# How many turbos is too many? logistically and physically speaking

I want to build an engine simply to create as much horsepower as possible; Not necessarily to put into a car, just to have a fun project that is powerful. so of course I'm trying to max out all the aspects of engine performance. and the only thing I have not been able to come to a consensus on is forced air. I'm wondering if a system of parallel-sequential-in-series turbos would be feasible. size and weight are not a problem at all. I simply just don't know where to start with this project. not sure what sized turbos are best, or where they would sit. I do know that it would be 6 turbos. 3 turbos on each side of the engine (custom V6 engine that can handle a bomb explosion). any input about any part of this process would be great.

• Within any SE, you'll note that one should be asking a single question, not opening a discussion or pursuing a development. Consider to create a single focus and proceed in segments or locate a forum in which such discussions are permitted. engineering.stackexchange.com/help/how-to-ask Commented Apr 24 at 13:39
• My input is that number of turbos is not an appropriate design consideration, determine how much air you are planning on pumping and then determine the turbo arrangement that will do that. Commented Apr 24 at 13:46
• Superchargers are also good. Commented May 1 at 16:00

Imagine an infinite number of lossless, perfect turbos and assume that every single smidgeon of work available in the exhaust is recovered by them, and added to the power output of the engine. This is equal to the thermodynamic power content of the free-flowing exhaust stream which can be calculated knowing its temperature, source pressure and mass flow rate, giving you a theoretical upper bound on the solution.

Now note that turbos are not lossless and there will come a point where adding more turbos in series will choke the flow rate through all of them, causing the upstream turbos to produce progressively less power for each added downstream turbo.

Adding turbos in parallel will eliminate choking but at the same time will reduce the mass flow rate through all the other turbos, causing each of them to spin slower, produce less power, and suffer significant efficiency losses (see below) so this trick doesn't help either.

This effect can be eliminated by increasing the size of the single turbo but then you get a new loss mechanism: the slower a turbo spins, the less efficient it becomes. So a big, slow-turning turbo will recover less power than a small, fast-spinning turbo can.

Of course, turbo manufacturers know all this and so for any given exhaust temperature and mass flow rate there is an optimum size for the turbo that you can solve for while designing it. This is why nobody cascades turbos in series or doubles them up in parallel.

The only exception to this dismal picture is when a manufacturer designs a turbo for, example, a 360 cubic inch, 4-cylinder aircraft engine. Then they use one of these turbos for each half of a 720 cubic-inch, 8-cylinder engine because this solution is cheaper than designing a single, larger turbo to handle all the output of the 8-cylinder engine.

• certainly manufacturers will use a turbo for each bank of cylinders in a v Commented Apr 24 at 22:27