Can you boost boost?

Question

with a twin turbo system (or super+turbo charged systems) the first charge is say 5 lbs boost, second charge is a 10lbs boost. will the engine recieve 10lbs?, 15? 12.5?

these would be set up sequentially not side by side split duty.

if one turbo sucks in sea level air(14.7psi) and spits out 10 psi boost (24.7psi) will that same turbo make 15 lbs boost if it is taking in charged air from a smaller turbo?(5psi boost)

Answer 1

It looks like you're looking at a compound turbo system, where the discharge side of one turbo feeds into the inlet of the next. With compound turbos, you have to think in terms of pressure ratios. One that puts out 5 psi of boost is raising the air pressure by a factor of 1.34 (from 14.7 psi to 19.7 psi), so its pressure ratio is 1.34:1. The one putting out 10 psi has a pressure ratio of 1.68:1.

The pressure ratio in a compound turbo system is determined by multiplying the pressure ratio of the two turbos. 1.34 times 1.68 is 2.25, so you are getting 18.4 psi of boost. And a turbo running at a pressure ratio of 1.34 is hardly even trying. Two turbos each running at 1.68:1 would put out 26.8 psi of boost.

Compound turbos are pretty complicated to deal with - you have to really know what you're doing to select the appropriate sized primary and secondary turbine, and need a lot of controls and cooling to manage it all. Pick a turbo the wrong size and you'll get ungodly amounts of heat in the intake, putting you at risk of trashing your engine. It's easier to hit 18.4 psi of boost with a single turbo or parallel twins.

Compound turbos are more useful if you want over 30 psi of boost, and really come into their own if you need to hit 100 psi. I know what you're thinking - that's nuts. And it is if you're running gasoline.However, it's quite possible on some diesels.

Answer 2

Yes, you can adjust boost -- depends on your system.