Show Notes
Single turbo or compound turbo: which one really makes more horsepower? This episode started with a comment from viewer @V8Lenny (youtube.com/@V8Lenny) claiming a single turbo will always make more power than the same charger run in compounds, so we put that claim up against years of diesel dyno numbers and real Cummins builds.
The conversation starts with results from 12 valve Cummins and 6.7 Cummins common rail diesel trucks. A 98mm charger that made 1100 horsepower as a single made 1500 to 1600 in a compound setup, while an S480 made the same power either way. It also covers why big singles are so hard to tune on a common rail diesel, from snuffing the turbo with too much fuel to adjusting rail pressure and timing so the fuel drives the charger.
From there it gets into turbo science. Diesels have less drive energy than gas engines, which is why sled pull turbos run huge turbine wheels and why high altitude makes everything harder. Compressor efficiency, pulse energy, and interstage cooling all come into play, along with The Godfather, a triple turbo 12 valve that made 2800 horsepower with four stages of densification.
Cylinder head flow is a big part of the picture too. Head flow decides where an engine wants to make peak power, and matching your turbos to the head matters more than chasing RPM.
Testing on the Junker drag truck shows both sides of the single turbo vs twin turbo debate: an S476 single that barely broke 700 on a 5.9 Cummins, an S369 single that beat a set of towing compounds, and a 12v Cummins build where compounds made way more power. The episode also explains why compounds spool better at low RPM and when they don’t.
It wraps with the 72 Fast class pushing near 1500 horsepower on a 72mm turbo and a final verdict on @V8Lenny (youtube.com/@V8Lenny) claim: compressor for compressor, a well tuned single usually wins, but compounds are way easier to tune.
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