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Old 01-11-2012, 10:40 PM   #21 (permalink)
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You also haven't said anything about testing your engine for back pressure to see if you really do have back pressure or what it is, that way if you do get a different exhaust you can again test the back pressure and see if it has changed.
The formulas that tend to be published in books on exhaust design tend to also be tested on real engines as well, they are also the same formulas that lead people to putting 2.5" exhaust pipes on their vehicles that they are using for racing with a 4 litter engine!
But the bottom line is, you want to keep your exhaust moving at around 240 feet per second and a larger exhaust pipe is not going to allow that to happen unless you have a much larger volume of exhaust as well, that larger pipe will end up with a much slower charge of exhaust moving down it so I have to wonder what good does that slower exhaust give you?
I really wish I could find an online calculator that did the math for you that allowed you to put in the engine size, RPM and get an ideal pipe size but Summit Racing has a crude version that you have to have to use a pen and paper to do your math.
How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos


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Old 01-12-2012, 03:08 AM   #22 (permalink)
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Quote:
Originally Posted by Daox View Post
Why are you taking this route with mods? You haven't even done any easy mods to the car yet. Start with a grill block, pump up the tires, and get a scangauge. Those few things right there plus a little driving technique will get you a 25% gain at least.
Ahh, but I have!
Tyres are at 40 psi, LRR tyres fitted, and I do a bit at getting good driving technique.
That's probably the 8%.

A Scangauge won't work in my car (No OBDII), but I spent my Christmas break busting out the Consult II protocol used in the car, and will be finishing integrating that into the OBDuino soon (it's probably more important now that I want to test different options).

I'm researching and looking into mods - is all, bouncing ideas around.
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Old 01-12-2012, 09:29 AM   #23 (permalink)
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...a "tuned" intake uses acoustic(length)/mass(diameter) inertia to cram as much air INTO the cylinders as possible.

...a "tuned" exhaust uses acoustic(length)/mass(diameter) inertia to extract as much exhaust OUT of the cylinders as possible.

...working 'together' it's possible to achieve >100% volumetic efficiency...but, ONLY at the "tuned" engine speeds!
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Old 01-12-2012, 10:29 AM   #24 (permalink)
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Quote:
Originally Posted by toc View Post
Ahh, but I have!
Tyres are at 40 psi, LRR tyres fitted, and I do a bit at getting good driving technique.
That's probably the 8%.
Well your garage doesn't say that.

I understand though, its fun to kick around ideas.
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Old 01-12-2012, 10:33 AM   #25 (permalink)
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Reading this thread has really piqued my interest. So yesterday I taped my exhaust pipe up with aluminum foil tape down to about 1". I'd like to say that I noticed an increase in low-RPM torque, but I have no actual data to back that up. What I did notice, however, is that because the tape can flex a little bit over the open pipe, it tends to "flutter" during engine deceleration and actually suck in to the pipe. It also sucks in to the pipe when I shut the engine off too. I am guessing that this has something to do with the mass of air moving at a set velocity through the pipe, creating a low pressure in the pipe when the amount of air produced by the engine is suddenly reduced and the momentum of the moving air tends to continue out the end of the pipe at a set velocity.

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Originally Posted by Ryland View Post
But the bottom line is, you want to keep your exhaust moving at around 240 feet per second and a larger exhaust pipe is not going to allow that to happen unless you have a much larger volume of exhaust as well, that larger pipe will end up with a much slower charge of exhaust moving down it so I have to wonder what good does that slower exhaust give you?
This makes me wonder if putting a reduction near the end of the exhaust actually has any effect on the velocity of the exhaust gasses inside the pipe before the reduction, other than simply creating backpressure in the pipe. A stock exhaust system is going to have a series of reductions and expansions anyway (cat, resonator, muffler, tailpipe). The Corsica has a resonator just before the cat, which I'm guessing is probably the best point to terminate the pressure wave from the engine, depending on which RPM and throttle that resonator is designed for. My understanding is that anything after the resonator is simply piping and any change should have nothing to do with pressure wave tuning. That doesn't explain why adding restrictions near the end of the exhaust system would aid in low-end torque. I'm thinking about adding something like the adjustable butterfly valve setup. But WHERE to put it is really the question.
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Old 01-12-2012, 10:41 AM   #26 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...a "tuned" intake uses acoustic(length)/mass(diameter) inertia to cram as much air INTO the cylinders as possible.

