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Turbo Exaust Theory

I read this info years ago and just came across it again.

See: http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

Notes that at 400hp+ a 3.5" tube is best.

Also notes problems to avoid with a turbo exhaust design. One might want to avoid cheated turns where a proper radius is not used to match the turbo with the muffler or crush- bent tubes. Also, any unnecessary turns like coming back under the turbo to exit the drivers side.

300hp likes 3" tube and is near its limit at 400hp.

Also discusses the advantage of a low back pressure exhaust system to improve boost response.

I am thinking a best practices 930 muffler would have a CNC machined tapered bend that transitions up to a 3.25"- 3.5" straight through muffler that exits the passenger side with a long radius and minimum turn tail pipe.

Years ago I had a popular aftermarket muffler that actually overlaped the turbo exaust opening adding restriction.

Old 03-04-2009, 05:01 PM
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Thats funny you bring this up- I am doing exhaust soon, Im doing- 3"-4" transition into a 4" muffler, out the back of the car and finally a 4"- 3" transition (to try to cancel out noise)
I didnt say anything because I thought that some may flame me for it. Im doing exactly the you are talking about except Im using 4" instead.
But also remember that we have the best situation with our short exhaust paths.
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Old 03-04-2009, 07:41 PM
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The paper supports your thought.

The difficulty I suspect is to come up with a decent transition that it curved. Years ago I read about I think it was a guy named Holcomb or something was carving his collectors out of a block using a CNC machine.

Check out Burns stainless. You might be able to stay at 4 inch and put a choke in the tail pipe for an option to remove it or tune you car by changing out the choke.
Old 03-04-2009, 07:52 PM
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Best exhaust is none.
There is always a compromise. If you want a stock exit you have no choice but to come back with a 180 bend. The bend I use expands larger than the 3"ID of the remaining system to reduce the restriction. My dual 3" outlet muffler addresses that issue as well by allowing a dump on the passenger's side.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 03-05-2009, 04:10 AM
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I am not an expert but I suspect merging out to two 3" tubes adds significantly to capacity so long as other issues are not created like a poor Y design. With two 3" pipes over one, there will be room for other less than ideal conditions with said tail pipes depending on how they are carried out.

I suspect going througt a muffer and making a 180 deg turn, then adding 4 foot of unnecessary pipe may help noise but will not be a pest practices system for making power.

I am not sure that a "best exhaust is none" on a turbo as suggested is correct.

Per the article:

"the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12 included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge"

I suspect a short tube like this that allows the hot gasses to exit in an orderly manner might be in improvement over none. Kind of like saying the best air performance air filter is none. An unfilitered Velocity Stack on the intake side will yeald better performance.

I wonder if we did this but quickly dumped it into a large chamber (muffler body), and then stuck two 3" tail pipes on that chamber, how well that would compair. We often hear a chambered muffler is not as efficient as a straightt through. However, this might be a bit different so long as there is not any baffling. If done right the can would be somwhat a substitute for the open atmospher.

Never know until theory meets the dyno.

Other than having at least a correctly sized tube I do not know if we are talking small differences of 1-3hp or somthing larger.

I can speak from experience that an expensive aftermarket SS muffler that came with a shorty header system as made by the top builder years ago had an internal 2.5" restriction in it when I cut it open. All I can think was to better control noise. Going to a custom straight through Borla muffler with simple tail pipe instantly gave me another 35hp on the dyno. That is huge.

Old 03-05-2009, 07:40 AM
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By "none" I mean a Zork Tube. A Zork Tube is not an exhaust system by street car definition.
If you are talking race cars then the theory is extremely simple - an short cone and large ID short exit pipe with minimal bends.

I have done dyno testing using stock muffler (no cat) vs my various mufflers vs the Zork.
On a stockish 292WHP Euro 930 engine you will gain ~15HP by replacing the stock muffler with a Zork. You gain ~12HP by replacing the stock muffler with my muffler - driver's side exit, dual out, passenger's side exit makes no difference. All are 3"ID.

Iterestingly the driver's side vs passenger's side outlet mufflers sound identical. The extra pipe does not quiet it down.

I also have one of those "Boxes with 2 pipes" you mention. That "muffler" sounded absolutely horrific! The box creates a resonance chamber that amplifies the sound. The tailpipe was huge 4"ID.

When you get above 400WHP bigger ID tube and muffler chamber is needed and design becomes more significant. At that point the turbo is pushing a LOT of air and any restriction becomes a noticeable problem.
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Old 03-05-2009, 08:41 AM
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Great into Brian.

For what we are doing, taking the bends out of the Zork is not likely make a significant difference.

Common sense, a straight through muffler, and proper sized diameter tubing of say 3" up 400hp and possibly a bit more after should keep it simple. Noise then being the only other concern.

I have always wondered about the RUF which I think was based on a stock muffler with outlets on each side. I suspect that was a modified chambered muffler. Also, do you know anything about there Yellow bird TT muffler. Was it chambered or stacked straight through?
Old 03-05-2009, 09:09 AM
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The Yellow Bird was a twin turbo. I don't think I have ever seen a picture of that muffler.

