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Discussion Starter #1
I have been running two high flow cats off the manifolds straight into an X and then dumped. Been running this set up for a while. Anyway yesterday I decided to add the mid pipes in that go around the gas tank and dump right after the diff to get rid of some of the interior noise. Well it worked great BUT it seems like I have lost substantial power in the 2k to 3200-ish rpm level. Is that normal? It doesn't seem like a few extra bends would make that much of a difference.. 3200 rpm and up seems the same as before tho.

I feel like I should check the pipes and make sure there isn't anything in there clogging it up.. Maybe it's just me? :zdunno:
 

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Discussion Starter #4
The pipes shouldn't be too big its only 2.5 inch. I mean I noticed a big improvement over the 2 1/4 pipe(s).

I've read some articles of scavenging in the exhaust... I can't remember exactly how it works but maybe when I added those extra pipes I moved part of the power band up farther? Idk I thought someone might know about this more than I would.
 

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I had a 85 5.0 Stang and one night at Music City raceway doing test and tune we decided pull the mufflers off and run with the h-pipe alone and after 3 runs it lost almost a half second . Then we replaced them and brought the time back we figure it needed that little bit of back pressure .....wierd :confused:
 

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Discussion Starter #6 (Edited)
Well I have heard of that before. Mufflers will have a larger impact on hp numbers than you think. That's pretty significant tho at almost half a second. Here's a link to a thread I started a while ago about different mufflers and their flow numbers.. http://forums.tccoa.com/showthread.php?t=137530
 

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Discussion Starter #7 (Edited)
Exhaust Savenging

Might be useful to some of you guys looking into a new exhaust system...


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/frequently-answered-dsm-questions/168578-exhaust-straight-scoop-backpressure.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/Miscellaneous/exhausttheory.htm
 

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I believe this scavaging effect also aids the induction of a new air/fuel mixture as additional vacuum is created in concert with the exhaust-intake strokes' during the brief moment where both valves are simultaneously open. Hense, the vacuum created in the exhaust system permaites to the intake manifold helping draw the succeeding air/fuel mixture into the cylinder much in the same way as a plunger on a syringe does when it is retracted.
 

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Discussion Starter #10
BUMP - More loss in lower RPM

Ok so today I finished the rest of my exhaust. I now have a complete dual exhaust from the manifolds all the way to the bumper. I put on a pair of super 44 rustmasters and turn downs. I will post pics tomorrow of it, maybe a video clip too. Sounds pretty good... really quiet.. :(

But since adding the rest of the exhaust I have noticed more top end and less low end power. When I was running two high flow cats into an X and dumped at the tail shaft of the tranny I could floor it and break the tires loose no problem. Even with the stock torque converter. When I added the pipes around the gas tank and dumped at the rear axle I found it was a little harder to break the tires loose but still easily doable. I then added the upgraded Mark 8 converter and I was able to break tires loose again no problem. Now here I am with the full exhaust with mufflers and I cannot break the tires loose without the brake pedal. I feel more top end but definitely a loss down low... :confused:
 

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How much do the mufflers weigh? Are you gauging the power of your car by how much it spins off the line? If your car is on the edge of losing traction, it could be tire pressure, temperature and type of pavement, weight in the trunk, how much gas you have in the tank. All kinds of things can determine traction.

edit. I just looked it up they weigh 11 lbs a piece, plus piping and tips from the x pipe back probably around 40 lbs total. Every little bit of weight in the back can help traction. Just remember the old saying...If your spinnin I'm winnin :D
 

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Discussion Starter #13
Cross over? As in X pipe?

No I'm talking same exact conditions. Same day within hours of each other. I can take the mufflers off and from a stop there's more power in the 2000-3000 rpm. Im thinking the mufflers/extra bends in the pipe moved some hp up around 3000-4000 rpm. Call me crazy..
 

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Have you considered going with an H-Pipe to get some of that low-end torque back?



Rayo..
 

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What's true for the exhaust is also true for the intake. Use less tubing with the intake and the rpm's of the power band will be in the lower rpm range but higher with a longer tube. Back when I had a Kia Rio I loved the long cold air intake that I put on it except during the rainstorms where I almost hydro-locked it. Somehow I seemed to lose all I had gained when I gutted the cats until I switched to a hot RAM setup during the next rainstorm.

You may want to try an H-pipe or go back to what you had before. Personally I think side exhaust has the coolest look even if it's a little annoying to those who pull up next to you and want to talk. I like the guy's car who had a lever that opened valves that dumped the exhaust right under the car whenever he wanted to. I think our stock intake besides the silencer and the old violin case is a great cross between a hot RAM and a true CAI that can easily be turned into a CAI if needed.

Do you still have the bung holes for the rear o2 sensors? If so, open them up and that might help some.
 

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Cross over? As in X pipe?

No I'm talking same exact conditions. Same day within hours of each other. I can take the mufflers off and from a stop there's more power in the 2000-3000 rpm. Im thinking the mufflers/extra bends in the pipe moved some hp up around 3000-4000 rpm. Call me crazy..
Well there's more than one kind of crossover, which is why I was asking which one you have.
 

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FWIW..Here's a dyno graph showing the difference in HP, and TQ with the "H" vs "X" pipe..

In the lower RPM range you can see the H-Pipe has a little more TQ than the X-Pipe..Then once you start getting into the higher RPM's the X-Pipe wins.. ;)




Rayo..
 

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I have been running two high flow cats off the manifolds straight into an X and then dumped. Been running this set up for a while. Anyway yesterday I decided to add the mid pipes in that go around the gas tank and dump right after the diff to get rid of some of the interior noise.
Unless he's replaced everything he's already installed, he's got an X. He didn't mention he replaced any of his existing setup, just adding to it progressively in stages.
 
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