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Old 01-11-2013, 07:33 AM   #1
rbpexman
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2012...Magnaflow good choice?

I need quick feedback as I may have a Magnaflow put on this morning. Please...what do you folks think about it?


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Old 01-11-2013, 10:15 AM   #2
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Cat back or just a muffler?

I'm trying to figure out if the 2.5" catback from an ST, modded to fit, would be a better choice than the standard 2.25" model for the N/A cars currently...
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Old 01-11-2013, 10:35 AM   #3
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Cat back or just a muffler?

I'm trying to figure out if the 2.5" catback from an ST, modded to fit, would be a better choice than the standard 2.25" model for the N/A cars currently...
The 2.5" would have a lower restriction than the 2.25" tubing, however if you go too large you reduce the backpressure too far and start to hurt performance.

Its been discussed that the 2.5" might be a better plan if you are planning on adding additional power adders down the line. If the limit of your modding is an intake and exhaust and nothing else a 2.25" exhaust may work better.
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Old 01-23-2013, 10:53 AM   #4
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The 2.5" would have a lower restriction than the 2.25" tubing, however if you go too large you reduce the backpressure too far and start to hurt performance.

Its been discussed that the 2.5" might be a better plan if you are planning on adding additional power adders down the line. If the limit of your modding is an intake and exhaust and nothing else a 2.25" exhaust may work better.
Back pressue is not a factor in direct injection engines, as fuel charge scavenging can not happen, becasue the fuel charge is delivered at full compresion.
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Old 01-26-2013, 07:58 PM   #5
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Back pressue is not a factor in direct injection engines, as fuel charge scavenging can not happen, becasue the fuel charge is delivered at full compresion.
No but you still need to clear the combustion chamber of burned gases.
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Old 01-28-2013, 10:39 AM   #6
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No but you still need to clear the combustion chamber of burned gases.
Yes, that's why we have such things as cam overlap. Back pressure would prevent some gasses from leaving the chamber.n

Remember these motors run 12:1, they are probably pretty good at clearing the cylinder anyway.
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Old 01-30-2013, 07:36 PM   #7
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Back pressue is not a factor in direct injection engines, as fuel charge scavenging can not happen, becasue the fuel charge is delivered at full compresion.
You want too still cram as much air as possible with a scavenging affect. Air is still a factor in power making.
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Old 01-31-2013, 08:29 AM   #8
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You want too still cram as much air as possible with a scavenging affect. Air is still a factor in power making.
Right, and having less exhaust backpressure, by removing restrictions in the exhaust system will increase this effect to a point.

People are getting all bent outa shape about loosing back pressure in these Focus motors, it's not as much of a foctor as say a Honda. Opening up the exhaust systems on these new Focus engines is really going to wake them up.
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Old 01-11-2013, 11:03 AM   #9
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Soooo if a 5.0L pushrod made 225hp from the factory with 2.25" dual pipes... Why does it gain power moving to a 2.5" system? I daresay that a 2.0L making 160hp from the factory is moving much more air volume than that weezy 2.5L per bank engine.

I honestly don't give much credit to the backpressure theories on modern cars where multiple cats are retained. The cats are a blockage, and force backpressure to be retained for the engine side of things, cancelling out most if not all pulsation in the exhaust flow, and anything post-cat system is made better by open pipes.
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Old 01-11-2013, 11:30 AM   #10
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Soooo if a 5.0L pushrod made 225hp from the factory with 2.25" dual pipes... Why does it gain power moving to a 2.5" system? I daresay that a 2.0L making 160hp from the factory is moving much more air volume than that weezy 2.5L per bank engine.

I honestly don't give much credit to the backpressure theories on modern cars where multiple cats are retained. The cats are a blockage, and force backpressure to be retained for the engine side of things, cancelling out most if not all pulsation in the exhaust flow, and anything post-cat system is made better by open pipes.
Pv=nRT, which is the ideal gas law will explain the principle of how the exhaust gasses flow at a very basic level.

The stock system on the 5.0 L was quite restrictive at the time.

Modern catylasts don't pose nearly the restriction on flow or increase in back pressure as the earlier systems did as well.

Only the PZEV models have two catalysts though for the 2012+ 2.0 L GDI Ti-VCT, the regular models only have the integrated exhaust manifold catalyst.
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