by "Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
Published 02/28/2017 at 16:15
by "Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
Published 02/28/2017 at 16:15
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So the math in here got me thinking about something I’ve suspected for a while... That I screwed up when I changed my muffler.
When I was thinking about exhaust mods, first thing I did was went to a well known local custom shop to have them look it over and give me some options. Seeing as I have no experience whatsoever, I proceeded to get sticker shock. Then, I latched onto two things they said—#1, the main pipe was already big enough for my power level and #2, they saw many people find significant benefit from mufflers alone.
So, I did my research, found what appeared to be the best one for the job, and found someone to install it—the shop I went to originally was, imo, too expensive.
Probably the reason they were more expensive was because they would have replaced the over-axle pipe as well as the muffler—I can’t remember them saying that, but bearing in mind their good reputation, attention to detail and what I found after the old muffler was cut out this assumption makes their price make a lot more sense.
So here’s the story. I measured the exhaust pipe before I ordered the muffler, but only the very end. It was 2 1/2". So, I bought a 2 1/2" in-2 1/2" out muffler... logical so far. But as it turns out GM had been up to funny business. After the fashion of the strangled intake runners on the Crossfire Injection 305, they had provided me with a 2 1/2" outlet... but the main pipe, and the muffler inlet, were 3" pipe.
Doing the math, then: According to the chart in that post the 2 1/2" on the outlet side can support 283hp. The 3" main pipe can support 339hp. My engine is rated at 300. An apparent contradiction, but for two things. #1 there’s going to be a little margin of error and #2, the constricted area is at the end of the exhaust system and will be a little cooler, and therefore denser, which means technically it can flow a little higher mass of exhaust. But overall, the takeaway is: This thing was set up right on the line from the factory, and while I wouldn’t have made it worse, I essentially took $250 and shat on it.
Also maybe explains why the tuner had such a hard time with it last summer.
I’d post pics but kinja doesn’t want me to
"RamblinRover Luxury-Yacht" (ramblininexile)
02/28/2017 at 16:29, STARS: 1
the constricted area is at the end of the exhaust system and will be a little cooler, and therefore denser, which means technically it can flow a little higher mass of exhaust.
You’d be surprised at just how much fluid mechanics can, to borrow a Britishism, play silly buggers. Exhaust, when extremely hot, has such low viscosity that it behaves worlds differently than intake, as an example. I wouldn’t automatically assume a late-exhaust position would be more favorable to flow related to size.
"crowmolly" (crowmolly)
02/28/2017 at 16:33, STARS: 0
Hellcat exhaust is a dual 2.8" system. If that chart were correct it would be over “capacity” by almost 150 hp. And you’d be giving up ~40hp in a 4th gen Camaro SS/Trans Am.
"RamblinRover Luxury-Yacht" (ramblininexile)
02/28/2017 at 16:40, STARS: 0
I don’t remember where I got it, but I worked off a more conservative sizing for my Ranchero, and got 2 1/4" dual pipes needed for a 4.2l engine and ~250hp or so, to be safe. NA, of course.
"crowmolly" (crowmolly)
02/28/2017 at 16:43, STARS: 0
Something about the sizing charts always seems off to me.
There’s another one out there that says you need dual 3.5" exhaust for a 400hp 454cid engine.
That seems a bit excessive.
"RamblinRover Luxury-Yacht" (ramblininexile)
02/28/2017 at 16:48, STARS: 0
I was looking at “2" will get it done, but”. Hence 2 1/4". My header situation is fucked up enough that I don’t need any extra restriction if I can avoid it.
"crowmolly" (crowmolly)
02/28/2017 at 17:02, STARS: 0
From what I recall the charts don’t take into account crossover pipes, either.
"RamblinRover Luxury-Yacht" (ramblininexile)
02/28/2017 at 17:07, STARS: 0
I’m not sure that crossover pipes really have that much of an impact at highest flow - though I could be wrong. At some point, the pulses entering from the primaries would tend to not be far enough apart for sum/aggregate pulse equalization to help, right?
