Confused about total comp. verses octane :/
11-05-2012, 04:47 PM
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850's will work fine. IMO, Thats the carbs i would choose. Even in the Holley/Weiand catalogs, if you were buying a 6-71 or 8-71, twin 750 for small block, twin 850's for big block.
Im running 850's on top of a 468 with a 420 B&M blower. Just slightly smaller than a 8-71. Engines made 804HP non intercooled and 6lbs of boost, and run perfect. Fuel Curve is great, not even boost referenced. Idle all day if I have to. Granted, 750's would work just fine for me, but the 850's sure arent hurting a thing.
Pretty much, the theory of too big a carb causing sluggish low end and soggy performance, you can throw out the window. Roots blower like lots of carb. Or let me rephrase that, they DONT like any restriction. Ever notice the twin 1050 Dominators on a merc 900SC? Merc Racing Engineers weren't dummies. A pair of 4150 style 850's might have helped it idle a little better, over the dominators, but i'd be very shocked if they made any more power over the 1050's on that engine. (540 W/10-71 and chiller). Overcarbing ( I mean like a pair of 1150's on your setup), would most likely cause tuning issues, more than anything. Too much carb causes poor signal thru the carbs, and you end up having to run alcohol jets to get a acceptable AFR.
BDS has a calculator for blower carbs. Based on a 509CI, turning 6000RPM, running 8lbs of boost, required CFM is 1364. They dont suggest more than 30% over carb needed on a blower. so 1364CFM, +30%, 1774CFM.
You see guys all the time with big roots blower stuff. They Gut their flame arrestors, put big old Pro Stock scoops on, trying to get more air into those motors. But then they want to run small carbs.
As for the 320cc RHS heads, vs the big Darts, tough call. I don't know much about the RHS Heads. While the 320CC port size works nicely for what you're doing, you need to think about flow also. I'd Talk to someone like JIMV here on the boards. If you have both sets of heads available to you, you might be able to weld up the floors on the Darts, to get the port velocity up, and smoke the RHS heads in performance. I'd hate to tell you to run the RHS heads based on the intake runner size alone. Those Darts move some air.
Figure out which heads you're gonna use on your build, and then have Bob Madera (RMBUILDER) here grind a cam for what you plan to do. If you take your time and do a nice build, I think you'll be pleasantly surprised, and make a lot of power. You may not even feel the need to spend the extra few grand to install a stroker crank and rods. The extra 30-40ci you gain, can easily be outdone with a properly set up 509. Then you can use that cash to beef up your drive.
Im running 850's on top of a 468 with a 420 B&M blower. Just slightly smaller than a 8-71. Engines made 804HP non intercooled and 6lbs of boost, and run perfect. Fuel Curve is great, not even boost referenced. Idle all day if I have to. Granted, 750's would work just fine for me, but the 850's sure arent hurting a thing.
Pretty much, the theory of too big a carb causing sluggish low end and soggy performance, you can throw out the window. Roots blower like lots of carb. Or let me rephrase that, they DONT like any restriction. Ever notice the twin 1050 Dominators on a merc 900SC? Merc Racing Engineers weren't dummies. A pair of 4150 style 850's might have helped it idle a little better, over the dominators, but i'd be very shocked if they made any more power over the 1050's on that engine. (540 W/10-71 and chiller). Overcarbing ( I mean like a pair of 1150's on your setup), would most likely cause tuning issues, more than anything. Too much carb causes poor signal thru the carbs, and you end up having to run alcohol jets to get a acceptable AFR.
BDS has a calculator for blower carbs. Based on a 509CI, turning 6000RPM, running 8lbs of boost, required CFM is 1364. They dont suggest more than 30% over carb needed on a blower. so 1364CFM, +30%, 1774CFM.
You see guys all the time with big roots blower stuff. They Gut their flame arrestors, put big old Pro Stock scoops on, trying to get more air into those motors. But then they want to run small carbs.
As for the 320cc RHS heads, vs the big Darts, tough call. I don't know much about the RHS Heads. While the 320CC port size works nicely for what you're doing, you need to think about flow also. I'd Talk to someone like JIMV here on the boards. If you have both sets of heads available to you, you might be able to weld up the floors on the Darts, to get the port velocity up, and smoke the RHS heads in performance. I'd hate to tell you to run the RHS heads based on the intake runner size alone. Those Darts move some air.
Figure out which heads you're gonna use on your build, and then have Bob Madera (RMBUILDER) here grind a cam for what you plan to do. If you take your time and do a nice build, I think you'll be pleasantly surprised, and make a lot of power. You may not even feel the need to spend the extra few grand to install a stroker crank and rods. The extra 30-40ci you gain, can easily be outdone with a properly set up 509. Then you can use that cash to beef up your drive.
As far as the heads go, I think I'll just stick w/ the 320's on the 502. I already have the 540 w/ Dart block and the stout rotating assembly I just need to bore it and ditch the 12.5:1 pistons for ssomething more like 8:1 and would like to go w/ a dry sump system on that motor. How well I can get carbs to work will help me deside if I'll go with EFI or not.
