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Rasp

13K views 28 replies 11 participants last post by  ajf909_specV 
#1 ·
I ran an ebay header for years until the rest of the system rusted beyond repair. I replaced the system with all 2.5 inch stuff and the rasp is unbelievably bad above 1/2 throttle. I have a Magnaflow spun universal 8" x 4" cat and 22" resonator and a flowmaster 50 series 3 chamber SUV muffler. The tone is damn near exactly what I wanted. Deep and mellow. The volume is almost twice as loud as I was anticipating and the rasp is horrible.

I'm thinking that a standard honeycomb cat may be a solution, as it will be larger and should slow the flow down so I'm not overloading the resonator.
Or I could bottleneck the the flow to 2.25 somewhere along the way.
Or I could add another, smaller, resonator to the existing lineup.
Or I could replace the muffler with something different.


I am not concerned with performance. The current setup was a failed attempt at creating a decent sound on a really tight budget.

What do you guys think I should do to fix the rasp? Replacing the entire system is not going to happen, even if the GB on Stromung would be the best solution lol. Just don't have the money for it.
 
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#2 ·
Vibrant ultra quiet. My exhaust tone has gone down about 40%. I had some rasp and it was quite loud even driving normally. Now it has a clean and modest tone. I had a magna 22" res, and there is no comparsion. I have a nismo header and megan dual tip exhaust.
 
#6 ·
funny as that sounds
its true
my experience too


4 exhaust systems later
stock
obx
and 2, 2jr systems

2 different resonators
and cats

my learnings
yay vibrant

but a ceramic cat [even race cat]
does wonders
even this worked


ceramic coated header


or wrapped exhaust


all helps

you can also encase muffler and resonator
and wrap the exhaust pipes
if you don't wish to change parts
https://www.heatshieldproducts.com/automotive/exhaust-and-header-wrap

also the quality of your rubber hangers
affects resonance and thus rasp

mine ended up this way
ceramic coated header, magnaflow 3" ceramic cat
na



and turbo
wrapped exhaust and same cat


use the thickest steel exhaust piping possible
avoid the really thin stuff

high quality ss has better thermal and resonance factors
and is preferred

good luck
hope at least some of this helps

honestly it took a lot of experimentation

now built in, to the later 2jr exhaust systems

easier just to buy one
thats for your next time
imo
 
#9 ·
With cars, everything is a compromise, if you want it quiet, you have to run a very restrictive exhaust, if you want high mileage, you have to tune, and run at low RPM's, and sacrifice performance. if you are willing to listen to the sound of ten thousand angry bumble bees, you can have a fast vehicle. Or, you can get your performance with power adders, N2O, Turbo, or Supercharging. It really depends on what you are willing to lose to gain the performance. Our 4 cylinders love to breath freely, both on the intake and exhaust side, anything that smooths out and quiets these areas will affect performance. Cold air intakes are good for a few ponys, wide open exhaust as well. Then you get into the drivability end of things. Are you willing to gain a few horsies at the expense of not being able to use your rear view mirrors? cut the balance chain... I regretted cutting mine until I had my whole build done by 2J, with the crank and everything balanced I am back to being able to use the mirrors. Keep in mind, I dont have the same goals as Yogi, I wanted my car to be a comfortable daily driver that would embarrass pretty much anything that would want to pass me, and still be a joy to drive the 6 hours to the beach from my house. To combat Rasp when I was NA, I had a High Flow cat welded in, a resonator, and a magnaflow (later, a flow master) muffler, This did a great job, no rasp, but it was still loud. Now, with a cat, and 2J side exit, I am not much louder than most mustangs on the road. It has been said that a turbo is the best muffler out there, and I believe it to be true.
 
#10 ·
I debated doing the BSR when I replaced the headgasket but decided against it. It is just a DD. It is odd that you had a HF cat, res and flowmaster and had no rasp. I have the same thing and such awful rasp. I have to conclude that its the resonator not working like it should, or is at a distance away from the exhaust valve opening that happens to be an optimal wavelength for the rasp frequency. Gonna go with the Vibrant and see how it goes.
It sucks cause I had the same-ish header, muffler and HF cat on my Neon and it had zero rasp, even without a resonator. And before you spit coffee all over your monitor, the Neon was no where near stock and a blast to drive. Still miss it.

Anyway, thanks for the opinions guys! Such a great community.
 
#11 ·
The cheapest option to try for you would be to wrap your exhaust, ( as mentioned already), as far as possible from the header back. It will be a little more elbow grease but you will not break the bank trying different exhaust modifications that may or may not work. I have mine wrapped through the midpipe and don't really notice the rasp.
 
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#14 ·
Interesting that this subject came up. Though I haven't done the exhaust for the B15 yet, I'll relate my experiences with the Honda Wagovan I built and daily drive, and you can draw your own conclusions...

