(I still advocate just buying a Thermal from Ron@whoosh - but I was impatient and staring at stitches on my hand so I reflex-purchased whatever was available immediately. I have been toying with the idea of building several resonators for people with mbrp exhausts anyways, maybe a replacement rear section or something, so this instance nudged me further in that direction)
I cut my hand pretty badly while fiddling (again) with my custom exhaust, so I hit up Ron@Whoosh to get something *now*, that was pre-made and would fit without trouble (except leveling the g*****n tips?!). Thermals weren't immediately available, so I grabbed an aluminized MBRP to play with (at that price I won't feel bad drilling holes in it). I've done my acoustic sampling and peak drone is at 3200 RPM - its enough that it causes that pressure-y fuzzy feeling in my ears, so it's definitely causing hearing loss. Until I get over to the fabricators to have my helmholtz chamber or quarter wave (I still haven't decided which. If a quarter wave works for thermal its good enough for me!) built I'll be wearing etymotic er20xs earplugs (*fantastic* ppe's, by the way) while driving.
Anyways, I know that I am way more interested in this stuff than anyone else, but since my first few iterations of the exhaust system I've used a thermocouple to better assess the air temperature inside the side branch resonator (it's basically ambient since there's no exhaust flow). My original calculations were a scant few inches off so I still got a tiny bit of drone, but now I have my exact target temperature to math it out properly.
Drone RPM - 3200 is the "peak" drone, so that'll be my target range. I'll add a bit of steel wool to the resonator chamber (just a bit) because it'll broaden the effective cancellation range by a little.
Input: Drone RPM 3200 and Exhaust pulses per RPM 2
so 3200 x 2 = 6400
6400 / 60 = 106.6666667 hertz (or 107, for the rest of this)
The resulting frequency of 107hz matches my acoustic sampling, so with double confirmation I am ready to proceed.
Input: frequency 107hz, speed of sound 1125.33
so 1125.33 / 107 = 10.5171028 (wave length)
10.5171028 / 4 = 2.629275701 (quarter wave length in feet)
2.629275701 * 12 = 31.55130841 (quarter wave length in inches)
SO - I need a side branch 31.55130841 inches long. Or 32 inches long because its easier
My hands aren't deft enough to do the tight 32" coil needed to fit in the ST's exhaust cavity, so I'm having my fabricator/favorite exhaust dude do it as soon as he has time. With July 4 coming up I'm not sure if I'll be able to get to him before he heads out to the desert for the holiday, but I'll update as soon as I get it done.
Notes: With the MBRP, it was nearly impossible to install without it touching the center brace, so I added washers to get ~5/8" of clearance between the mbrp and the center brace/plate/thingy.
I cut my hand pretty badly while fiddling (again) with my custom exhaust, so I hit up Ron@Whoosh to get something *now*, that was pre-made and would fit without trouble (except leveling the g*****n tips?!). Thermals weren't immediately available, so I grabbed an aluminized MBRP to play with (at that price I won't feel bad drilling holes in it). I've done my acoustic sampling and peak drone is at 3200 RPM - its enough that it causes that pressure-y fuzzy feeling in my ears, so it's definitely causing hearing loss. Until I get over to the fabricators to have my helmholtz chamber or quarter wave (I still haven't decided which. If a quarter wave works for thermal its good enough for me!) built I'll be wearing etymotic er20xs earplugs (*fantastic* ppe's, by the way) while driving.
Anyways, I know that I am way more interested in this stuff than anyone else, but since my first few iterations of the exhaust system I've used a thermocouple to better assess the air temperature inside the side branch resonator (it's basically ambient since there's no exhaust flow). My original calculations were a scant few inches off so I still got a tiny bit of drone, but now I have my exact target temperature to math it out properly.
Drone RPM - 3200 is the "peak" drone, so that'll be my target range. I'll add a bit of steel wool to the resonator chamber (just a bit) because it'll broaden the effective cancellation range by a little.
Input: Drone RPM 3200 and Exhaust pulses per RPM 2
so 3200 x 2 = 6400
6400 / 60 = 106.6666667 hertz (or 107, for the rest of this)
The resulting frequency of 107hz matches my acoustic sampling, so with double confirmation I am ready to proceed.
Input: frequency 107hz, speed of sound 1125.33
so 1125.33 / 107 = 10.5171028 (wave length)
10.5171028 / 4 = 2.629275701 (quarter wave length in feet)
2.629275701 * 12 = 31.55130841 (quarter wave length in inches)
SO - I need a side branch 31.55130841 inches long. Or 32 inches long because its easier
My hands aren't deft enough to do the tight 32" coil needed to fit in the ST's exhaust cavity, so I'm having my fabricator/favorite exhaust dude do it as soon as he has time. With July 4 coming up I'm not sure if I'll be able to get to him before he heads out to the desert for the holiday, but I'll update as soon as I get it done.
Notes: With the MBRP, it was nearly impossible to install without it touching the center brace, so I added washers to get ~5/8" of clearance between the mbrp and the center brace/plate/thingy.