Roots Blower Sizing
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10-23-2014, 06:33 AM
#23
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Nice informative thread Joe. I'd like to throw a little input in. Some guys including me don't have an open check book and have to shop for deals. If your stuck with a smaller blower that's not really the proper size for the application there is a way to make it work. Instead of running very low compression and over spinning the blower which will heat the charge and kill power. Raise the initial compression of the motor slow down the blower and keep the charge cool. In essence the outcome is the same for the most part. What is really being accomplished with the blower is adding cylinder pressure which makes more power. With higher compression in the motor you don't need as much added air from the blower as you would with lower initial compression. Hope this makes sense. Will a high compression low boost make the same power as low compression high boost? Not on pump gas. But you are better off putting more compression in the motor if you have a small blower and not spinning the heck out of it... Hope that may help some that have an empty wallet like me.
10-23-2014, 07:11 AM
#24
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Blower Sizing
The attached document reinforces what MT has posted. There is no set in stone supercharger sizing method, on this sheet it gives a general guides/sizing to compression, boost and CID. In my unprofessional opinion 1:1 ratio is a good starting point.
Another note, there are a few things that can be done to the roots style chargers to improve on efficiency and output (which would make the charts somewhat irrelevant). Closed discharge, hi-helix, and stripping come to mind. Coolers and chillers can throw another curve ball.
Of course this is all bogus garbage because there is only one manufacture of chargers that counts, starts with a W and it aint wieand
Another note, there are a few things that can be done to the roots style chargers to improve on efficiency and output (which would make the charts somewhat irrelevant). Closed discharge, hi-helix, and stripping come to mind. Coolers and chillers can throw another curve ball.
Of course this is all bogus garbage because there is only one manufacture of chargers that counts, starts with a W and it aint wieand
10-23-2014, 08:10 AM
#25
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iTrader: (3)
The attached document reinforces what MT has posted. There is no set in stone supercharger sizing method, on this sheet it gives a general guides/sizing to compression, boost and CID. In my unprofessional opinion 1:1 ratio is a good starting point.
Another note, there are a few things that can be done to the roots style chargers to improve on efficiency and output (which would make the charts somewhat irrelevant). Closed discharge, hi-helix, and stripping come to mind. Coolers and chillers can throw another curve ball.
Of course this is all bogus garbage because there is only one manufacture of chargers that counts, starts with a W and it aint wieand
Another note, there are a few things that can be done to the roots style chargers to improve on efficiency and output (which would make the charts somewhat irrelevant). Closed discharge, hi-helix, and stripping come to mind. Coolers and chillers can throw another curve ball.
Of course this is all bogus garbage because there is only one manufacture of chargers that counts, starts with a W and it aint wieand
10-23-2014, 08:19 AM
#26
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iTrader: (1)
Yeah, I know. That should have went in the "What I have learned on OSO" thread...sarcasm at is finest. Bang for buck the roots are 6-71/8-71/10-71 are hard to beat. I do like whipple products as I have the low pro intercooler which works great, but their prices are up there. The PSI screw chargers intrigue me, I watched a NICE set go in the swap shop for a song and dance.
10-23-2014, 08:25 AM
#28
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Yeah, I know. That should have went in the "What I have learned on OSO" thread...sarcasm at is finest. Bang for buck the roots are 6-71/8-71/10-71 are hard to beat. I do like whipple products as I have the low pro intercooler which works great, but their prices are up there. The PSI screw chargers intrigue me, I watched a NICE set go in the swap shop for a song and dance.
10-23-2014, 08:33 AM
#29
Another area, is carb sizing. If you are going to be running a supercharger, ESPECIALLY one that is a bit small for the application, you do not want to under carb the setup. When it comes to these old roots blowers, you need to throw the idea that "my engine only needs 950cfm, so a 950cfm carb will work fine" out the window. You really need to size the carb or carbs, with the consideration that it is no longer gonna be on top of a N/A setup. Things work a little differently on a roots.
