psi screw at 85% od vs a roots at unlimited od

How would a psi D or C at 85% od compare to an unlimited roots blower 14-71. Will the PSI make more steam if you spin it harder? or would they be close? appreciate any insight.

 

Is there an upper threshold RPM for a C screw either due to performance issues or mechanical issues?

 

There is a point where the screw blower can spin too fast for it's own good in compression, usually when the rotor's speed relative to the air it's compressing is supersonic. My first encounter with an outlaw PM with a C blower, I wondered why he didn't have a top pulley the size of the spines on the quill shaft, instead of the 112% OD he was running.. Same supersonic limiting factors as a roots blower, just much higher rotor speeds before it loses efficiency.

 

blower

The reason I asked the question was the class we had planned to run decided to let screw cars run with the roots. I am just trying to figure out which way to go.

c/d screw blower 85% od
roots hi helix non billet any od
billet blower 14 or 16 20% od
18-71 17 od%

 

If the screw rules are equal, go with a "C" for sure.

 

blower

The C,D are really hard to tell apart so there will never be a rule giving the D a break.
We looked at some of that TX racing and thought about heading south until we saw the payout.
The way I see it the roots blowers only exist to maintain parity with obsolete power adders. I understand the sport is in a tailspin and everybody wants car counts but it's frustrating that the most economical power adder is penalized because a few guys can dominate with one.
Obviously, the screw at 85% will makeup for slow overdrive if you spin the crankshaft faster. Then it turns into an expensive game. If somebody like Frankie or Gaylen shows up the OD will be 70% at the next race.
Just the same,if the roots dominates it will lose OD. And you have all the maintanence to keep it fresh.
I hope the outlaw roots deal is a fad that blows over. Everybody wants to make more power for less money and work. $15k handbuilt blowers are not the way to keep lower level promod racing alive.
sorry to hijack the thread

 

blower

All blower cars to weigh 2650 period and automatics only. Bad part sold all my screw blower stuff so I could change to a roots combo then bam they let the screws in. If a roots could weigh 2500 that would help alot. It will probably turn inot 2-3 cars dominating and everyone crying. So now I at a crossroads spent a shitload of money and screwed lol.

 

blower

Good question Mike

 

Mike

I was reading your post regarding od limits on hh and standard helix roots.
As far as i know fuel cars run standard helix blowers yet they spin faster than 9000 rpm how come?
Also on my own car the old engine was a bbc with a standard helix we started out at 11% od and ended up at 40% od so it must have been over 9000 rpm too, yet it made lots more power with the high od, i have since replaced the engine tho as it put a rod thru the block.

 

The rules should make decent parity to start. The outlaw roots and low OD screws are a little bit unknown on performance. I know Gaylen runs lower OD for the TOPMA stuff and it really knocks the wind out of his car.
The modern roots are designed for lower OD and they can put up big numbers at 2650lbs.
If you want to go race somewhere else with a screw you can pickup .2 with a pulley change and have fun. Especially if you've been running good at 85%.

 

The "critical RPM" is merely the point at which a blower starts losing it's peak efficiency. They still move air, it just costs more in the way of rotation,i.e. displacement per rev.

 

Blower cars with a 2 speed glide can weigh 2550. Should be a pretty close match with the limits on OD and weight. Hope to see you guys at some races.

Randy