Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. "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. For 400-450 hp, even 3" is on the small side.” 300 hp and you're definitely suboptimal with 2.5". Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.Īs for 2.5" vs. drive the compressor and make boost) with the available inlet pressure.Īgain, less pressure downstream of the turbine is goodness. This means the turbine is able to do the most amount of work possible (i.e. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. Remember, a turbine operates via a pressure ratio. 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. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Stick a Hoover on the tailpipe if you can. Just don't go so small on the header's primary diameter that you choke off the high end.ĭownstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. You want the exhaust velocity to be high upstream of the turbine (i.e. There is a happy medium here.įor turbo cars, you throw all that out the window. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. Go too big, and you lose velocity and its associated beneficial scavenging effect. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. 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. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems. Most of the facts have been covered already. For the rest of you, here's a link to it: !/p.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. I kind of just made cliff notes there of the important parts so non facespacer's (scheides lol) could see the meat of the conversation. We can do this unit for $650 exchange went ahead and built up a batch of 15 for the shelf so whenever people send in cores, we can ship same day. Not making a new color designation for this turbo, not going to build them as New Turbos either, just cheap core swap basis, something for the budgetarily challenged projects out there. Yeah, its my car so yes its not a short stroke budget motor, all my knobs are at 11 24/7 Its not maxed, that was only 151krpm on the turbo, it's not overerspeeding until it gets to 157krpm. 2.3, 4R cams, ported intake manifold, built moda, big FMIC, fuzzy dice on the rearview, whole nine yards.stock exh manifold, unported stock EVO9 turbine housing with a HP actuator. It's my personal EVO9, so its pretty much got everything on it. Since Robert FP apparently quit posting on forums, I've been stalking his facebook periodically for updates and he posted some today.
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