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2012 MP4-12C
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I've seen several videos of various downpipes and haven't heard the rasp. Maybe it doesn't come through as much on video, as mine was the case.
Yup. The rasp doesn’t come thru on video. Every car I have heard in person has it.
 

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I've got Fabspeed catless with sport exhaust and don't notice a rasp just a deep growl from 3500 and up. HOWEVER.....today I played with the ISG and set it on the highest level and I did get a 'Honda' sound from around 3200 to 4300 rpm's. It was gone when I set it back to the lowest level. Not sure what setting you run your car in but I hope this helps.
 

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Discussion Starter #23
I've got Fabspeed catless with sport exhaust and don't notice a rasp just a deep growl from 3500 and up. HOWEVER.....today I played with the ISG and set it on the highest level and I did get a 'Honda' sound from around 3200 to 4300 rpm's. It was gone when I set it back to the lowest level. Not sure what setting you run your car in but I hope this helps.
Mine has been set to the lowest volume since I discovered you could adjust it (hour 24 of ownership). I may double check to ensure that setting hasn't changed for some reason at some point (I haven't noticed the synthetic sound anyway). I drove it today to confirm the math and the reality that ~240 hz is the loud rasp, and it's about 6-8 dB louder in that range it's doing it than the rest of the exhaust. There are a few ways I can go about fixing it, I'm playing with ideas for the easiest should others run into it so they can possibly resolve it themselves.

This does ask the question though: Will the ISG delete eliminate the sound all together, or is the ISG volume all the way down the same as a delete?
 

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volume down is same as delete i think.
mine is turned down and can't hear a difference between sport or regular.
 

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Discussion Starter #25
volume down is same as delete i think.
mine is turned down and can't hear a difference between sport or regular.
You should only hear a difference where you have a volume difference. When I first got my car ISG volume was different between all driving modes, loudest on normal and quieter on sport and track. It confused the hell out of me as to why it was so much louder with windows up than windows down but I was also changing driving modes and didn't know that's what was changing it. So, about an hour after driving it I figured that out and turned it all down.
 

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Discussion Starter #26
Well, after doing some digging I've found the factory H pipe is likely why the rasp is there with catless downpipes. I know of a couple ways I could fix it by modifying what's there but I don't want to hack up the factory stuff (especially since mine is the rare valved type). So, I'm going to make an X pipe instead, which should resolve it. Note I said going to make, not going to buy, because there is no universe where that is worth $3,000 to me. Maybe if people like mine I'll produce them for sale, for not $3,000.
 

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2012 MP4-12C
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Note that these cars loose significant power with poorly designed exhaust rear sections. Not saying yours will be poorly designed, just a heads up. See video below, specifically the video details.

 

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Discussion Starter #28
Note that these cars loose significant power with poorly designed exhaust rear sections. Not saying yours will be poorly designed, just a heads up. See video below, specifically the video details.
I don't see a significant power change, but I do see a significant torque change. Are the Fabspeed LT style pipes air flow connected at all? Looking at the different systems I have some ideas where the differences are in results, but also worth noting that's on a 4.0, not a 3.8 and still catted, not catless. In any case, I'll gladly sacrifice a little torque to get rid of this awful sound (at least it gets reduced after exhaust temps are up).
 

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Discussion Starter #31
UPDATE: The company I was talking to about doing the exhaust flaked. I met with them, sent them all the photos they wanted, never heard back even after following up. So, I'm back to making my own. Pipe will arrive tomorrow, I've got the design more or less figured out so I may start fabricating tomorrow, I plan to have it operating again by the end of the week sans exhaust tips that won't be here for a few weeks.
 

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swap in an x pipe? maybe call and speak with magnaflow on speccing same.
 

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Discussion Starter #34
Update: This is what I've got so far. My tips haven't shown up yet so I left the factory shields in till the new tips come in, making it look more like a 675LT.

200471


The raspy sound is improved a lot, but it's still there some. I am doing a video on all of this, but I'll be unwrapping part of it and adding some side branch resonators to calm down that noise some, hopefully. I may get to that this weekend, we'll see how it goes.
 

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Cool, any pics of it unwrapped? How difficult is it to find someone who does cnc mandrel bending on a small scale?

You probably know this already, but the side resonators are only specific to very select frequencies, might be worth checking if those match.

love the fabwork
 

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Discussion Starter #36
Cool, any pics of it unwrapped? How difficult is it to find someone who does cnc mandrel bending on a small scale?

You probably know this already, but the side resonators are only specific to very select frequencies, might be worth checking if those match.

love the fabwork
200489


I bought pipe that was already bent and worked within what I had to make it work. Nobody locally has a mandrel bender except the place that didn't get back to me, so that was the next best thing.

I would be making my own resonators, and I made a spreadsheet to both calculate the resonance of the exhaust (which matched reality) and a branch to match that frequency, complete with various exhaust temperatures. Right now all I've gotta cut out is right around 250 hz, which is a 5.7" long pipe, I should have no problem getting 2 of those in there.
 

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pretty awesome, its nice to see people doing their own r&d. I did this back in the day on my m3 and it was nice to be able to tune the sound to your liking. Looking forward to the vids.

BTW I believe you can also tune some of the exhaust frequency by altering the exhaust distance either the exhaust tip length or the distance to the resonator.
 

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Discussion Starter #38
pretty awesome, its nice to see people doing their own r&d. I did this back in the day on my m3 and it was nice to be able to tune the sound to your liking. Looking forward to the vids.

BTW I believe you can also tune some of the exhaust frequency by altering the exhaust distance either the exhaust tip length or the distance to the resonator.
The tip will be longer than it originally was, and the pipe is already longer than it was so the frequency has changed. I'm looking into the distance to resonator to see if there is anything I need to keep in mind. What I REALLY wanted to do requires me having things mandrel bent on demand, so I didn't do that. If I had more room I'd just put resonators in there but that won't pan out too easily. I got the pipe for the resonators just now so I may work on that tonight, or tomorrow, then re-wrap and reinstall. Alternatively I may go for a drive tonight to get solid data on the frequency to be sure I'm doing the right things.
 

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My exhaust sounds great, but tinkering always helps! After watching this post the last few weeks I may end up making my own Akropovic X pipe. I have a Huth bender to make some of my sculptures so it might be time to have some fun! Congrats on the great looking exhaust you made.
 

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Discussion Starter #40
My exhaust sounds great, but tinkering always helps! After watching this post the last few weeks I may end up making my own Akropovic X pipe. I have a Huth bender to make some of my sculptures so it might be time to have some fun! Congrats on the great looking exhaust you made.
I did some more testing tonight since roads aren't so busy, actual testing now shows 254 hz resonance and math says 253 hz so I'll call that a success. I might do some testing with the modifications Sunday, though it will be 24 degrees colder and less desirable conditions.
 
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