Cold Air Intake, Headers, and Exhaust

[99LeCouch]

Perfect example just droned by. Some guy who is running what sounds like straight pipes on his Cobalt coupe. Buzzy and audible from 1/4 mile on a quiet night. Probably too large an exhaust not letting scavenging occur since he's (yes it's a he) flooring the thing through a nearby parking lot to move 15 mph.

[TOOSTUBBORN2FAIL]

ya, he probably has it way oversized, im running no muffler, but stock length pipe, stock diameter, using a delete pipe for the muffler, my car will hold idle in 5th up slight hills

[Christ]

When I get a diesel to run, I won't be using a baffled muffler, but it won't be loud, either.

PVC is acoustically inert, and it's fairly easy to make it reflect sound waves into each other so they cancel out.

[micondie]

I think these exhaust discussions tend to blur the distinction between stock exhaust manifolds and an aftermarket tuned header. The header is designed to give a scavenging effect at a specific RPM range while the stock exhaust manifold gives little if any scavenging at any RPM and just gives back pressure. Mods using stock manifolds should be aimed at getting the exhaust out as efficiently as possible in the RPM range normally seen which in our case means a not oversized pipe and a low restriction muffler.

[Christ]

...and headers are designed the same way that tuned exhausts are...

Furthermore, if you can show me a stock manifold that will provide back pressure when run to open atmosphere, I'll send a quarter to you in the mail.

Fluid dynamics is not nearly as cut and dry as that.

The way you posted that, it makes it seem like you're saying that if a manifold can't provide scavenging, it is providing back pressure, which is simply not true.

[micondie]

Anything bolted to your exhaust port that changes the direction of exhaust flow and restricts the exhaust gasses from expanding freely causes back pressure, even tuned headers. This back pressure can be minimized with proper design but always exists. The headers provide scavenging in a specific RPM range which is seen by the motor as reduced back pressure. Scavenging is produced by the expansion of gasses when the pipes join. This expansion produces a momentary pressure reduction and sends a low pressure sonic pulse back up the pipe. In order for scavenging to occur, this pulse must arrive back at the exhaust valve at the exact time the valve is opening. This timing factor is why scavenging is RPM dependent.
This is also why the guy in the Cobalt is not getting any scavenging from his "straight" pipe. Even if his pipe was straight off the manifold and not curved to fit under his car, his exhaust gasses would lose so much energy before reaching the end that any pressure pulse would be minimal and would not be able to travel back to the motor even if it could decide which cylinder to scavenge.

[Christ]

Originally Posted by micondie

Anything bolted to your exhaust port that changes the direction of exhaust flow and restricts the exhaust gasses from expanding freely causes back pressure, even tuned headers. This back pressure can be minimized with proper design but always exists. The headers provide scavenging in a specific RPM range which is seen by the motor as reduced back pressure. Scavenging is produced by the expansion of gasses when the pipes join. This expansion produces a momentary pressure reduction and sends a low pressure sonic pulse back up the pipe. In order for scavenging to occur, this pulse must arrive back at the exhaust valve at the exact time the valve is opening. This timing factor is why scavenging is RPM dependent.
This is also why the guy in the Cobalt is not getting any scavenging from his "straight" pipe. Even if his pipe was straight off the manifold and not curved to fit under his car, his exhaust gasses would lose so much energy before reaching the end that any pressure pulse would be minimal and would not be able to travel back to the motor even if it could decide which cylinder to scavenge.

That all relates to harmonic scavenging, which is only one type of scavenging. Reducing back pressure can be achieved even with a stock exhaust setup by increasing the pressure differential across the face of the valve, which can be done by simply increasing the temperature differential.

Hotter gasses will flow faster, and thus, have less pressure. The flow of gasses at high velocity will act as a vacuum, helping to lower the pressure diff across the valve's face, and allowing scavenging to occur.

In addition to, but not the same as, pulse scavenging.

[ncs]

And the practical application of this information is? A warm air intake will pay for itself on some vehicles. Are there any exhaust mods that will actually recover their cost over the life of the vehicle?

[bgd73]

Originally Posted by shovel

I have yet to ever see one single dyno sheet demonstrating a reduction in torque resulting from a reduction in exhaust restriction. I have seen countless dynos demonstrating an increase across the board, with a naturally greater increase at higher rpm's - but still a net increase at low rpm's as well.

When can we put that old story to rest?

I agree. Although, there is an error that can be gained with homogeny, especially japanese, it can be attributed to backpressure being to open. A small engine reveals this more than others. Some are just horribly hopeless without exhaust system somewhere in OEM math dimensions.

at 2.7 liters, it shouldn't matter. You may not even notice anythintg but vibrations of deeper fires. the same engine..opened, is all that is...
The cai helps everything, even a heat mixer version for carbs. Exhaust can stay one pipe or two, or headered, there is so many variables...

[Christ]

Originally Posted by ncs

And the practical application of this information is? A warm air intake will pay for itself on some vehicles. Are there any exhaust mods that will actually recover their cost over the life of the vehicle?

I guess that would depend on how much an individual were to pay for any mods, would it not?

Regardless of whether the payoff is there for everyone, the information is still valuable to someone.