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My Custom Hayward and Scott Cat-Back Design and Proving Introduction **If you don't want to read at the theory, skip to the end where there are pretty pictures and a video** Firstly I should point out that I am a bit of an old man and dont like noisy exhausts! LOL. Please dont take any of the following as a criticism to any aftermarket exhaust; I just wanted something that is quiet under full throttle until higher up in the revs. This then gives me some headroom to further modify the exhaust. My current exhaust, the Cosworth Cat Back, measures 92dB supercharged or 89dB N/A, so not particularly loud but when cruising on the motorway, 6th Gear, 3k rpms and going wide open throttle, I found it to be very boomy in the cabin when it was hot, particularly after a track day. Even with cruise control on when coming to a hill, the slight throttle was enough to annoy me on longer drives which are becoming more and more often in my GT86 these days. How on earth some of you live with 100dB exhausts is beyond me! Its not a criticism of the Cosworth cat back; I still think it sounds awesome and I haven't come across any other cat back for this car that is quiet when under WOT at low revs either, apart from stock. Certainly it's not any worse on the Cosworth 0.1 than other exhausts I have sampled in the 92dB range. Silencer Theory Straight though absorption silencers used in most aftermarket exhausts dont attenuate sound very well at low frequencies. It depends on the length, volume and packing material but generally they dont block as well at lower revs as they do the higher. The reason for using these silencers is that they dont restrict flow like a cambered silencer (stock). I decided to do a bit more research into OEM silencers and how they work. It turns out that a stock silencer typically has two or more chambers inside but each chamber often has a different target frequency range. I am not going to get into detail but here is a diagram of a typical 3 chamber silencer. Each chamber is a different design targeting different frequency ranges. And here is the insertion loss of the different straight silencer types: The tuned pipe, otherwise known as a branch resonator or sometimes mistakenly called a Helmholtz resonator is quite a popular DIY for helping drone. Its length is designed to reflect sound waves 180 degrees out of phase at the target frequency. The result is noise cancellation but the Q is quite high so it doesnt have a very wide range. At the target frequency of 100Hz and 150C gas temperature, the pipe would need to be about a meter long! Here is one such example (borrowed from the ft86club): I had noticed that some Cat Backs do have Helmholtz resonators, namely the ARK Grip and HKS HI Power Spec L. I havent heard either to comment other than to say, looking at my calculations and the images, it looks to me like they dont target 100Hz as the cambers are a bit small and both seem a bit loud for my goal. The Helmholtz does look appealing though, as it has a lower Q, so wider attenuation range and the packaging is much nicer requiring a big air chamber and a neck, bit like a bottle. Hayward and Scott I decided to speak to Ian at Hayward and Scott about a custom Exhaust as I have always been impressed with their work. Originally I asked Ian to make me a large single absorption silencer like the Q300 or Miltek but to connect up to the 3inch Cosworth mid pipe. The intension here was to add additional resonators in the front or mid pipe if it was still too boomy at low revs. After some discussion with Ian I decided that it was better to start with a whole new cat back rather than chop up the current one as Ian was sure he could get a much larger center silencer in the tunnel. We decided 70mm (2.75inch) pipe would be a good compromise and we should keep the Y design as it has the best flow. He also explained that they have a small chamber built into their silencers too which may help, so I left the car with them for a week while they made the system. Here are some pictures of the finished product: Ian has a catalog of exhaust tips but I liked a twin wall rolled and these with the perforated inner did the trick. Actually, originally I said I wanted a design similar to the Cosworth/Nameless/Perrin but Ian fitted the rolled and I decided to keep them when I saw how nice they look. Ian knows best! J The results were very good. Much quieter and a pleasing rumble, however on the drive home I wasnt quite happy with the 3k rpms WOT boom. Although far quieter than before, I wanted even less, but the top end was perfect. Helmholtz Design I decided to look in detail at a Helmholtz resonator design. First port of call was Wiki! https://en.wikipedia.org/wiki/Helmholtz_resonance Next I made a spreadsheet to calculate the required volume and neck lengths from the formula on the wiki page. At this point I want to thank the few engineers; Frederik, Dan and Matt who checked my sums weren't completely wrong! Also another good reference was a paper discussing using the muffling effect of Helmholtz Attachments to a gas flow path http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2200&context=icec I decided to copy the ARK style arrangement, with two Helmholtz resonators mounted in parallel to the rear silencers as this was the area under the car with the most space. Also two would work better than one. They would be quite a bit bigger than the ARK design though, targeting 100Hz. Once I was happy with the design on paper I made a mock up from cardboard and plastic pipe to check it could fit where I had space. The only real issue going forward that was highlighted could have been if I got the gas temperature wrong, as I had used a thermocouple in the exhaust pipe which wasn’t right at the point the resonators would attach. I measured 150C which I though was realistic near the back boxes. So back on the phone to Ian at H&S to get the Helmholtz chambers made up and welded on the existing pipework. Ian managed to tuck them up a bit so from behind you still see the nice H&S boxes Here are some pictures. I think it looks awesome! The result is a reduced 2-4krpms noise and just a nice background rumble when WOT @3Krpms. And to show the effect of the Helmholtz here is a graph of before and after: There is about a 10dB reduction downwards of about 100Hz (3k rpms) which equates to sounding about half the volume. In terms of how it feels, the noise power is 10 times less! Finally a little sound clip Final Specs are: · When hot 83-85dB @5k rpms (45degree, 1Meter), though at Bedford they told me it was 79dB. If you N/A reduce this by about 3dB. · 70mm internal pipe diameter splitting to 2x50mm pipes · Low level hum when WOT at 2k-3.5k opening up to a nice refined tone above 4k · Y design for a smooth gas flow · Stainless steel with a lifetime warranty (check website for terms and conditions or talk to Ian) · UK made · Personal touch being able to select from a range of exhaust tips Well thats just about it. This was much longer than I intended but I am very happy with the results and wanted to share my findings. Thanks to Ian for his craftsmanship and help designing this amazing cat back, and also for his quick responses to my emails over the last 3 months! I cannot recommend highly enough. H&S also do a selection of 2.5inch options with different middle silencers (or none at all) that are worth a look. https://www.haywardandscott.com/toyota-gt86.html If anyone has any questions, do feel free to ask away. **edit - added pricing** So pricing looks like this: Cat-Back with 70mm piping and the extra larger center silencer £650 + VAT Exhaust Tips of your choice: Varies but roughtly ~£75+VAT Helmholtz Resonators: £275+VAT Above priced are fitted. ease note this system differs from their standard offerings in that it has 70mm pipe diameter and the extra larger middle box is bigger than their "larger center silencer" on their website. If in doubt talk to Ian at H&S about your needs.