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Discussion Starter #1
I am wanting to get some test pipes for my Injen CBE, but i am wanting to know if it is worth the extra money to get theARK downpipes and straight test pipes or would just the test pipes being connected to the stock downpipes be enough to open up my exhaust some and give it a little more sound and power.
 

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There are two areas of restriction in your question. The first and the most being in the secondary cat, its is honey combed, think kind of similar to a sponge. Something similar looking to this
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Changing the cat to a test pipe will allow it to flow much better, period.

The second area of restriction, but not as much in comparison to the cat, is in the downpipe itself, it is slightly narrower than the rest of the piping. This restriction is not as bad as the restriction in the cat, but restriction nonetheless.

So, to answer your question. You will receive a benefit of increased flow by swapping out both. You will notice it mostly in the test pipe swap though.
 

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Discussion Starter #3
That is what i was thinking, thanks for confirming it for me.
 

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Downpipes ( mid pipes to be exact/proper) will not net you any power gain, as stock mid pipes alow good flow.

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The second area of restriction, but not as much in comparison to the cat, is in the downpipe itself, it is slightly narrower than the rest of the piping. This restriction is not as bad as the restriction in the cat, but restriction nonetheless.
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Baloney. The inlet on the standard V6 downpipes is the same size as the outlet on the standard exhaust manifolds: 2.25 inches.

Going to larger diameter downpipes does nothing to increase power. You will actually decrease exhaust gas velocity which reduces exhaust scavenging efficiency and therefore overall performance.

On an exhaust system for a naturally-aspirated engine bigger is not automatically better. This is very different from turbo engines where there is no such thing as too large of an exhaust pipe. On an NA motor you want the highest exhaust gas velocity possible, which means you want exhaust piping that is optimized for the airflow of your particular engine. Too small of a pipe creates unwanted back-pressure, too large of a pipe reduces exhaust gas velocity. Both hurt performance.

A good rule of thumb for modern naturally aspirated gasoline engines is 2.2 CFM per horsepower. A BK2 GenCoupe V6 makes about 350 horsepower, so that's 770 CFM. Decent quality straight exhaust tubing will support about 115 CFM per square inch of of cross-sectional area. These are somewhat rough generalizations, but good enough for spit-balling basic tuning ideas.

2.25 inch piping has 3.55 square inches of cross-sectional area, which means it will support 408 CFM. Divide that back into horsepower and you are right at 185 hp. Multiply by two since it is a dual exhaust and you get 370 horsepower. That's enough to support an NA V6 GenCoupe even with a significant amount of power mods.

So Hyundai was right on the money when they selected 2.25 inch down-pipes for the 350 horsepower Genesis Coupe V6. It's almost as if they are a massive car company that employs a legion of engineers to make sure they get this kind of thing right.

On a dual exhaust, even a small increase in tubing diameter results in a very large increase in cross-sectional area. Going up to 2.5 inch dual exhaust pipes nets you around 1020 CFM, which will support over 460 horsepower!

That is just way, way too much flow capacity for even a very highly tuned naturally aspirated GenCoupe V6.

Hyundai wanted the Genesis Coupe to be a tuner car, and it shows in how they designed the exhaust. They knew lots of enthusiasts would want to install catback exhaust systems, so they made a really high-quality front half of the exhaust system and a really shitty rear half.

They figured most enthusiasts were just gonna throw the rear half away, so they made it as cheaply as possible with that nasty, highly restrictive Y-pipe which has that flat spot everyone hates:



The stock V6 down-pipes, on the other hand, are excellent parts. Perfectly sized, stainless steel, with nice, smooth, large-radius bends. ARK Exhaust couldn't improve on the design, so they just copied it with 2.5 inch pipe because "PHWAR, BIGGAR IZ BETTAR"



The stock V6 down-pipes are fine. Leave them alone unless you are going with forced induction.
 

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Genesis OG
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According to your theory, Name ONE N/A platform that recommends keeping the stock downpipes (midpipes) and upgrading the rest of the exhaust. Trust me, we'll wait...

