Help! Going Dual Exhaust

[84Turbo]

What is "VE"? Before I pipe in here, I want to know so I don't misspeak.

Volumetric Efficiency.

 

[Evolution1980]

What law of gas physics or combustion efficiency backs up your last statement?

While I'm not educated enough to quote you specific laws within physics, I don't need to be. The end results are already out in en masse in the form of dyno results. Just the same, I don't have to have 100% knowledge of how a dyno calculates it's answers, but knowing that I can give it a repeatable 'input' and it gives back a repeatable 'output' within an acceptable percentage of error is enough to justify my statements.

In the end, if we had to go out and find results of testing your hypothesis, we'd find more more results supporting my statement than yours.
My car is a case in point. I had 1,5/8" primary headers (Hooker Competition) on my engine. I had the car dynoed. I then went to 1,3/4" primary headers (Hooker SuperComp). That was the only change made. I then had the car dynoed again. It showed a loss of low RPM torque and a slight bump in high RPM horsepower. And I'm not the only one to receive these same results with similar testing. Point proven, no?
One can find further evidence supporting the scavenging of gasses, balanced exhaust pulses, and such by reading studies done on the addition of an X- or H-pipe.

 

[84Turbo]

While I'm not educated enough to quote you specific laws within physics, I don't need to be. The end results are already out in en masse in the form of dyno results. Just the same, I don't have to have 100% knowledge of how a dyno calculates it's answers, but knowing that I can give it a repeatable 'input' and it gives back a repeatable 'output' within an acceptable percentage of error is enough to justify my statements.

In the end, if we had to go out and find results of testing your hypothesis, we'd find more more results supporting my statement than yours. Sorry, I'm an engineer. I need facts and figures, not just an opinion on what we'd find.
My car is a case in point. I had 1,5/8" primary headers (Hooker Competition) on my engine. I had the car dynoed. I then went to 1,3/4" primary headers (Hooker SuperComp). That was the only change made. I then had the car dynoed again. It showed a loss of low RPM torque and a slight bump in high RPM horsepower. And I'm not the only one to receive these same results with similar testing. Point proven, no?
One can find further evidence supporting the scavenging of gasses, balanced exhaust pulses, and such by reading studies done on the addition of an X- or H-pipe.

That's all fine, but it's immaterial. Nobody is talking about header primary pipe size, H-pipe, etc. We're talking about the the original claim that you need backpressure (caused by the pipes and mufflers) to get low end torque, and that reducing the backpressure will reduce the low end torque. I disagreed, and am still waiting for someone to offer up any physics or mechanical reason why you would want backpressure. I have offered up specific reasons why you do not want it. All I'm asking for is similar reasoning why you would want the opposite result.

 

[Evolution1980]

Sorry, I'm an engineer. I need facts and figures, not just an opinion on what we'd find

Dyno results are your "facts and figures", not "opinions".

That's all fine, but it's immaterial. Nobody is talking about header primary pipe size, H-pipe, etc. We're talking about the the original claim that you need backpressure (caused by the pipes and mufflers)

OK, then please give your definition of backpressure? What constitutes backpressure? Where does it originate and where are it's effective bounds within the system?

 

[KANE]

Dyno results are your "facts and figures", not "opinions".
OK, then please give your definition of backpressure? What constitutes backpressure? Where does it originate and where are it's effective bounds within the system?

Dre- I tend to remember something about scavenging, small cam motors, with low compression and low end torque. I see the point you're making and I know what you are after. Low end has an impact- it's not all about top end.


Ahhh- here is the article-

http://www.wheelsjamaicahost.com/wheels_forum/index.php?topic=846.0;wap2

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

The main point is this- there is a trade off between exhaust sizes. What works at low RPMs doesn't work as well at high RPMs. That means an owner needs to be honest and determine what he expects out of his engine. Torque is felt and makes a difference getting across an intersection from an idle. I'd opt for the low end torque since my motor spends far more time betwen idle and 3k.

Just my $.02

 

[84Turbo]

Dyno results are your "facts and figures", not "opinions". You didn't show any dyno results. You only offered up "if we had to go out and find results", which you didn't do. I merely stated that I need more than just what your opinion of the results would be.
OK, then please give your definition of backpressure? What constitutes backpressure? Where does it originate and where are it's effective bounds within the system?

