Where do I find a 3.5" downpipe for twins
 

Did a search on the forums, no dice.Can't seem to find any specifics, but I did find a few for single turbos.

SMB exhaust from Australia.

Just curious...why? Always thought that big heavy manifold negated the need/worth for anything bigger than 3".

On the old SMB website they stated that while developing the exhaust for the FD RX-7 SPs that won the production car series of the Bathurst endurace races they found a real performance benefit in increasing the downpipe diameter from 3" to 3.5" even with the rest of the exhaust at 3" Later for aftermarket exhausts they offered the entire length of the exhaust in 3.5" as well (though they did not state or imply and performance increase for this decision).

My guess is that since the exhaust gasses are very turbulent and also hot (more space between molecules) exiting the two turbines and "merging" the downpipe section needs to be larger or otherwise more free flowing than the following sections of exhaust as the same number of molecules of exhaust gasses naturally move into more laminar flow as well as cool and contract as they continue down the exhaust pipes.

The Japanese companies have had a parallel development where first they offered the twin 60mm tube downpipe which has ~20% more cross section area than a 3" downpipe (while retaining an 80mm flange to the midpipe) and then offered the linked 3.5" downpipes for stock twins as well.

My SMB, that Grant from SMB customize for my car, has a larger than stock or even a HKS bellow matching up to the turbo manifold. The bellow then matches up to 4" pipe that slowly tapers to 3.5" and then tapers to 3" at the flange. Grant stated to me two years ago when he had it for me that he believed this was the most ideal setup from his experience. The bellow he made for me was made even larger since I have a USDM car and therefore no crossing steering shaft. The rest of my system is 3" to the tailpipe except for my Vibrant metal cat which is 4.5" at the cat itself .
Mike

I have a Knight Sports dual tube downpipe on my car - goes from the turbo outlet to 2 65mm pipes then merges back into an 80mm pipe to go to the cat/midpipe.

I didn't dyno test it but butt dyno from running a US-made 3" downpipe to the dual tube I noticed a decent difference in power. It makes sense that having a larger downpipe as an "expansion chamber" after the turbo would be of benefit.

Also, I have never noticed a difference power-wise between JDM downpipes with the odd-shaped cast section at the beginning and US-made downpipes that are just 3" piping up to the flange. I do prefer JDM downpipes that allow you to get straight access to all 4 fasteners, it's a pain to get to the lower rear fastener on most US made downpipes.

Dale

It's shaped like that for the steering shaft. It allows clearance and also allows you to install and remove it without removing the shaft.

^Yep. Just wanted to point out that, even though the JDM downpipe looks odd, it does flow well and doesn't affect performance.

Dale

God help you, but I see "The Doctor" is doing something at present for a big twins exhaust.

Man, I just saw that. On Facebook I have FD specialist on my news feed, the guy who was talking about them specifically was Sasha.

He had pics of the pipe, offering 4" and 3.5".

300$ , I'm going to be the guinea pig on this and order one when it's available.

https://cimg7.ibsrv.net/gimg/www.rx7...8ae9c8b1b6.png
A screen shot of the ad on the page. Follow the page and you will see it.

I sent a message today to find out when they are ready for sale and to ship.

IMO, a few words of caution. I have run a SMB midpipe with their cat for many years and bought a SMB downpipe two years ago, which is installed but yet to be on the street due to finishing up my rebuild (hopefully two weeks from now). My SMB cat was a 100 cell and partially melted after about six months and the exhaust always smelled. If the cat is now only a 50 cell it will really smell and has no chance to pass any emissions testing if required in your state or country. I changed my cat out right before my rebuild to the UHO Vibrant 300 cell I mentioned earlier in this post. No smell and will likely pass emissions everywhere since it is CARB and OBDII rated. I is rated to 1600 degrees and won't melt. On the other hand, the downpipe is great if it is made the same as mine. Larger bellow to 4" pipe to 3.5' pipe to 3" pipe right at the flange.
Mike

There seems to have been a few experiments making pregnant SMBs, which I wouldn't be surprised, one of those guys was involved in. Looks a bit similar to what he's done recently - https://www.facebook.com/sasha.apostolov.9
https://cimg3.ibsrv.net/gimg/www.rx7...16ac33f0a1.jpg


Would have thought a lhd non crushed can approach, even if smaller diameter might even yield better results with the back pressure.

Two reasons the crushed look for the bellow is needed even for LHD. First, it must clear the twin turbo unit toward the engine with some nooks and crannies sticking out and still flush out on the flange. The outside, toward the frame, must be flattened in order for the pipe to be placed over the studs for clearance. If it is too wide here, you would never get it on with the turbos in place. Hence the reason for the widening or bellow look up and down. Increased area for improved flow with more of an oval chamber. Still more flow than simply a round pipe, but flow is more turbulent this way(non-laminar flow), which is not ideal.
Mike

Here is my partially melted cat and my downpipe before shipping to me and after ceramic coating.
https://cimg0.ibsrv.net/gimg/www.rx7...6fb3b8ad43.jpg
https://cimg7.ibsrv.net/gimg/www.rx7...7693174962.jpg
https://cimg3.ibsrv.net/gimg/www.rx7...3a2ec3250a.jpg
https://cimg6.ibsrv.net/gimg/www.rx7...b1ef598bd1.jpg

Marcus Read was posting on Facebook in the past few weeks that he can get SMB exhaust again.