I have REAL experience with downpipes not just a theory. I have ARK downpipes and test pipes and catback exhaust. I also have been custom tuned on a dyno. The problem with this and most ppl when it comes to HP is they only want peak numbers. You WILL see a gain under the curve over the stock downpipes. Now peak HP? Maybe maybe not. I was happy with the downpipes and recommend them for those wanting the most out of the exhaust.
 

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Cone Killa
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According to your theory, Name ONE N/A platform that recommends keeping the stock downpipes (midpipes) and upgrading the rest of the exhaust. Trust me, we'll wait...

I have REAL experience with downpipes not just a theory. I have ARK downpipes and test pipes and catback exhaust. I also have been custom tuned on a dyno. The problem with this and most ppl when it comes to HP is they only want peak numbers. You WILL see a gain under the curve over the stock downpipes. Now peak HP? Maybe maybe not. I was happy with the downpipes and recommend them for those wanting the most out of the exhaust.
Did you add the downpipes on their own or with all the other exhaust components at the same time? I guess I'm wondering how you know how much the downpipes helped.

Idk how much value there is to comparing the downpipes of other cars. The guy was talking specifically about the design of the BK V6 downpipes and how they seem to be well-designed.

@StrangeLiform backed up his position with measurable evidence but you didn't really say much. You just sound really... Defensive.

Subbed to gain some knowledge. One of my flex pipes on one of my stock downpipes is ****ed and I was actually just thinking last night about getting downpipes.
 

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I have ARK downpipes and test pipes and catback exhaust.
If you're still on the stock exhaust manifolds the ARK downpipes are useless. Stock manifolds have a 2.25 inch outlet, so your big down-pipes do nothing. Unless you go to after-market headers with a 2.5 inch outlet then using 2.5 inch pipe in the rest of the exhaust system is totally pointless.

Your dyno gains are from losing the cats and the crappy rear half of the OEM exhaust.

You WILL see a gain under the curve over the stock downpipes. Now peak HP? Maybe maybe not.
This is funny. The only place over-sized exhaust tubing ever helps is peak horsepower. Under-the-curve gains are driven entirely by exhaust gas velocity. Improved exhaust scavenging is how you get gains in lower-RPM power and torque.

Mike Kojima, who has been tuning cars for performance longer than most members of this forum have held a driving licence, had this to say on the subject:

"An old hotrodders tall tale is that engines need some backpressure to work properly and make torque. Well that is not true. What engines need is the lowest backpressure possible but with a high exhaust stream velocity. A fast moving but free flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve right as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping loss-inducing work. A too big in diameter exhaust pipe has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque. So remember fast = low backpressure + high velocity. Slow=low backpressure=too big of an exhaust pipe=low velocity."

This is from an article he and Sarah Forst wrote on DIY custom exhaust builds:

How to Make your Own Effective Exhaust System > MotoIQ - Automotive Tech, Project Cars, Performance & Motorsports

According to your theory, Name ONE N/A platform that recommends keeping the stock downpipes (midpipes) and upgrading the rest of the exhaust.
Pretty much all performance NA platforms. If you don't upgrade the exhaust manifold(s) to after-market headers there's never a reason to change the downpipes. Mustang, Camaro, Corvette, Challenger just to name a few. Exhaust tuning in all those communities and more revolves around either cat-back exhaust systems, or full exhaust replacement including headers. The only real variations on this trend are "axle-back" exhaust kits, and upgraded cross-over pipes (H or X pipe) that generally take the place of the secondary cats and accompanying tubing in that section of the stock exhaust.

This pans out when you look at products from super-reputable, motorsport-involved exhaust manufacturers such as Kooks headers. Their products consist of axle-back kits:



Cross-over pipes:



Catback kits:



And headers, with or without included complete exhaust kits:



None of their products, and hardly anyone in the performance tuning world that I've seen, changes out pipes that go in front of the secondary cat locations but behind the OEM exhaust manifold. Down-pipes like that are normally only removed when headers are installed. Usually, the new headers take up most or all of the length that the downpipes occupied because long-tube headers make a way better power curve.