I'm not trying to be impolite or disrespectful, but backpressure is a basic phenomenon in exhaust system discussions. I'm incredibly confused why you would repeatedly say my statements are incorrect, when you openly admit you're unaware what VE and backpressure are. It consumes more time than I wish to devote to this subject if I have to explain basic concepts of the internal combustion engine. I enjoy a good technical discussion on matters of hi-perf cars, but you gotta meet me halfway here. As I said before, I've offered up engineering and mechanical explanations of things that are happening in the cylinder and exhaust system. All I ask is that you offer some technical info to bolster your disagreements with me.
This isn't an ego thing. I don't win anything at the conclusion of this thread. I'm just trying to have an interesting discussion about a hobby we share.

 

[84Turbo]

............


Ahhh- here is the article-

http://www.wheelsjamaicahost.com/wheels_forum/index.php?topic=846.0;wap2


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Just my $.02

A good article. Thank you for searching and retrieving it.
The first few sentences pretty much sum up my basic thought on the "Low end torque requires exhaust backpressure" premise.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T.

The way I've been taught is: Torque (at any RPM) is dependent upon cylinder (combustion) pressure. Reduced exhaust backpresure allows better scavenging (purging) of the burnt gasses. A reduction in the amount of residual exhaust gas left in the cylinder allows two things. 1) An increase in VE of fresh air/fuel which will create higher cylinder pressure, and 2) an increase in combustion temperature (due to less inert exhaust gas in the cylinder) which will push the cylinder pressure a bit higher, increasing torque.

 

[minifridge1138]

A good article. Thank you for searching and retrieving it.
The first few sentences pretty much sum up my basic thought on the "Low end torque requires exhaust backpressure" premise.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T.

The way I've been taught is: Torque (at any RPM) is dependent upon cylinder (combustion) pressure. Reduced exhaust backpresure allows better scavenging (purging) of the burnt gasses. A reduction in the amount of residual exhaust gas left in the cylinder allows two things. 1) An increase in VE of fresh air/fuel which will create higher cylinder pressure, and 2) an increase in combustion temperature (due to less inert exhaust gas in the cylinder) which will push the cylinder pressure a bit higher, increasing torque.

OK, I'll chime in, but before I do: I am not attacking anyone or calling names. I am just stating the facts as I know them.

Here is how back pressure can increase power at lower RPMs (as I read about it). On the exhaust stroke, the exhaust valve opens and the cylinder rises. Depending on the duration, there is a chance that the valve will reach TDC and start to move down (the intake stroke) BEFORE the exhaust valve closes completely. If there is back pressure in the exhaust manifold, then additional air is forced BACK into the cylinder. The air is dirty, but there is still more air than there would be if there was no pressure. This added air increases compression, improving VE, and producing more power at lower RPMs.

Here is how free-flowing exhaust can increase power at high-rpm (as I read about it). With no back pressure the exhaust gas exits at higher velocities. At high RPM the piston moves faster and therefore so does the exhaust gas. As stated above, there is a period during the intake stroke where the exhaust valve is still open. If the exhaust gas is moving fast, it can create a low pressure area (also known as a vacuum). This vacuum helps suck clean air from the intake runner into the combustion chamber. But, this really is only going to happen at high RPM because the exhaust gas has to be moving fast enough to create the low pressure area (i.e. improved power at high RPM).

With a forced induction setup, you want free flowing exhaust. You already have a blower forcing additional air in during the compression stroke. Any backpressure at all is only reduce the amount of air being forced in; and reducing VE in the process.

Ultimately, an engine is an air pump. The more air that gets in and out, the more power it will produce. Playing with cam lift, duration, overlap, headers, combustion ratio, etc will determine how high it peaks and at what RPM.

 

[84Turbo]

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Here is how back pressure can increase power at lower RPMs (as I read about it). On the exhaust stroke, the exhaust valve opens and the cylinder piston rises. Depending on the duration, there is a chance that the valve piston will reach TDC and start to move down (the intake stroke) BEFORE the exhaust valve closes completely. If there is back pressure in the exhaust manifold, then additional oxygen depleted air is forced BACK into the cylinder. The air is dirty, but there is still more air than there would be if there was no pressure. This added air increases compression Compression is negative work. The expansion is where the power is produced. , improving VE VE only makes sense, power wise, if it is combustible fuel and air that is in the cylinder. Counting inert molecules that do not produce power in the cylinder does not make sense. , and producing more power at lower RPMs. I do not see how the laws of chemistry or thermodynamics can support that theory.

