Custom R36 Cold Air RAM intake - Almost $0 DIY or small fee to get it done

[Elmura]

Hi R36 and Passat CC V6 fans.

I wanted to share with you a DIY mod that improves performance of your R36 that only costs time, patience and the right tools.

I inspected and reviewed a few intakes for the R36 / CC V6. The Evoms produces a nice sound but ultimately has a couple flaws. First, it lets underbonnet hot air in near the exhaust manifold. This is a big No-No for performance and economy. Secondly, the filter is smaller in surface area than the original filter, which would outweigh any restriction improvement.


The net result is loss of performance and a waste of money (excluding the improvement to sound). The Gruppe unit is quite expensive & hard to find. I haven’t seen the filter nor the inner design to assess it.


So, I had a good look at the factory setup. It's pretty good mostly. I started by removing the AF sensor, then removed the metal screen behind the plastic grille (https://dl.dropboxusercontent.com/u/90236454/2015-04-02%2009.07.53.jpg). The metal screen serves to protect the AF sensor from stones etc but I will never have the Air Filter removed while the engine runs. It creates unnecessary turbulence and in one test I saw on Autospeed, they found an airflow improvement when removed. I then used electrical cleaner spray over the sensor electronics inside before reinstall.
I didn't remove the plastic grille as this controls the airflow over the sensor at lower RPM and would disrupt the performance.

The pipes leading to the airbox inlet over the radiator is made up of a few pieces. It’s an odd design, likely for Europe where snow / sleet and sub-zero temperatures. This part is not great for performance as it sucks both hot air and cold air. Also, the cold air path has to do a loop and so is not ideal. With my son, we had a look at how we can improve it. We came up with a great solution that has three benefits:


No more hot air induction
Ram Air at speed – The possibility of slight supercharging effect
When installed with cover, it looks completely original.


Benefit – More torque, more power, less fuel consumption due to needing less revs for the same amount of acceleration.

How it works- The hot air intake is sealed off, whilst the cold air airflow is straightened and opened for less restriction. Now the engine only sucks cold air from the front of the car - and at speed, this is slightly forced in (Ram Air). It’s a Win-Win.

HOW TO: (PS – Very hard to describe)


Unclip and remove the top cover of the inlet pipe
Unclip and remove the inlet pipe that fits between the air box inlet and the front ‘scoop’. It has a multi-inlet design
Cut (with a hack-saw / dremel / small grinder) along the edges where the cold air blade of the inlet points up. The idea is to enable the cold-air ‘blade’ to flex.
With a heat gun, protective gloves, patience, and good air-flow (toxic fumes), you heat up the ‘blade’ and apply pressure to make the blade fully flex down to the front inlet, whilst reshaping the outlet pipe back to original shape as the pressure causes it to deform.
Clamp with a vice (or grips) the blade to the inlet and fill all small gaps with a sealant (exterior grade silicone or similar) whilst using it to fasten the blade to the inlet. Images: https://dl.dropboxusercontent.com/u/90236454/2015-04-02%2012.10.52.jpg and https://dl.dropboxusercontent.com/u/90236454/2015-04-02%2012.10.38.jpg
The big gap created needs a different approach – I used expanding foam then cut it back once cured. I think heat resistant tape may be suitable as well.
Leave it for a day to cure. You can still drive your R36, it will be sucking hot air so performance will drop slightly.
Refit. https://dl.dropboxusercontent.com/u/90236454/2015-04-03%2009.36.12.jpg Then fit cover


I’d be willing to do this for owners for $80. Just mail the part to me and I’ll mail it back. You can still drive ur R36 while u wait - No harm. PM me if you’d like me to do this for you. Otherwise, if you have the right tools and skills, DIY.

Bonus tweaks – Overcome the overkill designed for non-enthusiasts


Use the same sealant to block a small hole at the bottom rear of the airbox. I believe it’s a secondary drain hole.
Improve the airflow through the ‘scoop’ in your grille. This has two parts, on top of the grille, and between the grille fins. Using fine side-cutters, cut off all the scoops’ grille to reduce turbulence and restriction for an enhanced RAM Air Effect. These are designed to minimise large foreign materials (leaves, sticks, stones) entering you airbox, but most manufacturers don’t have them. The airbox will hold this rubbish fine, just clean out every service with a vacuum. IMAGES: from top https://dl.dropboxusercontent.com/u/90236454/2015-04-17%2016.58.47.jpg AND from front https://dl.dropboxusercontent.com/u/90236454/2015-04-17%2016.59.27.jpg

[snerlo]

Hmmm,
Bit hard to follow without actually doing it mate, but good right up all the same. I have bought a cheap BMC filter which I am waiting to install when I get a chance so I might have a look at this while I have it apart and see what's going on. Hopefully it makes a lot more sense when you see things in real life. Good effort mate.