...a "tuned" exhaust uses acoustic(length)/mass(diameter) inertia to extract as much exhaust OUT of the cylinders as possible.

...working 'together' it's possible to achieve >100% volumetic efficiency...but, ONLY at the "tuned" engine speeds!
Wait wait wait! you are talking about using math, math is full of theories and we all know that theories are not real! don't you have some speculation and guesses that you can give us? maybe a few old wives tails.
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Old 01-12-2012, 03:35 PM   #27 (permalink)
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Quote:
Originally Posted by Old Tele man View Post
...a "tuned" intake uses acoustic(length)/mass(diameter) inertia to cram as much air INTO the cylinders as possible.

...a "tuned" exhaust uses acoustic(length)/mass(diameter) inertia to extract as much exhaust OUT of the cylinders as possible.

...working 'together' it's possible to achieve >100% volumetic efficiency...but, ONLY at the "tuned" engine speeds!
and only at WOT and only when the exhaust temperature is "correct".
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Old 01-12-2012, 03:58 PM   #28 (permalink)
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...and only with the same/proper fuel (methanol and gasoline different combustion waste temperatures).

...and, please NOTE - no digital or anlog numbers were harmed as they were being crunched through the obligatory mental equations of my brevit explanation above; thus, accordingly, the Numerical Nonsense Naysayers Society was not informed (wink,wink).
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Old 01-12-2012, 08:17 PM   #29 (permalink)
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Old 01-12-2012, 09:19 PM   #30 (permalink)
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...an excellent analogy is a 'slinky' sliding through a clear piece of tubing:

The whole length of the slinky represents a "slug" of exhaust gas from one cylinder.

At the moment the slinky leaves the exhaust value, it is tightly compressed together, representing the sudden, high-pressure, explosive out-rush of exhaust gas out/past the exhaust valve into the 'freedom' of the exhaust manifold header pipe. As the slinky moves away from the exhaust valve, it's length expands quickly forward as it quickly moves forward, within our 'glass' pipe, away from the valve and toward the open-end of the exhaust system. The compressed slinky represents the compressed exhaust gas at high-pressure. But, when the slinky finally reaches the open-end of the exhaust pipe, it is no longer under as much pressure, as represented by slinkys' "stretched-out" length.

At the moment the slinky reaches the open end of the exhaust pipe and "pops-out", a "low-pressure" (rarefaction) wave is created/reflected which propogates BACK up the still exiting slinky length (the forward/backward compression of the individual slinky coils)...back toward the exhaust value. When timed correctly this low-pressure reflection arrives back at the just-closing exhaust value and literally "sucks" the last remaining vestages of exhaust gas out of the cylinder...and if there's sufficient valve timing "overlap" it also helps suck "in" the fresh Air/Fuel-mixture from the intake manifold, past the just opening intake valve, helping "suck" more A/F-mixture into the engine than it normally would ingest.

The above process is acoustic (length) tuning, since it's about pressure/rarefaction waves within an air medium (acoustics). Engine speed, exhaust gas temperature and exhaust/intake value timing are the major controllers here.

The physical movement of the gas slug (slinky) down the exhaust pipe is mass (diameter) tuning, since it is the front-to-back passage of the gas slug sliding within the confines of the exhaust pipe area. Engine speed and the squared-ratio of pipe diameter to piston diameter are the major controllers here. A maximum gas movement of ~ 300-350 fps (mach limiting) is optimum.


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