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Talk about engine building here. New products, tricky questions or showcase your work - If it's engine building related it's welcome here.
As much as I have searched on compound turbo sizing and setups, most kits and builds use the stock turbo of a diesel engine as the smaller of the compounds and add a massive turbo for high end power, now I was wondering has anyone ever put a smaller turbo than stock size to get the boost threshold even lower for drivability?
Something like a 5.9 12valve Cummins has max torque at 1700rpm which I assume boost comes on just a little bit before that, would it be possible/beneficial/damaging to have boost able to come on at 1200rpm or even 1000rpm?
In the Cummins case I dont see how making 400-500ftlbs of torque at 1000rpm (essentially lugging) would be more damaging to internals than making 900hp and over 1600ft-lbs of torque.
You may find you need more pump volume to maintain good oil pressure at those revs. The natural VE peak of the head/cam arrangement will still have an impact too. Might need to be mindful as to if timing of mechanical pumps is compromised on old engines but electron8c injection shouldn't be problematic, again of going nuts you may find a fuel limit if there is a base bypass flow on the fuel system.
Even with good wastegating of the smaller turbo you will likely find a top end compromise if you go very small too. A small step down may be fine though as both turbos are likely to be at higher turbine and compressor efficiency than a factory turbo in the top end.
On that, the lower rpm will produce less hydro-dynamic wedge - oil support thickness - to support the higher loadings, so going up a grade in oil viscosity would give more protection.
However, there is a sub-forum dedicated to diesel, so perhaps if you posted there you'd get much better advice?