[GidyupGo]

ORIGINAL: Toad

A dirty air filter allows less air to enter the MAF, where the air flow is measured. Since there is less air going into the MAF, the computer reduces the fuel flow to keep the fuel/air mixture correct. This means your engine is making less power, so you have to put your foot into it more to get the same amount of power to make the car go. More foot, open the throttle body wider, sucking more air through the dirty filter. The engine must use more of its' power to suck the air in through the dirty filter to get the same amount of power at the wheels. Hence less fuel efficiency.

Fuel efficiency, or MPG, is a tricky calculation. When most of us speak of our fuel usage we are using the computer built into the car, right? Well, where does it get its' information? How does it compute MPG? I can't answer this for sure, but I know basically how it works on most cars:

Most new cars have a fuel flow metering device. These are usually small tubes with turbines in them, a turbine being a sort of fan. When a fluid is passed through the tube, it causes the turbine to spin. There is also a small magneto-sensing device which can count the small turbine blades as they pass a certain point on the wall of the tube. Each time a blade passes the magnet it produces an electrical pulse. The faster the turbine spins, the more pulses you get per time segment. Now we can measure fuel flow.
Speedometers work in similar fashion...there is a device somewhere on the drivetrain that can count how many times the drvieshaft is spinning, giving us driveshaft speed which usually equals tire speed which usually equals car speed.
Now we just compare the two, fuel flow versus car speed, and we get miles per gallon. You'll notice that MPG goes way down as you accellerate from a stop. This is because there is a lot of fuel passing the fuel flow meter, but not a lot of speed yet. Once you get to speed, MPG goes up.
You can do your own MPG calculations, based upon very simple inputs. Keep track of your mileage at every fuel up. Let's say you had 10,000 miles at your last fuel up. You drive for a week or two, and it's time to fill her up again. You put 12.0 gallons in, and your mileage is now 10,216. You have driven 216 miles since your last fuel stop. Now all you have to do is divide the miles you drove by the gallons you just put in, assuming you always fill to the same level. (that's important - otherwise the results will need to be measured over several fill-ups.) 216 / 12 = 18. You got an average of 18 MPG on the last 12 gallons of gas.

But here's the problem: If you do this, you may find that your calculations differ from what the computer tells you. Who's right? If done correctly and accurately, the DIY method I described above is actually more accurate than the computer in the car. Why? Because you're measuring exactly how much fuel it took to go so far. The computer is using the fuel flow meter, the speedometer, and various iinputs to PREDICT the current fuel efficiency. It's not an exact measurement.

Now, how does a CAI and a re-tune improve MPG? I personally have seen a small reduction in MPG since I installed my C&L and tunes from Brenspeed. I have watched it very carefully, using both methods. In basic terms, an engine uses the same amount of fuel to make the same amount of power, regardless of air flow, fuel octane, or anything else. This is because gasoline, or any fuel, has a finite, measurable amount of "power" stored in it. And it basically gets all used up in the combustion process.



If you change the timing, are you really getting more power out of the same amount of fuel? No. You're getting a quicker throttle response. But throttle response accounts for very little of the power curve.

A tune can however change the fuel/air mixture, and we know most of the tunes out there do this. I've read where something like 13.5 to 1 is the factory ratio, I don't know where. If you reduce the amount of fuel, or "lean the mixture", you'll use less fuel per mile. But the exhaust gases will be hotter. Hotter exhaust gases reduce engine life by wearing down the valve seats sooner, and making the valve systems hotter which cookengine oil more. If you increase the fuel/air mixture the exhaust gases become cooler, because not all of the fuel gets burned, and acts to absorb some of the heat created by combustion. If you set the mixture so that ALL of the fuel gets burned, and ALL of the oxygen gets used up, the exhaust gases will be so hot you can ruin the engine in a short time. So this is never done. There is ALWAYS some left-over fuel in the exhaust! This is what causes the most pollution, and that is an entirely other subject.

The other way tunes could increase MPG is by changing the way automatic transmissions shift. The harder you shift, the more power gets to the wheels, and therefore the better MPG you'll get. I know the tunes in my car have done this.

So the bottom line is, with a new tune, you may get better gas mileage, but it's not because the engine is making more power. That would actually cause the reverse. The reason you may see better MPG is because your engine is running leaner, therefore hotter, and you need to make sure to change your engine oil at short intervals. Shorter than factory recommended, in my opinion. You may also see better MPG if you have an automatic transmission due to the harder shifts it makes, but this is probably very small to negligible.

Harder shifts mean less slipping and more wear from shock but less power loss. I was speaking mainly about the OP ? about a CAI ONLY without a tune. It sort of migrated off topic.