This is a pair of Barry Grant Mighty Demon Blower 750s on a Blower Shop 8-71 billet blower
Scott Duenow, Detroit Lakes, MN: I'm putting together a blower setup for my 440-powered Charger. What mods are done to the carbs to run on a Roots blower? I'm undecided between Holley and Carter carbs. The motor has home-ported 906 heads with 2.14/1.81-inch valves, a Mopar Performance 292-degree-duration cam with 0.509-inch lift, forged TRW Six Pack replacement pistons on stock rods, and a reground stock steel crank.
The blower is a 6-71; the headers and exhaust will be replaced. I'm thinking either Hedman 1 7/8-inch or Doug Thorley tri-Y's along with a full-length 3-inch exhaust with good quality turbo or Flowmaster mufflers.
Jeff Smith: I've noticed a couple of things in your engine description that you should consider before bolting the blower in place, and then we'll get into your carburetor question. First, let's talk about the camshaft. This is the classic Mopar Performance Purple Shaft cam with a single-pattern design, meaning both intake and exhaust lobes use the same lift and duration with a 108-degree lobe-separation angle (also called lobe-displacement angle, or LDA). This is not the best camshaft for a supercharged application. The main reason is this cam's tight lobe-separation angle. This means there is a lot of overlap during the transition period when the exhaust valve is closing and the intake valve is opening. While overlap helps a normally aspirated engine breathe, on a supercharged engine, the pressurized incoming air and fuel will push right around the exhaust valve, resulting in lost cylinder pressure. If you want to retain this cam's 248 degrees of duration at 0.050-inch tappet lift, choose a cam with a wider lobe-separation angle of around 112 to 114 degrees. This reduces the amount of overlap, allowing more of the incoming charge to be retained in the cylinder, which will make more power.
Because you're using stock iron heads and these heads have a relatively weak exhaust port, we would suggest going with a dual-pattern camshaft with more exhaust duration and lift to help rid the cylinder of residual exhaust gas before the next incoming inlet charge arrives. The closest off-the-shelf cam we could find that would work is a Crane PowerMax H-312-2 hydraulic flat tappet that specs out with 242/252 degrees of duration at 0.050 with 0.528/0.552-inch valve lift and a lobe-separation angle of 112 degrees. This cam offers much more lift, so you will need to check valve-to-piston clearance, coil bind, and retainer-to-seal clearance as well. You will most likely also need good dual valvesprings. We suggest breaking in the new cam on the outer springs and then installing the inners after the cam is fully broken in with a can of break-in lubricant, such as GM's Engine Oil Supplement, along with 5 quarts of Shell Rotella-T diesel engine oil that's high in zinc additives to help the cam and lifters survive.
We also suggest going with the larger 1 7/8-inch headers because this big cam and the blower are going to push out a lot of exhaust, and those larger headers will help get the job done. We've heard good things about the Tube Technologies Inc. (TTI) headers for these cars. They're expensive but have a reputation for an excellent fit. TTI also sells a complete 3-inch exhaust system for B-Body cars like your Charger. This size exhaust is virtually a requirement if you expect to make a minimum of 550 to 600 hp with your supercharged 440.
Now, as for your original question on carburetors, we have to look at how a Roots 6-71 blower operates. This blower was designed to move air, not build pressure. This means it demands an unrestricted inlet. That's why you always see a pair of carburetors on Roots blowers. Running one small carburetor is too much of a restriction on the inlet side. For this application, you would be best served with a pair of 750- or 800-cfm carburetors. Both Holley and Barry Grant (among many others) sell specific draw-through blower carburetors that are modified to block off the undercarb manifold inlet for the power valve, since a supercharged engine may see relatively high vacuum levels such as 4 to 5 inches of vacuum (just below the carbs), even at wide-open throttle (WOT). For this reason, most draw-through blower carbs are modified for an external power-valve source below the supercharger. This ensures that the power valve will open under boost when additional fuel supplied by it is really needed. Previous to this modification, many tuners resorted to blocking off the power valve and increasing jet sizes on the primary side of the carburetor by six to eight jet sizes. This made the part-throttle air/fuel ratio excessively rich, which kills throttle response and mileage. There are also many custom carburetor companies that offer blower carbs, which gives you plenty of carburetors to choose from.
Crane Cams; Daytona Beach, FL; 386/258-6174; cranecams.com
Demon Carburetion; Dahlonega, GA; 706/864-4712; demoncarbs.com
Holley Performance Products; Bowling Green, KY; 270/781-9741; holley.com
Tube Technologies Inc.; Corona, CA; 951/371-4878; ttiexhaust.com