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Unread 05-06-2006, 06:09 PM   #1
Rich Z
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Question Exhaust back pressure?

OK, I'm confused yet again. When I was considering which headers to get, there was some talk that getting the 1 7/8 inch header pipes compared to the 1 3/4 inchers would possibly be a step down in performance for a basically stock motor. Something to do about the pipes being too big, which honestly didn't make sense to me based on what I had "learned" years ago. Maybe no longer relevant to newer engines, I suppose.

Then lately someone said that my getting the 3 inch pipes from collector to the catback muffler pipes was possibly a mistake as well because they are "too big" for optimum performance. So I may have gained low end power at the expense of top end power.

But if this is the case, what purpose is served by these electric cutouts I see advertised? Don't they COMPLETELY bypass the exhaust system from right after the headers when activated? Conventional (and possibly outdated) wisdom seems to say that uncapping your headers gives you more horsepower because of the unrestricted exhaust.

So obviously I am hearing two directly conflicting opinions about the effects of exhaust mods, at least on the C5/Z06.

How much back pressure is "good" back pressure, if any at all? Would taking the pipes off of the header collector give you MORE or LESS overall power, or would it be a power band thing where power would be gained in one band of the engine's RPM range at the loss of somewhere else?

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Unread 05-06-2006, 10:19 PM   #2
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Ok Rich, I'm not the mechanic some of these guys are, so reserve my post here for further review. What I have heard, aside from all the, what for's and why for's, is that if you don't run a supercharger, stick with the 1 3/4's. Can't give the the educated reason, just what I've seen posted many times before. Now if you're planning to go supercharged at a later date, go ahead an get the larger header pipe, no harm done, but if you're sticking to stock motor go with 1 3/4, and I think that's all you have to make a decision on. From x-pipe back, take your pick.............Tom
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Unread 05-06-2006, 10:29 PM   #3
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Actually I have seen headers with 1 5/8th inch pipes as well, so there seems to be three you can actually choose from. I guess I just like to know the "why's" of some things. I guess this stems from the number of times I have heard "information" that stemmed from someone else making a wrong guess at what they thought should be the answer. And it snowballed from there. Take the 160 degree thermostats, for instance. A perfect example of that.

So I guess my question here boils down to why is ANY back pressure good? Isn't this the force the pistons have to exert in order to force the exhaust gases out of the cylinders and into the exhaust system? I would think that this has to be robbing the engine of some energy, since some of the energy created by the igniting fuel in other cylinders is being used for this process. Logic (maybe flawed) would indicate that zero back pressure would be the best solution to reducing that additional work load on the motor.
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Unread 05-06-2006, 11:40 PM   #4
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Yes some back pressure is good, too much=no, not enough=no. Sort of like the goldilocks formula...

Have to find the right combo for your needs of mods.

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Unread 05-14-2006, 02:12 PM   #5
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I guess what I don't understand and would like to know is WHY is any backpressure beneficial? I would think that it is wasted energy produced by the combustion in the chambers. Some of that energy is being used to force out the exhaust gases rather then applying that energy to the rear wheels where it belongs.

So is it simply a matter of timing being set to accommodate this backpressure, meaning that with NO backpressure and adjusted timing you could regain that lost power?

So why the electric cutoffs I see some people have installed on their vehicles? And doesn't the new C6 Z06 have automatic muffler cutouts at WOT? Heck I can still remember from my days going to dragstrips those cars with just straight pipes coming from each cylinder's exhaust sticking up in the air and angled backwards with flames shooting out of them.

Something just does not compute about all that I see and hear. And yeah, I'm one of those kind of guys who just doesn't accept things just because most everyone else does.......
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Unread 05-14-2006, 04:16 PM   #6
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I think it comes down to where in the power band you want the power.

Coming from turbos, smaller diameter headers help velocity which helped spool up time alot. so low end was really nice but it will "choke" in the higher rpms.
bigger did the opposite.
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