Exhaust 2.5" VS 3"

[Tim_C.]

Thanks to StarQuestSir for this article.

I would like for people to post their own EXPERIENCES, not opinions, so those thinking about a new system can have accurate info to help them decide. Remember, there is a pretty good difference in price between 2.5 & 3", whether you do it yourself, or have a shop do it. I have edited some unrelated (to us) bellmouth wastegate dialogue

Here is the article:

 

Exhaust Design 101

So what exactly constitutes a properly-designed turbo exhaust system?

Many things. Among them being controlled management of exhaust gas boundary layer, minimal exhaust back-pressure at the turbine outlet, an absence of exhaust diameter changes/steps and the minimizing of exhaust gas turbulence and friction. In a perfect world (that is, one without sound level, practicality, emissions or packaging concerns), an exhaust system for a turbocharged engine would be nothing more than a short, horn-shaped exhaust pipe that extended outward from turbine discharge. Think of a horn speaker or a megaphone and you’ll get the picture. This will make maximum horsepower.

 

That's right.  The ideal turbo exhaust system would smoothly taper in the other direction- from smaller diameter (the same size of the turbine discharge outlet) to larger diameter over a short pipe length. The length of the exhaust system would be short enough to minimize exhaust gas friction while long enough to incorporate a suitable horn-shape with a diameter rate-of-change aggressive enough to minimize exhaust back-pressure while gradual enough to keep from inducing boundary layer disruption (when the exhaust flow no longer smoothly flows along the surface of the pipe but instead separates from it and becomes turbulent).

The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on www.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.”

[Tim_C.]

Here is the rest of it:

 

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.”

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. So 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.

[Tim_C.]

It has been my experience that the StarQuest stock engine & turbo sees absolutely no benefit, going from 2.5 to 3" exhaust, except a deeper tone sound.

According to this Garrett engineer, it would take 300 HP to outgrow the 2.5, and need 3".

That means you will need mega mods to ever really NEED a 3" system. It takes more than what most of us would admit to, to go from stock 188 flywheel HP, to 300. We can make a list of mods that would get 300 HP. MPI alone, and 3" exhaust won't do it, unless you raise the boost way up there. You would really need a bigger turbo. See how much money one would need to spend to actually justify a 3" system? Is it worth it? Well, after all that, I'll say yes! I love the tone, plus now I am at the edge of needing the 3" system I have, and am ready to do the last mod to get me over that for sure, if I'm not there now.

Non intercooled flatbody? 2.5 will be all you will ever need. Remember too, that 3" systems are also heavier, so if there is no performance gain, you are just adding weght, and potentially getting less power to the wheels.

[Boosted_One]

3" all the way.

 

If I can dig up the dyno runs I have a stock exhaust, a 2.5" exhaust and a 3" exhaust all ran on the same car.

 

The 3" waxed the 2.5" even a stock 12A.

 

We need to keep in mind our motors are very restricive to begin with, 2V head design and restrictive turbine housing. There is a large window to open things up before we see noticable drawbacks.

 

I'd say anything over a 16G should have a 3" to optimize it.

 

Here's one graph I dug up... 3" on a 12A vs stock. Lots of gain overall and no loss anywhere in the band. And this run is on STOCK boost levels.

 

http://a8.cpimg.com/image/14/53/11136788-6cce-02000192-.jpg

 

I can't see a 2.5" set up pulling higher numbers in the powerband.

[s.kodai]

umm, devils advocate here.

 

is that your dyno sheet.  it looks like the one from the cnm site.

 

having dealt woth james, i simply don't trust that the exhaust is the only difference.

 

if it is yours, i apologize.  i just don't trust cnm to not "fudge" a dyno run promoting a product that they sold.  were the runs "corrected"?  were they "correted " with the same baseline?  there could be a big difference just in the air temps.

 

hell, the tuneup might account for the diffence, if it was really bad before.  

[Boosted_One]

umm...it's John Beyers car and his dyno run.

 

It says SAE HP and SAE TORQUE which means it's corrected.

 

Judging by power increasments of other cars with 3" exhaust I don't find this run to be questionable whatsoever.

[WTFISIT]

I'm just curious about the big torque spike at 4k rpm.  Is that where the 88/89s jump to 10psi boost?

 

Thanks

Josh

[GoldNBlack]

I'm just curious about the big torque spike at 4k rpm.  Is that where the 88/89s jump to 10psi boost?

 

Thanks

Josh

 

Yes,  on the manual transmission 88-89s, Under 4k the boost is limited to 7, over it goes to 10.    There is a faq about disabling this "feature" by telling the ignitor that you have an automatic.

 

Steve

[spoolinturbo]

Starquestsir: I hear that the 4k limit is not on all 88/89s? If so...I am changing my 89 auto to standard so this 4k limit would not exist right?

jeff

[GoldNBlack]

All of the the 88-89's had the three port wastegate with the boost level control solenoid.  It was there for Knock protection also. However, if someone has changed the vacuum hose routing to the actuator it won't work right.

 

Only the Manual trans cars have the 4K switch over.  Without looking over the diagrams, I'd assume the wire that tells the igniter what tranny the car has may go all the way to the transmission itself.  So it's possible that as soon as you unplug the auto trans it will think it has a manual one.  It's just something somewhere that sends the ignitor a 12Volt signal.  

[ihavenoname]

Cool thanks for all the info i think i will go with a 2 1/2 exhaust i just found a downpipe for cheap and i dont think im goin over 300 hp so all be good.

 

Thanks,

Marcus

[jr_ss]

Even if someone doesn't think that they are going to break the 300hp boundary, it is still good insurance just to get the 3 in exhaust. I know most people on the boards like power, they start making the power they want more, and that leads to more mods. I went with the 3in for my intentions to grow into it. I didn't want to purchase a 2.5in exhaust and then later have to purchase a new system at 3in, what a waste of money. In my eyes the 3in exhaust is the best bang for the buck even if you aren't going to be reaching that 300hp mark...

 

Just my .02

 

Glenn