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Discussion Starter #1
So I was planning to run 4” downpipe on the built engine but there ain’t enough room so I have to run 3” until it clears the manifold.

Will this cause any problems?
 

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A member recently posted up some CFD modelling from various down pipe/mid pipe/exhaust diameter combos, and the results contradicted the long held belief that bigger is better.

Ive always thought that a 4" DP, leading to a smaller Mid pipe or cat back would be the most beneficial for a typical Supra street car, but this wasnt the case, as a 3" DP into a 4" cat back outperformed it.

Ill try and find the thread and post it in here for your perusal.
 

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^^That makes sense, the smaller cat back would cause a little backpressure. The "bigger into smaller" is to keep up velocity for a NA car and maintain low end torque.

Al
 

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A member recently posted up some CFD modelling from various down pipe/mid pipe/exhaust diameter combos, and the results contradicted the long held belief that bigger is better.

Ive always thought that a 4" DP, leading to a smaller Mid pipe or cat back would be the most beneficial for a typical Supra street car, but this wasnt the case, as a 3" DP into a 4" cat back outperformed it.

Ill try and find the thread and post it in here for your perusal.
That hasn't been my anecdotal experience. I've seen the best spoolup and top end power from 4 in DP/MP to 3in/80mm/3.5in catbacks vs 3in DP/MP into a 4in catback. That is speaking generally across a wide variety of manifold and turbo combinations, which also factor in.
With very precisely designed transitions that are optimized for a particular turbo and manifold, I find it plausible to see some magic happening there, ala F1 engineering tricks and such. But I believe that would have to match up rather precisely with a turbo and manifold selection.

Reith, I don't suppose you've got a link to that CFD modeling? My google fu and search skills are weak this morning, apparently.
 

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Hi Jeff :) How are you? I am sure I am not alone in saying that its so good to have you back here posting more often, and sharing your wealth of knowledge with the somewhat smaller Supra community. We are all very lucky to "have you"

Yes, I tend to agree with everything you have said above, mainly because its always been considered in turbo land that the "big to small" DP/MP/CB set up is the best overall way to reduce back pressure and lower EGT's etc, and I must say that the data from the CFD modelling had me second guessing everything I had learned previously, and almost questioning my own existence hahaha

I will try and find it now and post the link if I am able to find it. From memory, it was posted by one of our Scandinavian members.

Will report back soon :)

P.S When you mentioned F1, all I could think about is INCONEL, INCONEL, INCONEL!!!!!

EDIT: Found it!!! Racelab exhaust size simulations and results
 

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That hasn't been my anecdotal experience. I've seen the best spoolup and top end power from 4 in DP/MP to 3in/80mm/3.5in catbacks vs 3in DP/MP into a 4in catback. That is speaking generally across a wide variety of manifold and turbo combinations, which also factor in.
With very precisely designed transitions that are optimized for a particular turbo and manifold, I find it plausible to see some magic happening there, ala F1 engineering tricks and such. But I believe that would have to match up rather precisely with a turbo and manifold selection.

Reith, I don't suppose you've got a link to that CFD modeling? My google fu and search skills are weak this morning, apparently.
This has been my experience as well. When I saw the source was CFD modelling, I began to have my doubts. I've tested a lot of stuff, mostly exhausts, but bigger always spooled better and made more power (see Clash of the Titans I and II).


Ken.
 

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Im sure you are not alone there, Ken. It has been a common theme for as long as I have been a member on this site...bigger is better :)

I wonder why the CFD modelling contradicts real world experience? Maybe it is due to the test mule being solely done with a straight pipe with no mufflers of bends. Either way, I think we can say with some certainty that this myth has been busted!!!
 

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Looking at that post I think the software is good but the problem was in the simulation setup.

-Exhaust energy does NOT enter the downpipe from the turbine housing as linear flow. The turbine wheel directs it radially and all over the place so backpressure is massively higher right at the turbine exit than it is even a few inches downstream. This simulation has it running in a perfectly linear fashion upon DP/MP entry.

-It calculated all things at 1000hp of flow, and ignored behavior below that. A proper simulation would also include backpressure measurement from 100hp upward to max HP and my experience would indicate that spool will obviously be improved at that crucial threshold from the reduced overall backpressure of the 4in DP/MP, which will show significant gains in spool well below the threshold of a 3in catback becomes a significant restriction.

-It massively oversimplified the lengths and bends. A 4in DP/MP extends below the car for several feet under the car before getting to the catback, and the catback is not nearly as long as shown in the simulation. If the DP/MP/catback lengths had been properly configured, the restriction of a 3in DP/MP would have been significantly higher, and vice versa for the 4in DP/MP. It also oversimplified the transition from 3in to 4in. If anything, I would have loved to see this simulations results of doing a gradual conical transition from 3in to 4in compared to the typically 2-3in long adapters so often used.
One may assume that the gentleman who ran this simulation was simply assuming a 4in DP but not a midpipe or catback, but nonetheless I'd love to see the lengths used for each section and compare that to a real Supra exhaust system.

