My suggestion is pull midpipe, make sure nothing bouncing around inside it. Once that is ruled out, assuming you really have checked all heat shields and O2 sensors as potential sources, then likely dealing with engine noises that are getting unmasked by the highflow setup and the noise is just coming down the tubes and taking residence in the Helmholtz. In this regard I reached out to a few customers yesterday who reported unusual noise issues over the years with this setup. We sell a few hundreds of these per year, so we have seen a few unusual cases like this one and I had to meet with Rocky and reach out to a few customers in the file to get reminded on all the details.
Three customers got back to me to explain that the sound calmed down with more carbon, that's the most common response and you have read the same repeatedly at Miata.net. Another guy who reported unusual noise responded that his did not resolve until he replaced the motor. Apparently in his case the noise was a bent valve and eventually he lost the motor, his motor builder said the wear indicated the valve had been bent a long time, perhaps since it left the factory, never installed right in the first place and noisy from the start yet not noticed with restrictive stock exhaust system. He reports his rebuilt motor with bent valve replaced makes no usual noises through the exhaust system.
Finally, there is Nader's car, perhaps the most interesting case. That's Nader's car below, I have been instructor for autocross days here for him and his daughter driving the same NC.
Nader ordered up same system you have but he noticed odd noise getting broadcast through the helmholtz, so much like yours in my recollection that I sent him your video yesterday to ask if it sounded familiar. He reported back: "So what's on the video is not the same sound; similar but not the same. My noise was much more high frequency." In Nader's case also had an idle issue and that proved to be a bad EGR and Rocky replaced it. We hoped that would cure the exhaust system noise too but Nader tells me yesterday the sound did not change. Talking with Rocky I am reminded we changed Nader's car to our standard midpipe, which has fiberglass resonator, and noise was better absorbed but still there (the notes on Rocky's file remind me that I drove the car with him and we could both hear it through the car even with fiberglass resonator midpipe). Ultimately determined his noise is coming from inside the motor and any use of highflow header is going to release it to audible levels, midpipe with fiberglass resonator would mask it better than Helmholtz version but with both it was still there. So Nader went back to stock header and stock midpipe as the only way to keep that sound below audible levels.
What makes Nader's case even more interesting, is that the Helmholtz midpipe that was broadcasting his unusual motor noise was still here in the box yesterday with his name on it. So we brought my 2007 to the shop yesterday and installed that same Helmholtz midpipe that was broadcasting noise in his car and did our own video on my NC with holding rpms at 2000, then 2500, the 3000, then 3500. Results? It's perfect, no odd noises. I drove the car home, took the long way through the hills, it clicked over the 11k mark during the journey, and the same helmholtz that was allowing odd sound to escape on his NC is perfect on mine, because the highflow setup is just a carrier for the upstream sounds and not the source, absolutely nothing wrong with it. So, again, Helmholtz is not to blame for the noise, it's just playing the Canary in the Coal Mine and when properly installed on normal car it works perfect.
Notice in my video the unit is perfectly installed and alignment of hanger for the midpipe is ideal. This unit is same batch as yours, these all come from same JIG, it will install like you see here. So my suggestion is pull the midpipe and shake it and make sure nothing stuck inside that can rattle, and then try again on the install to get the hangers as you see in my video. If nothing else, it will make me more confident you got nothing inside that is jumping around and making noise AND make us more confident you have it installed right and it is not simply hitting something somewhere that you have not noticed because the point of contact is above the assembly, etc. Sometimes being just too close to things can result in noise even if no contact, items can move and vibrate in sympathy if close enough. After that, if it doesn't calm down with carbon loading then I think we need to conclude you have one of these cases of odd engine noises and a highflow catless header setup is letting that escape down the system and the no fiberglass helmholtz will release that more than any other setup. As with Nader we can swap you to the fiberglass version of midpipe that will do more absorbing of such engine noises, but ultimately like Nader your car may just be one of the special ones with some odd engine noise that any catless header is going to send down the pipes when the highly restrictive stock header would continue to hide it from you.