The Genesis Coupe is an anomaly in that there are after-market companies making "upgraded" down-pipes that match the length of the factory parts. They do this because most of the after-market headers available for the Genesis Coupe match the length of the OEM exhaust manifold. These "short tube" headers are designed not for maximum performance, but ease of packaging. In all motorsport-based tuning circles, short-tube headers are considered inferior to long-tube headers for this reason.

In short, after-market down-pipes on the Genesis Coupe only exist because of the popularity of short-tube headers. They are meant to be paired with after-market headers that have a larger outlet, not as an "upgrade" for cars that still have the OEM exhaust manifolds. Not that any of the after-market companies are in a hurry to point this out because, duh, money. If someone wants to drop cash on extra parts they don't really need, that's just a bonus for them.
 

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Genesis OG
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Well this escalated quickly.... Not defensive. It just erks me sometimes when someone asks for help and gets replies like "good rule of thumb" (is that GC specific??) No offense here, but me personally if I wanna know something about a product I wanna know from someone with experience with that product. Not theories. But that's just me. "They knew lots of enthusiasts would want to install catback exhaust systems, so they made a really high-quality front half of the exhaust system and a really shitty rear half." is where I kinda give up reading. If that were true they would have copied the VQ platform front to rear.



Individual piping into a collector into a bolt-on cat into a rear exhaust that is dual all the way out exhaust.




Stock manifolds have a 2.25 inch outlet, so your big down-pipes do nothing.
Again, by your theory all aftermarket catbacks with stock downpipes and 2nd cats are useless or stock downpipes to test pipes are because they go from 2.25 inch outlet into a 2.5 inch exhaust. Better flow is better flow. Simple as that. I didn't come in here to reinvent the exhaust or argue one way or the other. I came in to give my experience with downpipes because I have ARK downpipes. If that's not your thing by all means no offense taken.
 

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Cone Killa
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Well this escalated quickly.... Not defensive. It just erks me sometimes when someone asks for help and gets replies like "good rule of thumb" (is that GC specific??) No offense here, but me personally if I wanna know something about a product I wanna know from someone with experience with that product. Not theories. But that's just me. "They knew lots of enthusiasts would want to install catback exhaust systems, so they made a really high-quality front half of the exhaust system and a really shitty rear half." is where I kinda give up reading. If that were true they would have copied the VQ platform front to rear.



Individual piping into a collector into a bolt-on cat into a rear exhaust that is dual all the way out exhaust.






Again, by your theory all aftermarket catbacks with stock downpipes and 2nd cats are useless or stock downpipes to test pipes are because they go from 2.25 inch outlet into a 2.5 inch exhaust. Better flow is better flow. Simple as that. I didn't come in here to reinvent the exhaust or argue one way or the other. I came in to give my experience with downpipes because I have ARK downpipes. If that's not your thing by all means no offense taken.
Pretty sure when he used the phrase "a good rule of thumb," he was referring to CFM per horsepower for modern engines. There are usually exceptions to the rule so it would be smart to use the qualifying phrase to avoid suggesting that 2.2 CFM is the exact number per horsepower in every situation.

He said that he believes that the Hyundai engineers designed a good front half of the exhaust system and a poor rear. You then go on to say if what he believes is true, then Hyundai engineers would've done the same as Nissan engineers and built an exhaust that is well-designed on both halves. That makes no sense lol.
 

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Cone Killa
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Anyway, OP. I personally think there are gains to be had with larger diameter piping downpipes even on stock headers as long as you're also upgrading everything behind them (test pipes, catback) and getting tuned of course. What you want to avoiding is "upgrading" your exhaust system to where you're going from larger piping to smaller piping the further downstream you go in your exhaust system.

If you can afford downpipes and test pipes to go with your Injen catback then do it, but if you can only do one do test pipes.
 

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Genesis OG
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I disagreed in that the front half is good. The whole thing is pretty much flawed. As a fellow Engineer I wouldn't design any part I did not think was beneficial to the whole system.

Finally back on subject. I agree with what @ApexEight said
 

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This is how I see it...
Replacing "super restrictive" exhaust with aftermarket cat back net you about 7 whp. Most of the gain is at the top end.

