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Maestro · 05-18-2010, 11:32 PM

That is why I said to imagine that the hole could even support the engine!

To go to an analogy: Let's say your engine is a pool, and it needs a very particular supply of water to keep you relaxing in your backyard at peak efficiency. Let's say we're supplying this water with a garden hose. And on the end of that hose we have one of those old-fashioned metal sprinkler nozzles that you can screw shut/open. So we run the hose to the pool and adjust this nozzles to provide this perfect amount of water. Then, your annoying neighbor goes and steps on the hose back by the house. A restriction is created, the pressure at the nozzle drops, and you have to open it more to maintain the same flow. The intake issue is the opposite of this. Your intake is like the hose with the neighbor already stepping on it, except in this case, while he's stepping on it everything is perfect. Your pool is happy and your Saturday is wonderful. If you now go and knock your neighbor off (i.e., reduce total intake restriction) you will now have to go back and close up that nozzle (TB) to stay at the flow rate you were at previously. If you don't go close up that nozzle your Saturday will be ruined because your engine ran too fast and you got horrible MPG's.

In general, the restriction of the TB compared to the restrictions of the rest of the intake is huge. Therefore any change in the rest of the intake will likely result in a very small change in TB opening to maintain load. The main point though is that any small change that occurs will not improve your economy, because ultimately you're still filling that pool at the same speed.

To go more technical:

In any case, if we can agree that this hole supports a theoretical engine at idle, that is sufficient for our purposes. You are correct in saying that this hole has a certain limit to the air passage it will allow (given a certain pressure gradient), however, this limit is not directly relevant until we reach it. Let's say that at 500rpm, the engine requires 3/4ths of the air this hole can pass. That would indicate that at 500rpm, this hole is a considerable restriction. Because of this, we do not want to create a great deal more restriction with our throttle body. If we do we will not get sufficient flow. So, let's say we have no idle air control and must supply idle air with throttle opening, and that our hypothetical engine--without this restrictor plate--requires a 2% opening at 500rpm. Now, if we put on this restrictor plate and leave our throttle at 2%, we will no longer be getting enough air into the engine. The reason for this is that while we have not reached the maximum flow that the 1/2" hole can support at atmospheric pressure we have created a significant restriction in front of the throttle body (Incidentally, the effect would be almost entirely the same if we created the restriction behind the TB, though that may just confuse things for now). Because of this restriction we have dropped the pressure gradient across the TB, and therefore we have reduced the amount of air it can flow at 2% opening.

Imagine an intake manifold at vacuum, and the air outside your air filter at atmospheric pressure. There is no single point where this pressure switches; as we move along the intake tract towards the intake valve every restriction to flow is causing a reduction in pressure, eventually subtracting all the way down to your manifold vacuum. On the typical engine, a nearly closed throttle body is by FAR the largest restriction in this system. So, the pressure outside a nearly closed throttle body can be very close to atmospheric, while the pressure just behind it is high vacuum. However, the further away from low-flow conditions (i.e. Idle) that we go, the more other components begin to contribute to restriction. Components are commonly rated for flow at a certain pressure. What is important to note is that this rated pressure is not always the pressure gradient they actually experience in use. Let's say our TB is rated for 600cfm at 14.7psi (atmospheric pressure). That would mean that if we took off every part of the intake aside from the TB and could maintain perfect vacuum in the manifold at 600cfm consumption (which, for the record, we really couldn't) that TB will flow 600cfm. However, as soon as we stick an air filter on that (which, for kicks, lets say has the same flow ratings) we will no longer flow 600cfm, because the pressure across the TB is no longer 14.7psi, it is lower.

The issue with the reductions in intake restriction and economy/throttle position is directly related to this. If we make the intake overall less restrictive, we need to shift more of that restriction to the TB in order to maintain the same flow, i.e., we need to provide less throttle.

