CFM vs boost

So this is a help me understand thread. I’ve ran a blower for a couple years now and really enjoy learning more and understanding how my equipment works.

Trying to wrap my mind around blower CFM first. The way I understand is CFM should be dictated by rotor length (cubic inch/ Revolution = rotor length 31.53 for HH). So for a 14-71 looking at about 520 c.i. Or .300 cubic feet per revolution.

Talking with another racer friend and he said he has an older 14-71 high helix (delta bottom opening) and said it can’t keep up even close with newer models, only about 4300 cfm. Doing my math on his car at 27% over and 8000 rpm He’s only using/getting 3048 cfm on his 14-71. So assuming he doesnt intend to go more than 33% over doesn’t want to turn his motor any faster is a “modern” blower going to do him any Good?

It makes sense I guess for the TA racers that turn 13k rpm plus and getting really high boost but is there any benefit to the T/D racer turning 8k rpm and looking at 24-28 pounds of boost.

Since most of the 14-71 blowers have the same length, delta bottom, similar top opening sizes, 120* rotor twist , and inner/outer strips what separates the premium blowers from the others (boost, cfm,?). Initially I thought that maybe it would get its boost earlier but once I started running my blower I found that my boost stayed very flat through my whole run from 3800 rpm - 7700 rpm. Now I’m currently only running 16-18 lbs of boost, does the higher boost range not the same between high end and regular HH blowers?

Thanks for the formation, really enjoy coming here and expanding my knowledge of these things!

 

I'm no expert, but I have seen first hand what some minor blower mods did for CFM and boost on the dyno with a blower I was running. Increase from 4990 to 5577, and from 39.6 lbs to 44.6 lbs of boost (both taken at 12000 blower speed). It's all about how the air flows into the blower, coupled with the rotor design, and of course back out of the bottom. Very important for the path of incoming air from the hat opening to the top of the rotors to be smooth so no turbulence is created (everything has to match).

Monitoring intake manifold temp is a must if you want to know how fast you can spin any given blower. Large top opening (i.e. 11.5") can be spun a lot faster than a smaller opening (i.e. 10.5"). My experience with running blowers with the larger top openings, the boost curve was always fairly flat, but I never spun them over 35% either. I've seen really nice looking boost curves from blowers with smaller top openings, that really like the 26-28% OD range. But that's the difference between a $5k blower and a $10k blower.

If he doesn't want to spin the motor faster, spin the blower faster. As long as you stay below 130-140* you're still making power.

But, like everything else, there are a lot of variables involved. I don't think there is a simple answer for all of these questions.

 

Blower performance comes from a few things, How much hp does it take to turn it at a given rpm, outlet temps, and how much air it moves and when.

An example is a PSI 2" Roots liked 42-43 percent od and made 50 lbs of boost but took 480 hp to turn, and the outlet temp was lets say (Example) 140 degrees.

Now move over to an SSI F-3 blower that liked 38 percent od and made 49 lbs of boost but only took 280hp to turn and the outlet temp dropped to (Example) 120 degrees. The boost number is deceiving because it is how much air is in the intake not going into the motor and that number changes with the outlet temps. So the SSI in this case saved 200hp, was driven slower allowing the motor to rev higher before the rotor tips go supersonic and lose efficiency (12,300 or 12,700 rpm) and had cooler outlet temps therefor making it a better blower.

Top of the line blowers are designed to be more efficient and run at lower overdrives such as 14.5-29 percent and still make good boost, some of this is accomplished by shrinking the inlet opening and adjusting the length of it with inserts/Shoes, ceramic bearings for reduced friction, coatings, more efficient inlets, and tighter clearances. Billet cases help to keep the strips in them longer and do not add much boost as per Critchley. At 29 percent my cast case F-11 blower will make about 42 lbs of boost, take approx 220hp to turn, and the outlet temps will be cooler yet.

Depending on the design, blowers can be more or less efficient during different parts of the run but make the same peak boost. Lets Say both Blower A and Blower B make 40 lbs of peak boost at the same od and have the same outlet temps, However blower A makes more boost at the hit and through the mid ranges while peaking the same as Blower B. Obviously blower A would perform better assuming you can get it to the ground.

The ticket to efficiency now is to slow the over drive down and bring the compression ratio up. Depending on the given blower type, car setup, and inlet size will give you a sweet spot. My F-11 set the record at 38 od in a dragster, we will be slowing it down to 25-29 for the pro mod as per Critchley, I am told this has to do with car weight.

As for TD depending on your engine and setup you can run 6.0's with any modern Setback rotor blower and even some of the older units like the SSI D rotor, Kobelco non superman, Littlefield units with the Kobelco rotors (Can't speak for BDS's units). A good used 6.0 blower should be able to be picked up for $3,000-$4,000.

a 540 cid conventional headed BBC went 6.20s with an old stock Littlefield cased K-8 non setback rotor blower in a heavier dragster.

A 464 cid fathead 5 hemi went 5.90's with (basically the same blower) a Kobelco K-8 non setback rotor blower with inlet and outlet mods in a light car.

Hope this helps,
Trev

 

Thanks for the reply’s, that’s exactly the stuff I’m looking for!

 

this is a late reply , I have a question , I am Running DMPE M-4 blower @ 32 % , with the large 11.7/8 opening , would I benefit by going to the smaller 10. 7/8 opening ? and would my boost come in faster ?