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post #1 of 3 (permalink) Old 06-27-2009, 10:28 PM Thread Starter
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Opinions on Exhaust Dump Location

It will still be a few months, but I plan on fabricating the exhaust for my car using the mandrel bends back half for a true dual setup. I will have to make a set of downtubes from my shorty headers up to my di/do magnaflow, but I have highly been considering putting on a set of these style dumps. I will only be using them for the track and any local cruising.

http://www.jegs.com/i/Dynomax/289/88340/10002/-1

My question is, if I decide to run them should I put them in before or after the muffler? At this point I am leaning towards putting them after to avoid too much lost back pressure and so it doesnt get too noisy.

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post #2 of 3 (permalink) Old 06-28-2009, 06:16 AM
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Put them in the front - just after the cats. Like this guy did.
Putting dumps after mufflers so that they're not to noisy defeats the purpose of having dumps. Dumps are designed and ment to be used for racing and off road use. I'm pretty certain they're not street legal. (You can have them on the car, you just can't open them on the street.)

So, forget about opening them up at the local cruise unless you're sitting in a parking lot and just want to show them to someone. Otherwise, you run a good chance of getting a ticket. (Check your local laws for which law applies.)

Oh, and FYI...

Torque loss due to loss of back pressure is an Urban Legend / Myth.

Here's another good explaination from a fellow member here.

Quote:
Originally Posted by MadMikeyL View Post
ARGGH! The whole "backpressure in the exhaust" thing is another one of my pet peeves!

You never want backpressure in the exhaust, plain and simple. If you had any pressure in the exhaust at all, your power would drop like a rock. Ever drive a car with a plugged-up cat? It probably wouldn't go more than about 30mph, right? THAT is backpressure in the exhaust, and as you can tell, it didn't help the power of the engine at any point along the way.

You want that vacuum I was talking about for the scavenging effect, and vacuum is the opposite of pressure. The thing is that vacuum is only created by the velocity of the exhaust gases leaving the pipes. If you open up the pipe without increasing the quantity of exhaust gas that is going through it, you have just decreased the exhaust gas velocity. Decreased velocity causes decreased vacuum for scavenging, which is technically the same as increasing the pressure in the exhaust. So going to too large a pipe is technically increasing the pressure in the exhaust system. I bet nobody on the internet has ever told it to you that way before! Increased pressure (or decreased vacuum, since they are really the same thing) means that the exhaust has to be pushed harder to get out of the engine. This means more of the power the motor made is wasted pushing the exhaust out the tailpipe instead of pushing the car forward.

So, ANY pressure above ambient atmospheric pressure is going to drastically hurt your power and torque curve all the way across the line. While it is true that putting too large an exhaust on a car can hurt low end, the reason is not because the stock system has backpressure, but quite the opposite it is because the stock system has enough velocity at the lower rpms to create a scavenging effect. On a high perfrormance car, it might be beneficial to you to sacrifice a few lb-ft of torque down low to gain a few hp up top, and that is what the larger diameter pipe does for you because it moves both the rpm at which the scavenging occurs and the rpm at which the exhaust becomes a restriction up higher in the powerband. If you have a completely stock engine, and you don't rev it higher than the factory set rev limiter, and you go put dual 3" exhaust on it, you have just lost some low end torque, but you haven't gained anything up top because the operating range of your engine and the operating range of your exhaust system are now nowhere even close to each other.

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Last edited by Trunk Monkey; 06-28-2009 at 10:09 AM.
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post #3 of 3 (permalink) Old 06-28-2009, 12:51 PM Thread Starter
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Hey thanks for the info trunkmonk! I shouldnt have worded it the way I did lol I guess I opened myself up for that one, I should have said, to avoid loosing too much low end power.



Thanks!
Mike

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