Replacing OEM mid pipes that are not restrictive will not going to get you anything tangable on NA 3.8 with stock rev range.

Once you get your valve train redone to rev to 8k, then you might see benefit going larger.
There is no single dyno showing any gain with midpipes.



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It just erks me sometimes when someone asks for help and gets replies like "good rule of thumb" (is that GC specific??)
It's physics-specific. As in fluid dynamics. There's nothing so special about the engine in a Genesis Coupe that you can't use general knowledge about modern gasoline engines to make decisions about tuning a GenCoupe. Internal combustion is internal combustion. If you want to get beyond the rough estimations of using a horsepower-per-CFM conversion then you sit down and do the math properly:

The mass of air the engine breaths in plus the mass of the fuel equals the mass of the exhaust.

You can get the mass of the air by multiplying the engine's displacement by the RPM it's running at for this particular calculation. Then divide that number by two (four-stroke engine requires two RPM to draw in and exhaust air). Then you convert that volume into mass, accounting for air temperature and density due to elevation.

Then you get the mass of the fuel. Again, this is RPM-dependent. With a modern engine, you can do this by looking up the numbers in the ECU's fuel map. Then you can add that number to the mass of the air.

Now you convert the mass into gas volume. You need to know typical exhaust gas temperature at the conditions you are calculating for in order to get an accurate conversion.

This is still a flawed process because you have to assume perfect combustion efficiency and then back in to a more realistic number using efficiency estimates after the fact.

For an "engineer" this sort of math should be child's play. However, for an average Joe it is a complicated pain in the ass, and off-the-shelf exhaust tubing is only available in set increments anyway (2 inch, 2.5 inch, 3 inch, etc). There's no need to get super-specific when you don't have perfect control over exhaust diameter, so a horsepower-to-CFM conversion is accurate enough for basic tuning. HP-to-CFM estimates are based on real-world results and plenty of experts agree that a conversion factor of 2-2.2 CFM-per-horsepower is a good enough number to get you in the ball-park when trying to understand exhaust theory.

Here's a great video by /Drive where Richard Waitas, Senior Manager at Magnaflow and general performance exhaust expert, uses that same HP-to-CFM conversion to figure out appropriate exhaust tubing diamaters. Skip to 12:39 for the relevant part:


I disagreed in that the front half is good. The whole thing is pretty much flawed.
Why? Because it's ugly? Nissan uses an admittedly nicer-looking short-tube header on the 370Z and Hyundai uses more of a log-style manifold on the GenCoupe. Different designs, but they get similar results.

This is easy to see by looking at dyno gains from after-market header upgrades. If the GenCoupe's OEM exhaust manifolds are crap, then you would expect much bigger gains from an after-market header on the Gencoupe than you would get on the 370Z.

However, that's just not how it is. Stillen makes a short-tube header for the 370Z that is quite similar to ARK's short-tube offering for the GenCoupe V6. The Stillen 370Z header is well-documented as producing about a 10 wheel-horsepower gain:





This test was performed by semtex, a well-respected member of the 370Z tuning community:

Stillen Headers installed - Review/Pics/Vid/Dyno - Nissan 370Z Forum

Likewise, ARK's short-tube header for the GenCoupe is well-documented as producing about a 10 wheel-horsepower gain:



Those results are from the GenCoupe forum's own user Predat0r, who did an excellent build thread and performance analasys of the ARK headers:

http://www.gencoupe.com/3-8-v6-discussion-2008-2012/23508-ark-headers-installation-results.html

You can easily find similar results for a variety of 370Z and GenCoupe V6 header tests. Hell, look at the manufacturer's own claims. Stillen claims a 14.5 peak horsepower gain on their 370Z headers and ARK claims a 15 horsepower gain on their GenCoupe V6 headers. These numbers are higher than what most third-party testing has seen, but that's typical (manufacturers always cherry-pick their data) and, more importantly, the numbers are still the same relative to eachother.

