Blower Overdrive Limits

I Have Read On This Site About Psi Blowers That Guys Are Spinning 95 To A 100 Percent Overdrive And More I'm Sure There Are Rule Limits. At What Point Does The Psi Blower Become Inefficient, Does A Screw Blower Cavatate.what About A Roots Blower How Much Overdrive Before They Become Inefficient Lets Say A Hi Helix Retro That's What Most Who Run A Roots Are Using. IF NHRA let the TAD Class run more overdrive When would the blower stop being inefficient. Other That Slow It Down Just Curious Thanks Jamie

 

92% overdrive is the maximum NHRA allows in TA/FC and yes, they do check. Auzzies can run 125% over.

Back in the good 'ol days before the brilliant minds met, we were allowed 125% over. That all came to an end in 2001.

 

Thanks Randy For The Repy,i'm Sure People Have Read This Post And Asked How Could I Ask This Question That It Depends On The Intake And Cylinder Heads Etc. I Not Thinking About Whats Below The Blower Its The Supercharger Itself I'm Thinking About. How Much Air Can It Actually Move Before It Stops Moving The Air And Just Spins In It (cavatates) Just Wondering If Anyone Knows. My Thing Is If Nhra Lets The Blower Cars (tad) Run More Overdrive When Does The Blower Itself Become Inefficient?

 

My uneducated observation is as follows.

On the back of a PSI is a label with a formula for CI X OD% / # of Cylinders or something to that effect with a maximum Blower RPM rating. That is for maximum overdrive for each combination. For example lets say my formula will allow me to spin the blower 130% and be under the safe RPM zone but yours might be higher or lower depending on shift points and cubic inch. With that being said I dont think there is a place where a PSI becomes inefficient or there would not be a formula telling how fast the rotors can turn safely.

 

From what I have been told the roots blower quits at 72% over. Now how does it quit? Cavitation? And 72% of what? Doesn't engine RPM figure in here somewhere? Dave Germain

 

Hey Dave These Are Things That I Also Want To Know, Not That It Really Matters To What I'm Doing But Because I Want To Know. I Have Had People Tell Me I Need To Pay My Dues And That I Ask The Wrong Questions That They Have Had To Pay Lots Of Money To Learn. I Just Want To Know Because I Can I Dont Think I'm Asking For Secrets Or The Ol Story If I Told You Then I Would Have To Kill You Deal Just Curious See Ya Jamie

 

We don't run our roots over 58% O.D., have been told by many that for most combinations it starts to hurt more than help at anything over 60%. Just heats the air up too much.

 

At some point, I think that the amount of air that could flow thru the injector would come in to play. I think that a low pressure area would take place at the top of the blower. There is no movement in a vacum. (Pascal) Like a jet motor does when it goes it to a stall

Thanks
Bob

 

rotor speed

not sure on the psi where it loses efficency but on the roots something to think about is engine rpm vs. blower overdrive. i have always heard the same as dave and jamie but i think it primarily has to do with engine rpm,engine cubic inch and static compression. tractor pullers commonly run the most overdrive on roots blowers 60 to 72 over but they usually also don't twist there motors over 7800 rpm to often and have some where between 11 to 12 to 1 compression. that should be approx 13416 blower rpm. no take a pro mod 20 over 526 and say 13 to 14 to 1 compression and 9800 to 10,200 thru the lights 12240 blower rpm. i think the primary thing to watch with a roots blower is manifold temp. hot air is usually not good air. if you have a blower at 55 over making 140 degree air on your motor and you slow it down say to 48 over and it cools the intake to 105 and drop the boost a few pounds i probaly would look at richening the fuel system and running the blower slower for the cooler intake charge. roots blowers are basically a air compressor next time your shop compressor is running go over and touch the compressor usually pretty warm.