I wasn’t planning any crossover pipes, myself, because I’m going out and up with the pipes (behind the b-pillars, long story) and have no place to put such a thing with the lowered drivetrain.
"CobraJoe" (cobrajoe)
02/28/2017 at 17:08, STARS: 0
I wouldn’t trust the “hp to CFM” conversion to be more than just a rule of thumb. Every engine makes power at different RPM and RPM affects flow speeds and flow speeds can affect power at specific RPM.
A large diameter pipe can actually reduce power at low RPM (slower exhaust flow makes it harder to get all the exhaust gasses out of the cylinder at low RPM), and a small diameter pipe will choke the engine at higher RPM. Just like nearly any other thing you can change, it’s a trade off.
I think the bigger restriction would be the type of muffler you chose. A straight through or baffled muffler will almost always flow better than a turbo style muffler.
"crowmolly" (crowmolly)
02/28/2017 at 17:15, STARS: 0
I think firing order and cam duration come into play, but generally a crossover pipe (even a simple H) can free up some horsepower due to less turbulence and smoother flow. I seem to recall that not that much exhaust gas (in an h pipe setup) really crosses over. My experience with them is that it knocks down raspiness and seems to smooth out exhaust flow. But that’s just anecdotal, I’ve never done any type of real testing.
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
02/28/2017 at 17:39, STARS: 0
I think the chart is off... doing the “rule of thumb” math myself gives me ~400hp for the 3".
Please decode your last sentence for me though. I’m not exactly sure what you’re saying.
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
02/28/2017 at 17:44, STARS: 0
Actually, no.
2.8/2=1.4 ->radius
1.4*1.4=1.96
1.96*3.14159=6.1575164
6.1575164*115=708.114386 ->CFM
divide by 2 for the HP it can support and then multiply by 2 for dual pipes, obviously arriving at the same number.
Hellcat is 707hp, pipes can support 708.114386hp... bearing in mind that this is just a rule of thumb and not 100% I’d say that’s damn accurate
"crowmolly" (crowmolly)
02/28/2017 at 17:47, STARS: 1
This chart is what I was disagreeing with. And I think the Hellcat’s 2.75" pipe is an OD measurement, not ID, so you need to adjust for wall thickness.
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
02/28/2017 at 17:49, STARS: 0
I understand that concept, and the fact that it’s a rule of thumb... however the fact that it says that it can support ~20hp less than I’m rated for tells me I’m quite close to the edge. Hence tuning difficulties, waste of money etc. Eventually I’ll do it over from front to back, so... $250 lesson learned.
As far as muffler design... that’s another thing. I went for a straight-through (Borla) that seemed to produce the best results in an actual back to back third-party dyno test. Now the OEM is HUGE... imagine my surprise to find that it was a straight through as well, albeit with a much larger bend inside...
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
02/28/2017 at 17:50, STARS: 0
well then...
excuse me while I slide back into my corner
"RamblinRover Luxury-Yacht" (ramblininexile)
02/28/2017 at 19:38, STARS: 1
Decoded: a decrease in temp and corresponding rise in density doesn’t necessarily make the flow easier. Specifically, the Reynolds number, a number which can be used to describe the type of flow a fluid is undergoing in a tube is described thusly:
It increases linearly with density, and mean fluid velocity. That is, the same matter flowing at half the density and twice as fast (e.g. the same mass rate) will have The. Exact. Same. Reynolds number. Therefore, the matter will flow just as smoothly in an ideal tube of a given diameter and will NOT automatically flow more smoothly as it cools - your main thesis. Worse, while obstructions experience drag... drag is proportional to the density times the square of the velocity *AT* the obstruction - so if something only impinges slightly into the stream, it may *not* experience reduced drag at reduced *mean speed* due to a variety of factors, because flow within a tube has a range of velocities, and density itself raises drag.
In such a case as that in which the mass distribution within a tube is affected by speed *as well*, it will affect the pressure distribution within the tube and in turbulent flow can have a range of effects. If you have a crimp on the inside of the tube in a sharp bend, it may not affect a hot flow of reduced mass in the same way - it’s likely to slow it less, if I’m not mistaken.
