Help! Going Dual Exhaust

[Evolution1980]

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.

Hopefully that comment is just to minifridge, as I think we're getting somewhere good and I for one would definitely like to know the answer.
As you said, "I'm just trying to have an interesting discussion about a hobby we share." We've got more people chiming in...now we're getting there!

 

[84Turbo]

Hopefully that comment is just to minifridge, as I think we're getting somewhere good and I for one would definitely like to know the answer.
As you said, "I'm just trying to have an interesting discussion about a hobby we share." We've got more people chiming in...now we're getting there!

You are correct. My comment is directed to just one individual.

Regarding the previous statements of mine pasted in a post just slightly earlier, let me offer a little clarification. Several variations of exhaust systems were discussed, and each required reasonably unique answers. But, let me offer a short broad answer to the subject. With a header equipped system, the backpressure of the tailpipes and mufflers is an undesirable item, causing a loss of performance. With stock exhaust manifolds, there is usually such crappy flow through them that the flow restriction is closer to the ports compared to a header equipped system, and the resulting backpressure similarly impairs performance. In conclusion, backpressure, wherever it occurs in the system, is an undesirable thing, reducing performance.

 

[minifridge1138]

I was also trying to engage in a conversation about the hobby.

A quote from earlier in the post from 84Turbo:

"... and am still waiting for someone to offer up any physics or mechanical reason why you would want backpressure. "

I was just trying to describe a situation where someone would want back pressure.

I wasn't saying that every car should have back pressure. C4C5 was absolutely correct: every engine is different and there is no single answer that will be right all the time. I openly admit that there are times when any back pressure at all will hurt performance. I was just saying that there are some instances where back pressure can be good.

I am the first person to admit that I could be wrong. If I thought I was always right about everything, then I would never post any questions.

If someone could point me to some material that explains why I am wrong, then I would like to read it. It feels good to learn.

 

[krscholz]

Holly Crap - what did I start? Okay boys and girls, the system has been for a week now and I hear what you're saying about the low end torque situation, but I will tell you that I can definitely feel the difference in performance both off the line and on the road. I love my new pipes.

 

[minifridge1138]

Congratulations!

That is what the forum is about: helping people have fun.

Enjoy the drive.

 

[minifridge1138]

I was wrong.
84Turbo was right.

Most websites simply advertise that their smaller primary headers increase low RPM torque, but don't explain why. It is usually "explained" as back pressure. That is not true, and I fell for it.
I finally found some information that explained why smaller diameter exhaust primaries CAN (but not definitely) increase low end torque.

I'll summarize because the actual explanation was rather long:
Exhaust gas leaving the combustion chamber has 3 distinct phases
1) The high pressure head. This is just as the burned gas leaves the combustion chamber and is caused by the piston traveling up. The air being forced out of the chamber is of a higher pressure than the air in the manifold.
2) The medium pressure body. As the pressure in the combustion chamber and the pressure in the exhaust manifold equalize.
3) The low pressure tail. The manifold and header now have the same amount of pressure, but the exhaust gas is still moving away from the chamber. This creates a low pressure vacuum near the exhaust valve.

If timed correctly with overlap in the cam, it is possible for the low pressure tail of one cylinder to coincide with the overlap (period where both exhaust and intake valves are open before the induction stroke) of another cylinder. This will suck air into the combustion chamber through the open intake valve BEFORE the piston begins the intake stroke. Means more clean air enters the combustion chamber = improved VE = more power.

This effect is referred to as exhaust scavenging. This is the goal of tuned headers and X or H pipes.

If the primaries are too big, then the exhaust gas expands and slows down. This reduces the vacuum, and little to no air gets sucked in.

If the primaries are too small, then back pressure is created. This results in the piston having to work harder to push the exhaust gas out of the combustion chamber = loss of power.

