5, 6 or 7 blade compressors

[RD TRCTR]

What's the opinion of the experienced on CompD?

Looking for a turbo for a full body 06 QCSB with a well built 5.9. Truck is a street truck that will be raced. Truck will not be used to tow. I am looking at either a 476/96 1.15 T6 or a 480/96 1.15 T6.

[Smokem]

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.

[mhuggler]

I've always loved how the 6-blade sounds.....Does that help?

[wwentzler]

Originally Posted by Smokem

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? I've never liked the low blade count stuff. I also notice where Holset transitions from 6 to 7 to 8 (main)blade wheels on their factory comp wheels.

[Smokem]

Originally Posted by wwentzler

How does this translate across different diameter wheels?

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.

[J-Pipes]

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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[RD TRCTR]

Originally Posted by J-Pipes

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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From reading what he has shared, your understanding is correct.

[wwentzler]

Originally Posted by Smokem

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.

Right that makes sense. How much does efficiency shift with with blade count(like on a comp map)? Lets take a s591sx for example (because numbers make more sense in my head) which is a 90.7/120mm 8+8 compressor. Peak isentropic efficiency is 0.78 with the tip in the 2.5pr/85lb min area. 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?

[zfaylor]

Not all compressors will work well in all turbos. 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. Then you have to take turbine design (housing and wheel both) into account as you have to make sure it is efficient in that range as well.

There is more to take into account than just compressor efficiency as there are other parts in the turbocharger that must be happy as well. I know smokem knows this. I am posting this so somebody doesnt read this and go on ebay and buy a random x+x blade count compressor expecting everything to be wonderful.

[Smokem]

Originally Posted by wwentzler

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?

If your question is referring to removing blade pairs with no other design alterations, then your assumption would be on track. But it would seem obvious that doing such a thing and expecting an increase in performance over the same operating range would be quite naïve.

For example let's compare the cast 7-blade S400 SX3 and the FMW 10-blade S400 SX-E compressor wheels. The difference here is not as simple as changing the overall blade count. The tip height, extended tip angle, exducer/overall diameter, and blade height are altered from one variation to the other. And these design differences were geared toward increasing mass air flow albeit with less pressure ratio capability.

[Smokem]

Originally Posted by zfaylor

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.

Comparison.jpg

[wwentzler]

Originally Posted by Smokem

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.

Attachment 74388

Interesting. Had noticed that the 72sxe maps plotted out to a lower efficiency all the way right than the 72sx but maybe that was just a byproduct of the higher wheel speed plotted. Dumb question, but what do you mean by supersonic threshold as in how is it used?