2.5" exhaust
 

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? https://uploads.tapatalk-cdn.com/202...7a8e705367.jpghttps://uploads.tapatalk-cdn.com/202...cfb3b4d0bc.jpghttps://uploads.tapatalk-cdn.com/202...bb26700214.jpghttps://uploads.tapatalk-cdn.com/202...3a887cbeff.jpghttps://uploads.tapatalk-cdn.com/202...a0c07fd266.jpg

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Niiiiiiice. :cheers:

Thanks man!

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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

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?

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

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.

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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 .

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.

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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.

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.

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.

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 .

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.

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Also just a fact ! :confused:

On that note... https://uploads.tapatalk-cdn.com/202...2953201ca5.jpg

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

Fun facts.....

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

Dude, what are you trying to get at? I'm not denying your point that increasing the diameter of the exhaust will slow the flow. You're hurting your own line of reason by citing boyle's law though - as that volume expands the pressure will drop across the surface - it has to, it's conservation of energy. This is not physics 101 though and there will undoubtedly be more factors at play - CFD is stupid complicated. Flow patterns (laminar vs turbulent flow), boundary layers, vortices, standing waves and resonance patterns, all of this stuff will play into what actually happens in the real world. You clearly understand general physical principles so resistance should not be a foreign concept here. Resistance is the "thin line" so to speak of when it comes to flow. Too much and you choke the system, too little and you lose patterned (laminar) flow which ultimately will siginificantly disrupt the system.

If you don't believe this, take a tour of a major engineering complex (like GE for example). They have an entire team of engineers with impressive credentials whose job it is to model flow systems on computers that rival deep blue.

This part of my project started with the fact that the stock exhaust system is undersized for the hp rating of the vehicle at baseline, and the VE of the stock setup is nothing to write home about. This is my attempt at getting out of the motor what stuttgart left behind 20 years ago. You clearly understand physical principles, so don't you think it's a bit shortsighted to look at one aspect of this entire system from the viewpoint of one principle and then call me dumb for it?

Nevermind, please don't even bother answering that question. I am truly over this thread and I apologize to everyone who has read it.


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I didn't call you dumb, a cardiovascular surgeon? clearly you are not. I was referring to the flippant joke you made to BYP's comment about a fact.I am not prone to using emojis and LOLS. So carry on I still say you'd be better off not disrupting the flow especially slowing and disrupting the scavenging effect.

Time for the update - it took a while to sort out some coolant issues but I've been running the straight pipes for about 50 miles... It's too loud... Way too loud... Like permanent hearing loss by the end of summer too loud. Not only that, but dirty too. At 100mph you don't notice any fumes, but you will reek of fumes every time you drive. This may be because the exhaust terminated under the car and not behind the car, but never the less the whole setup wasn't a good fit for a DD.

So, here's what I've done. I added high flow universal cats at the end of the 2.5" pipes. Quieted things down but my wife still refused to get in the car lol. So I stuffed some bored out internal baffles (db killers) into the pipe before the cat. And now my wife (and I) love it! It's not quiet by any means, still on the loud side (probably will get your name brought up in an HOA meeting) but it's got a phenomenal deep growl and you can have a conversation while standing behind the car.

On an interesting side note, while the car was straight piped, there was still a drone! Exactly where it ought to be - 2-2.8k rpm. Unbelievable! It's clear to me now that this a feature of the engine harmonics and not the exhaust system, and the only way Porsche was able to kill it was with a choked off set of pipes and a gas tank sized muffler. Ahhh the m96 motor, what a marvel of engineering this machine it. If anyone has ever ported their head, I would love to know what effect that had on the drone.


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Update 2

The idea of the internal baffles bothered me, so I removed them and added shorty glass packs following the cats. Good thing I did! The damn things had dislodged and were eating through the cat matrix (I should've seen that one coming. Oh well, ya live and ya learn).

Interesting side note - the heat wrap works. I had a bit of trouble lining up the pipes perfectly when I installed the glass packs so I left it and figured I would straighten out another day. The top 1" or so of the passengers side muffler overlapped the bottom of the bumper with about an inch between the two. Well, that was close enough that it melted the bumper! The melting point of fiberglass is 2050 deg F so, like I said, the heat wrap is clearly working lol. This gives me a good excuse to now make the "aero mods" to the rear bumper I've been scheming for a while now.


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