-It complete ignored wastegates. Open dump WG's will obviously dump exhaust outside of the DP/MP entirely while recirculated gates either make things work better or worse depending on how well the gate is positioned on the manifold and how the WG recirc is merged into the DP/MP flow. For the sake of simplicity we could ignore this, but we would need to make a semi-accurate estimate of how much HP worth of exhaust energy was being dumped out of the open WG and thereby not involved in the DP/MP/exhaust at all.


Overall I think reality probably very close to the simulation as-ran. But to do this right, we would need to do a 3D scan or at least accurate measurements of lengths and angles of the exhaust manifold, turbine wheel, turbine housing, DP/MP and full exhaust before trying again.
 

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^^^My nomination for post of the year. You heard it here first 🙃.


Ken.
 

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Thank you for the kind words, guys. 🍺
 

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I've always understood the biggest restriction of the exhaust to be the turbine housing itself, considering how incredibly turbulent the air passing over that wheel must be. Then there's the aperture of the exit of the wheel itself. I'm guessing that if you give the air a nice big area immediately after the exit of the housing to sort itself out, that helps?

I mostly play with small turbos, so it may not matter as much in my case.
 
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So I had read the previous post with the info that started this discussion, but prior to reading that I was convinced that 4in all the way from turbo to exhaust was the right choice. Given the informed discussion in this thread, maybe that is not the case if turbo size plays a factor. As recently discussed in another thread on cast manifolds, I have an FSR kit that I was looking to mate with a Garrett GTX3582R Gen 2. I also got the 4in downpipe with the kit. I do not have a mid-pipe or exhaust as of yet, but again, assumed I would go 4in all the way. Perhaps the 4in dp/mp into a 3/3.5 inch exhaust would be better for my goals? Keeping in mind this will not be a track car and I'm not looking to win any hp contests either.
 

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So I had read the previous post with the info that started this discussion, but prior to reading that I was convinced that 4in all the way from turbo to exhaust was the right choice. Given the informed discussion in this thread, maybe that is not the case if turbo size plays a factor. As recently discussed in another thread on cast manifolds, I have an FSR kit that I was looking to mate with a Garrett GTX3582R Gen 2. I also got the 4in downpipe with the kit. I do not have a mid-pipe or exhaust as of yet, but again, assumed I would go 4in all the way. Perhaps the 4in dp/mp into a 3/3.5 inch exhaust would be better for my goals? Keeping in mind this will not be a track car and I'm not looking to win any hp contests either.
For pure HP and spool, yes, straight 4in all the way is absolutely the best bet, and this is regardless of turbo size. The higher the HP level overall, the bigger the difference one will see going to straight 4in.

But the NVH and ground clearance that comes with a 4in catback is unappealing to a lot of folks. So using a 4in DP/MP with a smaller catback that's quieter and has better ground clearance is an effective way to split the difference, and get most of the benefit of a 4in system without the noise of a 4in catback.

This is why I'd love to see a new 3.5 or 3.75in system with an effective resonator, especially in titanium.
 
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Hopefully FSR/Driftmotion will do a Black Friday deal. If so I'm definitely jumping in. 4" downpipe. Not sure about which turbo, leaning towards 6266 or 6466 DBB. I'm still considering what exhaust, and the Tomei titanium really looks nice.


That comes with a 4" titanium midpipe ...
Combine that with a 4" y-pipe, like a QTPYPIPE-400 and an exhaust cutout, either the SP $479 one or an electric like the QTEC40. The midpipe will require surgery to install the y-pipe, but that's a simple cutting out of a section and a couple of flanges.

I spoke to <don't remember name> at FSR early in the year, and he recommended against the electric exhaust cutouts - said that under high boost the flap would just bend. The SP one is much thicker and tougher. Yet I believe Stu Hagen runs with the electric ? Don't remember where I read that.

It would be really nice to add a No-Drone pipe if it would fit


I've been in a non-Supra that had one. Definitely made a difference. Trouble is it's stainless steel - can't really weld that to titanium.
 

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Discussion Starter #17
I’ve got the 6766 dbb with billet wheel upgrade.

I didn’t think you can just buy a down pipe as it’ll need to be fabricated to fit
 

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Hopefully FSR/Driftmotion will do a Black Friday deal. If so I'm definitely jumping in. 4" downpipe. Not sure about which turbo, leaning towards 6266 or 6466 DBB. I'm still considering what exhaust, and the Tomei titanium really looks nice.


That comes with a 4" titanium midpipe ...
Combine that with a 4" y-pipe, like a QTPYPIPE-400 and an exhaust cutout, either the SP $479 one or an electric like the QTEC40. The midpipe will require surgery to install the y-pipe, but that's a simple cutting out of a section and a couple of flanges.

