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Crotchety Old Bastard
 
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Thanks Chuck/Gabe.
Quality is my defense against slave labor rates. Equal length tubing requires precision fixturing; then there is the 321 stainless that no one else uses. I doubt if they can duplicate the merge collector at all, that requires true craftmanship:



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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 06-20-2009, 07:40 AM
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M&K 993 h/e system as installed on my '91 Turbo:
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Old 06-20-2009, 07:55 AM
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Love the 993HE's. Good tube size, small volume, and factory quality.

Apologies and I understand but still not a fan of the transition between the turbo and muffler.
Old 06-20-2009, 12:01 PM
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Crotchety Old Bastard
 
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The transition between turbo and muffler is deceptive (that is one of my mufflers). The angle going into the muffler body allows the neck to open up to about 3.5"ID. The turbo flange ID is 3" so in less than 4" length the neck cones out from 3" to 3.5". The muffler body has a partially packed core that allows expansion right at the point where the neck enters the body.
I've tried to take pictures but it's a bit tricky at that angle.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 06-20-2009, 12:48 PM
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Brian,

I love the large can muffler.

So that is a transition cone off the turbo not a piece of 3.5" straight pipe?

That would alow the gasses to expand in an orderly manner instead of creating turbulence right at the turbo discharge from just dumping right into a 3.5" pipe.

Steppin up to a 3.5" muffler, even with the cheated turn, is likly better that a full 3" system.

Packaging an exhaust around a 930 and most cars often require some type of compromise. Lucky we do no have a long run to the back of the car to create back pressure like most cars.
Old 06-20-2009, 01:56 PM
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No, that is the illusion. The neck itself is 3"ID, the "cone" is created by the back-cut angle into the muffler. The cross section at that spot is 3.5". This was done purposely to make the best compromise due to the angle/relation of the turbo to the muffler body. The muffler opens up internally into a 5"ID shell with a 3"ID perforated core. The core contains packing material that settles out to the remaining 3/4 distance to tailpipe. The space occupied by the packing material is influenced by the exhaust gas flow resulting in a cavity at the proximal end of the muffler right where the "bend" of the neck occurs.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8

Last edited by RarlyL8; 06-20-2009 at 02:14 PM..
Old 06-20-2009, 02:11 PM
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An interesting tidbit on turbo exhaust design:

Quote:
The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on Impreza.net regarding exhaust design and exhaust theory:

“Howdy,

This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems.

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

"As for the geometry of the exhaust at the turbine discharge, 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 7-12į included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.

A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.

If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.

Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.

Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.”

"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.

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 backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.

As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”

"Here's a worked example (simplified) of how larger exhausts help turbo cars:

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

o here, the turbine contributed 19.6 psig of backpressure to the total.

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would.
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Old 06-21-2009, 06:48 AM
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Obviously, this response leaves many of us unprepared to evaluate this man's credibility and/or accuracy, but I get the basics of his points. He certainly sounds like he knows his stuff. He's into engineering territory...so I'll be interested in what the "experts" on this forum have to say. Us lay people will have to sit back and watch.
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Old 06-21-2009, 08:08 AM
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Kenik,

That is from the link I listed Carlye. Much better to see it posted.

As I noted Carlye, we are not trying to create savaging w equal length. However, it dose create for a more orderly movement of gasses for increased efficiency. The cost is increased volume like using a larger tubing size than needed.

Per the expert listed above, the following are suboptimal turbo design elements:

-tubing larger than the turbo outlet at the turbo (big no no),

-cheated turns,

-sharp turns,

-to small or restrictive exhaust after the turbo.

For me, any time I see these elements in an aftermarket exhaust I become suspect. None are the end of the world and sometimes they become necessary do to packaging or cost considerations.

However, why go to the effort and not do it right when the goal is building a performance exhaust. Some reasons are probably:

-cosmetic attributes chosen over function,
-cost considerations, or
-lack of knowledge relating to turbo exhaust design.

