Its important to note more boost doesnt always mean more power, even with a blower. If the AE tanks then its just going to be making more hot air. Density Ratio is the important number, not Pressure Ratio, and it depends heavily on how [adiabatically] the blower is.

 

Curios why the TVS over the Whipple? Also since volume matters apparently, would a bigger blower at a lower psi be preferred or does it just take too much to drive? The C7 ZO6 has a smaller blower than the ZR1. Why the switch, is a bigger blower lazy at low rpm? I am really enjoying all the information and flow theory here...definitely great info! Gettin' edumacated.

 

A bigger blower at a lower psi would move more air so probably make more power. However, drive power is a function of both volume and pressure, but more strongly with pressure. This is why its important to have good cylinder heads with a blower. If you can max out the blower and only make 2-3 pounds you are moving the max airflow of the blower but the drive power is very low because the pressure is low.

A centrifugal blower (or turbo) is a bit different, it wants to be at a higher pressure to achieve a better efficiency.

Another thing is as boost goes up, the VE of the ENGINE goes up because the air is more...motivated to fill the cylinders. So more boost is better, in that case.

 

Although I agree with the majority of the information regarding pressure, volume, effeciency, etc as it pertains to roots blowers, I feel a piece of the puzzle is missing as it all pertains to twin screw compressors. That is the internal compression ratio thru the rotors. This is a crucial contributing factor as it relates to efficeincy and drive power of the compressor. With a higher icr, the compressor will be more efficient at manifold pressures above that icr than an equally sized compressor with a lower icr. The inverse is also true, that the lower icr compressor will be more efficient at lower manifold pressure than the higher icr equivalent.

Since Atomic likes math so much, here is a snap shot from my excel spreadsheet as it pertains to my current setup. First set of calculations use only engine size, blower size and pulley ratio. Second set uses airflow calcualtions and third adds Blower VE to the second method. Engine VE is estimated to make the final boost calculation match with what I actually see. Blower VE is a close approximation from dyno tests run on the 2.8H by Kenne Bell. However the calculations are still slightly skewed by the fact that intercooler pressure drop is unknown. The theoretical boost numbers would be at the compressor discharge.

Crank pulley dia (in) 8.25
Blower pulley dia (in) 3.5
Pulley Ratio 2.357142857
Engine Displacement (L) 6.1
Blower Displacement (L) 2.8

Theoritical Boost (psi) 17.10983607

-----------------------------------------

Engine RPM 6500
Blower RPM 15321.42857
*KB rated at 18,000

Blower CFM 1515
Power (CHP) 947
Power (WHP@30%loss) 663

Engine Displacement (ci) 371.6
Engine V.E. (%) 0.93
Engine Demand (cfm) 649.9774306

Theoritical Boost (psi) 19.56450377

Theoritical Used CFM 1515.044498
Engine Output (HP) 946.9028111

--------------------------------------------

Blower V.E. (%) 0.87
Blower Actual CFM 1318
Theoritical Boost (psi) 15.11011828

Theoritical Used CFM 1318.088713
Engine Output (HP) 823.8054457
Power (WHP@20%loss) 659

 

I see your theoretical boost number, do you have a real observed boost number to go along with it? I made a much better spreadsheet than the one I was using earlier to guess the boost, and I think it reflects reality a little better since it takes temperature rise and intercooler efficiency into account. I emailed it to a couple blower guys to try and ground it in reality.

Its all to be taken with a grain of salt because everything changes with RPM, and with TIME. So the best we can really do is design around a specific point in time and RPM.

 

It was fairly accurate for me. The real question is the efficiency of the engine. It can change fairly drastically from one set up to the next.

 

The third calculation matches my actual manifold pressure of 15psi. Playing around with it a little more, assuming a 2psi pressure drop thru the ic, an engine VE of .87 seems a little more realistic.

You are right, a lot of this is guess work. You can have all the right formulas and math but the results are only as good as the data you plug in. A lot of times it is just easier to build it and get the real world results but it is fun for me to try and figure out the science behind it.

 

Since it appears we both like math, let me know what you think of the spreadsheet I made. Some of it is a wild *** guess, but other parts are at least based on real compressor maps and real physics.

Oh and your comment about the internal compression ratio of twin screws vs roots. I believe this fact is absorbed in the better adiabatic efficiency of twin screws vs roots blowers. Notice how TVS blowers look an awful lot like a twin screw? That negated a lot of the classic differences. Now the biggest difference is really which direction the rotors spin.

 

Contrary to having enough college credits to have a minor in math, I don't particularly enjoy it.
As to your spreadsheet, it is interesting but seems compartmentalized. For instance, changing the ambient temp, I would expect that to effect data other than just discharge temp. As for temp, are the numbers in that column post intercooler and are based on the ic efficiency data? Also the VE data seems to stem from the third order polynomials on the second tab. I presume that is the portion derived from the compressor maps and real physics you mentioned? From my experience, I feel the boost psi increasing linearly with rpm is a little skewed but can see how the calculations produce those numbers based on the ve and ae data presented. I feel the blower ve is plausible for a twin screw with a higher icr along with the engine ve curve considering a certain cam profile.

 

I never get a lot of feedback regarding 1/4 mile times. I did have a guy with a Denali send me an engine bay picture and he had a built motor with our blower and a wet kit on top of that. I'm sure there are some fast ones running around out there.