Ram air mileage increase

[z3pilot99]

I have a question and thought. Over on the focus fanatics forum a guy came up with a cold air intake.
Link...
2012 Focus Custom Ram Air How-To - Focus Fanatics

People that have made the mod have seen a decent increase in highway mpg. It harms city driving mpg though. I think the city decrease in mpg makes sense based on some posts on here; Cold air causing the computer to add more fuel to keep the ratios in check.

The only thing I can come up with on the higher highway mpg is due to a possible positive/neutral pressure in the airbox. Could this be simulating a (very) low positive pressure turbo effect which has been discussed here on the forums? I plan on doing this and will do some a-b-a comparisons.

Any thoughts?

[ChazInMT]

Generally, anything that increases HP lowers MPG.

In the case of a cold air intake, the only effect on something that affects MPG would be the throttle position and thereby pumping loss. The energy required to drag air between the restricted opening that is caused by the throttle plate is what causes a decrease in engine efficiency. So anything we can do to make this restriction go away by opening the throttle helps, anything to close it hurts MPG. Warm air intakes increase MPG's by supplying less dense air, which contains less O2 for a given volume to the front of the throttle, in order for the engine to supply the required power for a given load, the throttle must open a bit in order to supply enough O2.

Conversely, a cold air intake supplies denser more O2 rich air, requiring the throttle to close a bit.

That makes some difference in MPG in both situations, warm air increases MPG, cold air decrease MPG.

This is why messing with the intake system to "Get More Power" and "Improve Fuel Efficiency" are usually things diametrically opposing each other. Pick one, lose the other.

Turbochargers change this game by providing high power at high loads by increasing the intake density and available O2 dramatically giving a small engine more power than it would have were there to be no turbo charger. But when operating at lower loads, the turbo does not operate, requiring the small engine to run with the throttle closer to wide open in order to allow enough air into the engine, this decreases the pumping loss, making the engine more fuel efficient.

Also variable valve timing can knock the stuffing out of pumping loss, but that requires a lot to explain. The subject is covered here on ecomodder somewhere.

[oil pan 4]

Originally Posted by ChazInMT

This is why messing with the intake system to "Get More Power" and "Improve Fuel Efficiency" are usually things diametrically opposing each other. Pick one, lose the other.

Or get a Diesel.

[ChazInMT]

Originally Posted by oil pan 4

Or get a Diesel.

Ya know for a long time I thought an efficient diesel was really great but 2 things changed that.

1st) The cost of diesel fuel makes it pretty much on par with gasoline for cost per mile.

2nd) A forum member from England, can't remember who, a year or so back came out and said the complexity and reliability of diesel engines make it to where when things go wrong...which they do, it is very difficult to diagnose and fix yourself, and the cost of repairs is very high. His take on it was a big "Run Away!!" when it comes to diesels.

So this had me thinking that if every 30-40K miles you have to sink $1-2K into something to keep it running, it isn't worth it. Not to mention the stress of knowing it may let you down at any moment, and if you're in the middle of nowhere, who's going to have the expertise to fix it, and what are you going to have to pay for that?

Just my opinion alone here.

I'll stick with a reliable gas engine for now.

[jeff88]

Originally Posted by ChazInMT

In the case of a cold air intake, the only effect on something that affects MPG would be the throttle position and thereby pumping loss. The energy required to drag air between the restricted opening that is caused by the throttle plate is what causes a decrease in engine efficiency. So anything we can do to make this restriction go away by opening the throttle helps, anything to close it hurts MPG. Warm air intakes increase MPG's by supplying less dense air, which contains less O2 for a given volume to the front of the throttle, in order for the engine to supply the required power for a given load, the throttle must open a bit in order to supply enough O2.

Conversely, a cold air intake supplies denser more O2 rich air, requiring the throttle to close a bit.

The OP mentioned people with CAI having increased FE on the highway. Are you saying the CAI is not a reason for that and only the more open throttle plate is the reason for the increase?

[z3pilot99]

I do understand the theory/reasoning of why a cai harms gas mileage. The thing that I question is the increase in mileage reported on the highway with this particular set up. This has been stated by many people that did this mod on the focus forum. I hesitate to call it ram air. So is there a confirmation a positive pressure in the airbox give a simular effect to a low pressure turbo, with possible mileage gains? Also, if I read the second post correctly it would be best to have a heated positive pressure in the airbox, right?

[oil pan 4]

Originally Posted by z3pilot99

I do understand the theory/reasoning of why a cai harms gas mileage. The thing that I question is the increase in mileage reported on the highway with this particular set up. This has been stated by many people that did this mod on the focus forum. I hesitate to call it ram air. So is there a confirmation a positive pressure in the airbox give a simular effect to a low pressure turbo, with possible mileage gains? Also, if I read the second post correctly it would be best to have a heated positive pressure in the airbox, right?

A ram air effect might give you a few inches of water worth of pressure, nothing like the effect you get from a turbo.
Put it this way, the positive pressure effect from ram air is slow low I have not been able to measure it.

[ksa8907]

If you try this it would be hugely beneficial for this debate/issue if you can keep track of absolute pressure in the manifold as well as intake temps.

[z3pilot99]

Funny you should say something about intake temps. The other day on the way to work I checked the intake temp reading on my ultragauge. It was within a degree or two of the ambient temperature as measured by the car. The temperature was in the very low 60's. At 55 mph.The car is completely stock at this time. I will see if there is a manifold pressure reading available.

I wish I could figure out why they get the increased mileage. It does go against what I understand and have read. I have to drill a hole in the airbox-don't want to do that without some ensurance it will indeed raise fuel economy. A new replacement is over $125.00.

[ksa8907]

For reference, my wai will sit at 120- 140 at 60-70 degrees ambient. Last week when it was in the 90s with the ac on I was seeing 200 degrees in the intake. I'm surprised it didn't pull timing but it stayed at 38-40 btdc