Good Idea???????

I don't have my car yet, but I am trying to decide what I am going to do with it whne I get it. I am thinking about just hacking the exaust system off and running a 6in. pipe out the back. Also putting high performance plugs and a homemade tam air system. Just poor mans mods. Are there any other poor mans mods that I could do?

a 6inch pipe? thats gotta be one hell of an engine. . .

no I just want it to sound big=)

well u will lose a ton of back pressure and your car will be slow as shit

yea, not to mention it's also illegal

you could gut the interior by removing the rear seat and any fabric and whatever you find behind the fabric. you could also gut the doors. the stuff in doors weighs a lot. my friend just made a turbo setup for his civic for about $400 by using left over stuff from some of my other friends, you could think about that.

well u will lose a ton of back pressure and your car will be slow as shit
Backpressure doesn't make an engine run... It's the combustion inside the cylinders that eventually put power to the wheels. Backpressure is something that you don't want.

ok i know how an engine runs dont insult me, and doesnt back pressure produce scavaging?

Your right, you don't want backpressure*, but it does have something to do with exhaust velocity, which is something that you do want. You can't just bolt on a huge pipe so you don't have any backpressure, as that would kill exhaust velocity and exhaust gasses would hang around the exhaust ports a lot longer. Low velocity exhaust doesn't scavenge very well (or at all) and leaves you open to reversion, which will dilute your intake charge and can very easily ignite it prematurely as the exhaust is very hot. This diluted intake charge, in combination with the lost scavenging effect, is what makes a motor loose power in the absence of any backpressure.

*Technically you don't want any backpressure, but I am using the term loosely (read incorrectly) to describe both actual backpressure and delta pressure. However I didn't feel like flinging around the term delta pressure to describe it more accurately, so just remember, when you bolt on huge exhaust you loose velocity.

Also, dude, 6 inch diameter piping is huge.

Are you saying that without any backpressure, the exhaust will go back into the engine? Don't the valves stop that?

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.

as an extremely general rule of thumb for naturally aspirated engines, a bigger exhaust will actually make you loose a little torque at low rpm but gain it back at high rpm. some backpressure actually helps to make low end torque by doing the things already posted. this is why if you put a turbo on a car and then put it on the dyno you sometimes see increased torque at low rpm before the turbo even starts to come up from vacume, because of the restriction of the turbine blades.

I'm sure someone will say I'm wrong but this is what I have seen in my limited experience.

Phhffft! Ha LOL

Sluttypatton wins the prize for the best attempt to answer a complicated question.

Reed wins the prize for Not Getting It.

1) backpressure is NOT the issue - VELOCITY is what you get from smaller pipes. VELOCITY contributes to scavenging. Backpressure impedes it.

2) a 6" dia straight pipe will drag the pavement. Even professionally built headers will often drag the pavement.

I did my best to answer it as simply as I could, but I should not have used backpressure in place of delta p. I thought it would make it easier to understand, but they are two completely different things. For the sake of clairity, delta p is the pressure drop between two points, backpressure is the resistance to flow in the exhaust tract. So you can see that delta p isn't pressure at all, it is the difference of pressure between two points.

I have been working in Engineering, or more precisely 'doing the dirty work' for Mechanical & Piping Eng's (draftsman & designer), for over 25 years and I STILL have trouble conceptualizing & visualizing a 'chunk' of hot, high pressure gas flowing down an exhaust tube and dragging a low pressure wave behind it.

Watching how traffic 'flows' (I use the term loosely) on the freeway (another term used loosely) helps me to grasp it. No kidding, a perfect model of what a fluid system would look like if you could actually "see" pressure.

It gets even more complicated when you try to visualize how that high pressure wave bursting into a collector creates a low pressure wave that 'sucks' on the other tubes of the header set or manifold.

Whoa! Boggled mind! And to think, I'm actually considering welding up my own set of headers for an engine & body combination for which there are none commercially available.

Thermodynamics & fluid flow is a fascinating subject, but not easy to grasp without spending a great deal of time carefully studying all the related subjects.

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.

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.

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.

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.

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).

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.

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

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

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

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.

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?

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.

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.

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 :D

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?

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.

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.

Can I ask what scavenging is?

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.

Are you saying that as RPMS rise, the only thing that changes is the ignition timing?

Can I ask what scavenging is?
When we say scavenging, we mean how fast it takes for the exhaust gasses to leave the engine. When there is no scavenging, the exhaust gasses would just hang around outside the exhaust valve, when there is lots of scavenging, the exhaust gasses would shoot out the pipe.

most normal engines maintain an ignition timing that is a set degree before the piston hits top dead center. on more modern engines with computers and whatnot you can have the ignition timing advance or retard on the fly but ignition timing especially on engines with magnetos and for the sake of argument stays a set degree before tdc.

We aren't talking about ignition timing, we're talking about valve timing. Unless the valvetrain incorporates some sort of variable valve timing mechanism, the valve timing will remain the same. I am saying that since valve timing is set in degrees relative to crankshaft rotation, and the camshaft is driven by the crankshaft, if the crankshaft speeds up, so will the camshaft and therefor everything that the camshaft controls. The valves will still open and close at their preset degrees, but will do so proportionately quicker. As long as the camshaft maintains it's speed proportionate to the crankshaft, if the crankshaft speeds up, so will the camshaft. So everything happens more quickly, but still at the same degree of crank rotation.

Ok, so nothing changes as RPMs rise? Unless you got V-TEC?

I know I'm a visual learner, so maybe a diagram would help you get it (if you haven't already).

http://files.automotiveforums.com/gallery/watermark.php?file=/500/63345Crank_rotation.jpg

Lets say for the sake of the example, that a motor could run at 2 RPM. At 2 RPM the crankshaft will rotate 720 degrees in 60 seconds (2 rotations in one minute). Since the camshaft is geared to the crankshaft at a 2 to 1 ratio, the camshaft will only rotate 360 degrees in that same minute. Let's say that the intake valve closed 45 degrees after BDC on the compression stroke (225 degrees). This would mean that the intake valve might close somewhere around 18.75 seconds into the minute

Now, if the engine speed were doubled to 4 RPM; the intake valve still closes 45 degrees after BDC on the compression stroke, however, since the motor has doubled in speed we can now see that the intake valve would close for the first time around 9.375 seconds into the minute. We can see that the engine has doubled in speed, so the intake valve is closing twice as soon. However, this doesn't change the fact that it is closing at 225 total crankshaft degrees (or 45 after BDC).

As for Vtec, I'm not sure if it has other parameters that need to be met in order for it to be actuated, but I know it is hydraulically actuated, meaning that it isn't directly engine speed that dictates when the Vtec changes the valve timing, but oil pressure.

In short, valve timing and overlap do not change with engine speed unless you have some sort of variable valve timing mechanism. Your dead on.

Hopefully that answers the question, and if you already got it, sorry. Also, I'm no artist so if that diagram isn't the greatest, sorry. I'm sure all the times for the valve closing are wrong, I was just using them to illustrate the point.

Alright, understood now, thanks :D