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Ultimately, an engine is an air pump. The more air that gets in and out, the more power it will produce. Playing with cam lift, duration, overlap, headers, combustion ratio, etc will determine how high it peaks and at what RPM.

You are absolutely correct that an engine is an air pump. However, it is a matter of how much clean, combustion supporting air that gets in the engine that determines its power, not just the amount of burned/dirty/diluted air.

 

[minifridge1138]

You are absolutely correct that an engine is an air pump. However, it is a matter of how much clean, combustion supporting air that gets in the engine that determines its power, not just the amount of burned/dirty/diluted air.

I agree, but exhaust gas still contains some clean, combustion supporting air. That is the premise behind EGR. Exhaust Gas Recirculation was introduced because they realized there was unburned fuel escaping into the atmosphere. Solution: mix exhaust in with the intake charge.

I know what you're going to say, EGR reduced engine power. I won't argue that. BUT it reduced engine power because it uses exhaust gas IN PLACE of clean air. It mixes EG in with the clean air in the manifold BEFORE the intake stroke. So now you had clean and dirty air entering from intake runner. This reduces the amount of clean combustion supporting air.

In the back pressure situation, you still get 100% clean air on the intake stroke, combined with a small increase in compression due to air being forced back in through the exhaust valve. This small push of dirty air increases the volume of air WITHOUT reducing the amount of clean, combustion supporting air.

 

[84Turbo]

I agree, but exhaust gas still contains some clean, combustion supporting air. That is the premise behind EGR. You are incorrect. Exhaust Gas Recirculation was introduced because they realized there was unburned fuel escaping into the atmosphere. Solution: mix exhaust in with the intake charge. No, No, No. EGR was introduced to reduce the pollutant NOx. By introducing an inert, non combustion supporting gas into the cylinder, the combustion temps were dropped (by the action of the inert gas molecules acting as extra mass heat sinks) below the level that it takes for the chemical reactions that will combine oxygen and nitrogen into NOx. Period.

I know what you're going to say, EGR reduced engine power. I won't argue that. BUT it reduced engine power because it uses exhaust gas IN PLACE of clean air. It mixes EG in with the clean air in the manifold BEFORE the intake stroke. So now you had clean and dirty air entering from intake runner. This reduces the amount of clean combustion supporting air. Yes. This is the same bad effect as a restrictive exhaust with excessive backpressure. Think about it.

In the back pressure situation, you still get 100% clean air on the intake stroke, combined with a small increase in compression due to air being forced back in through the exhaust valve. This small push of dirty air increases the volume of air WITHOUT reducing the amount of clean, combustion supporting air.

No, No, No, again. The cylinder volume is a set amount. You cannot have more of one thing without having to have less of another.

 

[minifridge1138]

No, No, No, again. The cylinder volume is a set amount. You cannot have more of one thing without having to have less of another.

No, No, No.

Cylinder volume is set. But cars do not run at 100% volumetric efficiency. If they did, then the term wouldn't even exist. Most naturally aspirated cars usually run at around 85% of their rated volume (so a 350 cubic inch small block usually only moves about 297.5 cubic inches of air at 85% VE). High performance engines are high performance because they get closer to 100% VE. That is why you can get a lot of power out of cam upgrades. They don't increase the volume of the combustion chamber, bore or stroke of the piston, but they increase the valve duration and in doing so increase VE.

Super chargers and Turbo charges can actually achieve VE of greater than 100% by forcing more air into the cylinder than the piston could possible pull in during the induction stroke.

So the back pressure forcing air back into the combustion chamber WOULD improve VE without making any change to the size of the cylinder volume.

However, if the intake system and piston were capable of 100% VE, then an exhaust system with back pressure would act as a restriction. Bringing the net VE of the engine to less than 100%.

Forced induction engines are capable of 100% VE or more, so you want free flowing exhaust.

But naturally aspirated motors do not make 100% VE on the intake system, therefore some back pressure can increase VE.

 

[84Turbo]

No, No, No. You can't be serious here.

................................