I am not an expert, just what I believe to date.
Old 06-21-2009, 08:21 AM
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So if going by what this man is saying- my idea for a turbo back is perfect with the exception of the 4-3 reducer on the end. But I have to do something to not get a noise violation ticket every 5 feet I travel. I got to figure something out for sound deadening...
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Old 06-21-2009, 08:31 AM
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Here's his original post:

http://forums.nasioc.com/forums/showpost.php?p=3149108&postcount=87

BTW, what is a cheated bend?
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Old 06-21-2009, 08:42 AM
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Quote:
Originally Posted by RarlyL8 View Post
The neck itself is 3"ID, the "cone" is created by the back-cut angle into the muffler. The cross section at that spot is 3.5"...
Brian,

Sorry, I do not mean to pick in your muffler. Bottom line it is much better than anything from Porsche and most of the units out there.

So yours is not a 3.5" muffler system. It is a 3" from turbo exit to the end of muffler. The turn into the muffler was make by angle cutting the tube to make the corner. Bevel cutting the tube would do nothing to increase "cross section" at the turn.

With a chambered muffler I do like the idea of an expansion area as the gasses enter the "can". However, I am only guessing that not having packing the full length of the can on a straight through 'might' add turbulence and interfere w velocity through the muffler section. How significant is this. It might not have any effect at all, or only after a given point.

On the plus side, the angle cut and chambered part of the muffler may work to moderate sound intensity to some degree.

However, 3" already seems to be on the small side for the HP levels some 930 owners are expecting and anything that might create turbulence might be more of a concern than if the tube sizing had more room to work with.

------

Another note, I once cut open my old brand name SS muffler from the top shorties exhaust maker. I can say even thouugh they had a proper elbo bend out of the turbo and into the can, your muffler system has to be much, much better preforming!

I could not believe my eyes when I did this as the muffler was restricted down to a the section area of a 2.5" tube.

I picked up 40hp per the dyno w no other change by just putting a straight through 3" Borla muffler exiting the passenger side on my car.

I love these cars and only wish they achive they achive there potental in the best way possable.

Again, we are likly spliting some pretty fine hairs here.

Old 06-21-2009, 09:13 AM
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Quote:
Originally Posted by 930gt-40r View Post
So if going by what this man is saying- my idea for a turbo back is perfect with the exception of the 4-3 reducer on the end. But I have to do something to not get a noise violation ticket every 5 feet I travel. I got to figure something out for sound deadening...
Actually I think he was accepting of reducing size where needed so long as it is as far from the turbo as possable. Hense, you seem to be spot on.
Old 06-21-2009, 09:46 AM
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Yes I use a cheated bend on the muffler neck but that fact alone is not the end of the story. The back cut angle on the neck opens up into an expansion chamber which allows the flow to transition more smoothly through the 3"ID muffler.
Everything I have designed is for CIS steet 930 engines. 3"ID will support 400hp which is about the top capacity for CIS. The vast majority of ethusiasts cars are 300-350whp which is my focus.
The theories are very simple and I have followed all that can be incorporated into a street car having about one square foot of room for an exhaust system, ha!
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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 06-21-2009, 10:50 AM
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Quote:
Originally Posted by kenikh View Post
btw, what is a cheated bend?
+1

May be secret exhaust builder talk, nudge nudge, wink wink.
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Old 06-21-2009, 09:15 PM
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I believe it means that cut a bend sharp and continue it with another pipe or bend thus creating a realy sharp corner.- I could be wrong
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Old 06-22-2009, 04:56 AM
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I would not call Brian's outlet configuration a cheat turn. A cheat turn is two 45 degree slices welded together.

Look ALL exhaust manifolds installed in a street car will incorporate compromises. Every system Iíve seen posted do an excellent job of packaging and compromising.

Most of Jay's posting relates to turbine outlet conditions however there were several other statements that run counter to most popular 930 manifolds:

1) It's been somewhat overlooked that here is another advising that unequal primaries are not acceptable in the pursuit for low volume and short path to the turbine.

