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Old 05-11-2007, 03:09 AM   #1
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Will an exhaust leak post cats cause power loss?

Just wondering if an exhaust leak after the cats would cause any power loss. Reason I ask this is because I've noticed a number of Cayman and 987S folks have been doing a mod to their factory exhausts to get more sound- as their exhaust is designed as a 2-can exhaust system with two crossover pipes from one can to the other, some have loosened the sleeve from the top crossover pipe and slided them over to allow a gap between the pipes up top, allowing exhaust to escape from there.

The sound is pretty amazing after this mod...I've driven a friend's car with this mod done, but I was wondering if this sorta muffler bypass causes power loss. As it is post cats, it shouldn't, right?

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Old 05-11-2007, 05:45 AM   #2
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you are correct in your assumption.
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Old 05-11-2007, 08:32 AM   #3
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Hi,

Actually, you may be incorrect. Too little Back Pressure can result in a loss of Torque in the low RPM range. Its a very complicated subject, and any Exhaust System is a compromise...

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Old 05-11-2007, 08:42 AM   #4
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Actually, I've read somewhere that the Boxster needs some backpressure to produce it's power, hence me asking the question. One thing I did notice was that when I drove the Cayman S (3.4L 295hp) and then my friend's 05 Boxster S (3.2L 280hp), the Cayman felt noticeable stronger than the Boxster S. I'm not sure if this is because of my friend's self modded exhaust or not, or if it was the difference btw the engine sizes.
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Old 05-11-2007, 09:33 AM   #5
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As I understand the exhaust backpressure issue (dealt with this a bit when I changed out the exhaust on my previous car... replaced headers and catback, and had a high flow cat installed), some is good for N/A engines.

Some backpressure is good for N/A cars, as it helps with low end torque, but can end up reducing high end horsepower. Reducing backpressure can reduce low end torque, but should help with high end horsepower.


Of course, actual changes in actual cars may differ, so it's pretty subjective.

When you say 'lose power', it's also subjective. 'Power' as in low end torque, or 'power' as in peak horsepower could be different things.


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Old 05-11-2007, 09:42 AM   #6
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Absolutely. I do recall now, that what I read indicated that with N/A cars, a little back pressure is good for torque, while removing back pressure will probably decrease torque and increase hp up top in the rev range. I believe that with force induced cars, reduced back pressure gives good gains in both torque and hp.
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Old 05-11-2007, 10:31 AM   #7
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Quote:
Originally Posted by Boxtaboy
Absolutely. I do recall now, that what I read indicated that with N/A cars, a little back pressure is good for torque, while removing back pressure will probably decrease torque and increase hp up top in the rev range. I believe that with force induced cars, reduced back pressure gives good gains in both torque and hp.
Hi,

Only when On Boost, otherwise, it is an NA car...

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Old 05-11-2007, 11:42 AM   #8
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I wish there was an exhaust which had butterflyu valves which opened at a certain RPM... you know, best of both worlds.

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