[Elmura]

I've just installed the app Torque for Android and bought a Bluetooth OBD2 adapter from a Ebay (Awesome). Anyways, on this cold Sydney night the app was displaying Ambient at 18-deg C. When the car was sitting idling for several minutes, you could see heat soak on the Intake temperature climb to over 50-degrees. Start driving and this temperature varied between 28-deg to 30-deg depending on throttle opening and speed (only tested to 60kph in local streets).

I pulled over, removed the intake pipe cover, and drove the same lap. This time I was getting 31-32 deg. So, even on this cold night, my cold-air-ram intake mod was benefiting on average 3 to 4 deg C. On a hot day, the difference should be significantly more. Note that I wasn't comparing original to modified (not possible) but something half-way between.

If any interested parties want to do a direct comparison, you can arrange a time to bring ur R36 and we can do a swap from ur stock to my modified version. I'm in the Liverpool-Fairfield region of Sydney. I reckon we could do a test at speed (100kph on the M5). PM me.

[gldgti]

A note on maf grilles -
Yes, they cause a restriction. No, its not there to protect the sensor from stones! Mafs have a mesh grille not to cause turbulence but to eliminate it. In order to be able to calibrate a hotwire maf over a large flow range, eliminating reducing turbulence in the pipe at the sensor improves the accuracy (a lot!)
By removing the grid you will decreasing the accuracy of the air mass flow measurement, at least at some flow rates. The consequences will vary, but on principal, feeding your ECU incorrect data should be avoided unless you can predict the error and account for it. FYI

[Elmura]

To be sure. Are you correcting me? If so, my post did not state that the grille was there to protect MAF sensor from stones; and I did state that removing it would not be wise for reasons of controlling airflow over the sensor.

I stated that the fine metal screen (behind the plastic grille) unnecessarily adds turbulence, and is likely there to protect the MAF sensor.

[Elmura]

In my post, I've shown a link to the plastic grill being kept

[gldgti]

Not so much correcting you as being pedantic. The grid doesn't increase turbulence,it actually is there to make the flow laminar. Your advice about what to do was sound, I just wanted to clear up the reasoning. Turbulent and laminar flow are specific fluid dynamics terms and the engineer in me likes to see them used correctly, that's all :-)

[Elmura]

It's an interesting perspective - I'm an engineer myself. I can understand how the plastic grille would make the flow laminar (straighten the airflow for non-engineers) hence why it's being kept, but the metal screen is similar to flyscreen. If it would make the flow more laminar, why not just make the plastic grille with a finer grid? Hence channelling the air more finely? Think about how the air would flow through that screen.
At higher airflows the screen is a restriction - that's a given (and proven).

Hence, I am convinced the screen is there for protection of the electronics and engine from worst case scenarios. When you design a product that has to cater for all possible scenarios (damaged or worn air fliter, no air filter, incorrectly sealed air box etc) you have to over engineer.

Just like the cold air intake scoop grille, water drainage points, shape of the intake pipes - they're designed for worst case scenarios. If you were engineering an intake for a Pagani - you have different constraints - Formula 1 - different again.

[gldgti]

My allroads maf only has the metal grid, and no plastic one... But I have seen both too. I agree it looks like a fly screen. Perhaps some further investigation is in order!
Consider though, that the plastic screens I've seen look like they would do a better job of stopping rock than the metal one...

[gldgti]

So after a bit of reading, it seems there is good evidence that flow through a wire screen will indeed serve to change the velocity distribution through the pipe, making it more constant from the middle out towards the wall (e.g before the screen it will be slow at the wall, fast in the middle, after the screen the difference in velocity will be less).
If you google " airflow through a screen" you will come up with quite a few papers on it, some of which explain. Unfortunately I'm without internet other than ony phone at the moment, so posting links is beyond me :-)

[Elmura]

gldgti - I searched that phrase and visited every link for two pages of Google results. Nothing in those stated what you state above. Anyways, we're getting too far off topic, probably best for a new thread to discuss the engineering merits of different approaches. Suffice to say, for the purpose of this thread - a small performance tweak without detriment can be had by keeping the MAF plastic grille but removing the screen mesh behind it on the R36 due to the small restriction it applies. Other vehicles without a plastic grille probably should leave the metal screen in.