So the back pressure forcing air back into the combustion chamber WOULD improve VE without making any change to the size of the cylinder volume. Nonsense. Pure nonsense. I've tried to explain the airflow and chemistry of what's going on in the cylinder, and you are either not understanding the principles, or are choosing to believe folklore nonsense. Residual (or forced) exhaust gas is essentially a negative component of VE. There is absolutely no performance value to allowing a bunch of exhaust to remain, or be forced into, the cylinder. (In fact, it reduces performance. Exhaust gas remaining in the cylinder reduces the vacuum level in the cylinder, reducing the pressure differential between the intake manifold and the cylinder, and thereby reducing the amount of fresh air that will pass from the intake manifold into the cylinder. Less fresh air and fuel equals less torque and horsepower.) I'm not losing any sleep with the fact that you choose to not believe the physics I've explained, but I am using up time that I could use more productively elsewhere. I recommend that you do some serious reading about how an engine works, and the chemistry of combustion. Perhaps then you will understand what I've been saying.

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But naturally aspirated motors do not make 100% VE on the intake system, therefore some back pressure can increase VE.

Again, pure nonsense. You don't understand an engine's airflow or the chemistry of combustion. It doesn't matter how fancy the intake system is, if the exhaust system cannot match that flow out of the cylinder, performance will be mediocre. Please, do some reading.

 

[JohnZ]

Incoming charge dilution by exhaust gases is essentially a non-issue at or above normal operating rpm due to the gas velocities involved; it becomes significant at idle, which is where most emission testing was done in the days when EGR was introduced as a "targeted" emissions device which would reduce combustion temperatures and thus reduce NOX formation at idle and off-idle where testing was done.

Large exhaust pipe diameters (like 3") all the way to the tailpipe tip do more for cruise-night bragging rights than they do for performance; exhaust gases cool VERY rapidly as they travel through the system, and their density and velocity are reduced substantially before they get to the mufflers.

Although the slight benefit of smaller primary header pipes for low-rpm torque production (like 1-5/8" vs. 1-3/4" or 1-7/8" on a small-block) can be measured on a chassis dyno, there's very little actual scavenging or extraction effect at any rpm from headers with a standard-type exhaust system in place, as the backpressure works against any productive reversion and wave resonance effects that can be achieved with a truly "open" exhaust system, which is what headers are designed for.

 

[Evolution1980]

Keep in mind, the trade-off with freer exhaust and less back pressure will be less torque at the lower end.

That's fiction. Improving the engine's VE improves torque and horsepower. It does not impair it.

Just to be sure we have focus on what the original statement and counterpoint was... I believe what krscholz was getting at is that having a less restrictive exhaust, such as that of larger primary headers will decrease low RPM torque. Is this still where there is a disagreement? (Short of having 1" diameter exhaust tips, the difference between a 2" and 3" rear exit would likely be negligible at low RPM.)

JohnZ, do you agree or not? It looks like by the first sentance in your last paragraph, the answer would be that you do agree. Correct?

 

[JohnZ]

JohnZ, do you agree or not? It looks like by the first sentance in your last paragraph, the answer would be that you do agree. Correct?

Countless chassis dyno runs comparing header sizes have shown conclusively that larger-than-necessary primaries reduce low-rpm torque production, but the numbers aren't dramatic.

 

[minifridge1138]

Again, pure nonsense. You don't understand an engine's airflow or the chemistry of combustion. It doesn't matter how fancy the intake system is, if the exhaust system cannot match that flow out of the cylinder, performance will be mediocre. Please, do some reading.

I love that you're argument comes down to: "You're wrong. I'm right. Go read something that proves it!"

I agree that if the exhaust system cannot match the flow (of the intake system) out of the cylinder, then performance will be impacted. Never in any of my posts did I disagree.

But what if the exhaust system can flow MORE air than the intake system? Perhaps the intake system is one of the reasons the engine is operating at 85% volumetric efficiency? In this situation, it is possible for the "back-pressure" to force additional air into the combustion chamber and increase VE. Even if it brings VE to 86%, then it has made an improvement in power. You'll need a computer to calculate the difference, but it will be a difference.

In the beginning of this post, back pressure was undefined. Later it was defined as "resistance to flow."
That is not the definition of back pressure I am using.
(You could argue that I'm changing the question to make my answer fit. I am still answering the question of: can back pressure increase low end torque and power. Yes it can.)

How can this happen?
Unequal length headers.
They cause the exhaust gases from cylinders to reach the collector at odd times. Sometimes the exhaust from all cylinders reach the collector at the same time. This creates "pulses" of pressure in the exhaust manifold and pushes air back towards the exhaust valve. There can be certain periods at which cylinder X has passed TDC and beginning the intake stroke. The exhaust valve has not fully closed. Cylinder Y is also on its exhaust stroke. Cylinder Y increases pressure in the exhaust manifold. Cylinder X is actually sucking air back in through the open exhaust valve just as cylinder Y creates this increased back pressure. This forces more air into cylinder X through the exhaust valve than it would normally suck. The result is the combustion chamber has a higher VE.