I spoke to <don't remember name> at FSR early in the year, and he recommended against the electric exhaust cutouts - said that under high boost the flap would just bend. The SP one is much thicker and tougher. Yet I believe Stu Hagen runs with the electric ? Don't remember where I read that.

It would be really nice to add a No-Drone pipe if it would fit

I've been in a non-Supra that had one. Definitely made a difference. Trouble is it's stainless steel - can't really weld that to titanium.
I'd encourage you to listen to a Tomei 4in system - I strongly doubt a cutout of any kind would make any performance difference if we're talking about a 4in DP/MP into straight 4in Tomei Ti. The cutout would only make different and less pleasant noise than the Tomei. I would be surprised if the Tomei Ti system were even 5db quieter than an open downpipe. It's rowdy as hell and if you're looking for rowdy and titanium it's an excellent choice. I just wish they made a resonated 3.5in system out of the same stuff with a similar muffler.

The QTP cutouts are hit and miss. I've installed a lot of them, and like FSR I've seen them fail more than once.
I really like the Armageddon Turbo 'Power Cutouts' since they're actually heat coated with a reinforced valve, and they're directly boost/vacuum referenced and controlled by a VSV powered on and off by a dash switch. No wiring down by the exhaust and no crappy power window motors or weak valve parts to fail.
But again, in the discussion of a Tomei 4in Ti I think a cutout would be totally superfluous unless you're making 1200+whp and have a 5in DP/MP to the Tomei or a similarly crazy setup.

No-Drone pipes and similar branch resonators work pretty well when the length and diameter is correctly tuned for the rest of the system. This involves some math to get right, or a lot of expensive trial and error.
To implement one, the best setup I ever saw used a custom 'test pipe' sort of section between the catback and midpipe with V-bands on either end for easy in and out removal. Weld a 1ft stub for your branch resonator on there in a safe direction Then find a piece which has an ID about the same as the OD of that material, then cap off one end and cut relief notches in the other end so you can adjust length with a simple exhaust clamp while you're fine tuning it. Build different lengths as needed until you find something that 'works'.


I’ve got the 6766 dbb with billet wheel upgrade.

I didn’t think you can just buy a down pipe as it’ll need to be fabricated to fit
Depends entirely on the turbo manifold you've got. Many shops offer a 'drop in' downpipe and other ancillary bits for popular turbos like a 6766 when installed on that shop's preferred or in-house manifold.

Seeing as you're in the UK and you likely have a RHD Supra as a result, the RHD steering parts definitely complicate the turbo setup and many 'drop in' parts designed in the US for LHD cars will not properly install. 4in DP/MP's are tricky in RHD Supras to begin with, so starting from scratch with a custom setup is probably the best bet.
But the bigger you can go right out of the turbo, the better.
 

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Discussion Starter #19
Depends entirely on the turbo manifold you've got. Many shops offer a 'drop in' downpipe and other ancillary bits for popular turbos like a 6766 when installed on that shop's preferred or in-house manifold.

Seeing as you're in the UK and you likely have a RHD Supra as a result, the RHD steering parts definitely complicate the turbo setup and many 'drop in' parts designed in the US for LHD cars will not properly install. 4in DP/MP's are tricky in RHD Supras to begin with, so starting from scratch with a custom setup is probably the best bet.
But the bigger you can go right out of the turbo, the better.
Thanks dude
 

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I'd encourage you to listen to a Tomei 4in system - I strongly doubt a cutout of any kind would make any performance difference if we're talking about a 4in DP/MP into straight 4in Tomei Ti. The cutout would only make different and less pleasant noise than the Tomei. I would be surprised if the Tomei Ti system were even 5db quieter than an open downpipe. It's rowdy as hell and if you're looking for rowdy and titanium it's an excellent choice. I just wish they made a resonated 3.5in system out of the same stuff with a similar muffler.
I currently have a 3.5" custom (from 1998, jeez that's a long time ago) through a Random Tech cat and light muffler. BPU on stock twins. Totally non-standard, so no "mid-pipe" connected to "cat-back" in the standard locations. So, it won't work when I put in the FSR kit, hence ... new exhaust.

I've heard that the Tomei isn't quiet, and that the Tanabe is too quiet (also only 80mm). But of course those are all subjective ... I guess worse comes to worse, a resonator or cat can be put into the mid-pipe.

Hrm, maybe not. Getting the FSR kit with a 4" downpipe, the Tomei midpipe is 3" at the downpipe, expanding out to 3.5" at the cat-back joint.
255658


Are the single-turbo downpipes reasonably standard in their joint location for the mid-pipe ? So, given an FSR setup, would the Tomei mid-pipe bolt up, or need massaging ?
 
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