Actual volume of exhaust gas
 

Does anyone have a chart of exhaust gas volume at various RPM + throttle combinations for one engine?

One of the most common modifications people (on this forum and others) want to do for FE is to put in a performance exhaust system, and they claim that if it flows more freely it will waste less power. I can't believe that a system capable of flowing the result of WOT@redline could restrict low throttle/low RPM flow at all...and it's hard for me to believe that the engineers who design vehicles are so inept that they can't figure the appropriate amount of flow. Sure, sometimes there are noise or cost related design compromises that could restrict power made at the top end, but flow is so much less under normal (and FE-driving) conditions such a system would still flow fine.

I would ask the same thing about intake air, but that one is easy to measure with my ScanGauge so I'll try to remember to do that.

not sure about the voumes but as far as the engineering behind intakes and exhaust:

I have been told (and it makes sense) that the car makers not only care about efficiency/power but also about sound volume. that is a big reason for all the things that take power away. the windy intake hats that are on some of the vehicles that you buy now. the sometimes several mufflers/resonators on cars.

my old truck had true dual flowmasters with a cold air intake on it mated to a ported and polished throttle body. it was loud to say the least. nobody looked at my truck until I cranked it up.

I can't say that free-er flowing exhaust would make that big of a difference for us or not but the concept is there.

I guess every little bit helps.

there are three differences between performance exhausts and the typical oem exhaust. 1. larger diameter tubing, 2. straight through muffler, 3. mandrel bent tubing.

For most of us here, I do believe that the first one will actually hurt us. We are the type that likes that low rpm backpressure since we spend most of our time at partial throttle low rpms. As for mandrel bent tubing and straight through mufflers I do believe they will help us as they keep consistent flow velocity. But where you fine an perforated core, straight through muffler in stock diameter?

Regarding backpressure....no, it does not help. You were on the right track with flow velocity, though.
See also https://www.2gstratus.org/forums/inde...howtopic=20754
As for mandrel bent tubing, I can see that mattering. I'm less confident that OEM mufflers are necessarily so bad. The last time I checked, straight through mufflers were available in a variety of sizes, though I doubt you'll find one for some of the dinky little exhausts on some older economy cars.

Drat, this discussion is making me want to put a Cherry Bomb on my pickup. :p

I know what you mean HC. This thread is making me want to order the headers and the Flowmaster 70 for the Durango already! But I need more tanks before I start modding.

I actually really like the stock exhaust pipes on it already tho so only a manifold and muffler change will be done. I think the single pipe after the factory y is a 2.5".

i do not have specified data. the way i see it, you want exhaust velocity and even if an exhaust system can flow WOT to redline, that does not mean it does it well. some 3in truck exhaust with ugly bends and an over baffled muffler and 2 cats might flow as much as a 2.5 in that has smooth bends 1 highflow cat and a straight through muffler. guess which is better for fuel economy. i know which one i would put on my car(if it was naturally aspirated, cars with turbos are a totally different story).

that is a good post you referenced, btw.

In general, larger diameter, mandrel bent, and lower-restriction will all help MPG. The less work the piston has to do on the exhaust stroke, the less parasitic drag on the engine.

The only exception to the logic above is when you consider that exhaust cools as it travels through the exhaust, and a larger-diameter exhaust system will have slower moving exhaust which will therefore be colder by the time it leaves the system. A colder exhaust will have a *slightly* higher viscosity than it's hot equivalent.

Larger diameter means:
- Slower exhaust gas speed inside the pipe (reducing sheer stress)
- Lower pressure gradient (i.e. PSI/distance) to push

On the other hand, the cooler exhaust will also contract (remember your PV=nRT stuff), which kinda means that the diameter at the end of the exhaust system will have less of an affect than the hot (more voluminous) exhaust at the beginning of the system near the engine. On my V8-455 Buick, I have 3" exhaust until the muffler, but only 2-1/2" tailpipes.

