My Custom Hayward and Scott Cat-Back Design and Proving

[Ade 517]

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, 6^th 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.

 

[nikndel 509]

is this the common design for a H&S exhaust for a 86

[Ade 517]

is this the common design for a H&S exhaust for a 86

The twin back box Y design is the same with 50mm piping. The middle silencer was custom made for me with a bigger silencer and with 70mm piping rather than 63mm piping.

They currently offer a 2.5" (63mm) cat back with choice of no middle silencer, a small one or a larger one. Using stock tips or a single wall option on their website.

Ian calls this one an extra large middle silencer.

The Helmholtz Resonators as per the thread were designed by me but Ian is able to make and fit them.

[nikndel 509]

8 minutes ago, Ade said:

 

 

The twin back box Y design is the same with 50mm piping. The middle silencer was custom made for me with a bigger silencer and with 70mm piping rather than 63mm piping.

 

They currently offer a 2.5" (63mm) cat back with choice of no middle silencer, a small one or a larger one. Using stock tips or a single wall option on their website.

 

Ian calls this one an extra large middle silencer.

 

The Helmholtz Resonators as per the thread were designed by me but Ian is able to make and fit them.

ah ok so it is a one off then

[Ade 517]

8 minutes ago, Ade said:  

 

The twin back box Y design is the same with 50mm piping. The middle silencer was custom made for me with a bigger silencer and with 70mm piping rather than 63mm piping.

 

They currently offer a 2.5" (63mm) cat back with choice of no middle silencer, a small one or a larger one. Using stock tips or a single wall option on their website.

 

Ian calls this one an extra large middle silencer.

 

The Helmholtz Resonators as per the thread were designed by me but Ian is able to make and fit them.

ah ok so it is a one off then

Well Ian is able to make it for you with your choice of tip options. Im just in talks with him at the moment regarding pricing.

[Deacon 1357]

Do you have a weight for it @Ade? Looks like there's quite a lot to it.

[Ade 517]

Do you have a weight for it @Ade? Looks like there's quite a lot to it.

Not sure. The Helmholtz resonators are hollow chambers so very light. It'll be alot lighter than stock. The extra weight will be the mid pipe silencer as that will be full of packing/damping materal.

[Varelco 211]

How much did this set you back Ade? Really interesting read and alot of work has gone into it but I can't see why an off the shelf exhaust wouldn't do the same job. An Invidia Q300 would of given you a similar result.

[S18 RSG 429]

Seems like good work and good research done.

Funny enough, my car is almost silent with no cats and no silencers. My exhaust guy fitted his own design backbox to quieten mine down, and of course the turbo helps quieten it even more. Without the screamer pipe I wouldn't be surprised if it's quieter than stock.

Interesting consideration for people stuck between supercharging and turbocharging. The lengths at which you have to go to achieve what you can get for free in a turbo setup.

[Ade 517]

11 minutes ago, Varelco said:

How much did this set you back Ade? Really interesting read and alot of work has gone into it but I can't see why an off the shelf exhaust wouldn't do the same job. An Invidia Q300 would of given you a similar result.

Q300 is 60mm ID this is 70mm ID.

Also the Q300 is quiet everywhere, more so at the top end because of the use of absorption silencers. I wanted more attenuation low down, hence the use of the helmholtz resonators. 

I spoke at length with Ian, and as per my write up above, originally I wanted a big single box like the Q300 but the sharp 90degree turns are not great for flow so he convinced me to go with the Y design and he can make a big central silencer. 

 

 

[TTR 509]

Might need to look at this. My exhaust is currently driving me fucking mad below 3000rpm 

[Ade 517]

41 minutes ago, S18 RSG said:

Seems like good work and good research done.

Funny enough, my car is almost silent with no cats and no silencers. My exhaust guy fitted his own design backbox to quieten mine down, and of course the turbo helps quieten it even more. Without the screamer pipe I wouldn't be surprised if it's quieter than stock.

Interesting consideration for people stuck between supercharging and turbocharging. The lengths at which you have to go to achieve what you can get for free in a turbo setup.

I remember the fensport demo car. They had fitted a quiet exhaust (possibly blitz) and the AVO turbo kit. It was eerily quiet as the sound generator was also deleted which is where most of the sound comes from stock. 

Its true a benefit of the turbo is lower exhaust noise, but then you get that whine from the SC. Each to their own. Its good we have options!

Actually that's a good point to make. I can hear and subtle whine from the SC now and its lovely 

[Ade 517]

2 minutes ago, TTR said:

Might need to look at this. My exhaust is currently driving me fucking mad below 3000rpm 

Yeah its always a problem for 4 pot engines. 100Hz @ 3k rpm is not easy to suppress with small absorption canisters. It is compounded by cabin stand waves too (cabin gain). The half wave length of 115Hz @25C is 1.5meters which is about the distance between our door windows..... Anyone into car audio will know all about that. 

[S18 RSG 429]

1 minute ago, Ade said:

low end torque, and throttle response from the SC.

