The turbine housing is the housing that covers the turbine and directs the exhaust gasses into the turbine wheel. The turbine housing is one of the most crucial parts of the turbo-charger, to determine how the turbo will peform. For this reason we cast our own turbine housings for most of our DPS Turbos.
Turbine housing science can be broken down into 5 different main categories.
Wastegated Housing: A turbine housing with a wastegate has valves that control the exhaust around the turbine so a portion of the exhaust bypasses the turbine when it is open. This is the simplest form of a variable geometry turbine housing. It can allow between 10-15% more exhaust gas to bleed off past the turbine. This helps reduce exhaust drive pressure, it can control boost and exhaust pressure and decreases turbine speed.
Non-Wastegated Housing: In non-wastegated housing there is no way of regulating the boost or turbine speed from the exhaust end of things. These housings are typically a little larger and are much cheaper, but they do not have as wide of an operating range as wastegated housing does.
Volute/Scrolls: Scrolls or volutes are snail shaped passages of the turbine housing. If you were to stretch the scroll out, you would find that it is a cone that is wrapped around the turbine and has a slot in the center that allows exhaust to pass through into the turbine wheel. The A/R ratio is measured starting at the tongue or inlet and is the AREA over the RADIUS from the centroid of the turbine scroll (notice I did not say “center”). A smaller A/R means that the “cone” is tighter and smaller which allows exhaust gasses to pass through at a faster rate, which helps the turbo spool faster. This is more conducive for low end drivability and quick spooling. A ratio of 1.1 to 1.35 is on the larger side of ratios, and will be indicative of slower spooling with exhaust flow being high on the top end but not as quick spooling or nearly as drivable.
Divided Scroll: In a divided scroll, on a 6 cylinder Cummins for example, the front 3 cylinders are separated from the back 3 cylinders by a divider. This means that only 3 cylinders have to pressurize in exhaust manifold so the exhaust pulses are much stronger as they pass into the turbine wheel. This allows for faster spool up.
Open or Non-Divided Scrolls: In an open scroll, the volute has only one open hole and it is not divided. This means the exhaust gasses have pressurize through the entire manifold to fill up the turbine housing before it can spool the turbo, and exhaust pulses are less powerful. This means the spool up will be much slower. However in theory an open scroll can flow slightly more at top end, although our experience does not necessarily support this theory. In other words, we like a divided scroll on a most diesel engines. We believe that this theory is may be correct on gas engines, where RPM’s are much higher.
The ideal combination is a divided manifold with a divided turbine housing. With this option on a 5.9 Dodge Cummins you will see typical spool ups of 100-200 RPMS quicker. For this reason, we only build full divided scroll manifolds. we highly advise any 3rd gen Cummins owners who purchase our turbos to also purchase our 2-Piece Exhaust manifold because the combo makes your driving experience more enjoyable.