Because exhaust travels at different speeds depending on engine RPM, exhaust systems are "tuned" for a specific RPM range.
At high RPMs the exhaust gas is traveling fast enough that it is possible to expand to large diameter primaries and still create travel fast enough have a scavenging effect.
At low RPMS the exhaust gas is traveling too slow to create a scavenging effect. So smaller primaries act as a venturi and increase the speed of the exhaust and create a scavenging effect.
This is why small exhaust primaries can show increased low end torque. However, those same primaries would not have the same power at a high RPM as would larger primaries tuned for high RPMs.

I will again admit that 84Turbo was right and that I was wrong.
I hope that we can all still get along and make sure CAC is THE place to go for Corvette knowledge

 

[krscholz]

Up Date

The two inch dual exhaust were installed on my 75 and I have noticed a considerable increase in power. The sound resonates very nicely. This was certainly a good move. The job included removing the air pump and putting it in storage, taking about two hours to install the pipes - that's it, no carburetor adjustments or timing adjustments needed and there is no reason to have a muffler shop involved! I recommend Corvette Central for the best deal on pipes.

 

[Vette79]

and there is no reason to have a muffler shop involved!

Depends on the exhaust configuration and the pipes involved. I needed a custom exhaust shop for my chambered exhaust. Glad the exhaust setup worked out for you. Did you install headers?

 

[krscholz]

No, not yet. I want to take down the top end of the motor first and install a new cam (winter project) and do the header's after that (maybe this summer). My situation for installing the new exhaust system was an unexpected expense due to the fact that the 33 yr old pipes literaly fell off the car.

 

[Evolution1980]

No, not yet. I want to take down the top end of the motor first and install a new cam (winter project) and do the header's after that (maybe this summer).

I believe the generally accepted belief is that our engines don't really come alive until we allow them to exhale easier, as the OEM exhaust manifolds are very restrictive. So you may not notice the full benefit of your cam until you get yourself some headers.

 

[wfm0269]

wfm0269

I spent last weekend installing my dual exhaust system from Corvette Central. It took roughly four hours and the system fit perfectly and included everything I needed. The package included Magnaflow mufflers and the car seems to have more power but I haven't tested it yet. The sound is more fuller now but not overbearing that I can't hear the radio. If you're thinking about doing the same for your car, contact Gus at Corvette Central, he is a wealth of information and was very helpful to me. His e-mail address is as follows: mail@corvettecentral.com Any other questions contact me as well.

I read your old post on your exhaust system rebuild and found it very useful. It just keeps on giving. My L48 goes in to the machine shop tomorrow for a rebuild. I wish it was my 1967 that I sold. Thanks for the help.

 

[baxsom]

I had read in a few places that the reason why people believe that larger exhaust causes a power loss is because that the large pipes allow the hot exhaust gas to cool down and become denser forcing the engine to work harder to push the gas out. then someone puts a bend or a pipe reduction etc in the mix and due to the bernelli (sp) effect the gas speeds up again. it was then incorrectly deduces that the bend in the pipe caused backpressure to the engine which brought the power back up. exhausts are a fine line between shape and size. too big and you lose power, too small and you lose power. kinda like a carb.

how close am i, there are wayyyy too many old wives tales and theories in the world of engines to believe anything without multiple points of reference and proof.

 

[73shark]

wfm0269: Welcome to CAC.

baxsom: I've read where there's an optimum amount of exhaust back pressure.

 

[84Turbo]

wfm0269: Welcome to CAC.

baxsom: I've read where there's an optimum amount of exhaust back pressure.

Yes, and that amount would be ZERO!

 

[73shark]

That might be true if the engine is designed for that, e.g., top fuel dragster. Otherwise it depends upon a lot of factors such as cam, timing, CR, etc.

