2.5" exhaust - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Hey gals and guys, here's my (almost) finished 2.5" straight pipe exhaust I rigged up. I know this sounds rather excessive, but there is a method to the madness lol. So, without further adeu... Gut the stock system. Yuck. Seriously, this is an awful exhaust. So.much weight, such little pipes, I get it (emissions, sound ordinances, yadda yadda) but geez come on. 1.5” pipes after the second cats? I'm amazed the poor girl can breathe at all. After thats out, clean er up real nice nice and get ready for the goods - Chinese headers of course! Painted with high temp grill paint and then wrapped. (everything is painted, not for thermal effect, just for rust prevention). Here's the tricky part - the collector on these is only 2". Not big enough. The 987 collectors are 2.25". Getting better but at a grand a pair that's just ridiculous. So what to do? Fab an adapter! This was the hardest part, finding a 2.5" "collector reducer" that I could then weld a 2" flange on the other side. That took 10 seconds to type out but it took me 2 weeks of endless searching to find the dang part that would work. Anyway, I digress... These were too short to wrap but did get a triple coat of paint. After the 2 to 2.5 adapter comes the 2.5 pipes. Offset was needed here. Again this took quite a bit of searching but hedman made just the right set. Painted and wrapped just like the heads. Last but not least the pretty part h simple 2.5" chrome turndowns. They're tucked up under the car to far to see currently, but they won't be like that for ever. Now for the assembly. All joints get aluminum crush gaskets with a thin coat of the copper silicone gasket goop. Adapters in the headers, headers onto block. First snag encountered - O2 sensors too long to fit in first sensor bung on passenger side. Thanks china. No problem, moved down to second bung, got some slack on the harness, and used the postcat sensor (which as far as I can tell is the same sensor but with a longer harness). Problem averted. From there the offset pipes go on, sneaking past the strut tower and over the sway bars. Good clearance on driver's side, super tight on passenger side. Now the tips went on (with a generous coating of antisieze) and pointed down and out. Lastly the diagonal bars and aluminum plate went back in. The trapezoidal bar that supports the aluminum plate in the rear touches the passengers side pipe (grrrrr) but clears the driver's. Everything torqued to spec and voi la. Now, I would like some opinions. I do not like the fact that the pipes touch rigid structures. My concern is that something will break. I think that I need put a donut with spring bolts between the 2-2.5" adapter and the offset pipes. This will suck donkey nuts because I will have to disassemble everything, unwrap the pipes, and then put everything back together. But, I sure as hell don't want to crack anything, especially at the block (go ahead, ask me how long it takes to drill one of those out and retap the hole lol). What are yalls thoughts about this? Anyway, I hope everyone enjoys! Best, Ike Oh, one other thing - the aluminum plate came off in the beginning without effort. It took a bit of oomph to get back on. I was gonna have the alignment checked before going crazy, has anyone else encountered this? Sent from my POCOPHONE F1 using Tapatalk

Niiiiiiice.

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Thanks man! Sent from my POCOPHONE F1 using Tapatalk

The parts rubbing will make a knocking noise but I'm not sure you'll hear it because that car will be LOUD. You're going to wake up the neighborhood with that exhaust. lol

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Our boat has a 454 bbc and it definitely rivals that! I haven't had. It back on the road yet, we'll see what is sounds like at that point. If it's so loud that I can't hear myself think then I'll probably put some single chamber mufflers on it. I figured it would rattle a bit through contact, but you don't think it will actually crack anything? Sent from my POCOPHONE F1 using Tapatalk

Header wrap and water do not play well together. Even worse if you live in an area where they salt the roads.

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Is that because they trap moisture against the pipe? I anticipated this, that's why I painted everything. Or does it wear down the wrap? I don't drive much in the rain but it will definitely happen at some point. No salt though, that's what the truck is for. Sent from my POCOPHONE F1 using Tapatalk

I like what you have done except the no muffler ........ that is going to be loud . Report back after you have driven it a while .

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Will do. Ive gotta check alignment and push 996 row tune on it before I can start running her around but I will keep the post updated. Sent from my POCOPHONE F1 using Tapatalk

While you ultimately wanted to reduce back pressure by using the 2 X 2.5" adapter that actually will increase pressure. Expanding the diameter reduces velocity of the gas, reduce velocity increase pressure. Just my .02.