Add also that fucky things occur regarding surface roughness in turbulent flow WAY outside what you’d expect and are strongly affected by density... it’s not weird or even uncommon for the flow to be less sensitive to problems at the hot end than the cold end.
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
02/28/2017 at 21:22, STARS: 0
Ah. Thanks for explaining.
Btw not my main thesis... I don’t know that I had one, really, beyond me screwing up. I was trying to pick out things that could possibly make it outflow what the diameter supposedly allowed, while still leaving me right on the edge of “you ain’t gettin’ any more than this, boy” in the power/efficiency department. It was one possibility I’d heard and it sounded plausible.
So...in broad terms what you’re saying is, if the hot air flows best through diameter “x”, than “x” will also be ideal for that same mass of air even when it cools and takes up less volume? And obstructions inside the pipe will affect flow more at the cooler end? Please correct me if I’m wrong, it usually takes me a few tries to get things the right way around :)
In any case, that still leads me to think that my weak link is the narrow bit at the end... not to mention the s bend (crush-bent, at that) section they added to join the new muffler to the existing pipe. Any way I look at it I fucked up by being cheap.
"CobraJoe" (cobrajoe)
02/28/2017 at 22:33, STARS: 1
Being “sized for 20hp less” than what you’re rated for does not mean you’d gain 20hp by upsizing, nor does it mean your current system is undersized. Unless there’s some constricting bends or you absolutely want a peak horsepower number to brag about, you likely aren’t losing much power by having a slightly undersized muffler.
I just think you’re beating yourself up over something that doesn’t affect your power that much. Tuning could be have been difficult for any number of reasons (though I have no idea of what your actual setup is), and if the muffler wasn’t that restrictive, then you wouldn’t have seen a gain with any other muffler in its place. A full system might give a bit of power, but if the stock muffler was that free flowing, then you’ll probably find more power for cheaper with other mods.
"Smallbear wants a modern Syclone, local Maple Leafs spammer" (smallbear94)
02/28/2017 at 23:26, STARS: 0
That’s not what I said. What I’m saying is the muffler seems to be the sticking point, I tried to address it and what I did was pointless because it was strangled at the tailpipe and I left it that way inadvertently—waste of money.
You’re right, it wouldn’t affect my power much on it’s own. But if you use a step stool in a 7' basement you bang your head on the ceiling. If you use it on the main floor where the ceilings are 8' or 9', you can have comfortable headroom. This is like being on the main floor—and then whacking your head on the kitchen bulkhead. And then removing the bulkhead, and building a new one in the exact same place but painting it a different colour. It’s a different weak link, but doing the same thing and it’s got to go before I can do anything else if it’s going to be worthwhile.
And fwiw, considering the gains others with my engine have got from tuning and exhaust I’m convinced that that’s the weak link. You could argue that the tuner didn’t do a good enough job, but I doubt it. He owns a 4.3 powered Sierra that he’s gotten some awesome numbers out of with little more than headers and tuning. He did another truck identical to mine, also stock, he had similar issues with it. Lightly modded trucks seem to do better. There’s actually another on the forums identical to mine, did the same thing as I did... except that he did a 3in-3out Borla instead of my 2 1/2 and did the over-axle as well. No tuning. Net result—couple % better economy, something I haven’t seen. As far as the stock muffler being restrictive, like I said it literally downsizes in the middle of the muffler. Other than that, good design. But it’s a deliberate strangling and I’m convinced that’s the trouble spot.
TL(or too jumbled);DR All I’m saying is I thought I did something stupid because I didn’t think it through completely and this math confirms I did.
"RamblinRover Luxury-Yacht" (ramblininexile)
03/01/2017 at 08:29, STARS: 1
What I was calling your main thesis was your “I really hope this is the case” - :P. In broad terms, yeah - mass flow rate at the two different temps will be roughly similar in the same size pipe, with outright obstructions (like inside a muffler) maybe affecting hot flow more, but outside edge obstructions, pipe roughness, and slow-downs from bends very likely affecting cold flow more, unpredictably.