The straight-back works under the batt box with a 2.5" pipe, and that's PLENTY of flow area for about any streetable 1.6L Mini motor, even at full chat on a pulley.
I was determined to make this configuration work. In so doing, I ran, extensively, over a DOZEN different muffler configurations in the straight-back configuration on my R53 (15% pulley, Alta CAI). All were crazy loud, even with a spiral "muffler", not glasspack, upstream. Crazy loud.
And resonant: that length pipe has a resonance about 2800-3200RPM, right AT the 68-73MPH range, not that anyone spends a lot of time there. And it doesn't matter the mufflers, I've tried Magnaflows in various lengths and diameters, Summit aluminized glasspacks in various lengths in both 2.5 and 2.25" diameters, Summit stainless glasspacks, a memorable RedHot glasspack (insane loud, basically hollow), along with two variations of spiral core mufflers, one of which held too much heat under my shifter box and about melted the bottom off it - even with heat shielding there! You should small fresh exhaust-roasted nylon. Mmmmm), and I ran many in several combinations, long muffler upstream, long muffler downstream, change out one muffler for another, re-run.
Even my exhaust hangers are tired! Two of those 6 bolts that hold the chassis brace on (keep it on, BTW), had to be replaced once!
At least most of my exhaust bolts are well anti-seized.
Lessons:
- Loud. Power, but loud. Mainly from resonance at the above ROPM range, and it's nearly impossible to cure unless you go with a Heimholtz Resonator, basically a capped specific-length "T" off the exhaust that wraps around the battery box and cancels the resonance - a bit - at the desired rev range. Difficult to calculate the length and diameter, heavy, and little performance gain.
- Put the higher-restriction muffler upstream of the lower-restriction one (you need a pressure differential to make them work).
- Exhausts get HOT - either relocate the battery box (difficult) or use some heat shielding on the bottom (easy), and a heat-shielded batt box works fine with even a 2.5" on a somewhat-lowered car (I'm down about 1/2", you can go up to about 1", but measure). You can also flatten one side of the aft pipe (or ovalize) for some additional clearance.
- Exhausts get HOT (2): Don't put a nice, dense, spiral muffler beneath your battery box. It's fine while running, but the retained heat will eventually toast the bottom of the shifter box, making it fall off. It'll then heat your cupholders such that a hot cup of coffee will be waiting about 10 minutes afer you buy the iced frappucino. And if you've already installed the short shifter with the shifter box bottom missing, just plan on heat.
- Resonance is a BEAST to get rid of. Putting the ac on recirculate made a difference, opening a window made a difference, but in winter with neither of these options available, I still had to open a window. You know that feeling when your ears need to pop? That. Plus noise.
The quietest least-uncivilized system I ran in this config was a 2.25" system fabbed from a stock front pipe into (2) Summit stainless glasspacks. Found a bunch of horsepower (~10 using some calculations on acceleration rate on a known long straight hill, same weather conditions, passenger load and gas), but it'd still drone a bit at highway speeds, MUCH noisier than the Borla Race system I'd been running, and I can't compare it to the JCW I currently run because, well, the JCW is relatively silent (and honestly doesn't provide much if anything in the way of horsepower gains).
I had a total of 14 different configurations on, including the stockie and the Borla Race (easily the sweetest aggressive-sound system out there). I miss them. They made POWAH. They sounded awesome in tunnels and under bridges and in parking structures.
But I like my hearing, and my Harmon Kardon, more.
Hope this helps!
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It's not that I think it's better; I'd rather run a straight pipe. But with it straight, with a cat, and a big magnaflow muffler in front and back, it was too loud on throttle. Car was making "only" 250WHP, and the drone and noise at high speed was too much. At civilized speeds it's was ok. I still have the magnaflow up front, and JCW cans in back, this works well. I'd rather go the Milltek route for the cans and keep it 2.5 the whole length, but it's not a big deal.