I'd be happy to discuss the fluid dynamics on another thread with links to test reports. Just link me in on the new thread somehow (or PM me).

[gldgti]

Quite right too. Search devahldavis air flow through screen.

[eurobahn5]

Hi, I was inspired by your idea so I decided to give it a go. I did'nt do every step but hopefully what I did will make some difference. http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=15947&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=15948&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=15949&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=15950&stc=1:cool:

[Elmura]

Great stuff. Some questions:

1. What generation is your car / engine as my R36 is a MY10 and doesn't have the mesh screen shown in your duct. I'd strip / cut that away. Another engineering overkill. You have a filter and a box to catch rubbish.
2. Does your car have a top mounted intake over your grille? If so, opening this up reduces restriction further. If not, leave it.
3. What adhesive did you use
4. What did you use to seal off the big gap created after bending it down? Did you contour whatever you used to prevent turbulence?

[eurobahn5]

Hi, my R36 is the old 2008, it did'nt have the mesh, i put it in, but i might take it out, i'm just overly cautious about debris entering the engine, although i guess the filter will ultimately block it out. As for the top intake, i might again still chop those vents out as well. I used areldite to hold the re-postioning of the flap, plus another piece of black plastic cut into shape, as for smoothing the area in regards to turbulence...well, i havnt done anything about that yet, i'll have to think about it...

[snerlo]

Eurobahn,
I have an 08 model as well so mine will be the same as yours. I see that you have a BMC filter there. Make any difference? The reason I ask is that I bought one second hand for quite a reasonable price but I am waiting to purchase a cleaner kit before I put it in mine. Just curious as to whether they make any difference. Main difference is maybe fuel economy but you never know!!

[Elmura]

Is there any difference betw the BMC & K&N? I've just bought a Passat CC V6 with K&N fitted. There are rumours, don't know if proven, that the oiled cotton filters aren't safe because, according to rumour, they let more dust in by virtue of their higher airflow.

[eurobahn5]

I bought my BMC filter for $50 new. It works fine but no real noticeable power gain to rave about !

[Elmura]

I just examined a GruppeM intake. Besides sound, and a slightly better airflow, these are NOT cold air Ram intakes. They are not sealed from engine bay heat. Ram intakes, by definition, force cold air into intake. They have to be in a position of high air pressure as you drive and be SEALED. The GruppeM is neither a cold air, nor Ram intake!!! I think I'm going to design and make a proper high performance intake for the R36, Passat CC V6, CC V6 2012+, and Passat V6 .
Just need to find a manufacturer. Worldwide potential.

[kamold]

I just examined a GruppeM intake. Besides sound, and a slightly better airflow, these are NOT cold air Ram intakes. They are not sealed from engine bay heat. Ram intakes, by definition, force cold air into intake. They have to be in a position of high air pressure as you drive and be SEALED. The GruppeM is neither a cold air, nor Ram intake!!! I think I'm going to design and make a proper high performance intake for the R36, Passat CC V6, CC V6 2012+, and Passat V6 .
Just need to find a manufacturer. Worldwide potential.

Colour me interested in the first batch...

[XXX-1.8T]

What about the one Forge makes for the R32 apparently fit the R36 as well?

[Elmura]

Updated on next post

[Elmura]

What about the one Forge makes for the R32 apparently fit the R36 as well?

Thanks for pointing this one out. Although, I haven't found a kit to suit the R36 / CC V6 yet. If you know of such, I'd like to review it. I did find this image and it looks quite decent. The elbow is a bit sharp and the air filter may not have large enough surface area, but is much better than the GruppeM and EvoMs as it is a proper Cold Air intake and, (appears to be) a RAM intake.

http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=16155&stc=1

[XXX-1.8T]

They use K&N panel filter

[Elmura]

Update:


Further proof that removing the wire mesh behind the plastic grille is a good idea, I just pulled my (recently acquired) MY12 Passat CC V6 intake apart to perform the same operation to find it had no wire mesh in the MAF sensor! The MAF sensor is different from the R36 and is an update for the BWS engine.

