Turbo Technology 102...
Going to highlight a few topics that are most relevant to GT-R guys.
Back-pressure is a measure of restriction in a turbo system and efficiency. Exhaust has a maximum air speed. That speed is very close to the speed of sound (its actually slightly above it) What happens is as boost and airflow goes up, the exhaust speed goes up. The smaller the turbo system, the higher the speed will be with the same volume of air. Next what happens is that air naturally wants to go a certain speed (the speed that it exited the engine). Now its being forced into a much smaller opening, the air speed goes up, and a restriction begins. Now that high air speed is great for turbo feel and spool. But it is not good for back pressure related restriction. Once the exhaust cant go any faster, pressure builds to force it out (and it also forces it BACK IN a motor through reversion, and contaminates the incoming charge of air)
In general the Backpressure measurement is generally expressed as a pressure ratio between manifold pressure (AKA boost pressure in the engine) and Turbo pressure in the turbine housing. In a Stock Turbo GT-R, this is actually fairly high, at relatively low boost pressures. At 15psi of boost in a stock turbo GT-R (at lets say 7000rpms) the back pressure is above 35psi. It is in the 2.4-2.5 to 1 range. In general the higher the rpm (or airflow of the motor) the higher the back pressure will be.
So if you want to optimize an RPM range, turbo housing sizes play a big role, and effect back pressure directly. If you want a drag racing motor, going to a GT35 with a 1.06 AR might make more sense (Ratio of inlet and turbine size, the higher the AR Ratio, the more flow the housing has in a given size)
With a stock manifold GT-R you are stuck with the size of the inlet and turbo nozzle. So when you go bigger the ratio changes actually smaller. Since the manifold did not change and the turbine got bigger. A typical AR Ratio for a stock frame turbo is around .40 AR (or very small), This is great for spool, but not great for pressure build up and subsequent heat build up.
NOT all AR ratios are the same. Since its a ratio, if both numbers change, you can maintain a Ratio, but have a different size and flow. Lets take for example a VBand housing for a HTA3794 (67 Inducer Compressor, 65mm Exducer turbine) And compare that to a PTE 6766 T4 housing.
Lets say both have the Same AR Ratio, but since the PTE T4 housing is larger in both measurements, it has much higher flow and much lower back pressure, and will make a good amount more power. (And require cutting the frame to fit them LOL)
The other BIG topic with back pressure is Camshafts. If you change how an engine pumps air, how long the valves are open, how long they are overlapped, etc you can effect back pressure directly. Also RPM range and boost effect back pressure a lot too.
A lot of guys think I am a miracle worker when I retune a car with lower boost, lower timing, richer AFR, and make more power and or go faster at the track. It is mostly the understanding of back pressure that is the key here, and its effects on how an engine does work.
I am looking for the Goldilocks number, of maximum boost with minimum back pressure.
Ok well thats back pressure.
When guys refer to "Billet wheels" in a turbo they are referring to the compressor wheels. Its "Technically" a Machined Forged Billet compressor wheel.
By going billet, it allows to change the blade design, blade pitch, number of blades, wheel height, hub thickness, Curve and wheel dimensions. These are all great things and allows a small batch change (its cost effective to make as little as 10 forged billet wheels at a time)
For example, On my billet wheels, I do custom dimensions and wheel height, and custom hub thickness. You can notice in this photo, the bottom is thinner, the wheel is taller and thinner, the extended tips are wider.
This is something a guy like me can do. The wheel on the left is made by garrett a HUGE company, and because of modern technology a little guy like me can make something that can compete with them. (all be it on a MUCH smaller scale)
That same type of technology, small production, optimized for a certain size, engine, or even camshaft can be applied to the turbine wheel as well. As far as I know I am the 1st GT-R "Tuner" to put a "Billet" Forged Inconel material turbine in a GT-R.
This allows me to optimize blade height, blade dimensions, hub dimensions, Number of blades ect. So I can make the perfect turbine for the application. This is an even lower volume type of operation because of the machine time involved.
Any turbine wheel from a large manufacture is going to be cast. The costs involved in doing a machined turbine are relatively high, and the break even point on doing a casting is as little as 25pc. So this is a very low production method of manufacturing, but it does allow for designs that cannot be cast.