just a little something posted up by one of my local forums' members
There is always a discussion in regards to ball-bearing vs. non-ball bearing turbochargers and the performance and reliability aspects of one to another.
Question: Everyone is asking about ball-bearing turbo's and do they spool quicker than non-ball bearing turbo's. Should I pay the extra $425 which is the difference between a non-ball bearing RPS turbo kit and a ball-bearing unit?
ANSWER: All right! This question has been asked many times and I have yet to answer, so here goes. The main factor that determines when a turbo will start to spool hard is the A/R of the exhaust housing. On a side note, the weight of the exhaust wheel is a factor, i.e. a ceramic wheel. Unfortunately ceramic wheels are very, very expensive and are not readily available in the sizes we need.
Everyone needs to remember that a turbo is basically just like a kid's pin wheel that you can hold out a car window and make spin. If you increase the velocity of the car (i.e. the exhaust going into the turbo) the wheel will spin faster. If you put your hand behind the pinwheel, the wheel will slow down because air cannot flow though it. Let's talk about an engine running at 3,000 RPM, full throttle. The engine will produce a constant amount of exhaust gases as long as the RPM is held constant. So the only way to spin our "pinwheel" faster is to increase the velocity of the air. The only way to increase the velocity of the air is to use a smaller A/R ratio. The problem with too small an A/R ratio is that it will choke off air flow at higher RPM. This is why matching the turbocharger's size, CFM, exhaust back pressure vs. intake pressure, and A/R ratios is so important.
The ball bearing option was developed to improve the reliability of high output turbos. When high boost is used, there are great stresses on the centre housing bearings to keep everything in place. The ball bearing option is a great way to handle these added stresses and improve reliability.
As a side effect of using the ball bearing option, drag cars saw immediate improvements in et's. The reason for this is the ball bearing improved the rate of acceleration of the exhaust wheel, thus boost. Remember, the A/R ratio determines when the boost will start, a freeing spinning wheel (less friction with the ball bearing) will make the boost rise at a faster rate.
It's that simple. The A/R ratio determines when, the friction in the bearings affects how fast.
NOTE: If you plan on running your car hard with boost in the 23+ psi range we recommend for longevity purposes the ball-bearing option. Also, if you are a serious drag racer and want to squeeze out those better ET's that are associated with quicker spooling then this is a good idea. If you just want a fast street car that runs most of the time at 20psi or less on normal fuel, then the non-ball bearing option is a good one for you.