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Old 05-11-2008, 01:44 PM   #1
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Default Again with the backpressure = torque

Backpressure is a side effect of small runners in a header in order to get the velocity (actually kinetic energy) up, to effect scavenging. Backpressure in and of itself is undesirable. Headers are usually designed to maximize midrange or upper midrange torque, thus you can see how "backpressure improves torque" myth came about.

There is one other side effect, and this my conjecture. With old school V8's and their poor port flows, they need a lot of cam duration to get decent power. The effect of this duration is that at low RPM with good exhaust flow the exhaust backs up into the cylinders. Some backpressure probably alleviates that.

With modern 4-valve heads the flow is so good they don't need a lot of cam duration, and so the low end will be good with a good flowing exhaust.

Read this by a Garrett engineer:

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.

Hope this helps.

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Old 05-11-2008, 02:11 PM   #2
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good read.
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Old 05-11-2008, 03:29 PM   #3
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Nice...good find/post.
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Old 05-11-2008, 08:17 PM   #4
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Jason, thanks for clearing that up for everyone here, it is always good to hear from the pros who do this for a living.
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Old 05-11-2008, 09:00 PM   #5
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Jay Kavanagh (who I'm assuming Jason is quoting here) is the father of the disco potato as well. Also a turbo miata guy. I believe one of his former motors is in Jason's car right now.

I think he's engineering editor at edmunds now?
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Old 05-11-2008, 09:16 PM   #6
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Ah **** for no reason at all i just remembered i forgot to get the recent GRM while i was running around town all day.

Good write up jason, much appreciated info.
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Old 05-11-2008, 09:23 PM   #7
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I think something good to add is why there is the myth to start....

when you put larger runners on an n/a setup, not only does the AFR mixture change (lean) but the EGTs drop. This drop in EGTs slows the velocity and allows the gases to expand to back up in the pipes (backpressure), even though they are larger...the engine, now has to waste energy pushing they exhaust out instead of pushing the pistons down. once the rpms raise the effect lessens and the pipes allow for better flow, this is why you see higher top-end gains. its the backpressure down low that's actually lowering the torque...
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