The Fuf muffler I am familiar with. Not a fan. Too heavy and restrictive in my opinion.
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Old 03-05-2009, 07:00 PM
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I need to make my setup soon, and then take pictures for citique- I am going to tack it together and post the pics before I fully weld it.
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Old 03-05-2009, 07:35 PM
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Brian, thx. I do not like a heavy tail either.

Kris,

Here is the "chokes" I was taking about. You can tune your exhaust by changing them out to different sizes. The are fit to slide into the tube, not be welded in-line.

http://www.burnsstainless.com/dynosys/dynosys.html

Old 03-05-2009, 09:01 PM
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Quote:
Originally Posted by 911st View Post
Kris,
Here is the "chokes" I was taking about. You can tune your exhaust by changing them out to different sizes. The are fit to slide into the tube, not be welded in-line.
Now those are a good idea, Im only thinking about using the 4-3 transition at the end because they are only $39 and if I want to make it louder, Ill just hack it off. However, if the wallet would permit, your idea would kick ass becuase I could tune it. I have had some experience with tuning open headers with that setup, works like a charm. However it does get complicated when you factor size vs. length vs. transition... and then how they impact the car...
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Old 03-06-2009, 05:12 AM
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I think it might look cool to just put a transition at the end. Kind of like an expansion chamber.

How about making a tail pipe insert or nozzle. Could cut a donut out of sheet to fit inside you tail pipe and roll your own cone to weld the small side of the cone to the inside of the donut.

Then stick that inside the tail pipe tip and recess it a little. To trip people out you could drill some added holes in the donut. Could use a screw from underneath to hold it. ???

Old 03-06-2009, 07:49 AM
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I always love a chance to show my exhaust

A couple weeks ago I changed the muffler from a 3.5x4.5x12 to 3.5x6.25x17 so I have some new pics. Both mufflers are straight through so there shouldn't be much difference in flow and the new one's a little quieter.

The exhaust flange was water cut to match the turbo housing, then the flange ID was taper machined to perfectly match the K27HF2's 2-3/8" outlet to the 2.5 to 3.5 tapered pipe. A short piece of 3.5" elbow is welded to the tapered pipe to point it to passenger side:







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Old 03-06-2009, 02:09 PM
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Old 03-06-2009, 05:29 PM
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Per the article referenced above on Turbo Exhaust Theory, a 3" pipe's limit is about 400hp. That migh be with a front to back exhaust system where gasses cool, slow down, and create back pressure so I am not sure about us with our short exhaust path.

However, he also notes a taper just out of the turbine to I believe aid in flow and inhibit turbulence.

With many passing the 400fwhp range on our 930's, a 3.25 or 3.5" system may have some measurable benefit.

I really like what 125 shiftr did here to perfectly match his flange to the turbo and immediately transition to his chosen tube size.

My old B&B was not anywhere as well fit and had more of a step out of the turbo. Most of the builders seem to cheat this area.

Quote:
Originally Posted by 125shifter View Post
The exhaust flange was water cut to match the turbo housing, then the flange ID was taper machined to perfectly match the K27HF2's 2-3/8" outlet to the 2.5 to 3.5 tapered pipe. A short piece of 3.5" elbow is welded to the tapered pipe to point it to passenger side:


Old 03-08-2009, 08:10 AM
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Boy I make muffler just lkie that for someone on this board
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Old 03-08-2009, 08:20 AM
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Your larger muffler can with the same transition detail out of the turbo could take 930 exhaust's to a new level.

If it would fit, just one long radius turn at the tail pipe would be nice.

Old 03-08-2009, 08:36 AM
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David's design is perfect.
There is a fitment problem using the cone on our mufflers; I looked at that early on. The compromise was to use a slightly oversized pipe exiting the turbo outlet that quickly goes to the muffler can which is an expansion chamber.
Unfortunately the costs outweigh the benefits when applied to my design.
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Old 03-08-2009, 12:53 PM
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Quote:
Originally Posted by RarlyL8 View Post
...The compromise was to use a slightly over-sized pipe exiting the turbo outlet...
Intuitively that sounds like a reasonable compromise and I do not know if we are talking big or little differences.

As to that, the above Turbo Theory article says:

" There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to back-pressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge. "

Also most of us do not seem to be concerned with the increase in exhaust back pressure that comes with the small hot sides and big compressor wheels that are the rage. They make for early boost and extended the boost up top but possibly at the expense of lost VE or HP. It would be interesting to see a back to back or measured back pressure using the 7200 v 7006 turbine wheels. The fact that we are seeing some over-boost with the newer big turbos with some using a .8 bar spring to deliver 1 bar boost is kind of part of this I suspect.

Old 03-15-2009, 01:09 PM
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Quote:
Originally Posted by 125shifter View Post
I always love a chance to show my exhaust

A couple weeks ago I changed the muffler from a 3.5x4.5x12 to 3.5x6.25x17 so I have some new pics. Both mufflers are straight through so there shouldn't be much difference in flow and the new one's a little quieter.

The exhaust flange was water cut to match the turbo housing, then the flange ID was taper machined to perfectly match the K27HF2's 2-3/8" outlet to the 2.5 to 3.5 tapered pipe. A short piece of 3.5" elbow is welded to the tapered pipe to point it to passenger side:
Looks great David.

Did you do the fab work yourself? I am looking for someone locally to do a custom exhaust instead of my B&B.

Cheers.

RT
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Old 03-15-2009, 01:51 PM
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