What happens 75% over on a roots blower?

Anyone ever try and run a 14-71 roots at 75% over or more? Just curious, have seen them at 65%, but never any higher than that.

 

I have seen them in the high 60 -70 over anything more was illegal do , so the answer is no to 75% . Why would you is a better question , the rotor speed becomes excessive, the heat , detonation and fuel delivery are erratic and the power will start to decline rapidly . There are very efficient roots blowers that will make big power @ lower overdrives now , but they have to be engineered to suit the weight and application of the car/motor/trans to allow for the biggest gains in power and reliability . .

 

you can only pull so much air through an opening of a certain size is one thing, another is the amount of heat created by spinning the blower that fast would cause a ton of detonation issues. there is NO need to spin a blower more than 60%. I used to run 5.80's at 57.7% over and with todays blowers you shouldn't need more than 50% on an unmodified blower.

 

We used to run 68% over and were succesfull doing so. The rules than slowed us down.
Generally speaking 12-13,000 seems to be the max rotor speed for all the reasons already mentioned.
12,500 blower RPM / 1.75 overdrive = 7140 Engine RPM

So there might be a time and place for really high overdrive. Maybe your rules limit rearend gear ratios and the motor only gets to 7000 or thats just what you choose to do. Maybe your engine combination doesn't allow it to spin faster, that's what I had 8-10 yrs ago.

 

Rotor Speed ??

I have Customers that Have in excess of 15,400 rotor Speed and they make big power in their application , I also have Promod Configs (legal ) that do not exceed 12,500 Rotor speed with a much different hp to torque than the high O.D application and these cars are very powerful in a different way . The Blower must be right for the application after modifications otherwise you will have to change the car configuration and setup to satisfy the blower/fuel system you have .

 

Norm Drazy told me that anything over 72% OD causes more problems than it fixes. Something about the air going supersonic and heat generated were the last things I remembered before I fell into a coma from too much information thrown at me too fast for my miniscule human brain. Dave Germain

 

Ol' Norm

Dave; A lot of us know exactly what you mean...........
It was easy to imagine a "beanie" on him, and when he really got to explaining things, the propeller on the beanie would go supersonic

 

Is Whipple Superchargers making a TAFC legal screw blower or is it just PSI? I have left Dustin Whipple a bunch of messages and emails about it and have not received an answer.

 

Maybe some of todays blowers are better higher than 57 over but my experience is that anything over that wasn't worth it. 63 or 65 over made no improvement.
Dale

 

Les who!!!

75 over! That would be good at stage! Supersonic shaft speed at .25

 

high overdrive

we used to have a low compression engine. (9.9 cr)
due to time & money limits we started at 57 over. also we did not want to turn it past 8000 rpm. it ran good. (12,500 rotor)
went to 63 od and it went quicker and faster. (13,000 rotor)
finally tried 69 over and it was slightly quicker. (13,500 rotor)
i suspect that the point of deminishing returns was near. rotor rpm is the key. if you can afford a "low od" blower, you don't need as much rotor speed. pullys are cheaper than anything !
good hunting chuck