4, 8, or even 12 turbochargers?

[nist7]

As the title implies, is it feasible to use one small, efficient turbo for each cylinder in the engine instead of one or two big ones?

Or how about have two turbos in series for each cylinder? 24 turbos? :surprised :confused :cool:

[Jonathan]

IMO it's better to have one or two large ones. With putting a turbo on each cylinder, you are relying on multiple turbos to produce the same output as the others, which may or may not happen. You also aren't evening out fuel mixture to all cylinders like a single turbo would. You wouldn't drive a turbo as well with only one cylinder either, remember a gasoline engine doesn't produce a steady flow of gasoline, it has multiple pulses of exhaust flow and it would affect the RPM of the turbo impeller to an extent, opposed to having 3 or 4 cylinders on a bank basically simulating a constant flow. The beauty of a single turbo on a cylinder bank is that it assures that the distribution of air to each cylinder evens out and is near equal to each other. Plus, the turbo basically chops up the fuel/air mixture and reduces parasitic heat and friction. They improve the combustion efficiency of the engine because the manifold pressure "sweeps" the old air out through the exhaust valves and makes for much better mixture in the cylinders. If done right a turbo can improve fuel mileage, assuming you can keep your foot out of it.

[GlassWolf]

Pretty much what Jon said.
Firstly, most turbochargers need a certain amount of exhaust velocity to spin the turbine that drives teh impeller shaft to compress air and force it back into the engine. In order to get enough compression to make good boost, you need the turbo snail to be of a certain size. You couldn't really reach that sort of output with a turbo on each cylinder. Also, you'd run into a similar problem to what we see when over-carbureting a motor using small single barrell carbs per cylinder. a motor set up with small dedicated carbs has to use one carb per cylinder and one carb or barrell per runner with separate intake manifolds. This proves to be far less efficient than using a shared plenum intake manifold with say, a single 4bbl or dual quads forcing large amounts of air and fuel into a "community" chamber then allowing each runner to suck in as much air/fuel as it needs per cylinder or set fo cylinders (depends on manifold design)
Weber downdrafts encountered this problem on some earlier English engines that had dedicated runners and carbs, and actually lost power because of it. The same sort of thing would happen with separate turbos.
One reason twin turbos are used is to put one turbo on each half of the engine, and thus use smaller turbos so they'll spool faster without the need to provide as much boost per turbo, but there are pros and cons to both large and small turbos that I won't delve into just now as it's not directly relevant.

Simply put, a turbocharger works by scavenging the expended heat and velocity of exhaust gases. The more exhaust the turbo can use, the more boost you're going to be able to get from it, so if that turbo has to divide up the finite amount of expelled gases, it's not going to be able to produce as much resultant output.

It's like having a limited amount of food, and being given the choice of feeding one strong worker, or a bunch of weaker workers. each of the little workers will use a larger percentage of the food because a person, regardless of size needs to eat a certain amount of food to sustain life and health, so you'll get less work and more resources used to do so. Efficiency suffers.

[GlassWolf]

if you want an example, look at motors like the Gail Banks 350 Chevy small block twin turbo he sells that produces 1200HP and nearly as much torque, on regular pump gas for street cars, or any of the turbocharged top fuel funny cars and NHRA supercomp cars. They usually run 3 stage N2o and twin turbos that produce about 49psi of boost each to attain power ranges of ~3000HP

They also go through 5 to 8 gallons of gas each run though, in a quarter mile.