Surely you've driven a TD turbo car by now? In which case, you would know those features are not exclusive to supercharging.

[Ade 517]

Just now, S18 RSG said:

Surely you've driven a TD turbo car by now? In which case, you would know those features are not exclusive to supercharging.

Im not going to get into a debate and take this thread off topic. You are happy with your choice and good for you 

[S18 RSG 429]

Just now, Ade said:

Im not going to get into a debate and take this thread off topic. You are happy with your choice and good for you 

Where's the debate? I'm just setting the record straight. We can't be putting out false information about things of which we are blissfully unaware.

[TTR 509]

Christ can't you two ever get along 

[Ade 517]

Just now, S18 RSG said:

Where's the debate? I'm just setting the record straight. We can't be putting out false information about things of which we are blissfully unaware.

Its a debate until someone actually measures it for comparison. I drove the AVO car with a smaller, faster spooling turbo and wasn't convinced it was for me. That's my opinion on the matter, now back on topic please! 

[S18 RSG 429]

1 minute ago, TTR said:

Christ can't you two ever get along 

All I did was mention how interesting it was that this much work is required to quieten a supercharger setup, complicated compared to turbo setups. That's useful information for prospective buyers.

I didn't feel the need to mention the advantages of my choice of setup, because this is a thread about exhausts, but I'm not gonna let false information go unanswered

[Ade 517]

 

6 minutes ago, TTR said:

Christ can't you two ever get along 

We just both like to argue our choices are the best options 

[Ade 517]

3 minutes ago, S18 RSG said:

All I did was mention how interesting it was that this much work is required to quieten a supercharger setup, complicated compared to turbo setups. That's useful information for prospective buyers.

I didn't feel the need to mention the advantages of my choice of setup, because this is a thread about exhausts, but I'm not gonna let false information go unanswered

I've removed the "false information" for you. Back on topic please.

[S18 RSG 429]

Just now, Ade said:

I've removed the "false information" for you. Back on topic please.

Listen, I'm all for a good debate as much as you are, but I mentioned turbo vs supercharger here not to have a debate, but because you've gone to great lengths and expense to expose an obvious point of consideration for future owners. Your research will potentially save people money in the future who can't decide between turbo/supercharger, but want a quiet exhaust. I just wanted to point that out, as you've provided a good service here.

The point scoring was unnecessary, and that's why it went unneccesarily off topic.

Back on topic, like Jeff mentioned, it would be interesting to see if you can get the system weighed, as I'm sure that might be a concern to others looking for a similar setup.

[Rich 378]

18 minutes ago, Ade said:

Yeah its always a problem for 4 pot engines. 100Hz @ 3k rpm is not easy to suppress with small absorption canisters. It is compounded by cabin stand waves too (cabin gain). The half wave length of 115Hz @25C is 1.5meters which is about the distance between our door windows..... Anyone into car audio will know all about that. 

2-3k did my head in on my unresonated Milltek, the resonator seems to help a lot there.

[Ade 517]

8 minutes ago, Rich said:

2-3k did my head in on my unresonated Milltek, the resonator seems to help a lot there.

Absorption silencers do attenuate, but more so at the high rpms. So yes it would help for sure in the same way the H&S system is quieter than my cossy exhaust. 

10 minutes ago, S18 RSG said:

Listen, I'm all for a good debate as much as you are, but I mentioned turbo vs supercharger here not to have a debate, but because you've gone to great lengths and expense to expose an obvious point of consideration for future owners. Your research will potentially save people money in the future who can't decide between turbo/supercharger, but want a quiet exhaust. I just wanted to point that out, as you've provided a good service here.

The point scoring was unnecessary, and that's why it went unneccesarily off topic.

Back on topic, like Jeff mentioned, it would be interesting to see if you can get the system weighed, as I'm sure that might be a concern to others looking for a similar setup.

It wasnt intended to be point scoring in the same way your comment about turbos having the benefit of lowering noise to be point scoring. 

TBH I doubt it will ever get weighted unless I need to remove it from my car for whatever reason. I've pinged Ian the question to see if he knows. 

Moving on...

The point with this system, is that with the helmholtz resonators, its still got volume higher up in the rev range, but the lower range where you cruise on the motorway, noise is attenuated more so by the helmholtz chambers which are designed to target 3k rpms. 

So its not necessarily a system for people that want a quiet exhaust, like a Q300, rather its a high flow 70mm design with a high pass filter for noise so that it still sounds fairly meaty up top. 

Pricing will be coming soon too.

[S18 RSG 429]

10 minutes ago, Ade said:

It wasnt intended to be point scoring in the same way your comment about turbos having the benefit of lowering noise to be point scoring. 

Well, the thread is about the volume of exhausts. I think many people would be surprised to hear that an unsilenced, uncatted exhaust system on a turbo car is perfectly livable, whereas a supercharged car may require a lot of extra silencing. I saw that more as useful consumer advice than point scoring.

I'm not that much of a dick that I'll find any opportunity to belittle your opinion or choice, just trying to make a useful contribution