Good Idea???????

[Doug Rodrigues]

The theory about back pressure being necessary for more power makes no sense to me either. Even the "Tuned Exhausts" only function properly at a certain designed RPM range to have that effect of dragging exhaust pressure out of the system by way of a pressure pulse. Any higher or lower RPM range from the calculated length of the "tuned exhaust" manifold would make that exhaust system no better than the stock cast iron one.

[MagicRat]

Originally Posted by 59elcooldsuv

I can't hold it against anyone for not "getting it" - only suggest that they make some effort to try to think about it before jumping to a contrary conclusion.

Although I (mostly) understand the theory and principles, most people 'don't get it' including the header manufacturers and customers.
For most street applications, a Tri - Y header scavanges the cylinders better (over a wider rpm range) and is easier to install than the traditional 4 into 1 style (which usually are tuned to only scavange cylinders effectively in a narrow very high rpm range.
However, Tri Y headers traditionally have sold so poorly, few are made today. Why?? As much as I can tell, people see the 4 into 1 type on all the race cars so they want them for the street.
The concept that a race application and a high performance street application are different things does not occur to most header buyers.

The plague of 4 inch rice rocket fart cans out there has not helped either. They just popularize the false notion that bigger+ noisier= more power, with no understanding of the principles involved.

[Reed]

Without trying to start an argument I kind of take offense to the statement that I "don’t get it". I like to think that I have a pretty good understanding of how exhaust gasses work. I understand that header length has everything to do with where exhaust gas pulses meet and pull one another that is all basic physics and Bernoulli’s principle ( VELOCITY increases pressure drops, yeah I know) but back pressure and impedances in the exhaust flow have a ton to do with velocity. And where was my contrary conclusion. I basically said that a big tube is not going to give you more power except at very high rpm and especially not a 6 in tube which is just beyond impractical.

[SaabJohan]

Exhaust scavenging is mainly an issue for the exhaust manifold. However, to get the lowest pressure loss (backpressure) from a pipe both the velocity and pipe diameter should be low, the pipe diameter has to do with that with a larger diameter pipe the flow tends to be more turbulent which increase the pressureloss (backpressure).

[Doug Rodrigues]

I understand the theory about the "Tuned Exhaust," but so far no one has given any logical explaination here for back pressure. At a given amount of exhaust produced, a smaller diameter pipe should have low volume, and high pressure. A larger diameter pipe should have high volume and low volocity. I know what Bernoulli's principle is, but I believe that it relates to free airflow and not exhaust pressure pumped through a pipe. Bernoulli's ideas had to do with a curved surface speeding up the flow of air and therefore creating a low pressure area. It didn't have to do with packing pressure into a pipe from a pump called a piston and cylinder. "Tuning" an exhaust has nothing to do with Bernouli, but everything to do with timing an exhaust pulse. The range of that "tuned exhaust" is usually limited to about 300 RPMs. Any faster or slower than the range at which the exhaust was "tuned" would negate any gains from that "tuning." So if someone installed a "tuned exhaust" to make their engine run better, but they only drive around town at lower RPMs, money spent on that "tuned exhaust" was money wasted. Ahhhh, why lose sleep?: I only use stock parts for repairs anyway.

[Reed]

actually when bernoulli made his principle he was using a metal tube with a dent in the side thus restricting airflow. his principle talks about the gas that is directly in the narrowest part of the tube where it is indented not the gas before or after the indention. So yes bernoulli made his principle using airflow over curved surfaces but the statements he made apply to all air all the time.

bernoulli's principle basically just gave us this:

as temp increases-pressure increases and vice versa

as velocity increases-pressure decreases

there are a few others that are basically the same as this but they arent that reletive to this discussion

so exhaust scavenging has a lot to do with bernoulli's principle but more to do with pulse timing and gas velocity.

if you hvae two pipes coming together in a Y, and you send a pressure wave through one of them with the end of the other Y closed ( valve closed ), and then you open the valve right after the pressure wave passes the junction in the Y, the pressure wave will help to suck the exhaust out of the other cylinder. But for this to work the headers need to be the right length and diameter and the exhaust system has to move just the right amount of exhaust gas or else the pulses wont be at the right place in the Y junctions at the right time (or if its a 4-1 same idea) to help suck the gas out of the cylinder. Also any restriction ( even tubing diameter) in any of those pipes will create a high pressure area that will reduce velocity and throw off the whole system. (restriction = backpressure) so too big of a pipe will move air too slowly, and too little a pipe wont move enough air.

wow thats confusing

[Doug Rodrigues]

I agree: You've just described a "Tuned Exhaust."

[travsstuff]

well guys thanks for the help. I might consider a smaller pipe maybe 3 1/2-4 inches

[Reed]

you didnt mention what kind of car you are getting but if it is a naturally aspirated 4 or 6 cylinder then 3in should be plenty even for a turbo 4. if its a built or turbo 6 then you could go with dual 2.5 or 3in pipes.

[Auto_newb]

Originally Posted by Sluttypatton

Valves would stop it if the motor had zero valve overlap, however all modern 4-stroke motors have some degree of valve overlap. Valve overlap means that the intake valve will be open for a fraction of a second during the end of the exhaust stroke in order to further facilitate scavenging. Normally what would happen is that the high velocity gas creates an area of negative pressure behind it as it leaves the cylinder, then the intake valve opens allowing fresh fuel and air to be drawn in by the negative pressure and help flush out the remaining exhaust gasses. Then the exhaust valve closes and the intake stroke begins. However, if the exhaust gas has little velocity it will hang around the exhaust ports and will not create the negative pressure area behind it in the cylinder that high velocity exhaust does. This means that it doesn't scavenge, and during the intake stroke there is no negative pressure in the combustion chamber to suck in fresh fuel and air through the intake valve.

Sorry for bringing up an old thread, but at least I didn't make another one. Something just came to my mind, doesn't valve overlap become a REAL serious problem at higher RPMs?

[Sluttypatton]

Anyone who ever gets upset at you for bring back a post for a valid reason should shut up, that is okay, as long as you don't add mindless drivel.

Please explain what you mean by problem? Why do you think it would be a problem? That would help me answer the question a little.

[travsstuff]

soory about not mentioning the car it is a 1998 dodge stratus ES. I will definetly take the advice. If you guys have any other ideas on how to make my car better w/o spending a bunch of money please tell me. any help would be appreciated.

[Auto_newb]

Originally Posted by Sluttypatton

Anyone who ever gets upset at you for bring back a post for a valid reason should shut up, that is okay, as long as you don't add mindless drivel.

Please explain what you mean by problem? Why do you think it would be a problem? That would help me answer the question a little.

Ok then, were good

I mean that since the exhaust gasses don't scavenge, valve overlap lets in some exhaust gasses if the engine has a big exhaust right? But at lower RPMS, isn't there just the smallest degree of valve overlap? Then at higher rpms, there is major valve overlap right?

[Reed]

thereis always the same DEGREE of valve overlap regardless of rpm unless you are running a system that changes cam timing like vvtl-i or v-tec or something. the actual time the valves are open decreases with rpm cause they everyhting is moving faster.

[Sluttypatton]

Exactly, valve timing is measured in degrees. Each cycle in a four stroke engine is 720* at the crankshaft and 360* at the camshaft. Since the timing is set in degrees, when the crankshaft speeds up only angular velocity is increased, not the lead and lag angles.