Look at the flow chart for the heads. That is positive air flow through the heads and it all goes into the exhaust. There have been many ways devised to lower the back pressure an exhaust system makes. There will never be a vacuum when you are blowing air into a pipe. Not unless you hook a pump to the other end that sucks more exhaust than flows through the heads. Just think about it. If there was vacuum anywhere in the exhaust that would mean negative flow. Positive flow means no vacuum. Positive flow with any restriction at all causes pressure.
I like that. You never see an arrow point inward from the exhaust. All out. Only potential for pressure. Simple physics. If anyone did find a way for an exhaust to be a vacuum it would be a great power adder. Like a turbo......
That would be true if all cylinders were firing at the same time or all exhaust valves opening. The vacuum or negative pressure occurs in the pipes that are not firing.Helping to clear them of spent gases.
Think of your header collector or manifold out pipe as like a sand blaster head, as the air pressure goes by the pickup tube the sand is drawn into the stream by vacuum. Same as in the exhaust.
Vikki has got it. It helps pull but there's no vacuum. Just lessening of the pressure. The engine is a big pump. You will never overcome that big pump. Lots of ways to reduce pressure but no vacuum.
We were talking in the context of an exhaust system causing a vacuum. An exhaust system will always have pressure not vacuum. There may be some push and pull near the heads but once you get to the end of the manifold it's all push. Pulsing push, but all push.
suction...but only very little change from atmospheric pressure in this case. It seems that at high rpm the pulses may be so close together as to not have much beneficial effect; it appears that too large a tubing at low pulse rates would also lose the scavenging effect. You can certainly feel distinct exhaust pulsed out the tailpipes of my car at idle, and is definitely pressure, but between pulses the pressure is much reduced.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching
Exactly, some reduction in pressure but all pressure. It has to be. Any vacuum in that system would 'increase' the pressure in other parts. Ideally, it would be a nice even low pressure in the exhaust. That's why you have the x pipe. To even out the pressure.
If you won't belive me maybe some of these people can help. Got thies from PY.
(Will)
A properly sized header will do something similar to what bastero is talking about with a 2-cycle engine. In a 4-cycle engine the idea is to use the dimensions of the header to work with the cam and intake tract to produce a scavenging effect on the cylinder. What happens with this scavenging effect is that the exhaust pulse heading down the headpipe creates suction on the cylinder behind it so that when the intake valve starts to open during the overlap period the fresh intake charge gets pulled into the cylinder by the suction created by the exhaust pulse.
In general terms, shortening or enlarging the diameter of the head pipes will increase the RPM range at which effective scavenging begins, and the opposite is also true.
Mr. Pebody Bruce nailed it. The ONLY purpose for blocking the crossover without isolating each port, is to keep the intake cooler. This may offer a little gain, but not much. By isolating each cylinder from the others, the scavenging effect, as Bruce noted, is improved. _The sound wave and vacuum signal are also much less disrupted. This can increase power at a specific point as much as 20-25 horsepower.
(Tom Vaught)
Tuning of ports works on the helmholtz principal. You get a wave form from the exhaust traveling down the header pipe to the collector discharge. If the header discharge is designed correctly (with the pointed triangle at the center of the 4 connecting pipes and the right sized collector diameter) you will get a strong reverse wave back up the pipe acting on the exhaust valve. You time the pipe length/ resonance wave correctly you get a mild form of scavenging
Oh yes. There's some 'mild scavenging' going on near the head(in a perfect system the expert said). But there isn't any vacuum in an exhaust system. Didn't you ever see a smart old mechanic put his hand over the end of the exhaust pipe to check for valve problems? If it's not all blow then there's a bad valve.
Yes. That makes sense. I understand about the scavenging near the head but that's not where this started. What I said I didn't believe was that one exhaust system, past the header, caused vacuum where another did not. It's a myth. A pressure differential makes sense. There's no arguing that.
I think the part that was the least 'palpable' was the thought stated that a smaller exhaust pipe caused more vacuum. What vacuum and why more because the pipe was smaller? Doesn't make sense. It would make more sense that the larger pipe allowed more flow which should have been countered by adjustment to use the better flow.