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No way dude, I used a level 5 hex to actually cause the gravitational pull of the crankshaft to accelerate flow at the level of the expansion... Joking aside, I'm not sure I follow you there. Bernoulli's principle certainly dictates pressure inversely proportional to flow velocity over a surface, but that is not the same as pressure within a cylinder. I don't disagree with the fact that this will slow gas velocity at the expansion, and a decrease in flow velocity will certainly change flow dynamics, especially at the boundary layer (between turbulent and laminar flow). But, minimizing heat loss through the system (hence the wrap) should mitigate this and maintain the desired pressure gradient per the ideal gas law (PV= nrt, keep the nrt equal and P and V move in opposite directions) and Pousielle's law (restriction of flow through a tube is inversely proportional to the the 4th power of radius) If your statement is correct - i.e. change in pressure is directly proportional to the cross sectional area of flow, this would mean that an infinitely large expansion (i.e. from an exhaust tube to the atmosphere) would create an area of infinitely large pressure. If that were the case, then I think we just turned our exhaust pipe into a rocket booster?!? That is, unless I have completely misunderstood what you were getting at.

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Wow. I like the KISS principle. Real world example, a centrifugal pump or compressor works by taking a fluid and increasing it's velocity (impeller) then routing to a volute (fancy name for wide part of the pipe) which decreases velocity and increases pressure. P&V inverse relationship. As far as temperature is concerned expanding gasses cool. The wrap otherwise is a good idea.

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I admit that I had not thought of an exhaust expansion like a volute but I really do not think that they are equal. Lets try to think this through though. The flow dynamics of an exhaust system and centrifugal pump are not the same. Exhaust systems are pulsatile, centrifugal pumps rely on constant flow, and these two phenomenon do not behave the same. Even if exhaust were a constant flow, the geometry between the two are markedly different - small diameter increase over relatively short distance versus marked expansion over relatively longer distances. Look at the role that A/R plays in turbocharger applications - reducing the length of the volute substantially decreases the effectiveness in the design's ability to build top end pressure. Since this is a straight line, the R would be approaching infinity which means that little number divided by really big number is a teeny tiny number. Where there may be a marginal increase in pressure at the expansion joint, the reduced resistance of the pipe to follow it would more than make up for it. Here is another real world example (I'm a surgeon who sees a fair bit of trauma so bear with me here) - someone comes into the ER bleeding to death. I have a choice - an IV catheter (the part that goes in the vein) that is smaller than the tubing that goes back to the bag, or an IV catheter that is larger than the tubing that goes back to the bag. If you are correct, the larger IV catheter would not flow as well since there is an increase in relative diameter in the system. Trust me when I say that this is not the case. That catheter (called a Cordis) saves peoples lives by replacing blood faster than most injuries can lose it. Shorter and fatter is the way to go when flow matters. Just ask Ron Jeremy. That is, until you disrupt gas scavenging (complete disruption of laminar flow), and then you've gone too fat. I know I sound like a smart ass, I am just having fun with this though. Your statements are valid. I wish I had a setup to test it truthfully. I always learn the most when I'm dead ass wrong.

In a perfect world you would have done before and after chassis dyno runs . Start with stock tune and exhaust and then start with mods . Being able to monitor any changes ( positive or negative ) is the only way to truly know the results . But very few have access and $$$ to blow on multiple dyno runs .

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There's actually a shop not to far from me that rents a dyno trailer for a day, which I'm gonna try to get my hands on once it warms up. I didn't dyno before because, well, I just didn't have the time to get down there to do so. I am gonna do a bunch of runs though to test the whole package (intake and straight exhaust with 996 row tune), and then try different tunes (row vs us) and also some minor changes (resonance flapper deleted, single chamber mufflers, etc). I will definitely post them once it happens, not sure when though. Sent from my POCOPHONE F1 using Tapatalk

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Also just a fact !

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On that note... Sent from my POCOPHONE F1 using Tapatalk

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See the First Law of Thermodynamics (Law of Conservation of Energy) and Boyle's Law.

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Fun facts.....

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And this really? According to the first Rule of Thermodynamics it is a fact.