Big turbo will have bigger ports, larger turbines and compressors and so on, so at high power levels will flow more generally. But the conundrum is that we measure the boost at the manifold. So surely 1bar in the manifold is 1 bar?
Yes an no.
If you make high boost on a small turbo, you revs the nuts off it. The problem with that is, the exhuast turbine will still act as an impellor (centrifugal compressor) if you spin it enough. So there comes a point where you're spinning the turbo fast enough to make say 1.5bar boost. Except the exhaust side is spinning so fast, that the has trying to get out the middle is being forced out, and back into the turbine housing. This will create a big increase in exhaust back pressure.
Symplistically, the exhaust back pressure will rob power. Firstly it will leave burnt ex gases in the cylinder so you get contamination of the inlet charge. Secondly, because there's gas in the cylinder when the inlet charge is entering, there is already a higher pressure in the cylinder to overcome. So although you're reading high boost at the manifold, you're net boost, relative to the cylinder pressure is lower.
But what about density? The harder a turbo has to work to compress the air, the more heat it will put into it. There will always be a rise in temperature due to compression, and while you might get it back post intercooler, it's still extra work. So if you have a smaller compressor side, like on a small turbo, and you do eventually make the boost, you are going to have to drive the compressor harder to do it, this means more work, and nearly all the work done on turning the shaft goes into heating the air, which will artificially raise its pressure. You may get some of that temperature back after the I/C, but unless it's very efficient, you're more than likely going to have a reduction in air density, and therefore mass flow, when compared to a physically larger turbo.
Paul
