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Old 01-30-2009, 09:51 AM   #1
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Question 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.
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Old 01-30-2009, 10:16 AM   #2
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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.
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Old 01-30-2009, 10:35 AM   #3
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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?
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Old 01-30-2009, 11:12 AM   #4
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Regarding backpressure....no, it does not help. You were on the right track with flow velocity, though.
Quote:
Originally Posted by dkjones96 View Post
I think you were also running under the impression that back-pressure causes better fuel economy or that just having extra exhaust in the chamber upon intake stroke will increase economy. Lets explain both of these.

Back-pressure helping fuel economy. Gross myth that needs rid from the internet and old people's heads. Back-pressure is ALWAYS the enemy of people trying to get fuel economy or power. Exhaust Gas Velocity or EGV is what causes good economy and low-end grunt from engines using a small exhaust diameter, not back-pressure. A high exhaust gas velocity increases an exhaust system's scavenging effect and pulls more waste from the chambers.
See also http://www.2gstratus.org/forums/inde...howtopic=20754
Quote:
The reason for this is because of Exhaust Gas Velocity (aka EGV.) Mainly people commonly feel that bigger diameter is better, less back pressure is the most common attribute to their reasoning. As you increase your pipe diameter size you also the increase the amount of space on the inside of your exhaust...Volume. To an extent that is OK, but with the case of the majority of small displacement engines the exhaust system volume may be increased, but the the volume of exhaust coming from the engine hasn't. You are trying to fill more space with the same amount of exhaust gas. You are effectively reducing the EGV unless you are able to force more air into the engine to balance out the larger exhaust size. What I have described above in simple terms is more commonly known as Volumetric Efficiency.
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.
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Old 01-30-2009, 12:11 PM   #5
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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".
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Old 01-30-2009, 12:40 PM   #6
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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.
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Old 01-30-2009, 01:37 PM   #7
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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.
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Old 01-30-2009, 01:44 PM   #8
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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.
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Old 01-30-2009, 01:44 PM   #9
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Quote:
Originally Posted by bobc455 View Post
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.
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...
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Old 01-30-2009, 01:49 PM   #10
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Quote:
Originally Posted by theholycow View Post
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.
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.
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