Didn't try a resonator, I'm not sure anyone has had much luck with one at high HP, but it would be worth a try. Not sure with what you're running how well it'd work. Spiral cone insert may be the way to go for the loudness, but it's the drone that's going to kill you. You're going Rotrex, right? If you're going to try a straight pipe, please, please talk to Jan and Colin. There are ways to do it, but it's not easy. Coastfab is the way to go if you're dead set on this route. But it's much easier to split the cans in the back--it makes all the difference in the world. I may try a straight pipe again, I'd really like to put on a rear diffuser, but after my first foray into that, I might not.
Engine is done, or very close to being done and back in. Car is in Irvine; all the work is being done there. I'm just a spectator...
opcorn:
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Wellzy, hopefully I can save you some angst - been well down that road. I've used
- a straight back (LOUD, no power, not enough restriction and some restriction is necessary to gain some torques)
- various configurations of single can and fruit loop ("two-can") systems, and one memorable 3-can system, attempting various configurations of spiral mufflers, resonators, and glasspacks, in both front and rear positions
- 2.5" and 2.25" systems
The ONLY straight-back that worked sonically on the R53 involved a 2" ID muffler (off a Justa!) just aft of the cat, an 18" 2.25" glasspack, then a small 2.25" resonator off a Buick, for a total of (3) cans, (6) muffler clamps, and a lot of cussing... and that one didn't make any power. No losses, but no power. Sounded fantastic, but then again so did the Borla, and the Borla actually made a few hp while sounding good. And it fit better.
One that *nearly* got it was a 2.25" system using the stock front piping (to retain the bend and the center muffler mount), using (2) Summit Racing 18" SS glasspacks, one just aft of the flange and one between the chassis brace and the batt box. Sounded fantastic for a couple thousand miles, a *little* drone but not bad, note that any glasspack system will eventually "blow out", and all will leave some shiny dust on the back of our llittle aerodynamic toasters. Then we tried to weld it (it had been clamped). The act of welding caused some of the packing to burn and the alignment to change, then it got droney. We made some power with that one, but I didn't want to live with it, even in the former "OK" sound, long-term.
Lessons learned:
- It's less noisy if the higher-restriction muffler is upstream so you have the necessary pressure drop across each muffler
- Straight-backs will get you pulled over.
- Single glasspacks will probably get you pulled over.
- Double glasspacks will get you "the look".
- None are what you'd call "subtle".
- Exhaust diameter near as makes nearly zero difference on our little motors with a stock pulley, just not enough airflow except at wi-ide open for a long time. Some restriction actually seems to add to torque, which is good, we like torque around town. This was a strength of the Borla.
- Aligning exhaust systems is not really easy on jackstands, alone.
- "Real" muffler shops laugh at homebrewed systems, even if they work
- Summit racing SS band clamps need grade 8 fasteners and they come with grade 5. They can break. Dramatically ("BANG-ping-ping-ping" followed by "pfff" and, if left long enough, "waaaah")
For drone: As far as we can determine (using some solid engineering, good maths, and lots of practical experience), it's the length of the pipe as compared to the interior volume of the car and the resonant frequency of the chassis, meaning the R56 exhaust (which you can probably run if you cut off the battery box and fab up a center mount) would likely do the same thing. It seems EVERYTHING comes together ri-ight about 3000RPM at that pipe length, regardless of diameter (2.25" and 2.5" both do it, at very slightly different rev ranges but it's all at 70MPH +/- 5). It seems this *can* be addressed using a Heimholz Resonator, a specific length of capped exhaust pipe, welded at a "T" to the rest of the system, angled to drain, and of a length determined by which frequency you want to moderate. It's all very mathy, but it works - problem is, by then, you've thrown so much into it that a factory system starts looking like the Holy Grail.
Oh - as for testing, we used a full 2nd gear "pull" on a long, essentially empty onramp near the house. Not perfect, but with some maths it was directionally "good enough".
Hope that helps!
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