Will an Exhaust mod help fuel economy?

[Christ]

Adding to the mandrel bent exhaust tip (which, apparently, many GM cars have as OE exhaust systems), any curves in the exhaust should be kept to a minimum. Even if you have a mandrel bent exhaust, the flow around a curve is not at a constant diameter. As speed increases, the flow attempts more and more to continue traveling in a straight line. Changing the flow's direction suddenly causes it to "stack up" against the outer radius of the curve, just past the apex (highest point, or center of the arc).

In order to keep exhaust velocity constant (mostly) you actually need more shallow bends (if any at all) and any bends you do have must be smoothly designed and of a larger diameter than the entry pipe, with no more than a 15-18* (I believe) transition step angle (the expansion of the pipe can't be any more than that).

Getting a little complicated, yet?

[Ryland]

You asked about intake, intakes fallow the same rules as exhaust, not the exact same numbers to figure out the ideal size but the same basic rules, if this was not the case you would put your air cleaner right on your throttle body, if you do this you will get the least amount of "restriction" and your car will run poorly and installing a plate with some kind of restriction would not help, just like adding a clogged restricted air filter would not help "but my car needs restriction to run correctly!" is BS created by people who can't read and don't understand math.
Air has mass to it, when you have anything with mass moving it will want to keep moving, air entering your engine is moving in pulses, not a constant flow, your engine and intake are designed to work together in this area, putting to large of an intake on is going to have the same kind of affect as removing your intake altogether, that is part of the idea behind the VVTi, you have a runner for each intake valve and half of the runners are closing off completely at lower engine speeds.

[Maestro]

Originally Posted by Ryland

"but my car needs restriction to run correctly!" is BS created by people who can't read and don't understand math.

Wow. What an incredibly rude thing to say. I guess that will teach me to join a forum and try to be helpful. Thank you sir, for putting me in my place. Fortunately though, I can read and do understand math, so I guess that BS was created by someone else, not me. Your car does not technically need restriction to run "correctly", but it does need restriction to run controllably. Take off your throttle body and see how you do.

I stand by my original post, as well as my decision to largely ignore intake harmonics and focus on explaining total restriction. Intake harmonics are important for performance and can alter throttle position at a given load just as overall reductions in restriction can, however this falls right back into what I said before; if you improve harmonics at a certain load, you've just increased the average pressure outside the intake valve, and you'll just have to decrease throttle position to stay down to the same load you were at previously. You have not helped economy, only performance. Consider the fact that, ultimately, pumping losses are essentially created on the face of the piston, and the only way to alter this loss is to alter the pressure on the face of the piston. To do this, you must affect the amount of FA mixture entering the chamber, and it does not matter to your MPG meter whether this is done through harmonics or restriction, the effect is the same; pulse pressure at the intake valve is varied, and you have to ignite either a denser or thinner FA charge.

Since the majority of the people here are building for economy, not performance, I would say that most can ignore intake harmonics. Economy-wise, intake temperature can help, and realistically, that's it. The physics do not support anything else. A large increase in total intake restriction or alteration in the throttle cable-cam may also help by decreasing accelerator sensitivity, but this would be more an alteration to the driver than the engine.

[Ryland]

Originally Posted by Maestro

Wow. What an incredibly rude thing to say. I guess that will teach me to join a forum and try to be helpful.

Sorry if you were offended, it's just that the design of engine intakes has been studied and written about many times, there are SAE, hotrod engine tuning books, motorcycle engine tuning books, along with web sites on the same topics, all of these sources tend to agree, to big is not good.

[Chalupa102]

I would recommend not going anything larger. On my '05 Toyota Corolla A/T, i had an aftermarket 4-1 header, 1.25" custom exhaust piping from the cat, and an aftermarket muffler installed a few years ago. At the time i did all that, i wasn't really keeping track of gas mileage. A few months ago i switched back to the stock muffler/piping and exhaust manifold and f/e actually went up.

[Maestro]

Originally Posted by Ryland

my car needs restriction to run correctly!" is BS created by people who can't read and don't understand math

Originally Posted by Ryland

all of these sources tend to agree, to big is not good.

Now you are contradicting yourself--as bigger is almost always less restrictive--as well as missing the entire point on the discussion of intakes and economy. Which is that, for the most part, IT DOESN'T MATTER. Certain runner sizes are beneficial for PERFORMANCE due to air velocities and harmonics, which as I mentioned before are not going to affect ECONOMY. I never said bigger was better anyway, nothing remotely like it, in fact.

In any case, you have given me no reason to retract anything I said about total intake restriction and its relationship to efficiency at low loads. I would point out that "all these sources" you are citing are performance oriented, not economy oriented. So until you show all these sources demonstrating how intake modifications affect economy in any way other than the ones I already mentioned or derivatives thereof, I think I'm done here.

[Christ]

SAE is hardly a performance source...

[Maestro]

Originally Posted by Christ

SAE is hardly a performance source...

I'll buy that, to some extent. But for it to matter, you'll have to show a SAE quote showing how intake volume/restriction/whatever, improves economy, rather than (or in addition to) performance. I'm not being sarcastic to say that I would be extremely interested in such a source.

[Christ]

I was just pointing it out for the sake of those who didn't know any better. I don't have a source from SAE, that I know of, regarding intake restriction on any level.

I will note, however, that only for Otto cycle and 2-stroke gasoline engines is an air-throttle actually necessary, and there are actually DI-type gasoline Otto cycle engines which no longer require an intake throttle. Their speed/power is controlled by load and fuel input, like a standard compression ignition engine.

I'm not sure that it's entirely relevant to the OP's question, but you (Maestro) mentioned earlier that throttling restriction was necessary to make an engine run "in a controllable fashion" and didn't clarify any further.

Welcome to EM, by the way. Maybe try a proper introduction?

[Maestro]

Thanks for the welcome! Yes, I should introduce myself. Hello everyone, by the way.

I mentioned in my original post, but yes, it's worth reiterating that the discussion of total intake restriction only applies to the typical throttle bodied, NA engine (though arguably it's even more relevant to supercharging). For those interested, in addition to diesels and DI, there are some interesting gasoline designs employing variable compression and variable valve timing that can largely negate this issue.