Turbo Exhaust Theory

[RIPae86]

Taken from reputable source cited from http://www.tercelref...ust_theory.html

Most may already know this (Hence why, for a forum based on turbo cars I can't find much about exhaust size, back pressure or how to optimise an exhaust modification) but I thought I'd post it up for those who may find this informative.

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Turbo Exhaust Theory

I stumbled across this information on a forum.

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 7-12� 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.

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[luvpsi]

wow thats in so much detail that it becomes confusing

[R31Nismoid]

Been posted up a few times overthe years, good read either way.

[Converge]

Thanks for sharing, good read.

[MrStabby]

To the split dump haters - umadbro?

[blk94r33]

Just what i've been searching for for the last hour and bumping threads..... 2.5" is good for 250-300hp there-about. Which is more than my car can handle anyway.

Can anyone tell me the standard 33 GTST dump/front pipe size?

[wolverine]

To the split dump haters - umadbro?

I don't hate split dumps but the bellmouth made more power on back to back runs with a GT-RS.

I have a split on the R33 (ext gated).

So I consider myself a swinger.

[RIPae86]

Been posted up a few times overthe years, good read either way.

A sticky will end the repost woes

Just what i've been searching for for the last hour and bumping threads..... 2.5" is good for 250-300hp there-about. Which is more than my car can handle anyway.

Can anyone tell me the standard 33 GTST dump/front pipe size?

I was keen on a 2.5 as well aiming for an optimum size for my power output, the response would definitely be greater at low rpms as well which is what I want as my car is only dedicated for the street and wont be pushing too far past a stock power output.

I am not sure on the standard size but dumps are so cheap im sure you can just find an aftermarket one to suit your application.

I guess for the turbo to perform to its potential the exhaust system you musnt have any bottle necks, even if the stock dump is 2.5 inches you will still find some performance gains in a quality aftermarket one because of mass manufacturing realities

[R.3.2.G.T.R]

great info, thanks for posting

[Nee-san]

To the split dump haters - umadbro?

Pretty much all of the divorced dump pipe designs on the market are shit anyway, and yes this includes HKS etc etc.

So I wouldn't be surprised if a well thought out bell-mouth would pip them.

[Lithium]

I've got a divorced wastegate feed which stays split for some distance, seems to work really well - car appears to make respectable power (and spool) for a car running all stock manifolds etc. Sounds pretty awesome when the wastegate opens, too - and yes, it does merge back with the exhaust.

Out of interests sake, a mate of mine has a Honda Prelude which he's been building up over the last couple of years using just the skills and knowledge of himself and his friends, and after many an evening of drinking and plotting the "best way" of doing everything he's (within reason) implemented the hairbrained ideas we've come up with, one of which included a large tube header with a 2.5" collector and 3" exhaust on a 2.2litre naturally aspirated engine.

After I tuned it, he ran it on the local dyno most people use around here for reference and it basically has the most solid power delivery out of any NA motor up to that engine size on that dyno... its pretty loud, but being a 3" exhaust has certainly not caused it any issues.

[MrStabby]

Pretty much all of the divorced dump pipe designs on the market are shit anyway, and yes this includes HKS etc etc.

So I wouldn't be surprised if a well thought out bell-mouth would pip them.

Pip them in max power yes, spool (wastegate shut) no. And a well thought out split would be best for both.

I have xforce splits and the merge is very poorly done. Even with the short runners, i reckon a good merge (right at the front pipe flange) would equal the power of a bellmouth, with better spool.

[Nee-san]

Pip them in max power yes, spool (wastegate shut) no. And a well thought out split would be best for both.

I have xforce splits and the merge is very poorly done. Even with the short runners, i reckon a good merge (right at the front pipe flange) would equal the power of a bellmouth, with better spool.

I'm not disagreeing with you at all !

my point is that there are very few off the shelf divorced designs that are actually good.

If I can get one I custom designed manufacture I have no doubt it would be better then a bell mouth design.

[ARTZ]

Just what i've been searching for for the last hour and bumping threads..... 2.5" is good for 250-300hp there-about. Which is more than my car can handle anyway.

Can anyone tell me the standard 33 GTST dump/front pipe size?

You said you wanted 220kws that is over 350hp you realize....

Even with stock turbo and 10psi you are pushing 300hp..

Just get the 3 inch..

[Harey]

I was keen on a 2.5 as well aiming for an optimum size for my power output, the response would definitely be greater at low rpms as well which is what I want as my car is only dedicated for the street and wont be pushing too far past a stock power output.

So all the effort of posting this article up and not reading it properly yourself??

"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."

Please show me where it states that having a smaller exhaust after the turbo will help your response and be better at low rpms. The quote above I think is what you are referring to and it simply states that you wont gain much, it does not say you will loose anything.

[ClutchBurndout-:(]

good read

lol at bellmouth haters

[Beer Baron]

standard a R33 GTST makes nearly 250hp so on anything but a bone standard car 2.5inch is a waste of time. 3 inch (76mm) is the smallest exhaust you should be looking at for a 33 GTST. better even to go straight to 80mm which is a touch bigger.

[blk94r33]

You said you wanted 220kws that is over 350hp you realize....

Even with stock turbo and 10psi you are pushing 300hp..

Just get the 3 inch..

http://www.google.com.au/search?sourceid=chrome&ie=UTF-8&q=killowatts+to+horsepower

-1 killowatt=~1.34 horsepower

-So 220kw=295hp

-How is 295 more than 350?

If a GTST can run 9psi in standard form, how is the standard exhuaust too restrictive?

[ARTZ]

295 at the wheels is close to 350 at the engine ..

Your exhaust flows engine hp not wheel hp..

[blk94r33]

295 at the wheels is close to 350 at the engine ..

Your exhaust flows engine hp not wheel hp..

Ah, right, now I understand you.

Bit too late now, dropped off the car today.

The guy doing the exhaust knows his stuff, and recommended 2.5" over 3" based on my intentions with the car, eventual power goal, noise, and his previous experience with turbo cars and the results. He believed a good 2.5" exhaust would flow enough for ~400hp (flywheel) on a turbo car? If it ends up being too restrictive, i'll only be down ~200 to get the piping re-done, as the muffler and cat will be compatible...

I did a bit of digging before agreeing to the 2.5" system, there were posts by R31Nismoid (Ash) stating as a rough guide the size vs. power ratio, and it worked out to be roughly 1" per 100 flywheel kw, but f***** if I can find it now...

Will find out tomorrow how it goes...