05-18-2010, 11:32 PM	#27 (permalink)
Maestro EcoModding Lurker Join Date: May 2010 Location: Midwest Posts: 9 Thanks: 0 Thanked 3 Times in 2 Posts	Brace Yourself That is why I said to imagine that the hole could even support the engine! To go to an analogy: Let's say your engine is a pool, and it needs a very particular supply of water to keep you relaxing in your backyard at peak efficiency. Let's say we're supplying this water with a garden hose. And on the end of that hose we have one of those old-fashioned metal sprinkler nozzles that you can screw shut/open. So we run the hose to the pool and adjust this nozzles to provide this perfect amount of water. Then, your annoying neighbor goes and steps on the hose back by the house. A restriction is created, the pressure at the nozzle drops, and you have to open it more to maintain the same flow. The intake issue is the opposite of this. Your intake is like the hose with the neighbor already stepping on it, except in this case, while he's stepping on it everything is perfect. Your pool is happy and your Saturday is wonderful. If you now go and knock your neighbor off (i.e., reduce total intake restriction) you will now have to go back and close up that nozzle (TB) to stay at the flow rate you were at previously. If you don't go close up that nozzle your Saturday will be ruined because your engine ran too fast and you got horrible MPG's. In general, the restriction of the TB compared to the restrictions of the rest of the intake is huge. Therefore any change in the rest of the intake will likely result in a very small change in TB opening to maintain load. The main point though is that any small change that occurs will not improve your economy, because ultimately you're still filling that pool at the same speed. To go more technical: In any case, if we can agree that this hole supports a theoretical engine at idle, that is sufficient for our purposes. You are correct in saying that this hole has a certain limit to the air passage it will allow (given a certain pressure gradient), however, this limit is not directly relevant until we reach it. Let's say that at 500rpm, the engine requires 3/4ths of the air this hole can pass. That would indicate that at 500rpm, this hole is a considerable restriction. Because of this, we do not want to create a great deal more restriction with our throttle body. If we do we will not get sufficient flow. So, let's say we have no idle air control and must supply idle air with throttle opening, and that our hypothetical engine--without this restrictor plate--requires a 2% opening at 500rpm. Now, if we put on this restrictor plate and leave our throttle at 2%, we will no longer be getting enough air into the engine. The reason for this is that while we have not reached the maximum flow that the 1/2" hole can support at atmospheric pressure we have created a significant restriction in front of the throttle body (Incidentally, the effect would be almost entirely the same if we created the restriction behind the TB, though that may just confuse things for now). Because of this restriction we have dropped the pressure gradient across the TB, and therefore we have reduced the amount of air it can flow at 2% opening. Imagine an intake manifold at vacuum, and the air outside your air filter at atmospheric pressure. There is no single point where this pressure switches; as we move along the intake tract towards the intake valve every restriction to flow is causing a reduction in pressure, eventually subtracting all the way down to your manifold vacuum. On the typical engine, a nearly closed throttle body is by FAR the largest restriction in this system. So, the pressure outside a nearly closed throttle body can be very close to atmospheric, while the pressure just behind it is high vacuum. However, the further away from low-flow conditions (i.e. Idle) that we go, the more other components begin to contribute to restriction. Components are commonly rated for flow at a certain pressure. What is important to note is that this rated pressure is not always the pressure gradient they actually experience in use. Let's say our TB is rated for 600cfm at 14.7psi (atmospheric pressure). That would mean that if we took off every part of the intake aside from the TB and could maintain perfect vacuum in the manifold at 600cfm consumption (which, for the record, we really couldn't) that TB will flow 600cfm. However, as soon as we stick an air filter on that (which, for kicks, lets say has the same flow ratings) we will no longer flow 600cfm, because the pressure across the TB is no longer 14.7psi, it is lower. The issue with the reductions in intake restriction and economy/throttle position is directly related to this. If we make the intake overall less restrictive, we need to shift more of that restriction to the TB in order to maintain the same flow, i.e., we need to provide less throttle.