After my turbo D16 install, I took the Wagovan to the local Midas shop for a header-back exhaust install. Four hours later the backwards-hat wearing dufus from the back shop came out and said he couldn't figure out how to build a two and a half inch system for the car. By this time i was pissed off, so I stopped by Autozone (which, coincidentally, is 2 blocks down the street) and bought a flex pipe, some straight pipe, some bends, and two Thrush glass packs. I went home and built my own exhaust system in about six hours, and it's nice and QUIET... no rasping, very mellow, and it's just loud enough to let you know it's running when you jump on it. Secret here was to butt the two glass packs together with a short connector pipe in between. Also helped that I have a welder and could weld it all together. Not saying this is the answer to ALL the rasping problems you might encounter, but the car flows free, is quiet, and it still gets 38MPG on the highway.

The QR build might only get one glass pack, since we're going on track with it, but it'll be quiet enough to pass the DB meter trackside and not set the alarm bells off...
 
#16 ·
It is odd that you had a HF cat, res and flowmaster and had no rasp. I have the same thing and such awful rasp.

welcome to chaos theory and resonance

1st
what is rasp

well best i could find
its destructive resonance of a reed like structure

a vibration that literally shakes the whole pipe

like a bad note on a clarinet
where the reed flutters

yup
rasp is tail pipe flutter
or so say the physicists


thus wrapping, pipe length and change of pipe thickness
as well as
the ratios of valve to resonator and muffler to tailpipe volumes and lengths

alter this overlapping cyclic phenomena

people like the mad hungarian with long experience
almost invariably get the ratios of pipe length, volume and such right

because flutter frequently slows flow
interferes with power and destroys parts
like and similar to, wheel hop

tuned for power also means tuned pipes
at least they are aware of it

and choose ratios that deliver good power
thus no flutter


some short drag racing pipes may use this frequency to empty the combustion chambers
they may not have rasp but they sound terrible
[listen to stevenspecs 12 sec pass]

best i can figure out
imo

simple examples

thick pipes short lengths
solid mountings

loud but no rasp


well mounted with cat


huge resonator on long thick ss pipes
look under car almost touches the ground


coupled with big muffler
 
#18 ·
Another thing to check that would create more rasp, is to check inside the exhaust pipes at the flanges. On the lower quality exhausts you would see a small gap between the ID of the flange and the OD of the pipe. This would create, (as Barry mentioned), a reed like resonance and add to more turbulent flow. I believe the higher quality exhaust manufacturers either grind those edges down or are more careful in the welding process.
 
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#22 ·
Don't suppose you know a site or article on how to calculate the diameter, length and position for a J pipe for varying engine and exhaust makeups? lol
 
#26 · (Edited)
this is what science has to say
And from my perspective
THEY ARE RIGHT

from previously posted link
quote

There is, however, and exact science to eliminating drone: calculate the drone wavelength and cancel it with a resonator tube. You can also use the sound frequency to buy the correct sound suppression material since the better ones have different material for different frequencies.

Here's the formula to get the tube length:
((343)/(RPM * 4 * (1/60))) / 4

Just replace RPM with the RPM where drone is at its worst.
E.g. ((343)/(2500 * 4 * (1/60))) / 4 = 0.5145 meters



Tip: if you just want the frequency for buying the correct sound suppression, use this part:
2500 * 4 * (1/60) = 166.67 Hz (replace 2500 with your drone RPM)



For drone at 2500 RPM you need to weld a capped exhaust pipe 0.5145 meters in length before your muffler, as perpendicular to your existing pipe as possible (it should look like a T). It's as simple as that. Although it's often called a j tube, it's actually better if it's straight, as the more it bends the more sound bounces back before reaching the full length. The j shape comes from having to fit it in small spaces. If you can't weld, with this number you can tell any exhaust shop what to do.

For those who are curious, here's the formula breakdown:
343m/s is the speed of sound
4 is the number of pulses per rev the engine makes (a V6 would be 6, but some engines fire two at once, so a V8 would still be 4).
1/60 to convert from minutes to seconds.
/4 divides the wavelength by 4 to get the length of tube that will cancel out the frequency.
 
#28 ·
Update.
I had the Vibrant resonator installed Saturday. The rasp is gone! Woo!
SRS header from Mark ---> magnaflow metallic spun high flow cat ---> magnaflow resonator ---> vibrant resonator ---> Flowmaster super 50 series.
Sounds deep, like really deep. Idle is very mellow and not too much above stock.
Wide open throttle sounds wonderful. Aggressive and tuned, dare I say euro-ish.
Normal 4k shifting in traffic sounds nice. Not overly obnoxious or droning.
I still would like it more quiet though. Just a little bit more. Maybe switch to a regular honeycomb cat down the road. Or go with a borla muffler or something non-chambered.
Overall, I'm happy with the sound. Thanks for all the advice fellas!
Hopefully she'll live to see another 160k miles.
 
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