Air Intake on Geo Metro, Better Mpg?
 

Just wondering if putting an air intake into a 97 4cyl metro would give me a boost in Mpg? Im not entirely sure if it would. Would the ecu try to over compensate for the extra oxgen going in. Or would it give my engine better compression. Therefore using less fuel.

assuming a new air intake allows as much air into the engine as it can get, you're likely to increase compression a bit, sure.

But a higher compression ratio requires higher octane fuel, otherwise the ECU will retard the ignition and lower your MPGs.

Trust your ECu; if it's getting good info from good sensors, it's designed to get the best fuel mileage possible within the contraints of the engine design. If you trick the ECU to run leaner, you'll burn pistons, valves, your head, or all the above. Run too rich and you waste gas.

However, if you run warm air into the intake, you can sometimes improve things, all depending on the engine. Otherwise, without redesigning the engine, there's not much to be done.

this will NOT change compression in any way
 

there will be slightly increased pressure on the throttle plate
IF
you install some kind of ducting to direct air into the "intake" under pressure
it will have absolutely NO effect on engine compression
it will have absolutely NO effect on fuel trim .

it may have a slight effect on engine MAP
which is Baro - intake vacuum

and a slight positive effect on suction throttling loss

the overall effect on FE should be a benefit , but it will not be a large benefit

Yes, it has been my experience that engines run better with intakes on them. ;)

My personal experience on my car ('05 Toyota Corolla A/T) when i had an aftermarket intake my f/e actually went down. My Corolla does better with the stock air box. The f/e also got better once i switched back to the stock muffler and exhaust manifold.

Ok that clears things up. I've decided not to put one in since any gain in mpg would offset the price for replacing parts.

I admit an increase to compression likely doesn't happen to all cars. But i read an article that was linked here (wish I could find it....) where these aussies went to extremes to eliminate all pressure losses from the throttle plate to the intake horn. When it was all said and done, they suceeded but the car began to predetonate with regular octane fuel and they had to switch to premium.

If an engine is engineered assuming restriction in the intake system leading to the throttle plate, I could see how an increase in compression is possible.

If you have a Geo Metro LSi with an automatic, the best thing for fuel economy is driving habits, rolling resistance, and aerodynamics. Keeping the 3 speed auto below 60 MPG will net the best results.

My 3-speed seems to stay below 60 MPG automatically. I seldom goes over 60 MPH, either. :-)

A cleaned up air intake can only make a difference when the throttle plate is wide open, which is bad for economy because then the injectors get wasteful. Some mods are common to both high efficiency and high power engines, but this is not one of them.

compression does NOT change
 

..."I admit an increase to compression likely doesn't happen to all cars. ."... ..."" I could see how an increase in compression is possible."...

what ever you may think
(excepting the effects of EGR and variable cam timing)
the engine compression will not change on any car except some SAAB engines
which have variable compression due to a hinge strategically placed in the engine block

the words you seek are
peak Combustion pressure
but do not forget IGN timing will change based on MAP changes and
it is possible that the authors of the article you read were experiencing some of that

..."
A cleaned up air intake can only make a difference when the throttle plate is wide open, "...
i agree with this
but i believe the OP's intent was to fabricate something to pressurize the intake
like RAM AIR on an old Pontiac ...

however i may be INcorrect --
if the OP was referring to the brightly colored garbage with conical filters that ingest hot air from under the hood sold by the alleged "performance outlets".

I found the thread. I finally got desperate and searched through my history till I found it. After rereading the last bit, I found a few important details, especially the bit about this being a turbocharged vehicle. The folks doing the mod also went to extreme lengths to remove any hint of vacuum from the intake system. But using a pressure meter they detected a slight positive pressure where they once had negative pressure; however their setup was like a beefy ram air setup, so no suprise.

I apparently took the example of an extreme setup and applied the logic to a more common setup.

no vacuum in the intake ? i am standing beside myself .
 

..."The folks doing the mod also went to extreme lengths to remove any hint of vacuum from the intake system."...

if that were true "their" engine would not run.
=================
it looks like the article's author went to great lengths to increase flow of a pre existing duct that was 50% BLOCKED by the location of a fog light ?

his modification may actually remove flow from the "oil cooler" if that is what that was ,
which may not be such a wise thing to do

there are books out there on intake design and from my reading of them the large short after market intakes that you can buy are look like they might be the worst thing you can do, I don't have the math in front of me but there are formulas out there to help you figure out what size is best.