Jim, your quote,"past the header, caused vacuum where another did not".
This whole decision has been about the headers and the "pressure wave" in the header pipes and the size of the header pipes and the speed of the flow through them.
The system after the collectors can't cause vacuum.
I agree. I only was in disagreement to the comment about the different, smaller pipe causing better vacuum. This comment, "Reducing the pipe diameter in Banshee's applications apparently strengthened the vacuum pulses and generated more power."
It just didn't make sense to me. When you don't challenge things like that then one would assume they are true.
Well, you guys have been busy while I was gone! Can't begin to address all the different things that came up!
Okay, as decided, NASCAR engines will have different systems for different rpm ranges depending on the track but that goes with the complete engine package, not the same engine with different exhaust systems.
Tom's car: I will accept that back pressure in his case indeed helped but will continue to say that his situation was an anomoly and not generally how things work. I'd like to find out more about that specific combination.
Jim's insistence on an agreeable definition of the pressures in the header tubes is valid, especially if it helps someone understand the process. Just a note, drag headers sometimes have a one-way reed valve plumbed into the collector to make use of the pressure differential that exists in the collector between exhaust pulses to "vacuum" out the crankcase - up to 9" of "vacuum" - and reduce the amount of air and the resulting windage in the oil pan. That means that the pressure is about 4.5 psi lower than ambient atmospheric pressure and qualifies at least as a partial vacuum. GM also used this effect to pull additional air into the exhaust of the Cosworth Vegas so it could meet emission standards without the extra A.I.R. pump. Actually, the atmospheric pressure pushed the air into the partial vacuum in the header tubes. It was the P.A.I.R. system or pulsed air injection reaction system. Got pictures if you need 'em!
The comment about smaller tubing causing stronger pulses is true, because the air has to move faster to get the same volume of air out of the smaller tube. This faster pulse of air has a higher amount of kinetic energy and is denser than the pulse in a bigger tube. That means that the air behind the pulse is less dense or at a lower pressure, and that lower pressure will "pull" harder on the tube beside it as it goes into the collector. Also if that energy is not reduced before it gets to the end of the pipe, the resulting sound is louder than that from a larger tube because of the higher energy content in the pulse.
By the way, pulses do continue down the rest of the exhaust and give us that distinctive V-8 sound. The pressure fluctuations that you feel at the end of the tailpipe are a small fraction of the pulsing that you get right at the exhaust port.
One note to add to the pressure wave, a lot of people have trouble with the idea that a pressure wave can go back up the header tube even though the air is flowing the other way. Think of a river that has the water flowing downstream. A huge rock is dropped into the water. The wave from the splash will go upstream faster than the water is traveling downstream and if it were to hit the flat side of a boat, would slap against it just like a normal wave. This is what happens with the sound wave in the Heimholtz principle and is used extensively in 2 stroke tuning to slap or ram some fresh fuel-air mixture that has been purposely overscavenged back into the cylinder, effectively supercharging the cylinder with more air than it would have otherwise. The effect is very noticable when the engine "gets on the pipe".
I wonder if the average desktop dyno program compensates for the speed of the air column moving through the tube when it calculates for the Heimholtz wave traveling back up the tube. The higher the air column speed, the later the Heimholtz wave will arrive at the port. So by that theory, a small tube with a shorter length will behave similarly to a bigger tube with a longer length but will have less resistance to the air flow. That jives with present exhaust tuning theory, so it might be true.
Dual 2.5s would work fine, and if you want to fine tune even more, use 2.25 tailpipes after the mufflers. That will help to maintain the velocity as the exhaust gas cools and reduces in volume.
Dual 2.5s would work fine, and if you want to fine tune even more, use 2.25 tailpipes after the mufflers. That will help to maintain the velocity as the exhaust gas cools and reduces in volume.
LOL, guess what...my 2.25 pies coming out of the exhaust ,switching to 2.5 at the bend,, ends up with 2.25 splitters...
Any larger pipe in the system will help flow. It's that way with hydraulics, seems like it should be the same with exhaust. Even if you have smaller diameter, then larger, and then smaller again.
