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post #1 of 8 (permalink) Old 06-24-2012, 09:04 PM Thread Starter
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Open headers?

Anyone run longtube open headers, or do these engines need more backpressure than that? I can't claim to be very knowledgeable about exhaust theory. I'm building a car to drag race, so no concern about db level. Pretty much stock 5.0 for now.
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post #2 of 8 (permalink) Old 06-24-2012, 09:54 PM
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That would sure be loud. Isn't there danger of sucking a valve by doing that? I ran open pipes at the former location of the third cat for a few months on my old 95 bird...full throttle sounded like a jetboat going down the river, especially in a tunnel...fun, fun, fun...but it got old and I had a mufflerless exhaust with an X pipe welded together, exiting at the rear bumper...still nearly as loud, but not right in front of and/or under you.

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post #3 of 8 (permalink) Old 06-24-2012, 10:27 PM
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Quote:
Originally Posted by Ahapp88 View Post
I'm building a car to drag race, so no concern about db level.
Some tracks have a dB restriction, so be sure to check.

A sidenote about all that noise.....I once ran a '78 Cutlass with a built 350 Olds on the street for a bit while I was waiting for the chance to finish out the rest of the exhaust. I was cool for about 10 minutes. lol After that I absolutely dreaded driving it anywhere until the exhaust was finished because it was the most annoying sound in the world



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Originally Posted by Ahapp88 View Post
....Pretty much stock 5.0 for now.
And then no need for the open headers yet. Unless you just want loud.

Delete my account.
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post #4 of 8 (permalink) Old 06-24-2012, 10:41 PM Thread Starter
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At the very least I am taking off the cats & mufflers, but do I need to run a X pipe for equalization / backpressure? At the local track a good friend of mine runs a 632 Dart block w Big Chief heads in a 69 Camaro w open headers and man that thing will split your skull, so no, Idont think there are any db restrictions there! Thanks
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post #5 of 8 (permalink) Old 06-24-2012, 11:32 PM
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Don't Run Open Headers

Quote:
Originally Posted by Ahapp88 View Post
At the very least I am taking off the cats & mufflers, but do I need to run a X pipe for equalization / backpressure? At the local track a good friend of mine runs a 632 Dart block w Big Chief heads in a 69 Camaro w open headers and man that thing will split your skull, so no, Idont think there are any db restrictions there! Thanks
I'm sure your "friend" is making some big-time horsepower with that Engine set-up..



When you have an Engine that makes over 700 Hp..

It's beneficial to run open Longtube headers..Because they're pumping so much air through the Engine..




On a basically stock Engine it's actually going to hurt your Quarter Mile times..

You're never going to make over 700 Hp on your stock 5.0 block..

So just forget about running Open Longtube Headers..







You can run an H-Pipe or X-Pipe for Equalization..or..Straight Pipes all the way back..

That's entirely up to you..

An X-pipe might be the most beneficial powerwise with your current and future mods though..






Rayo..

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post #6 of 8 (permalink) Old 06-25-2012, 03:40 AM Thread Starter
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Thanks Rayo, that's what I needed to know. That Camaro I was talkin about was just in reference to the possible db restrictions that 94 Thunderbird XX mentioned above, I wasn't trying to compare performance aspects of the two. I think his car uses about as much fuel in one pass as mine has displacement!
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post #7 of 8 (permalink) Old 06-25-2012, 05:38 AM
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Also, there is this prevalent misconception that engines need backpressure to run properly. That is simply untrue. Take, for example, a car with clogged cats. Plenty of backpressure - but no power, if it runs at all.

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Originally Posted by Tbird1997 View Post
Might be useful to some of you guys looking into a new exhaust system...

Quote:
There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.
Found this article here... http://www.dsmtuners.com/forums/freq...kpressure.html


Also found this article (a bit more lengthy)... Some really good stuff in here but also some not so good stuff... http://www.nsxprime.com/FAQ/Miscella...austtheory.htm

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post #8 of 8 (permalink) Old 06-25-2012, 10:06 AM
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