Reducing backpressure will not negatively affect engine operation/tuning (unless you are talking extreme modification, like 6" long header pipes that dump directly to atmosphere). There are a couple of fears associated with lower exhaust backpressure, none of which are really a concern:
- Lower backpressure means the cylinder will empty more, requiring that more fresh air & fuel are brought into the cylinder. Reality: Only as much air will enter the cylinder as you let through the throttle, and the ECM will match that amount of air with the appropriate amount of fuel.
- Less residual exhaust in the cylinder will mean a higher ratio of new air/fuel mixture, leading to increased volatility and therefore combustion speed and therefore propensity to knock/ping. Reality: Not a concern for a daily-driver vehicle. (But you should still keep your EGR system working well)
(Anyone that wants to get into racecar cam design, then feel free to PM me and we can talk about that more.)

A colder exhaust COULD AFFECT THE OPERATION OF YOUR CATALYTIC CONVERTER which is why "cat-back" systems are fairly popular. And, BTW, if your car is fairly new and has an oxygen sensor in back of your cat, you could have issues with this. But after that last sensor, go ahead and put 8" HVAC ducting on...

Your biggest bang for the buck in improving exhaust flow (and the associated MPG gain) is the muffler. However there is a 3-way balance between flow/noise/price (pick any two).

-Bob C.

I am telling this the way I understand it (and I am no expert)

I was told that the cooling affect that you speak of is the reason for ceramic coating headers. the ceramic coating keeps the heat in the headers.

if you did see gains from this type of modification, it would almost never offset the price of ceramic coated headers. they are expensive

*edit*

dk,

if you ever want to really make your durango a monster, check out what they are doing at www.krcperformance.com they are nuts down there in florida.

I agree with that, but I don't believe that the amount of airflow at WOT@redline could be flowed at all by a system that provides the least bit of restriction under FE-driving conditions. I think if you make it so restrictive that 2000rpm is restricted, there's no way you'll get to 6000rpm at all.

This is why I want to find some numbers. If I see just how much exhaust gas is moved at various RPM+throttle combinations, it becomes easier to compare. If it's moving X units at WOT@redline, and the exhaust can flow well enough to allow that, then could it possibly be the least bit restrictive at Y% of X? I want to find and compare the values of X and Y...

You can't think of exhaust as a continuous flow. You have to think of it as a series of pulses. (The further forward in the exhaust system you are contemplating, the higher the "pulse" effect, the further toward the back the more those pulses get dampened). This is what makes an "x" pipe so brilliant for us V8 guys. It's not just a question of "flow capability", it is also a question of pressure gradient / pressure drop.

Those pulses actually have a high velocity, so even though there isn't as much mass as at high throttle/RPM, you still want to allow that pulse to flow freely.

-Bob C.

Ok, the pulses thing helps, but wouldn't the pulses be a whole lot smaller (from less air/fuel) at lower throttle/RPM and slower (from slower piston speed) at lower RPM?

Another thing to point out is a header only has a real benefit if the pipes are of equal length. This evens out exhaust pulses and improves scavenging.

krcperformance is a pretty cool site. I like the 408 stroker kit! 4" bore + 4" stroke = 451hp and 505ftlbs of fun!

https://www.gassavers.org/showthread.php?t=5668 Back pressure being a positive thing or not has been tested here.

**Spoiler Alert**
It isn't positive.

Of course, but they still have enough velocity (for part of the front pipe) that you have to think of them with a pretty high velocity where these little things can still have an effect.

But any part of the exhaust system that makes flow easier will help the engine breathe easier.

-BC

Actually since headers typically have broader bend radii and larger volume before the collector, the pulses will have a easier time flowing than through the typical cast exhaust manifold. Header tuning does not do nearly anywhere near as much as people think, it's about as effective as spark plug indexing (a tiny bit of benefit only). There's a lot of untrue hype with the "tuned" headers.