2) His overriding principle that there should be NO expansion of the exhaust gases before introduction to the turbine is true but needs qualification (none - most of you are not ricer drag car guys).
This may be possible when placing the turbine inlet flanges of a twin turbo installation on the collectors. However, with a single turbo 930 application this advises that the 3 into 1 collector outlet tube(s) should be the size of the primary tubes. Intuitively this is wrong and itís wrong in practice. You canít drop all N.A. header convention in the pursuit of quick spooling the turbo. Itís not inadvisable because it affects off boost response and more importantly will choke off the process of building boost. The engine must first breath N.A. before bootstrapping to turbocharged.
Old 06-22-2009, 06:46 AM
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Yes a cheated bend is a sharp bend created by coupling two back cut straight pipes.
I concede that I use this bend at the muffler inlet for the sake of argument but qualify it as a design constraint relieved by an expansion chamber.
Many folks are stuck on bigger is better when it comes to primaries. I hope the quoted expert puts this idea to rest. My primaries for the 930 are 1.5"OD with 2" secondaries. This is calculated to aid velocity without choking volume at the HP levels that the system is designed for.
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Old 06-22-2009, 07:36 AM
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Quote:
Originally Posted by RarlyL8 View Post
Yes a cheated bend is a sharp bend created by coupling two back cut straight pipes.
I concede that I use this bend at the muffler inlet for the sake of argument but qualify it as a design constraint relieved by an expansion chamber.
Many folks are stuck on bigger is better when it comes to primaries. I hope the quoted expert puts this idea to rest. My primaries for the 930 are 1.5"OD with 2" secondaries. This is calculated to aid velocity without choking volume at the HP levels that the system is designed for.
Mine, too. For my 2.3L, the exhaust ports transition perfectly into a 38mm (1.5") equal length primary header tubes, that then wrap toward the front of the car and back to a 3>1 collector of 2" diameter. I still think that for a smaller motor that doesn't benefit from the additional pumping force of large pistons, this kind of optimization will pay dividends. We'll see.





Was doing some preliminary fit this weekend for the turbo, so I could measure out the tubes from the ehaders to the flange. I'll say that getting everything fitted right and ensuring equal lenth from the headers to the turbo is pretty challening. Since I am using a twin scroll turbo, getting this nailed is going to be critical.

Also, the turbo I am using, an MHI TD05HR-16G6-10.5T (stock Mistusbishi Evo VIII turbo) is provin to be even more challenging as "clocking" the turbine for optimal positioning for the oil drain isn't easy. To get the compressor outlet lined up correctly has the oil drain hole perpendicualr to the ground, so looks like I will have to come up with a fabricated solution for routing the air from 90 degrees to the ground and back up.



Oh the things you learn when you go the non-traditional route...
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Old 06-22-2009, 07:51 AM
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Brian,

First, thank you for putting up with all the scrutiny and taking the time to help and educate the 930 community.

Your products make can make a huge improvement in a 930's potential and are equal to and better than most options offered by others.

I dought that not making the changes we are discussing would limit performance on any CIS 3.3 turbo I might build, in any perceivable or measurable manner.

I see our discussions as more idealistic in nature for the most part. Once we eliminate the stock restrictive 930 muffler and or cat with a straight through system such as yours, any change from making it idealistically perfect for a 3.3 CIS 930 motor it going to be pretty darned small.

Continuing--

As to the cheated bend, I am thinking any possible 'disruption in flow' from the cheated bend into the muffler will already have been done.

Trying to make up for it after the fact would probably not fully mitigate any disturbance it might add.

Also, I am thinking that adding a ďchamberedĒ section to the muffler by leaving out the packing material would have the effect of quickly slowing the exhaust gasses as they expand to quickly fill the void. As you know, for the most part a chambered muffler is not a best choice for a performance turbo. This might have the effect of stalling the exhaust gasses and may actuallyy work counter its intended goal and further add to potential back pressure.

In the real world, I do not see these points actually making any significant difference for most of us with CIS 930's.

Retaining CIS is much, much more detrimental to our performance potential than reaching to achieve idealistic exhaust theory goals.

Further we can not alwas think our selves throught and exaust system. Testing and validation are the only ways to see if we have made a good system.

I know you love these cars as most of us do and are working to make them the best they can be.

Thank you for your time and keep up the good work.

Old 06-22-2009, 10:13 AM
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