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Exhaust 5.7 HEMI Exhaust Discussion


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Old 03-02-2011, 02:44 PM
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Default back pressure?

I have a 2008 1500 5.7L and I have noticed some people worry about back pressure when it comes to changing out there exhaust. So I guess my question is what does back pressuse do and will effect my power? I have been thinking about cutting my cat out and putting a try x crossover in then running the pipes back to the axle
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Old 03-02-2011, 02:49 PM
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No back pressure equals less torque. An X-pipe will help if you run true duals, but the two mufflers also help.
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Old 03-02-2011, 04:18 PM
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The problem is, virtually all fuel injection systems were designed for engines that are muffled and have some back pressure (picture your 80yr old grandma driving a gas guzzling exhaust snorting big block to church on sunday). The result is many times when back pressure is reduced or eliminated the fuel curve is disturbed and the engines often loose low end torque because of the mismatched lean fuel curve. A simple jet change many times wont correct this. It requires a completely new fuel curve that is beyond most people's capability; this results in many people believing some back pressure is necessary.

Now, that being said, at top end when cam timing is the secret to getting the most HP out of the motor, a zero back pressure system is ideal, just dumped right out the headers and you're laughing, but at low speed/RPM, your engine is struggling to pull air into the combustion chamber

Consider the motor as being an air pump. Restrictions at either end (intake or exhaust) limit output as well as efficiency. So, in order to improve efficiency on the exhaust side, most folks go with a larger and lower restriction system. However, you can reach a point of diminishing returns when the exhaust system is too large a diameter pipe and the scavenging effects are reduced (might reduce the exhaust pulse)...

That happens because of exhaust velocity. Notice how a header is all bent funny? Wouldn't straight tubing be simpler?

Yes.

But, the tubes are made from exact lengths for a reason... when each exhaust pulse hits the collector at the end of the header, the pulses are timed in such a way that it creates a small vacuum (ie: Scavenging) that increases power by reducing pumping losses from the exhaust stroke, and helping to suck in the new charge of fresh air during valve overlap.

When your exhaust is TOO large, then the speed at which the pulses hit the collector goes down, reducing the scavenging effect. This is why some old farts say when they tell you that "Backpressure is NEEDED by the engine to make sure you valves work properly, blah, blah, blah". Truth is, most of them just don't understand the purpose of an exhaust manifold (header).
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Old 03-02-2011, 04:39 PM
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I just got schooled! Thanks Kurtis!!!
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Old 03-02-2011, 05:13 PM
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Okay, that actually made a lot of sense. Thank you very much
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Old 03-02-2011, 05:22 PM
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A bit more on this subject:

A constant lean fuel curve can result in exhaust valve burnout as it will not be cooled properly. The loss of torque assosiated with lower backpressue is due to not having enough air to burn the fuel, ending up with an air/fuel ratio below the ideal stoichiometric ratio. Most motors already burns below the ideal 14.7 A/F, using some of the unburned fuel as a cooling agent to lower the combustions temperatures to reduce the build up of NOx gasses.

As for the exhaust after the headers, most common is a 3" collector, however, routing a 3" true dual will actually decrease the backpressure (good) AND lower the exhaust gas velocity (bad). A true dual system is ideal; however, you need to balance the exhaust bank pressures or you can end up off-balancing the harmonics of the motor, such that you will not be drawing equal air volume/densities into each head resulting in unbalanced combustion across your crankshaft.

If I was to set up a true dual system then I'd look at a 2.75" with either an H-pipe or X-pipe set up. Benefits of an X-pipe are there's fewer 'sharp' corners and the system flows smoother, the downside is trying to find the room to do it properly incorporating our fuel tank and driveshaft. An H-pipe is a bridge between the two banks, normally of a narrower diameter than that of the main pipe. It will balance the pressures between the two banks, the down side is unless you join to the main pipe using a bellmouth you're inducing a structural ridge to the flow, which is not ideal to flow velocities. An H-pipe is much easier to build compared to an X-pipe.

I won't even begin to get into why this all started with carbeurators and back fueling
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Old 03-02-2011, 05:25 PM
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Quote:
Originally Posted by 916Timmy View Post
I just got schooled! Thanks Kurtis!!!
Quote:
Originally Posted by creed0423 View Post
Okay, that actually made a lot of sense. Thank you very much
Glad I can help... see mom... I still use my engineering degree from time to time
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Old 03-02-2011, 05:51 PM
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Leave the Cats on. I'm in the middle of designing a true duel with 2-1/2 and a cross over and two 4X9 Magnaflows. I'm taking the pipe on the driver side through the trans brace and turning it behind the trans under the yoke and run parallel on the passenger side. This will keep the exhaust heat from the trans as well.
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Old 04-11-2011, 09:05 PM
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what about leaving the cats and using a single exhaust with no muffler or resonators? this is what i was thinking of doing? any advise...?
i do plan on doing a duel exhaust in the future but want the sound now. i plan on leaving the cats coming back to a 2 in 2 out cherry bomb extreme then heading out the back at angle what r ur guys thoughts? good idea or not?
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Old 04-11-2011, 09:23 PM
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and also about the back pressure i was always under the impression that you needed it for the mds to work properly?
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