You get annoyed with people who use "rules of thumb" and I get annoyed with "engineers" who don't use data to make decisions. Agree to disagree I guess. :icon_rolleyes:

There is no single dyno showing any gain with midpipes.
Andbudzi says it all. Could have saved myself a lot of breath and just pointed this out.
 

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'13 3.8 Track
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Why? Because it's ugly? Nissan uses an admittedly nicer-looking short-tube header on the 370Z and Hyundai uses more of a log-style manifold on the GenCoupe. Different designs, but they get similar results.

This is easy to see by looking at dyno gains from after-market header upgrades. If the GenCoupe's OEM exhaust manifolds are crap, then you would expect much bigger gains from an after-market header on the Gencoupe than you would get on the 370Z.

However, that's just not how it is. Stillen makes a short-tube header for the 370Z that is quite similar to ARK's short-tube offering for the GenCoupe V6. The Stillen 370Z header is well-documented as producing about a 10 wheel-horsepower gain:
I want to punch myself for stepping into this argument...but unless I'm misunderstanding something, Strange, you're contradicting yourself a little with the above statement. Because you later provide a link to headers that do show a HP improvement, apparently across the entire curve, that provide a measurable and noticeable increase in power and torque.

The only reason I bring this up is that Ark has a fairly generic design of their headers for the Genesis Coupe. Maybe they pioneered the header design...I don't know...but the design is the same for the DNA headers I just installed on my GC. Maybe there's a difference in the collector...I don't know...but I can definitely feel a difference compared to the OEM manifold.

Anyway, I'm only replying to get clarification on the statement you made above. I've done a lot of research about the size of piping, and scavenging is a great benefit of exhaust gas velocity, but it doesn't appear to trump HP gains by removing all back pressure. Maybe I'm misunderstanding something with that...but it appears to be an argument of "if you have to have tube size restrictions, get some scavenging benefit...but if you can run 12 inch long straight pipes like a dragster, that's even better".

Thoughts?
 

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Discussion Starter #16
is there really any difference between ARK and CNT when it comes to the downpipe and straight test pipe combo, or even just the test pipes from either brand.
 

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'13 3.8 Track
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Test pipes are just empty, straight pipes. Any brand will be the same, in my opinion because they're not doing anything except removing the restriction from the secondary cat. From my perspective, if it fits well, buy the cheapest test pipes possible.

Headers are very much the same design if you go with shorty headers. I've heard there are fitment issues with CNT. But, if you compare them and other knock-off brands to Ark headers, the design (tube layout) is the same. The only thing you may get differently is round vs. oval inlets at the start of the header...and maybe the collector is different. Not sure how much this changes performance, but it may be a little. I wouldn't spend $1,000+ on headers...that's insane.

;)

Edit: I didn't even mention your downpipe question, but I think it's going to be the same as what Strange said, above. I got a set with the headers I bought, and they were identical in every way to my OEM downpipes. I opted to leave the stock ones on my car.
 

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...you later provide a link to headers that do show a HP improvement, apparently across the entire curve, that provide a measurable and noticeable increase in power and torque.
Yes, but it's the same gain in power that the 370Z gets from headers with a similar design.

You might have misinterpreted my stance on headers what with all the back-and-forth in this thread. I never claimed that headers don't increase power. Good headers totally increase power! I claimed that larger diameter downpipes, without after-market headers, don't increase power. As adbudzi pointed out, NOBODY has ever produced a dyno graph showing gains from bigger downpipes alone.

The argument about headers was pretty much unrelated to the downpipe discussion. I brought up the ARK headers for the GenCoupe and the Stillen headers for the 370Z in an attempt to refute the claim that the OEM GenCoupe exhaust manifolds are poorly designed. If that were true, then you would see way bigger gains from headers on the GenCoupe than you would on the 370Z which allegedly has a "better" designed front half in its OEM exhaust system.

It doesn't.

Looking at real-world gains, the "good" 370Z OEM exhaust manifolds and the "bad" GenCoupe OEM manifolds have very similar performance. The GenCoupe and the 370z have both been shown to gain 10hp from similarly designed after-market "shorty" headers. This is backed up by the manufacturer's claimed gains as well, which weigh in at 15hp and 14.5hp respectively. Like I said already, the GenCoupe would gain much more power from after-market headers if its OEM exhaust manifolds were significantly worse than those on the 370Z.

