Screw vs. Roots Short Block

What is the interchangeability on a short block to go from a screw to roots setup? Or vice versa? Has anyone had experience using a short block built for a screw and ended up using a roots deal on it? Pros/cons of doing this (if any)?

Thanks

 

Basically the only difference would be compression and camshaft. Each blower wants these variables changed but it will still run either way, just not at the top of the pack.

 

Thanks. That's what I have been told. Just wondering what the fine line is between having plenty of "safe" room for tuning and the motor just being a complete slug.

What is the target compression ratio roots guys are running? This a 451 hemi, 14-71 HH retro. I can probably get 10.25:1 with a .040 gasket. Maybe a little lower than what the sought after CR is but could it be made up for with OD%? Curious about cam duration being feasible at 296 in, 300exh also.

Thanks for the input

 

The more cr you can get the better it will run. The roots makes heat with more overdrive and at some point when it goes supersonic the air is garbage. RPM times overdrive. When the blower speed gets close to 15000 you are out of room.

 

Hi, Cam and compression is correct, also the roots blower guys typically run a 1.156" Pin as the roots is more prone to detonation than the screw and I am sure the added dome weight is probably not helping. Compression will vary depending on what blower, overdrive, engine rpm, and clutch or converter. Between 12:1 and 13:1 is what I have seen/worked on, the magic number for where to run a roots blower is either 12,300 or 12,700 rpm rotor speed (Can't recall but "Magic" Mike Canter was the one who provided those numbers) after that the tips go supersonic and efficiency dies a slow and painful death...lol. If you are running a clutch or a converter will help to determine your overdrive and engine rpms, each blower brand/setup has a sweet spot too and some take work to get them there others you can run off the shelf and still go pretty quick. A go to roots cam was the Crane 296-500-16-48 or comparable. I knew some guys that ran a 114 degree variation of that. With less overdrive you can run more compression. In your case with having no higher than 10.25:1 I would put a higher overdrive blower such as a PSI high helix roots or even better would be an SSI F series rotor with the properly sized inlet and run it at 38-42 percent od if its a converter car. The clutch seemed to like 48 percent or higher od psi roots blowers back in the IHRA TAFC days. Dale Brand ran a 5.60 flat with that deal. Jody Stroud has ran 3.60s in the 1/8 mile with his dragster using an SSI f-3 that I believe was untouched, the SSI deals are excellent off the shelf blowers. If I can help feel free to call.

Trevor Sherwood
727-364-4368

 

Thanks Trevor. I sent you a PM.

Mike

 

Ran both blowers same short block combo only thing I did was flip flop port nozzles to work with blower discharge point and added a little fuel for screw.IMO screw would be less detonation prone due to cooler intake charge. than a roots.

 

What kind of compression ratio were you working with?

 

I ran 12.5:1 and also ran the 296-500 cams.Heck think I still have cpl of the cams and one is in a injected hemi right now.

We found out that you could run faster with mild compression(12 vs 13.5) and not hurt as many parts.

If wanting mor compression sometimes rod length can be used,provided there is enoutgh piston to head.

Not sure how good SSI is now.I know one of first we had made 22 runs and qualified number 2 behind Newberry at New engnland.Heck only reason I stripped it then was Rich about had heartattack about me making 22 runs with it. Was hands down best blower we bought,well other than Whipple.Now this was 93 or so and they should have only gotten better.