L300 diesel exhaust ?

[ThumpinA2Z]

I need a new exhaust system for my 91 L300 diesel so I was wondering if its worth having a custom larger diameter better flowing system done.
I also plan on K&N Filter and a snorkel (with or without custom exhaust).

For those in the know what's the ideal diameter of the pipe what mufler and how important is the length (side exit vs out back)?
Would there be a reasonably noticeable difference in performance and/or fuel consumption?

Thanks
V

[Tojo]

http://www.delica.ca/forum/viewtopic.ph ... ss+mandrel

2.5" is an ideal diameter. Side shooting or out the back it up to you depending on the amount of noise you want. Side shooting reduces cost as well. Stainless is optional, mandrel bent is optional. K&N makes a difference, also reuseable. Notice a difference in performance, no difference in fuel economy for me. I don't have a snorkel, but others do so search the forum.

[FalcoColumbarius]

I have a two point five inch mandrel pipe. The flange is actually two point two-five. The thing you need to remember is you have a turbocharger that is affected by the back pressure of the stock one point five inch pipe. Also the stock diameter of the pipe not only affects the turbo but is also tuned to the exhaust timing from the cylinders so the exhaust travels through the pipe in neat packages that keep the flow consistent. I love my stainless pipe, it sounds and looks out of sight, man ~ but there is a trade off ~ as in any deal.

Regarding a snorkel, you know that the Starwagon comes stock with a snorkel, eh? It's a part of the air intake system under the driver's seat. This thread explains a little: http://www.delica.ca/forum/viewtopic.ph ... ue#p126060 ~ unfortunately the link to the thread that demonstrates the snorkel in action has broken* (will have to fix that). I've forded as deep as three feet without a problem. The link that is broken shows the Mystery Machine going through five plus feet of water.

Falco.

P.S.: I've moved this thread from General Discussion to L300 Technical.

*P.P.S.: Here's a link to that thread regarding what the stock snorkel can do that had the broken link (Mystery Machine To The Rescue): http://www.delica.ca/forum/viewtopic.php?t=1859

[Mimic4]

What sort of trade off? For going to 2.5".
I thought about crush bent 2.5 eventually to get better flow..

[Growlerbearnz]

The retaining flange that bolts to the turbo dump pipe comes in two sizes, the larger size will take a 2.5" pipe, but the smaller size (2.25") might require a step down or strategic application of a hammer to clear the bolts and springs. I'd check your exhaust flange size before ordering any parts.

Lots of lovely pictures and advice here: http://dinoevo.de/side-exhaust/

[FalcoColumbarius]

What sort of trade off? For going to 2.5".
I thought about crush bent 2.5 eventually to get better flow..

The thing you need to remember is you have a turbocharger that is affected by the back pressure of the stock one point five inch pipe. Also the stock diameter of the pipe not only affects the turbo but is also tuned to the exhaust timing from the cylinders so the exhaust travels through the pipe in neat packages that keep the flow consistent.

With the wider pipe you get more turbulence which interferes with the travel of the exhaust parcels emitted every time an exhaust valve opens. I've got a buddy who is a brilliant mechanic, working usually on VWs, Mercs & Audis, sometimes old Rolls Royces. Every-so-often I get him in a corner and I interrogate him on something. One day I was talking about my two point five inch mandrel pipe and he gave me a lecture on how a pipe diameter can affect the engine's performance in one way or another. Typically the engine is balanced to the weight of the vehicle, right down to how much air comes in to how much fuel, what type of fuel (original diesel vs LSD or ULSD) ~ every time you change something you have to consider the consequences elsewhere in the vehicle.

I'm actually a painter by trade and have discovered these things along the way, sometimes by a stroke of genius, more often by making a dumb-ass blunder and getting helped out by friends like my buddy ~ but I've learned a couple of things along the way.

Falco.

[Mimic4]

That's very interesting, did he explain the pros / cons of expanding the pipe diameter? And which other areas would suffer?

Everything is always connected isn't it...

[Growlerbearnz]

Exhaust tuning is a very complex subject, and well-intentioned wisdom that applies to some engines can be inappropriate for others. Diesel engines in particular have their own peculiarities, and rules of thumb that work on gas engines sometimes don't work so well on diesels.

As I understand it, exhaust sizing is one of those things that's different for diesels. Because they don't have a throttle, diesels flow a LOT more exhaust than a gas engine. Non-turbo diesel engines can use exhaust pulses to improve flow very effectively, but exhaust sizing and length is important. It's not a good idea to mess with a non-turbo diesel exhaust unless you have a way of testing your improvements (EG. dyno, EGT gauge, fuel economy comparisons.)

Turbo engines are far less finicky- exhaust pulses are still useful between the head and the turbo, but once the exhaust hits that tiny turbine the pulses get all mixed up and the flow becomes turbulent. The dump pipe (the cast iron bit directly after the turbine) is important for tidying up the turbulent flow, but after that there's nothing to be gained by restricting gas flow. (Gas engines and highly tuned diesels with large turbos can sometimes benefit from a tuned exhaust- but that's not our tiny 4D56s.)

Stock exhaust systems are a compromise between performance, noise reduction, cost, and ease of manufacture- Mitsubishi seem to have leaned towards cheap and quiet with the stock exhaust, but there's no reason why you can't go the other way.

As long as you leave the stock dump pipe in place you can use any size exhaust you like and I promise you there won't be any loss of power, though there's probably not much point going above 3". Reducing the backpressure will help your turbo spool faster though, so make sure your wastegate is working properly.

Again, standard disclaimer: Advice from internet. Worth every cent you paid for it. Credentials supplied upon demand. Your mileage may vary.

[Mimic4]

Exhaust tuning is a very complex subject, and well-intentioned wisdom that applies to some engines can be inappropriate for others. Diesel engines in particular have their own peculiarities, and rules of thumb that work on gas engines sometimes don't work so well on diesels. ...

Thanks again Growler,

You're such a useful pot of knowledge! :D I'm running around doing all my winter tune-ups and getting everything in place, and since I rebuilt my Turbo I've been looking for the knowledge to get everything running great, incl a little extra boost. ;)

Thanks again!!!!

[Growlerbearnz]

I considered my post for a while- exhaust tuning is one of those things that can turn into a flame war, but in this case I know I'm on the right track.

I found a neat, more detailed explanation allegedly by an actual expert (Jay Kavanaugh, Garrett turbo systems engineer) here: http://www.tercelreference.com/tercel_i ... heory.html

It's aimed at gas engines, so some of the theory doesn't apply to ours, but it's a much better overview than my piece.

Reposted below for the archives.
-----------------------
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 7-12° 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.