Maybe there's a difference in the collector...I don't know...
Power gains from after-market headers are almost all in the collectors. The collector is what helps smooth out the exhaust stream, turning the distinct exhaust pulses from each cylinder into more of a smooth, laminar flow. The collector is what sets up the negative pressure wave which is so critical to increased exhaust scavenging, reduced pumping losses, and increased power.

There's a great real-world test that proves this. Watch David Freiburger of Hot Rod magazine bash the hell out of the primaries on a set of headers and see practically no loss in power as a result:

(skip to 8:46 for the serious bashing)


It's all because the collector is the crucial component in a set of headers. The primaries really don't matter much as long as they are all roughly the same length and are around the correct cross-sectional area.

Anyway, I'm only replying to get clarification on the statement you made above. I've done a lot of research about the size of piping, and scavenging is a great benefit of exhaust gas velocity, but it doesn't appear to trump HP gains by removing all back pressure. Maybe I'm misunderstanding something with that...but it appears to be an argument of "if you have to have tube size restrictions, get some scavenging benefit...but if you can run 12 inch long straight pipes like a dragster, that's even better".
That all depends on peak vs midrange horsepower. If you only care about screaming high-RPM peak horsepower then, yea, bigger can be better. However, the loss in exhaust gas velocity will cost you when it comes to midrange torque. On naturally aspirated engines, a well-designed, correctly-sized exhaust system that produces high exhaust gas velocity will net you more "area under the curve" when it comes to power gains. Also, if that system has a very optimized configuration, it won't sacrifice an appreciable amount of high-RPM power either.

HotRod.com did a great test that illustrates this point. They ran a bunch of different headers on the same engine with wildly varying tube diamaters:

Headers Dyno Test - Hot Rod Network

Here's their results:



The black line is a super over-sized set of headers with 2 1/8 inch primaries. Notice how they get beat from 3500-5000 RPM by even the super under-sized 1 3/4 inch primaries (green line). The 1 3/4 inch headers clearly suffer above 5000 RPM, but notice that the 2 inch primaries (blue line) beat the larger 2 1/8 primaries even at the very high end. If bigger was always better, that wouldn't be the case.

This really shows that, for naturally aspirated engines, there is usually a real world "ideal" exhaust sizing that preserves enough exhaust gas velocity for good midrange torque but still has low back-pressure for high-RPM power.

Just to put an exclamation point on my assertion that less exhaust piping is not always better, check out this graph from the same article:



That's a set of of headers that just dump straight into the air (red line):



Versus a similar set of headers with 18 inch header extensions (black line):



The headers with extensions DESTROY the stand-alone headers, gaining 50 ft-lbs of torque in the midrange without any sacrifice at the top end.

This all goes to show there is a true science to exhaust design that can often be counter-intuitive. Real world testing is always the best way to find an ideal setup and, if that's not an option, then using solid math based on the most similar real world testing you can find is your best bet.
 

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Dude, that has to be one of the best posts I've ever read. Thanks for that. I learned quite a bit, and thanks for unconfusing me. ;)

But now I may have a problem! The headers I just put on my GC have a collector tube size of 2.375". It's only 1/8" larger than the stock tube diameter, but I didn't notice this difference when putting the headers on...and I used the stock downpipes. So, I've got a 1/16" lip around that flange which chokes down from the headers to the rest of the piping.

That's a pretty tiny lip, but it's going to keep me up at night...I'm not so concerned about the volume of air, but rather the turbulence that could be created at that point.

What are your thoughts on this situation?
 

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Ehh, not sure I'd lose sleep over it. The trouble is that you could go up to 2.5 inch downpipes to eliminate the restriction, but then you still have a turbulence-generating lip, it's just on the other side.

You could always file a little chamfer into that sharp lip, just to smooth it out and cut some turbulence. That said, I don't think it would make a difference that you could measure on a dyno, let alone with the seat of your pants.
 
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