As a further minor enhancement to the Elmura Cold Air RAM intake mod, I removed the water drain plug at the front base of the airbox and filled it with silicone, along with the two small holes that hold the plug.

I also double side taped the intake mouth / scoop to the back of the car's front grille to seal off the incoming air and reduce turbulence. And, I used wide tape to seal the gaps further behind, again, for aerodynamics.

PS - While the mods are fiddly and individually have minor benefit, they all add up to give benefits similar to a good aftermarket cold air intake.

[gldgti]

That's not really proof it's a good idea, as you just said the sensor is different. Despite your doubts I would still suggest that the sensor calibration would be dependant on the presence of the grille. IMO.

[Elmura]

Here are some Diagrams to show you what the main modification is and how it benefits. The diagrams are not to scale, they'e a rough cross section going by appearance only.
http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=16321&stc=1 http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=16322&stc=1 http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=16323&stc=1

[esskayy]

Hi Elmura, you have pm

[xR360]

Hi Elmura & fellow admirers, how did you go with the Forge and/or other comparisons to your design since the diagrams compare the stock to Elmura?
Any further updates or adjustments?


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[dvkent]

Might have to try this mod on my CC.

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[Elmura]

Hi guys,
Been a while since I updated this thread. I'm still using my "Elmura Cold Air Ram Mod" but now on my CC. The buyer of the R36 was keen to keep it on so I had to make a new one for the CC.

For the CC, other improvements included cutting out the "comb" plastic filter that sits just behind the grille. This required unscrewing the cover that sits between the radiator and grille and removing this long plastic piece (see pic). I then cut off the vertical pieces of plastic "comb" filter.
http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26702&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26703&stc=1

The idea behind this is to minimise obstruction (and thus pressure loss) for a superior RAM effect at speed. I also sealed up the gaps that adjoin the back of the grille with the intake scoop using strong tape that has held up thus far for almost 1.5 years.
There's a K&N panel filter in the air box that gets cleaned every 20,000kms

I siliconed sealed holes at the bottom the airbox to prevent hot air being pulled through.

I applied fibreglass heat wrap to barrier the airbox from the oil cooler & insulated the oil cooler pipes to reduce heat transfer to the airbox. Further, I heat wrapped the exhaust in the engine bay (a major task that I don't recommend) to minimise heating of the intake manifold. I also fitted a reflective blanket over the intake manifold portion sitting behind the radiator as I found this to be a massive source of heat.

#xR360 (http://www.vwwatercooled.com.au/forums/members/xr360/) As for the Forge intake, not only is it quite pricey, it's not designed to fit the CC or R36 engine bay. Also, the air box is quite small so I calculated that it would not flow as well as an optimised factory system.

However, I have started prototyping a similar version designed to fit the CC & R36 with larger ducts, straighter air flow, a much larger air box and very large pod filter. Early experiments with unsealed approach (sucking engine bay air) had performance loss in most street scenarios - but an improved highway speed performance. The problem is getting the air scoop above the radiator to connect with the rest of the system.
I had bell mouths fitted for smooth air transition and heatshield wrap around the "box". I tried aluminium piping & silicone. I haven't had time to complete this design though as I've been designing and owner building a house which takes up all of my time.

Off thread topic - I also ditched the centre "suitcase" muffler and fitted a customised exhaust bypass valve (2nd version of this experiment). With this open and at RPM >2500, all sonic hell breaks loose and it sounds wicked. I've been told it sounds like a V8 Supercar, a Lamborghini, and "insane". At low RPM it's way too loud & droney so I keep it closed except when I'm rolling up to lights. I wanted to automate it but it's a hassle so I keep the little remote at hand.

PS - In case your thinking I've completely lost it, I do have a Mechanical Engineering & Electronics Engineering background.

[dvkent]

Wow. I have also deleted centre muffler. Roughly what cost for the exhaust bypass valve set up?

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[ZR36]

I would have to say that VW would have a team of engineers and designed the intake that way for a reason, and performed test after test after test on such design.
If such a simple change would be a significant cost/performance/efficiency trade off then VW would have made the mod themselves as standard. You wouldnt get increase performance AND efficiency without affecting something else such as engine life.

