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Message: [QUOTE="me2, post: 359865, member: 3825"] Here is a performance graph of twin versus single turbocharger compressor setups that I wanted to use about 15 pages ago to explain why I was going with twins rather than a big single. I'm not trying to start a discussion or get off track. I just came across the graph and thought I'd post it while I had it to complete the discussion. [ATTACH=full]88693[/ATTACH] Multiply Kg/sec by 132 to get lbs per minute. 0.8 Kg/sec = 105 pounds per minute. 0.6 Kg/sec = 79 pounds per minute. 600 HP needs a good (ie not hot) 80 pounds per minute (0.6 Kg/sec) for reasonable EGTs. A PR of 4 is about 40 pounds of boost by the time air filter and intercooler restrictions are taken into account. PR of 6 is about 70 PSI. The Towing Twins and R700 use the Super B as the secondary turbo. Notice how much more pressure and the increase in flow that the twins setup achieves versus the Super B alone. The R850 uses the Super B Special as the secondary turbo. Notice how much better the R850 Twin setup does at LOW FLOWS compared to the Super B Special alone. This is because both the primary and secondary turbos are creating boost and their compounded boost is better than the boost that could be produced by the Super B Special by itself. At 0.3 Kg/sec, the Super B Special will push a PR of 3.5, which is 36.75 PSI under perfect conditions, less in the real world. The R850 twin setup achieves a PR of 4.5, which is 50 PSI in perfect conditions. The R850 achieves better low flow performance than either turbo could achieve on its own, because even at that low flow, the boost is compounding. A Cummins 6.7 making 1200 ftlbs at 1600 RPM needs 52 pounds of air per minute (0.4 Kg/sec) to stay cool. It takes over 50 PSI of boost to do that, which requires a PR of at least 4 in the real world to achieve. The Super B Special by itself will achieve that boost, but it runs out of pressure and is at the edge of its map before it hits the 0.6 Kg/sec to make 600 HP on the top end. The R750 and the R850 setups will easily achieve both the low end boost requirements (PR > 4 @ 0.4 Kg/sec) and the high end (flow > 0.6 Kg/sec). The Towing Twins achieves the 0.6 Kg/sec flow rate, but only just. The Super B is a 57mm turbocharger. I don't know what the Super B Special is. The Towing Twins use an S366 with the Super B as the secondary. So the compressors are about 66 and 57. The R750 uses an S471 and the Super B. So the compressors about about 71 and 57. The R850 uses an S475 and the Super B Special. I'm guessing the compressors are 75 and 60ish. I'm using an S475 and the HE351VGT, which has a 60 mm compressor. I expect my twin setup will perform somewhere between the RS750 and the RS850, hopefully with the good parts of each. The benchmarks for my turbo setup is 50PSI of boost at 1600RPM, 50 PSI of boost at 2700 RPM and no surging anywhere and cool EGTs everywhere. Of course none of this matters unless its on an engine installed in a truck. Back to the issues at hand. Edit. In case anyone is interested, a PR of 9 is 117 PSI under perfect conditions, probably 100 PSI in the real world. [/QUOTE]