Thanks for keeping up with the research and options. Not sure what the video shows... my car doesn't make the sound on steady load as I've indicated several times. It only occurs under acceleration/engine load.
I was notified of a parcel pickup at my post office. Left work in good time and swung by to pick up the envelope with the new glass pack inserts. Paid the duty and taxes and raced home. Took out the metal ones and replaced with the glass packs. I let the car warm up quite a bit then went for a drive. The glass packs greatly diminish the sound level generally (surprised at how much sound they suppress..... they work well). Sadly, the metallic sound, under acceleration, is still present albeit a bit less loud. I took my wife (without telling her what I've done only that I have a new fix) and she immediately shook her head under the first acceleration out of our street. This is not a sound that I'm willing to live with in "hope" it gets better with more carbon build up. I've put over 4,000 kms on this car approximately and there is no noticeable change yet.
So.... as much as I REALLY want this system to work as I love the no drone I think it's time to take you up on your exchange offer. But I'm intellectually very curious as to what's going on... thinking a lot about what you said about the header. The only way we can help solve this mystery is my following proposal:
I will install it. If the sound completely goes away we have a solution, I'll put the old midpipe in the box and send it back to you.
2. However, if the sound persists even a little bit with the new midpipe that means more sound is coming from the engine than the exhaust can handle (I'd have the rare cars you speak of). Then I'd buy a catted header from you and install that then test with both mid pipes. Hopefully the catted header would stop enough engine noise (if I have the rare car) protecting the sound from bouncing around the Helmholtz. Then I'd keep that and return the other midpipe.
Either way we both learn what's the cause and I can get the sound & performance I intended and you learn more about overcoming a potentially rare situation.
I hope you can work with me on this to solve the mystery. If you are unwilling then I'll just exchange it per your offer and we'll part ways chalking it up to experience.
I'll call your office tomorrow to try to organize the details. Sound good?
Start with testing stock header back on the car with Helmholtz midpipe you already have and SuperQ muffler you have. Nothing needs to be shipped across borders, no additional customs duties to pay, and it will have the same testing effect as adding fiberglass midpipe. My bet is your mystery sound drops way off, perhaps below level you can hear it, confirming something upstream of header is the source.
Thanks for the suggestion but that is not possible. When I removed the stock header I found the secondary oxygen sensor was frozen. Despite Perry’s techniques it wouldn’t budge so I had to replace it. I like to keep my shop clean so I hate old unused ugly parts lying about and I never had considered a need to use it again so I previously discarded the factory header.
I have called your office and left a voicemail. Can someone please call me back so we can resolve this. You’re busy trying to run a company and I have a complex job as well. I don’t want to spend more time on this then I have to.
Great chat with Ryan today. Really appreciate the flexibility and focus on making this work; whatever is the magic combination!
It's really easy to get snippy with each other over these types of things in the heat of frustration. But customer service as illustrated by you and particularly Ryan make a huge difference which creates loyalty. Looking forward to the old school midpipe to see if that solves this puzzle. Will advise as soon as UPS drops it off! Tools are at the ready. Gawd... loving this lift!!!
This evening I received my traditional resonated high flow mid pipe that I arranged with Ryan. I carefully installed it and with much excitement and trepidation I ventured out. It's PERFECT!!!! There is NO hint.... I mean ZERO... any metallic noise or anything unpleasant. It is a nice deep rumble at idle and low RPM's... and as the RPM's build it sounds.... great!!! Not too loud as the SuperQ keeps that in check. But nice little pops on shifts with no metallic resonance or noises. Just a nice sporty sound... and I retain the added torque of the Max Power header.
Finally, I'm one very happy customer.
I will be boxing up the Helmholtz and returning it this weekend.
Thanks to Brian and Ryan for helping me through this. I regret the HH didn't work for me but I love the sound now.