-BC

i dont know about that man.... some motors respond very well to aftermarket headers and others do not at all.

most often a header configuration that makes the best power does NOT give the best mpg. case in point: sequential pairing of runners vs opposing pairing. also important is runner (and exhaust for that matter) diameter. larger is NOT better for mpg unless it is down stream from a turbo ;)

The highly tuned headers (vis-a-vis nascar etc.) are tuned for a specific RPM range, and if you can keep the RPMs in a fairly narrow range then tuning the length (and equal length) can have more significant gains than on a street car that has a relatively wide range of RPMs (high - to - low). In other words, if you tune your headers for 7800 RPM +/- 10%, the "tuning" will have a lot more benefit than a car tuned for 2000 RPM +/- 50% (and nobody "tunes" headers for 2000 RPM anyhow). In the typical RPM range of a hypermiler, we're just looking to reduce flow restriction so that the engine has an easier time pushing the exhaust gas through a 15-foot long path.

There are some aspects of header design that are the same for hypermiling and high-RPM HP, but some are the opposite.

-BC

Some stock manifolds are better than others. The 4.7L found in later Durango/Dakota/Jeep models is said to gain nothing from headers because they engineered the stock exhaust manifold so well it performs the same. Chrysler engineers openly admitted that.

However, the 318/360 like I've got have disgusting log style manifolds that are far from efficient.

im getting the feeling that you are familiar with only certain engine types with narrowed aftermarket availability. i dont think you can speak in generics when making the suggestions you do because the dont apply to some of the 4 cylinder cars i have owned.

when putting an equal length opposed piston pair header with very mildly larger diameter primaries, secondaries and collector on my old civic i got 2-3mpg highway keeping the stock exhaust(pea shooter). when i put larger diameter exhaust on i lost said fuel economy gains. the same was true with my most recent civic, but i kept the stock exhaust to avoid attention and held onto the little more mpg. but larger is not better for fuel economy.

Jared-

Sorry if I'm coming across as some kinda knowitall, I'm sorry about that. I work with fluid dynamics every day, but there is always stuff to learn (I think everyone on this board would agree!)...

Of course not only are there exceptions to every generality, there are also consequences of going "too big". There's cam design to consider, as well as a bunch of other stuff. For example if a huge exhaust system cools the exhaust temp so that the O2 sensor doesn't read the same, throwing off the ECM, then it doesn't help afterall. And as we've all heard, have too big of a header can hurt low-end torque in a racecar, and the generality (again, only a generality) is that low-end torque happens in engines with better gas mileage.

-BC

i did not think you came across as a know it all. i just think you are off in what you imply about larger diameter being better. you are VERY knowledgeable in the area that is familiar to you and i respect that.

that said i think we are coming from opposite ends of the spectrum as far as our automotive experiences as a lot of what you have learned to be true comes from larger displacement v8s where i am coming from the 4 cylinder crowd. i think you are also correct in that the cars you know have a much different philosophy in cam design than those i am. i bet its honda that fights generalities as they are the most low-end-torqueless cars i have ever driven but they did maintain good fuel economy.

in short: i believe i didnt consider your automotive background when saying that larger is not not better because there are cars out there with a factory pipe that is way too small and larger would be better.(i cant site examples, but after looking at some cars from the 70s and 80s it becomes clear.

The thing to remember here is that air and exhaust gases have mass. You exhaust valve opens and the exhaust gases have to get whatever volume/mass of exhaust moving as is sitting dead in the pipe. Then if it's slow to get moving, due to having large volume/mass relative to the exhaust gas, the exhaust gas and the stack of it along the pipe might present resistance to the upcoming piston. It may eventually gain the same momentum per unit length as a smaller pipe, due to more mass moving slower vs less mass moving faster, but it's somewhat "stretchy" as it were, and cylinder scavenging will not be as efficient. You get it moving at high enough velocity down the pipe and it can actually pull on the piston slightly and as the intake valve cracks, transfer significant momentum to the intake charge. However, there's a sweet spot between the resistance due to viscosity and surface friction in the pipe and the resistance due to stagnant gases in the pipe from previous strokes. This sweet spot compromise is often tuned towards high RPM often higher than stock redline, or to forced induction motors, which do not have valve overlap and don't need scavenging. Ignore the "biggest exhaust you can afford" advice from forced induction folks, it ain't like that for N/A.

On the topic of engine efficiency and exhaust design, I found this page with several exhaust energy recovery technologies - pretty neat stuff:

https://www.heat2power.net/en__benchmark.php

Bill