To make the air flow more "laminar" a cross sectional grid would not do alot. Especially being so far way from the value intake, buy the time its gets there its most likely turbulent again. You would need the addition of a thicker guide such as a series of straws that go as close to the ignition point as possible.

[Mysticality]

I would have to say that VW would have a team of engineers and designed the intake that way for a reason, and performed test after test after test on such design.
If such a simple change would be a significant cost/performance/efficiency trade off then VW would have made the mod themselves as standard. You wouldnt get increase performance AND efficiency without affecting something else such as engine life.

To make the air flow more "laminar" a cross sectional grid would not do alot. Especially being so far way from the value intake, buy the time its gets there its most likely turbulent again. You would need the addition of a thicker guide such as a series of straws that go as close to the ignition point as possible.

Volkswagen has done hours and hours and hours of testing on each one of the components in our cars; yet we still modify them.
If everyone believed the engineers did the best they could for everything then there would be no aftermarket parts business at all; no forums... etc, etc. ;)

[Greg Roles]

I would have to say that VW would have a team of engineers and designed the intake that way for a reason, and performed test after test after test on such design.
If such a simple change would be a significant cost/performance/efficiency trade off then VW would have made the mod themselves as standard. You wouldnt get increase performance AND efficiency without affecting something else such as engine life.

To make the air flow more "laminar" a cross sectional grid would not do alot. Especially being so far way from the value intake, buy the time its gets there its most likely turbulent again. You would need the addition of a thicker guide such as a series of straws that go as close to the ignition point as possible.

They do indeed.

Then the emission team, the sound level police, and the bean counters all come in and make a mess of it. The typical intake is designed to flow enough air for normal use, be very quiet, and be cheap as possible and uniform across as many models as possible. It's a long way from optimal, and anyway, why not have a crack and do a bit of tinkering? The V6 would be an awesome motor to get more induction and exhaust noise out of, you should see how similar DIY mods transformed my ladies NA Veloster from sewing machine quiet, to something a lot more fun to drive, everyone who has a go loves the noises now, and it's still not very powerful!

[Elmura]

Wow. I have also deleted centre muffler. Roughly what cost for the exhaust bypass valve set up?

I did some of the work so costing for someone to do it may be hard. The valve itself was my second for this car. The first was a motorised unit from Ebay. It was slow to open & close, and the gears self destructed after a few months.
The new unit I got is vacuum operated so instant open & close, remote controlled. I had an exhaust shop fit a Y-piece and the butterfly valve at a spot just in front of the rear diff such that all equipment had space. I then wired it, fastened in the controller, vacuum hoses, T-piece etc.
The unit itself is, by memory, around $180.



I would have to say that VW would have a team of engineers and designed the intake that way for a reason, and performed test after test after test on such design.
If such a simple change would be a significant cost/performance/efficiency trade off then VW would have made the mod themselves as standard. You wouldnt get increase performance AND efficiency without affecting something else such as engine life.

To make the air flow more "laminar" a cross sectional grid would not do alot. Especially being so far way from the value intake, buy the time its gets there its most likely turbulent again. You would need the addition of a thicker guide such as a series of straws that go as close to the ignition point as possible.

You should re-read my post. I removed this grid which serves to block leaves, rubbish etc from getting into your airbox. This is designed to reduce the amount of filtering the air filter does. However, I'm not a typical owner, I optimise my cars from an engineering perspective, which means that I am aware of the implications - in this case, more frequent cleaning of the air filter (or replacement paper elements). I also spray cleaner on the MAF sensor every 20,000km. PS - It ain't simple to do these mods.

Similarly, the first mod of the intake scoop directing all air into the airbox does have an implication as well - the oil cooler doesn't get as cool an airflow over it. Again - I compensate by using premium synthetic oils changed every 10,000km, along with a new filter.

[xR360]

#Elmura
Haha, keep up the updates & I don't mind the crossover of topics, really keen on the centre muffler delete & now the vacuum bypass. Where did you get the quality one from?
I have seen the ones on eBay & wondered.. My Ol man is looking to get one for his hotrod also...
What sort of timeframe are you thinking for your prototype for the R36 / CC RAM design? After the house build of course.


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[eurobahn5]

I am also about to install aftermarket euro IV 200 cell cats and straight through rear mufflers this week. Also I have nearly completed my custom DIY foam pod cold air induction set up.

