Turbo Pathologies
Garrett engineer Jason Kavanagh points out that a lot of turbo application "no-nos" can damage bearings: "Since ball-bearing turbos (well, Garrett's anyway) have many times the load capacity of conventional bearing systems, they can cover a lot of these sins. Coupled with their superior transient boost response, this increased robustness has made Garrett BB turbos very popular in the aftermarket." Bottom line, sizing issue can definitely affect turbo longevity.
1) Oversized compressor wheel for an engine. The most notable symptom will be audible compressor surge when boost is coming up. Surge can damage bearings. Compressor surge in this manner is often accompanied by a lumpy power delivery. Boost response will also suffer relative to a suitably sized compressor.
2) Undersized turbine housing (too small A/R) will give more immediate boost response, to the point of jumpy boost response at throttle tip-in. It will also backpressure the engine a lot, which will make the power go flat early in the rev range, make the engine more detonation-sensitive, and in extremis more likely to overheat.
3) Oversized compressor wheel and undersized A/R will tend to exacerbate the compressor surge by making it come up on boost quicker. Surge can damage bearings. On the plus side the boost will come up quicker...
4) Undersized turbine wheel and large turbine A/R "Hard to say in general terms; depends on other factors (choice of engine, compressor)"
5) Undersized compressor wheel Good boost response but power will go flat earlier in the rev range because comp efficiency drops like a brick as you approach choke (which is what happens when a compressor is undersized for an engine), and because backpressure will be climbing up (the turbine has to do this to keep up with the plummeting compressor efficiency). When wound to redline you also run the risk of overspeeding the turbo and putting excessive loads on the thrust bearing. Also the compressor oil seal will tend to leak.
6) Hybrid turbos (say T3/T04) can work well as long as the wheel diameter mismatch isn't too extreme, and if the wheel aero is good. Some hybrids perform quite well. Some hybrids suck, though.
7) Compressor surge/stall With oversized compressors, surge can occur when boost is coming up, or when the throttle is closed when boost is up. The former case is harder on the thrust bearing, but neither one is really desirable. Surge can happen even if the compressor is sized correctly for the engine--surge can occur if the compressor inlet is too restrictive, and/or if there is a lot of pressure drop in the intercooler plumbing. Also see #1 above
By Corky Bell
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