This is the exact same principal idea as supercharging and turbo charging. It is forcing more air into the combustion chamber than the piston would suck in on its own.

What happens as exhaust manifolds become larger in diameter? The amount of pressure pushed back towards the exhaust valves by the pulses is reduced. Basic physics: the amount of gas being produced is constant, yet the volume of the container (the diameter of the exhaust system) has in creased. Reduced pressure will reduce the amount of air pushed back through the exhaust valve and reduce VE.

 

[c4c5specialist]

Wait a minute.
After monitoring this, what exactly are we talking about?
Which engine?
Forced induction or not?
EGR or not?
Computer command control or not??
Fuel injected or not?
EGR requires backpressure, to effectively work in correct volumes, for emissions control ON COMPUTER CONTROLLED VEHICLES!!!!!!!!
However, EGR also has a bearing on engine performance. The less EGR dilution due to lower back pressure changes air/fuel mixture. If there isnt enough dilution, detonation reduces ignition TIMING because the knock sensors pick up the excess detonation and therefore REDUCE POWER!
Especially on carbed computer control, because air fuel ratio changes cannot occur fast enough to compensate.
However, backpressure for NON computer controlled cars is less of a variable. Because you can change camshaft profiles, induction, exhaust to change it
Its much less of a variable there.
The truth of all this, is that there is NO one right answer.
Each engine, each generation, each system requires unbiased evaluation of the individual vehicle, powertrain requirements for the owner and what the owner looks for.
Header primary tubes control exhaust gas velocity for an equal sized collector.
So, that being said, your exhaust gas velocity varies with diameter and length.
Each engine, based on camshaft profile, piston design and heads changes the optimum power curve created by the header design.
There is NO single correct answer for this question.
Allthebest, c4c5

 

[84Turbo]

I love that you're argument comes down to: "You're wrong. I'm right. Go read something that proves it!" No, I'm trying to get you to improve your mind by opening a book. It's helped countless people throughout history. Please give it a try.

..............................................................

But what if the exhaust system can flow MORE air than the intake system? Perhaps the intake system is one of the reasons the engine is operating at 85% volumetric efficiency? In this situation, it is possible for the "back-pressure" to force additional air into the combustion chamber and increase VE. Even if it brings VE to 86%, then it has made an improvement in power. You'll need a computer to calculate the difference, but it will be a difference. I cannot believe this. A basic fundamental fact is an engine makes power by airflow going one direction (carb to cylinder to muffler), and one direction only. Anytime the flow goes the other direction (that would be inert, oxygen-deficient post-combustion gasses), even temporarily, the engine is going to be experiencing a reduction in torque and horsepower. I didn't write the rules. That's just the way they happen to work.

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.

This is my last response to you. Occasionally in life one runs into people who are unwilling or incapable of learning. It's time to cut my losses, and stop wasting my time trying to help you.
Save the wave.

 

[Evolution1980]

Wait a minute.
After monitoring this, what exactly are we talking about?
Which engine?
Forced induction or not?
EGR or not?
Computer command control or not??
Fuel injected or not?
...<snip>...

Paul, we'd be talking about a normally aspirated, carburated, 1975 V8 with no emissions. I was unable to determine if krschoulz had kept the OEM exhaust manifolds and just did a 'cat back' or if he did the whole shebang.

As for 84Turbo, I'll quote him again:
"We're talking about the the original claim that you need backpressure (caused by the pipes and mufflers) to get low end torque, and that reducing the backpressure will reduce the low end torque. I disagreed, and am still waiting for someone to offer up any physics or mechanical reason why you would want backpressure. "
Now if I read that for exactly what it says, the inclusion of "caused by the pipes..." would include the exhaust manifold / headers (i.e. "collectors"?) all the way back to the muffler tip.
However, he also said, "Nobody is talking about header primary pipe size, H-pipe, etc. We're talking about the the original claim that you need backpressure (caused by the pipes and mufflers)" thus meaning to me that he's talking about everything after the collectors.
But even that all seems somewhat irrelevant based on the bold-texted comment above.

So is it fair to say that regardless of what type of normally aspirated, carb'd engine, and no emissions, the question is: reducing backpressure will reduce low end torque. True or False, and the explanation why.
(For the record, I also qualified my comment as the torque produced at low RPMs)