 

[84Turbo]

I had read in a few places that the reason why people believe that larger exhaust causes a power loss is because that the large pipes allow the hot exhaust gas to cool down and become denser forcing the engine to work harder to push the gas out. then someone puts a bend or a pipe reduction etc in the mix and due to the bernelli (sp) effect the gas speeds up again. it was then incorrectly deduces that the bend in the pipe caused backpressure to the engine which brought the power back up. exhausts are a fine line between shape and size. too big and you lose power, too small and you lose power. kinda like a carb.

how close am i, there are wayyyy too many old wives tales and theories in the world of engines to believe anything without multiple points of reference and proof.

Yes, there are too many. And several of them are in your first paragraph.

Cooling is not necessarily bad in all cases. When a gas cools (in a contained volume), the pressure drops. Reduced backpressure is a good thing.
Regarding the dreaded change in density, the mass content of the exhaust is not changed due to the temperature. Mass is what requires force/energy to move or accelerate it. Unless the exhaust stream is trying to slow down for some reason, it generally doesn't need extra energy (in the form of piston movement induced pressure) to get it to keep moving.
Bernoulli's principle in an exhaust system is a tricky thing to do. Physics generally shows that there's no free lunch in the universe. To get the gas to accelerate through the restriction, there needs to be a pressure drop (delta p) across the restriction. To get this pressure drop, you first need to have higher pressure upstream of the restriction. This upstream location is always the cylinder. Additional pressure in this location is generated only by the piston, and the unavoidable side effect is a restriction on the movement of the crankshaft (ie: Less horsepower). Again, backpressure is a bad thing.
Too big and you lose power, and too small you lose power. An incorrect generalization. Too small of an exhaust pipe or too small of a carburetor will definitely lose you horsepower because you won't be able to get the air in or out of the engine without significant pressure drops. (See above comment about pressure drops.)
Too big of an exhaust losing power? Depends on where it's too big. On the primary pipes (headers) there are sizes that help tune an engine's power band to match the vehicle requirements, but a large secondary pipe can reduce the backpressure seen by the cylinders or pistons. A big carburetor will almost always make more horsepower (than a smaller carburetor) due to the smaller pressure drop seen across the venturi area. The down side of a large carburetor is usually just the drivability issue due to this larger throttle/venturi area.
These are general statements. Each individual situation needs to be evaluated to determine the best combination of parts to arrive at the desired result. But keep in mind, ignoring the laws of physics won't make your car faster.

 

[84Turbo]

That might be true if the engine is designed for that, e.g., top fuel dragster. Otherwise it depends upon a lot of factors such as cam, timing, CR, etc.

Can you show me any examples, or any laws of physics supporting your position?

 

[73shark]

Nope, like I said I read it somewhere. But if less back pressure is good, then why do bigger engines have larger headers than smaller ones do?

 

[Evolution1980]

Nope, like I said I read it somewhere. But if less back pressure is good, then why do bigger engines have larger headers than smaller ones do?

There's lots of threads in the forum here dealing with the technical nature of backpressure and it's application. An advanced search of the forum should net you a few examples.

In short, the application of backpressure is not a constant across all engines and all applications. Maybe it would help to think of backpressure as an additive. Sometimes the additive helps your engine and other times it has no effect or potentially a negative effect.

Simply stated, from my somewhat hazy recollection of it, backpressure is a good thing in lower RPM situations where the flow of exhaust gasses isn't as rapid and the pressures are lower. I believe that it has to do with the "scavenging" of exhaust from the cylinder after combustion.

 

[84Turbo]

There's lots of threads in the forum here dealing with the technical nature of backpressure and it's application. An advanced search of the forum should net you a few examples.

In short, the application of backpressure is not a constant across all engines and all applications. Maybe it would help to think of backpressure as an additive. Sometimes the additive helps your engine and other times it has no effect or potentially a negative effect.

Simply stated, from my somewhat hazy recollection of it, backpressure is a good thing in lower RPM situations where the flow of exhaust gasses isn't as rapid and the pressures are lower. I believe that it has to do with the "scavenging" of exhaust from the cylinder after combustion.

Can you quote any law of physics that would support this?