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Old 11-07-2021, 05:20 PM   #5 (permalink)
some_other_dave
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Depends on a number of things. How is the air flow being measured? There are three different ways this typically happens (or four if you add carburetors to the discussion).

One is "just guess based on the throttle position". This is the Alpha-N strategy, and I don't think it's used by any modern automakers.

Another is to measure manifold air pressure. When combined with the knowledge of how large the manifold is, the amount of air the engine takes in during a full cycle, and other information about the intake system, this allows you to guess pretty reasonably about the amount of air going in. The earliest electronic FI (Bendix Electro-Jet, Bosch D-Jetronic) used this, and a number of Honda products at least through the mid-1990s did as well.

The last uses a device in the air stream to measure the flow. Combined with the temperature, this gives you a very good idea of the mass of air flowing in. Most of these air flow meters are a fixed size, so changing the diameter of the pipe that goes to them or that leads from them to the manifold won't do much to help air flow. In fact, it may harm air flow in some flow regimes due to the tube changing size.

In a decent modern system, the O2 sensor will compensate for inaccurate measurements of air intake. But it will generally lag behind what is actually occurring, because it is reporting after the air is combusted and also takes some time to react after that point. So it will be a little laggy, which can produce odd transitional behavior in some circumstances if bad enough.


The upshot is: With a larger pipe you may gain a little top end power depending on all of the details of your motor and intake and fuel system. You are likely to lose a similar amount of mid-range torque, but you might not notice it. That's assuming you don't have a smaller section of pipe to fit your air flow meter. (If you do, you may find power losses in many places throughout the RPM/throttle ranges, and likely not much power.)

Pumping losses are likely to be similar to unmodified, because at throttle openings that we generally care about the throttle valve itself is by far the largest restriction to intake air flow.

That said, these are all generalisms. There can be and are specific instances where just putting a larger pipe on will help with power, or with pumping losses, or both. Direct experimentation will provide the answer in any specific case.
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