Will an Exhaust mod help fuel economy?

[comptiger5000]

Chalupa - What was hurting your FE was the 4-1 header. Those move the power up too far in the RPM range. A fairly long tubed 4-2-1 would be a better choice for FE.

[micondie]

[QUOTE=Maestro;174858] So, say your car runs at 1800rpm and 10% throttle at 55mph. If you then go and modify the intake to be less restrictive, you may now need only 8% throttle to maintain this same condition. And the condition that you are maintaining is a certain pressure outside the intake valve. Therefore, in order to maintain the same output you have not changed pumping losses at all, you've simply altered how much restriction is being created by each individual part of the intake. So you are correct in thinking that decreasing intake restrictions would reduce pumping loss at a given throttle position, but what you need to consider is that a reduction in pumping loss on the intake side means more air is now entering the cylinder, and you're producing more power, using more fuel. So to maintain the same RPM/speed/load etc. as before your intake mods, you need to reduce the throttle opening to get back to the same total restriction you had before the mods.

What jumped out at me while reading this is that if it takes less throttle opening to maintain the same speed then your motor is taking in less air and since you have the same air/fuel ratio, you are using less fuel!
Or is there a flaw in my logic?

[Maestro]

Originally Posted by micondie

What jumped out at me while reading this is that if it takes less throttle opening to maintain the same speed then your motor is taking in less air and since you have the same air/fuel ratio, you are using less fuel! Or is there a flaw in my logic?

Yes, I'm afraid there is a flaw in your logic. Read carefully. What I'm saying is that with reduced restriction elsewhere in the intake, you're now flowing the same amount of air at a lower throttle opening, so nothing really changes.

This is easily visualized if you take it to the extremes. Imaging blocking your intake prior to the throttle body with a plate with a 1/2" hole in it. Imagining you could even support the engine with such a hole, you would need to open your throttle very far to reach the same airflow you would have had without the plate at a smaller throttle opening, however once you open the throttle to achieve the same flow, everything is the same. It works the same in reverse with less restrictions instead of more.

[Christ]

Originally Posted by Maestro

Yes, I'm afraid there is a flaw in your logic. Read carefully. What I'm saying is that with reduced restriction elsewhere in the intake, you're now flowing the same amount of air at a lower throttle opening, so nothing really changes.

This is easily visualized if you take it to the extremes. Imaging blocking your intake prior to the throttle body with a plate with a 1/2" hole in it. Imagining you could even support the engine with such a hole, you would need to open your throttle very far to reach the same airflow you would have had without the plate at a smaller throttle opening, however once you open the throttle to achieve the same flow, everything is the same. It works the same in reverse with less restrictions instead of more.

To oversimplify the concept - The throttle plate is the single largest restriction in your intake system. Excluding small gains in flow velocity from smoothing pipes, and only adhering to basic flow principles, intake mods will only work at or near WOT because of this.

That said, there are other sciences involved that can improve the VE of the engine, thus decreasing it's BSFC via reduced pumping losses. The amount of gain achieved at our levels of operation by such things is essentially non-existent, though, unless the pump itself (engine and auxiliary systems) is tuned as a whole for a specific desired effect.

[micondie]

Originally Posted by Maestro

Yes, I'm afraid there is a flaw in your logic. Read carefully. What I'm saying is that with reduced restriction elsewhere in the intake, you're now flowing the same amount of air at a lower throttle opening, so nothing really changes.

This is easily visualized if you take it to the extremes. Imaging blocking your intake prior to the throttle body with a plate with a 1/2" hole in it. Imagining you could even support the engine with such a hole, you would need to open your throttle very far to reach the same airflow you would have had without the plate at a smaller throttle opening, however once you open the throttle to achieve the same flow, everything is the same. It works the same in reverse with less restrictions instead of more.

Your example doesn't make sense. If you have a restrictior plate with a 1/2" hole before your throttle no extra amount of throttle opening is going to make more air flow through that 1/2" hole. That hole is the limit. Your throttle at idle probably has less air flow than that 1/2" hole but once you reach the flow limit of the hole that's it.

[Christ]

Originally Posted by micondie

Your example doesn't make sense. If you have a restrictior plate with a 1/2" hole before your throttle no extra amount of throttle opening is going to make more air flow through that 1/2" hole. That hole is the limit. Your throttle at idle probably has less air flow than that 1/2" hole but once you reach the flow limit of the hole that's it.

That's exactly what the example said...

If you put a restriction in front of the throttle plate, you have to open the throttle further to get the same amount of air through it under vacuum (this does not apply the same way under boost).

Still, the point is that removing any restriction prior to the intake's throttle butterfly just means that you have to open the throttle that much less to get the same amount of air into it. You're still using the same amount of fuel, regardless.

[Maestro]

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.

[Ryland]

Maestro: if you read books on how an engine really works you will quickly see that how you explained why a "less restrictive intake" is good, why it doesn't really work that way, it would if the air was moving at a constant rate, but it's not so it does not fallow the same rules, the formulas that are out there hold true for people who are tuning for speed and for people who are tuning for peek mileage, why do they work for both? because they take in to account engine speed, an example of this being used in a car is the civic vx, the intake on the 1.5L engine that gets better mileage is smaller around then the intake on the regular 1.5L engine, leave the rest of the engine alone and just swap out the intake for a like intake from the "standard" engine that puts out slightly more HP at near red line and you will see your MPG drop.
Again, the formulas for figuring out your ideal intake and exhaust size have been figured out, tested and published.
I suspect to much time has passed for me to look at what books I've ordered from the library to tell you exactly what SAE books I got the information from, but it is out there and accessible to the public.

[Maestro]

I am well aware of the pulsatile nature of engine flow, as luckily I have read books on how engines really work. Steady flow is difficult enough for many to understand, therefore it is easiest--and quite valid--to ignore pressure waves and the like for an economy-oriented discussion. Also I would note that I never said that a less restrictive intake is good, as you're "quoting" me as saying. You're now quoting me as saying the exact opposite of what I'm saying. And what I'm saying is that reducing intake restriction will not help your economy.

Unfortunately Ryland, to insinuate that I have never read a real book on engines or have no idea how engines work just continues the insulting tone of your previous posts, and I have lost all interest in debating this with you, as it is becoming difficult not to stoop to your level. I wonder if you argue with people like this when not hiding behind a computer? If so, I expect you've been punched in the mouth a few times and deserved it. In any case, Micondie, Christ, others, if you would like to continue the discussion I would be more than happy. Ryland, you will be ignored.

[Christ]

I don't feel like it really needs too much more discussion, honestly... as was stated, the information is readily available to anyone who chooses to look for it, and well, aside from the "black arts" of tuning for pulse length, etc, we've covered it, I think.