Cal Club Autocross Forums

Okay, someone talk me through it. For daily 'cruise control' driving, why would a freer flowing filter (either clean, new or high performance) air filter increase MPGs?

My thought, the desired Air Fuel ratio is going to be maintained by the computer until your air requirment exceeds your air delivery. I'm pretty sure that unless your filter is seriously clogged, any old fliter would have no problem keeping up air flow in cruising speed range.

So, are MPG claims bogus, or am I missing something?

JayW

I hear you....

But when we removed the stock airbox on the 3.2L Intrepid ES and replaced it with a prototype three inch tube and big ol K&N cone we got a consistent 10% increase in highway MPG.

Also... I've heard the bed cover increases are bogus, but I clearly seem to get a similar increase in highway MPG with the cover closed as opposed to open bed.

Improved performance = less throttle = better MPG?

Seems like I get better mpg when I use 91 instead of 87 octane in my adaptive timing Toyota. The van is definitely peppier, so again, less throttle is required.

Mako Koiwai wrote:Improved performance = less throttle = better MPG?{/quote]But "Improved performance" for a filter = higher air flow, which may mean less throttle, but not less air into the engine. Less air into the engine should = less gas put in by the injectors, which would mean less gas burned (although you'll go slower too).

Mako Koiwai wrote:Seems like I get better mpg when I use 91 instead of 87 octane in my adaptive timing Toyota. The van is definitely peppier, so again, less throttle is required.

Well yeah, that's a real tune thing that'll get move umph out of the gas. Technically the 91 has less energy in it than the 87, but the more aggressive tune tends to make that up and more.


Maybe better air swirllling going on with the hipo filters? Mixing the air fuel mix for a better burn?

Jay W

Tom Wilson's reply to a similar question in Road & Track (a fine magazine; you should read it. )

Low-restriction air filters improve engine efficiency because less horsepower is wasted trying to drag air through the restriction posed by the air filter. The driver can take this bonus either as extra power--just mash the throttle and more power is made (and more fuel burned). Or extra economy is enjoyed at less than maximum power because slightly less power is lost drawing the air into the engine, so the throttle need not be opened as far. Thus, you can gain power or fuel economy, but not both at the same time.

Doug Kott wrote: Or extra economy is enjoyed at less than maximum power because slightly less power is lost drawing the air into the engine, so the throttle need not be opened as far. Thus, you can gain power or fuel economy, but not both at the same time.

Well max power is easy to see. More max air = more max power (so long as the fuel and engine can keep up).

But your fuel economy isn't directly related to throttle opening, unless fuel is being dumpped in based on throttle only, todays cars are smarter right? Fuel economy should be air entering the throttle, no matter how open or closed it is. The vacuum created isn't extra work on the engine, it's how you restrict acceleration.

I think of it like a water hose. I water my plants with a slow restrictive soaker hose (i.e. a restrictive filter) with the spicet full open. I could actually run it with a much more open soaker hose (i.e. a hipo filter) with the spicet half open. I don't think I'm losing any water here, and it gets done in the same amount of time.

Jay W

But the extra vacuum is extra work for the engine, i.e., pumping losses created when the piston moves down on the intake stroke. It takes energy to create a vacuum, at least here in the earth's atmosphere. That's where an unthrottled diesel gets some of its efficiency.

The throttle controls the flow of air by providing an adjustable restriction (or resistance to flow). Take the restriction of the air filter out, and you need to close the throttle slightly to maintain the same amount of air flow. If you allow the air flow to increase, extra fuel is injected and the car speeds up. So at a constant cruising speed, it doesn't seem to matter whether the airflow resistance comes from the throttle or the filter. No?

John Stimson wrote: So at a constant cruising speed, it doesn't seem to matter whether the airflow resistance comes from the throttle or the filter. No?

Yeah, thats what I'm trying to convey. Thanks John.

But to Steve E's message, it seems there's too many testimonials for them all to be false and/or placibo effect. {shrug}

Jay W

Are these fine magazines the ones who say that a limited slip differential allows 50/50 torque split? }:)

I think the answer may have to do with swirl/mixing. In theory, you have 0psi (absolute) in the cylinder and 14.7psi on the outside of the air filter. Swirl/mixing is going to be more effective with greater pressure drop across a restriction. If you drop more pressure across the air filter at the first point in the intake tract, you have less available pressure to drop and thus less available swirl energy at the valve where you need it. In reality, since we're asking for a certain amount of power at a certain speed in a certain gear and thus a certain amount of air flow, the free-flowing filter will require a smaller throttle opening and take a bit more of that pressure drop and thus swirl energy. But the throttle is closer to the intake manifold than the air filter and there is less time for the flow to settle into uniformity. This, BTW, is the real genius behind BMW's throttle-less variable valve system - almost all the pressure drop is across the valves all the time and even at part throttle when the valves are opened only a little bit, thus ensuring maximum swirl. I'd like to see the specific power numbers on those engines (hp per unit of fuel), especially at part throttle. Doug, tell Dennis this would be an awesome article topic....

All that said, full throttle is a whole 'nother ball game where ECUs are typically hard-mapped to expected airflow as opposed to actual flow for maximum power and response. A clogged up filter there will cost you power and force you to stand on it for a longer period of time and at a less-than-optimal and richer than expected a/f ratio (since it's hard-mapped).

on evo's at least, different airfilters screw up the maf calibration and cause the car to run leaner than it thinks it is. The o2 feedback system doesn't fully correct for it because the offset in calibration due to the filter change isn't linear across it's flow range.

i suspect you'd see a variable fuel mileage in a cruise control situation where if you stayed at the same load for long enough the fuel mileage would eventually return to it's original value.

This thread is another fine example of Internet Engineering Masturbation.

John Coffey wrote:This thread is another fine example of Internet Engineering Masturbation.

Well, if that post is the climax, I'm disappointed....

Will Kalman wrote:Are these fine magazines the ones who say that a limited slip differential allows 50/50 torque split? }:)

Motor Trend, perhaps?

Doug Kott wrote:

Will Kalman wrote:Are these fine magazines the ones who say that a limited slip differential allows 50/50 torque split? }:)

Motor Trend, perhaps?

Green Car Journal?