Dirty Air Filters Save Gas

[Shaun @ Tru6]

Originally Posted by lendaddy

If the filter is causing restriction at a given throttle position the throttle plate simply opens and negates it. This of course can't happen at WOT which is why maximum potential IS affected.

Take your garden hose example, you error is in assuming there is only one restriction (the sock). The reality is that there would be another valve right by your mouth. Without the sock on the other end you may be able to suck Xcuft of air per second with the mouth valve open 10%. But...with the sock on the other end you need to open the mouth valve to 30% to have the same restriction free flow of Xcuft per second.

Make sense?

you and your fancy talk.

Glenn said it best: in modern cars (I'm guessing ODBII), the gas pedal is an AIR pedal. It lets in air. the computer then lets in fuel. a clogged filter only means you don't have maximum acceleration.

[lendaddy]

Originally Posted by Shaun 84 Targa

you and your fancy talk.

Glenn said it best: in modern cars (I'm guessing ODBII), the gas pedal is an AIR pedal. It lets in air. the computer then lets in fuel. a clogged filter only means you don't have maximum acceleration.

Yes, but he has it in his head that the engine has to "suck harder" which is not true at all.

[Shaun @ Tru6]

Originally Posted by lendaddy

Yes, but he has it in his head that the engine has to "suck harder" which is not true at all.

Yup, many here, myself of course included, are experienced more with (random start year) 69 to 95 cars.

[VINMAN]

One of the best commercials ever!

[lendaddy]

Isn't that nihilist guy

[speedracing944]

Originally Posted by lendaddy

If the filter is causing restriction at a given throttle position the throttle plate simply opens and negates it. This of course can't happen at WOT which is why maximum potential IS affected.

Take your garden hose example, you error is in assuming there is only one restriction (the sock). The reality is that there would be another valve right by your mouth. Without the sock on the other end you may be able to suck Xcuft of air per second with the mouth valve open 10%. But...with the sock on the other end you need to open the mouth valve to 30% to have the same restriction free flow of Xcuft per second.

Make sense?

You know I thought about that as I was driving in to school this morning. The dirty filter is just another "valve" like the throttle plate.

The dirty filter would hinder the WOT max HP unless the filter area is so large that even with having it packed with crap it still is able to flow enough air to satisfy the requirements of the engine.

There is also the issue of once a filter is loaded up and the pressure differential increases across the surface there can be damage to the medium which would pull dirt through the filter which would otherwise not be able to travel through if the filter was clean.

IMO a dirty filter will NOT increase fuel mileage except to limit WOT max HP. It is better to run a clean filter and just place a block of wood behind the accelerator pedal for the same effect.

Speedy

[911pcars]

Here's my take on fuel consumption with a restricted air inlet.

The A/F ratio delivered by an EFI system is maintained at close to stoichiometric (14.7:1) by sensing the air flow signal through the inlet. If the air flow inlet is restricted, for a given throttle opening, the engine delivers less power.

At WOT, EFI systems are usually in an open loop mode. In this mode the ECU only relies on an internal map to determine WOT A/F mixtures and not the O2 sensor nor other system sensors. Thus, the mixture will be richer than designed due to the restricted inlet.

At throttle openings less than wide open, to obtain the same equivalent power with a restricted inlet, the driver can increase the throttle opening to allow more air into the engine. The A/F ratio will remain the same but the total fuel volume increases proportionally with the increased air flow.

Whether a restricted air filter uses more or less fuel depends on how the operator (driver) reacts to the restriction. He may settle for the loss in power and vehicle performance and thus use less fuel or compensate with the throttle to maintain a previous level of performance (vehicle speed, acceleration rate, etc.) and thus consume more fuel.

EFI systems rely on many sensors to determine/maintain a target A/F ratio. The air flow sensor is but only one of many sensors. For example, the ECU may determine that a lower-than normal air flow signal along with a wider-than-normal throttle position sensor opening may be considered an abnormal condition and trigger a diagnostic trouble code (DTC).

I may be incorrect, but read through this a couple of times and see if it makes sense.