[Passat R36]

had a quick look at the inlet, I think the "blade" serves it's purpose, to prevent water getting into the air box. Imaging you driving at 100km/p in heavy rain or using pressure water to wash your car...

[eurobahn5]

http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26813&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26814&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26815&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26816&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26817&stc=1http://www.vwwatercooled.com.au/forums/attachment.php?attachmentid=26818&stc=1
Hi guys, I installed my DIY home job air intake today, nothing too flash. I also had fitted high flow cats and thru rear mufflers. The sound is quite amazing, it kind of somewhat resembles the R36 with the supersprint exhaust on that youtube video clip. I will try and get some audio of it in action and post it on here...

[kamold]

Can't wait for the video!!

[ZR36]

I am also about to install aftermarket euro IV 200 cell cats and straight through rear mufflers this week. Also I have nearly completed my custom DIY foam pod cold air induction set up.

Aren't you selling your passat mate?

[eurobahn5]

No one interested so I think i'll keep it...

[Stevo36]

Hi - can anyone tell me an easy way to remove the plastic screen so I can remove the wire mesh?

I don't really want to damage the screens just in case if I want to put them back in.

Thanks
Steve

[Stevo36]

Don't worry - got it :)

[Elmura]

had a quick look at the inlet, I think the "blade" serves it's purpose, to prevent water getting into the air box. Imaging you driving at 100km/p in heavy rain or using pressure water to wash your car...

I did consider this also and have since tested it many times including pressure washing. Worst case scenario some water settles on the bottom front of your airbox which is very deep. This will evaporate with engine bay heat and be inhaled as additional water vapour like a humid day.
You would have to drive through a flood before it would cause a problem.

PS - the same scenario applies for Lamborghini / Ferrari / Mclaren / Koennigsegg etc. If it's OK for them, it's OK for me and should be OK for modifiers.

#Eurobahn - Have you monitored intake air temperature before and after this intake mod? In my prior custom intake mod version whereby I had a much larger POD filter fed with a directed stream of air from the factory scoop, I found IAT skyrocketed in traffic / idling. It would take high speed driving to get any benefit. It's not really a cold air intake.

Also, Eurobahn, did you get any errors from the Cat changeover? Did you have to fit resistors or ceramic bungs over the post Cat O2 sensors?

[eurobahn5]

Yes you are right Elmura, my air intake is now being revised, it has a great induction noise and can really gulp in lots of air but I need to capture and enclose the ducting right to the front grille intake. I am making a cardboard model then I will take it to my plastics fabricator. As for the cats, it hasn't tripped any lights yet...

[Elmura]

#Elmura
Haha, keep up the updates & I don't mind the crossover of topics, really keen on the centre muffler delete & now the vacuum bypass. Where did you get the quality one from?
I have seen the ones on eBay & wondered.. My Ol man is looking to get one for his hotrod also...
What sort of timeframe are you thinking for your prototype for the R36 / CC RAM design? After the house build of course.


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The type of exhaust valve I currently use with moderate satisfaction can be found on eBay with this search term "vacuum exhaust valve close". This type stays closed until activated when engine vacuum can instantly open it, and closes instantly as well.
The mechanism is tough & reliable. The problems with this product are:

1. The valve vibrates when the engine is loaded while it's closed. It's not a pleasant sound. I've tried glueing rubber strips, Sikaflex, weather sealant, increasing the spring tension, and washers on the end screws. The last two reduced the vibration, the others failed after a full throttle run.

2. To open it requires low or no throttle. So, at lights, coasting or cruising. To open the rest of the time, I have to release the throttle briefly, then get back on it. To overcome this, I designed a compact vacuum reservoir that mounted close to the valve. I bought all the materials but haven't gotten around to building & fitting it. Probably because problem 1 is far more annoying.

As for size, the electric motor operated valve I had first was 2.5". This was a little smaller than the exhaust pipe. After that failed (quickly) I went looking for a vacuum operated one as the electric motor & gearbox is slow to open & close. It vibrated during the transition.

The vacuum model I chose was 3". A bit larger than the exhaust pipe (70mm) which requires a bit of pipe expansion to mate up. The idea was better gas flow. In retrospect, maybe a 2.5" won't vibrate as much, and since it's too loud with the 3", maybe the 2.5" would be tamer.

So, my expense & experience is hopefully beneficial to others.

PS- Some American "exhaust cutout" look better engineered.

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