Main blade count is directly related to the efficiency of a compressor design in relation to wheel speed, more blades are ideal for lower wheels speeds and less blades are ideal for higher wheel speeds.
How does this translate across different diameter wheels?
So in basic theory, the bigger the inducer, the more blades you need? Also the higher the desired rpm range, the lower the blade count? Ex- a towing setup would favor an 8 blade, where a sled puller would go towards a 5? Might add this is pretty well over my head, just seeing if I'm tracking correctly.
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If you look at the supersonic threshold from two different compressor sizes with the same compressor trim, for example 63mm inducer vs an 88mm inducer there is a 35k RPM differential in shaft speed. So this will have a very large impact on the compressor design to be efficient inside the desired operating range.
Negating effects of blade design, what happens when we go to a 7+7? or a 6+6? My assumption is the efficiency peak is overall slightly lower(say 0.76) with lower blade count than optimal(8+8, i realize there is likely diminishing returns on higher blade count) and i would guess the peak is going to shift slightly to the right(more mass flow) and probably a little downward(less PR). Is that kind of on the right track?
For example you have to take surface speed at the bearings into account as well. While an S400 for example may be more robust, it can take less overall RPM than an S300 generally. The shaft is much larger and the bearings are much larger so at the same RPM the S400 has a much higher surface speed at the bearing. If the compressor you want to use in an S400 is really only efficient in a range where the S400 is going to overspeed you obviously do not want to use that compressor as it will result in repeated overspeed issues. It may make power on the dyno, or give you the characteristics you want, but at the cost of lifespan.
This is a good point, and I'm glad you brought it up because it is specific to the common failures seen in diesel applications with the S400 SX-E compressor designs. Borg Warner pulled a bit of a fast one that I don't think most people pick up on, if we compare the 76mm SX3 and 76mm SX-E compressor maps, you can see how the SX3 compressor map shows a peak pressure ratio of just under 5:1 whereas the SX-E compressor map shows a peak pressure ratio of just over 5:1. The thing to note is the peak wheel speed on the SX3 is 561 m/s yet the peak wheel speed on the SX-E is 605 m/s, which is beyond the industry standard for supersonic threshold.
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