Short of running the engine on the dyno and switching carbs, or running it on the lake, and switching carbs, you may never realize you are undercarbed. What happens when you are undercarbed, and lets say you are trying to make 6lbs of boost, but only seeing say 4lbs on the gauge. Naturally you simply install a smaller pulley, to spin the blower harder, to get to 6lbs. When, quite possibly, you just didnt have enough carb, or a restrictive flame arrestor, etc. So by installing a smaller pulley, you just introduced more heat than necessary into the charge.
True story here. I went on a fun run a couple months ago. My engines are 468ci, with 420 B&M blowers, and twin 850 holleys. While on the run, I had noticed my starboard engine was down on boost about 1lb. Never had that happen before. Normally they are both at 6lbs, but the starboard was maxing out at 5lbs. I started to think my blower was wearing, or belt was slipping, or something was going on. Back at the dock i started looking things over. What I noticed, was when I had the carbs apart for a jet change a few days prior to the run, I had pinched the secondary linkage by binding the accelerator pump arm. I put the throttles to wot, and went back and looked down the throats of the carbs. Yup, sure enough, the secondary on only 1 carb, wasnt opening all the way. Mind you, I still had one 850 carb fully opening, and the primary of the other one fully opening. Theoretically, I had 1275 cfm of air available. But, it obviously wasnt enough. Now, if I used that little carb cfm calculator, it would have told me that my engine would be just fine with 1275cfm of carb. The real world told me different. Fixed the linkage problem, went back out, and got my 1lb of boost back. How much is 1psi of boost worth on a setup like mine? About 25hp.
Would it be safe to say, that on my engines, going from 1 big ole 1350 single dominator, to a pair of twin 850's, would have netted a 25hp gain on the dyno? quite possibly. Its not uncommon to see guys running big massive throttle bodies, or some big twin 1050-1350 dominator carbs, on top of a psi style blower. Its not that the engine really needs 2300-2500 CFM of carb/throttle body, but its the blower that likes it. Basically, you want just enough carb, where the blower doesnt have to work harder than it needs to , to get the desired boost level.
Short of running the engine on the dyno and switching carbs, or running it on the lake, and switching carbs, you may never realize you are undercarbed. What happens when you are undercarbed, and lets say you are trying to make 6lbs of boost, but only seeing say 4lbs on the gauge. Naturally you simply install a smaller pulley, to spin the blower harder, to get to 6lbs. When, quite possibly, you just didnt have enough carb, or a restrictive flame arrestor, etc. So by installing a smaller pulley, you just introduced more heat than necessary into the charge.
True story here. I went on a fun run a couple months ago. My engines are 468ci, with 420 B&M blowers, and twin 850 holleys. While on the run, I had noticed my starboard engine was down on boost about 1lb. Never had that happen before. Normally they are both at 6lbs, but the starboard was maxing out at 5lbs. I started to think my blower was wearing, or belt was slipping, or something was going on. Back at the dock i started looking things over. What I noticed, was when I had the carbs apart for a jet change a few days prior to the run, I had pinched the secondary linkage by binding the accelerator pump arm. I put the throttles to wot, and went back and looked down the throats of the carbs. Yup, sure enough, the secondary on only 1 carb, wasnt opening all the way. Mind you, I still had one 850 carb fully opening, and the primary of the other one fully opening. Theoretically, I had 1275 cfm of air available. But, it obviously wasnt enough. Now, if I used that little carb cfm calculator, it would have told me that my engine would be just fine with 1275cfm of carb. The real world told me different. Fixed the linkage problem, went back out, and got my 1lb of boost back. How much is 1psi of boost worth on a setup like mine? About 25hp.
Would it be safe to say, that on my engines, going from 1 big ole 1350 single dominator, to a pair of twin 850's, would have netted a 25hp gain on the dyno? quite possibly. Its not uncommon to see guys running big massive throttle bodies, or some big twin 1050-1350 dominator carbs, on top of a psi style blower. Its not that the engine really needs 2300-2500 CFM of carb/throttle body, but its the blower that likes it. Basically, you want just enough carb, where the blower doesnt have to work harder than it needs to , to get the desired boost level.
