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Old 03-08-2015, 11:52 PM   #1 (permalink)
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Question Air intake restriction, manifold vacuum, fuel flow

Could a more restrictive than OEM air intake, such as my current WAI, increase fuel flow by increasing intake manifold vacuum? A significant manifold vacuum increase could pull more fuel through the injectors, no? Would this then confuse the ECU which would know about the added fuel from the O2 sensor and respond by cutting fuel trim? The result could be very unstable idle and really erratic fuel trims changes, no? I could read the MAP sensor data with my scantool to see if the WAI is even increasing vacuum, no?

I am a journeyman/novice mechanic at best ... what am I not understanding about these systems?

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Old 03-11-2015, 11:14 AM   #2 (permalink)
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Higher vacuum is lower pressure. That is seen as less air, meaning less fuel will be injected. Which is the opposite of what you are talking about.

It is also very unlikely indeed that an air intake would cause more restriction than a mostly-closed throttle plate. At cruise RPMs, it probably isn't enough restriction to even measure.

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Old 03-11-2015, 12:17 PM   #3 (permalink)
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You have a 100 mile tall column of air trying to crush you into nothingness. You're worried about a WAI?

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Old 03-11-2015, 01:44 PM   #4 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
You have a 100 mile tall column of air trying to crush you into nothingness. You're worried about a WAI?
More wondering than worrying.

Quote:
Originally Posted by some_other_dave View Post
Higher vacuum is lower pressure. That is seen as less air, meaning less fuel will be injected. Which is the opposite of what you are talking about.

It is also very unlikely indeed that an air intake would cause more restriction than a mostly-closed throttle plate. At cruise RPMs, it probably isn't enough restriction to even measure.

-soD
I see your point about the throttle plate, but I wonder how far I would have to open the throttle for the added restriction from a smaller diameter intake pipe to matter. At some point from closed throttle to WOT the added restriction would start to effect air flow and thus vacuum, since the throttle opening is of greater diameter than my intake airtube. I should be able somehow to calculate where that point is, no? 80% might do it, maybe.

On the injection and vacuum/pressure... maybe I don't understand the intake manifold that well. If vacuum effects both the air and injectors, then would'nt higher than expected vacuum in the manifold increase the effective fuel rail pressure by "pulling" more fuel from the injectors than intended because the vacuum is effectively added to fuel rail pressure?

The car was designed with specific relative vacuum and pressure in mind, and the ECU is monitoring those parameters and adjusting fuel partly based on them, maybe, no?

James
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Old 03-11-2015, 02:24 PM   #5 (permalink)
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The FPR uses manifold pressure as its reference.
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Old 03-11-2015, 07:27 PM   #6 (permalink)
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The engine does not suck fuel through the injectors, the injectors squirt fuel under pressure, 60psi or higher. Your air intake is irrelevant as far as the injectors are concerned. If your hot air intake is too restrictive, then the air volume will be reduced, the Mass airflow sensor will detect less air flowing, and the injectors will squirt less fuel and you will just not go as fast as you otherwise would have gone. Further, the oxygen sensor tells the ecu what it sees, if the ecu detects a too rich a mixture from data from the oxygen sensor, then the mixture will be properly leaned.

That's the big picture. Actually the ecu goes from lean to rich, to lean to rich, ad infinitum.
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Old 03-11-2015, 07:35 PM   #7 (permalink)
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Also, cars are designed to run in Death Valley which is below sea level and the run at the top of Mt. Evans in Colorado (14k feet ?), so how can the ecu experience manifold vacuums beyond what it "expects"? The system is designed to handle extremes as norms.
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Old 03-11-2015, 07:37 PM   #8 (permalink)
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A restricted intake sort of mimics a dirty air cleaner. If the air intake is restrictive enough the engine will not run. Until then the system operates properly.
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Old 03-11-2015, 10:20 PM   #9 (permalink)
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Quote:
Originally Posted by jakobnev View Post
The FPR uses manifold pressure as its reference.
Yes, and I have wondered about that. The vacuum line from the FPR to the intake manifold indexes pressure to the vacuum? That would keep effective fuel pressure in the injector rail within a narrower range than without the vacuum line, no?
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Old 03-12-2015, 08:18 AM   #10 (permalink)
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In this type of system, the pressure difference between fuel and inlet manifold is held constant.

There are other - returnless - systems in which the ECU compensates for the pressure difference. The O2 sensor has a lot of influence.

In addition to the effects of altitude and air cleaners of varying flow restriction, fuel pumps and engines vary, perhaps due to wear. The ECU is programmed to compensate.

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