I would really like to see more R&D put into compound setups. I know that you can just throw a couple of turbos on an engine and it will make more boost and flow, but what about efficiency, drive ability, and application. It seems like the mass of consumers in the diesel world are just beginning to look in depth at how turbos really work, and what will work best for them. The turbo gas guys (ricers too
) have been building “custom” turbos for themselves for quite some time. Its interesting to talk to guys that are selecting the compressor wheel, turbine wheel, design of bearing housing, size and type of turbine housing, and turbine wheel trim, that works best for their application. They would be quite hesitant to purchase a turbo that someone just told them would work, without knowing the details.
You need to know a minimum of how much boost is needed, the amount of airflow required, size of the engine, maximum engine rpm, and volumetric effecency to properly size a turbo, or turbos, for a certain application. Things are a little different when applied to a compound setup, but much of the information holds true. It is difficult to find compressor maps for some of the Holset turbos, but most turbo manufacturers have their maps readily available for us to use. Earlier in this thread there was mention of the 60mm HX40 compressor wheel - there are actually several versions. The most common are an eight blade, “high flow” six blade, and the “super sixty” six blade. Each are similar, but the maps show different flow, and pressure ratios. Its also just not about the inducer size of the compressor. The exducer, and stackup height also play a role in performance. A wheel with a larger inducer size can actually flow less than a slightly smaller wheel with a larger stackup height. For our diesel application, we generally run at a much higher pressure ratio, per lb/min of airflow. This makes finding the proper compressor wheel slightly more difficult. Turbine maps are not as popular as compressor maps, but if available do aid in selecting the correct wheel (P trim, Q trim, etc. ) for best flow. Custom steel turbine housings can also be used to get the right spool up characteristics, and needed exhaust flow. Some even have internal wastegates up to 38mm! That may be enough flow for a small A/R turbine housing on a small set of compounds without using an external gate. Quick boost, and acceptable drive pressure.
I think that matching the primary and secondary turbos could be the most difficult part of building compounds. Things like selecting compressors that work together, trying to decide what PR the small turbo needs to operate at when the large turbo is making max boost, choosing the correct size turbine housings and wheels so that the compressors operate within their most efficient island, etc. There are also other variables, such as what the engine is going to be used for - drag racing, pulling, towing, street. Since no setup can be perfect there will be compromises between spool up, max flow, max boost, smooth transition, etc. I hope to see more engineered setups soon that will really show what compounds can do. Its amazing how much I realized I dont know when I realized how much there is to know.
In the mean time, back to my 35/40 - 3B combo... Good luck to you guys!
Mike
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