changing. It's possible that the control program just doesn't work as well when a K&N filter is installed, and that the engine may run lean, on average. In the same manner, a dirty MAFS may cause the proram to run rich, on average.

I fully believe, from experience with my Ranger, that the K&N is the cause of the dirty MAF. Mine started causing problems within 10,000 miles of the cleaning and re-oiling of my K&N, and after a cleaning, had similar problems about 10,000 miles later. I think the MAF gets contaminated with oil from the K&N.

I replaced my K&N with a stock filter 40,000 miles ago, and had no further problem with my MAF since.

[Formula413]

A dirty MAF sensor would cause a lean condition if anything, since more air is entering the engine than the computer is seeing.

[Clev]

Originally Posted by Formula413

A dirty MAF sensor would cause a lean condition if anything, since more air is entering the engine than the computer is seeing.

Yup, with engine-damaging pinging.