Sherwood

[Tidybuoy]

Originally Posted by Superman

I have a friend

Is that an oxymoron

[9dreizig]

I"ve decided to only drive downhill,, much better gas milage that way...

[serge944]

I think a lot of your are overlooking a huge factor.

When your air filter is clogged to the point of causing additional restriction throughout the rpm range, you are operating at lower volumetric efficiency. Meaning, you are making less power at each point throughout the rpm range.

Now, given you have a certain driving style, you will have to go higher in the rpm range to extract the acceleration that you like to passively utilize.

If you have to operate at higher RPM to produce a certain amount of power that will propel you with a certain amount of acceleration, you will be using more fuel per marginal amount of power.

There is a reason 6-7 liter american cars can get great gas mileage. They produce so much power at low RPM that revving the engine isn't necessary. Back to the whole volumetric efficiency thing...

[island911]

serge, this thread ( http://forums.pelicanparts.com/showthread.php?t=423673 ) started (and I believe ended) the topic.

where I typed:

In modern cars the "gas pedal" controls (restricts) the amount of air. ...it's an air pedal.

If you have MFI, or carb's, your "gas pedal" controls the gas(oline) and not the air..

So hopefully you can see to ditch the "it's an air pump" model and see that restricting air will restrict the fuel (just like a partial throttle position)

[Jims5543]

Originally Posted by LubeMaster77

So I guess I should breath through my wifes dirty underwear in order to live longer. Yup, sounds bout right...

Fixed it for ya.

[Mule]

Originally Posted by serge944

I think a lot of your are overlooking a huge factor.

When your air filter is clogged to the point of causing additional restriction throughout the rpm range, you are operating at lower volumetric efficiency. Meaning, you are making less power at each point throughout the rpm range.

Now, given you have a certain driving style, you will have to go higher in the rpm range to extract the acceleration that you like to passively utilize.

If you have to operate at higher RPM to produce a certain amount of power that will propel you with a certain amount of acceleration, you will be using more fuel per marginal amount of power.

There is a reason 6-7 liter american cars can get great gas mileage. They produce so much power at low RPM that revving the engine isn't necessary. Back to the whole volumetric efficiency thing...

Can you tell me how the engine knows whether the air is being restricted by a dirty filter or the throttle butterfly?

[lendaddy]

Originally Posted by Mule

Can you tell me how the engine knows whether the air is being restricted by a dirty filter or the throttle butterfly?

There you go.

[sammyg2]

Originally Posted by serge944

I think a lot of your are overlooking a huge factor.

When your air filter is clogged to the point of causing additional restriction throughout the rpm range, you are operating at lower volumetric efficiency. Meaning, you are making less power at each point throughout the rpm range.

..

No,. That is not true unless the filter is so clogged almost no air at all can get through, or unless you drive full throttle all the time. Oh, never mind, I see you have a 944 Just kidding.

At partial throttle the restriction in the air filter is not a factor unless it is nearly epoxied and fiberglassed over. I don't think it's possible to get a filter that dirty.
The restriction is the throttle plate, not the filter.

The only time it will effect VE is if it is wide open throttle, higher rpm, and the filter becomes the dominating bottleneck.
Most filters are designed to flow much more air than an engine needs even if it's dirty so you really, really have to munge it up to make it the dominating restriction.

[island911]

Originally Posted by Mule

Can you tell me how the engine knows whether the air is being restricted by a dirty filter or the throttle butterfly?

. . .or high altitude...

[Mule]

Originally Posted by serge944

I think a lot of your are overlooking a huge factor.

When your air filter is clogged to the point of causing additional restriction throughout the rpm range, you are operating at lower volumetric efficiency. Meaning, you are making less power at each point throughout the rpm range.

Now, given you have a certain driving style, you will have to go higher in the rpm range to extract the acceleration that you like to passively utilize.

If you have to operate at higher RPM to produce a certain amount of power that will propel you with a certain amount of acceleration, you will be using more fuel per marginal amount of power.

There is a reason 6-7 liter american cars can get great gas mileage. They produce so much power at low RPM that revving the engine isn't necessary. Back to the whole volumetric efficiency thing...

You are confusing yourself. RPMs have nothing to do with it.