Twin-Turbo TerrorChris Cerce, Taunton, MA: I'm building an '86 Camaro with a twin-turbo small-block. I made my own headers, and I'm using two T04 turbos with Turbonetics wastegates along with an intercooler. The only piece of the puzzle I can't figure out is the carb. I have a back issue of Car Craft where a kid built an S-10 with two turbos and said he used a Holley 750 but modified it. I wish I knew what he did to it so I could do the same. I want to run 7 psi of boost and don't have $850 to spend on a CSU carb. There was also a back issue with a twin-turbo Camaro with a big-block, but I lost that issue.
Jeff Smith: There is an increasing amount of information on blow-through carburetor applications that are not just for turbochargers. If you think about it, the carburetor really doesn't care what device creates the boost, so the modifications for a blow-through carb will work regardless of the application.
We did a story in the Oct. '05 issue called the "Guide to Blow-Through Superchargers" in which our pal Tim Moore built an ATI ProCharger centrifugally supercharged 540ci big-block Chevy using a blow-through carburetor. We learned some very important things from that engine that will help you dial in your package.
The first and perhaps most important item is the fuel-delivery system. Even with 7 psi of boost, you're looking at making some serious horsepower, and it's likely that if your combination is successful, double-digit boost will not be long in coming. This will require a high-volume fuel-delivery system. We recommend a full-flow system with a return. This will require a pump with the capacity to feed a minimum of 600 to 700 hp. We had excellent success with an Aeromotive A1000 pump (PN 11101) that's capable of up to 1,100 hp in a forced-air condition. Next, you'll need a boost-referencing fuel-pressure regulator, such as the PN 113105. Both the main fuel-feed line and the return line should be no less than 11/42 inch in diameter, and the return line cannot be smaller than the feed line to prevent backpressure problems. If the return line is too small (especially on a carbureted application), it effectively reduces the fuel pressure the pump can supply to the carburetor. Be sure to return fuel to the tank in an opposite corner from the inlet so there are no fuel-aeration problems near the fuel pickup. This is important with a high-volume system where a majority of the fuel is returned at low engine speeds.
We're going to assume you will use a fuel hat on top of your carburetor rather than a full enclosure for the carburetor. Either way, the fuel-pressure regulator must be able to boost-reference. When the boost pressure is applied to the carburetor, the float bowls are subjected to this same pressure. That means the pressure regulator must increase fuel pressure in a 1:1 ratio for every psi of boost pressure created by the turbo. For example, if the turbo creates 7 psi, and under normally aspirated conditions the carb sees 5 psi of fuel pressure, at 7 psi of boost, the actual fuel pressure at the float bowls will climb to 12 psi (5 + 7 = 12). Effectively, however, the fuel pressure in the bowl is still only 5 psi, which ensures adequate fuel pressure and fuel volume to the carb. This is important and cannot be stressed enough if the package is to be successful.
As for the carburetor, it's actually quite simple. We learned much of this from our friends at Quick Fuel Technology. According to Quick Fuel's Marv Benoit, the key to success with a blow-through carb is to use a smaller-cfm carb with annular-discharge boosters. The trick is supplying enough fuel to feed the potential airflow. On our 540ci Rat motor, Quick Fuel supplied a 750-cfm carb with annular-discharge boosters, and we had excellent luck making 977 hp with virtually out-of-the-box jetting and only 11.75 psi boost. Everyone who witnessed our combination thought the carb was too small to make this kind of horsepower, but Benoit says this smaller size generates higher air velocity through the carb, which pulls more fuel from the annular-discharge boosters. We think this is the key. You might be able to get away with a standard downleg booster with low boost levels, but be prepared to run very large jets-on the order of high 90s-and you will probably have to drill out the power valve channel restrictors again to increase fuel flow. Smaller high-speed air bleeds may also be necessary to increase fuel flow at higher engine speeds.
If it's possible to find a used, annular-discharge booster, Holley carb that would work, we'd suggest sending it to Quick Fuel to let them tweak it for you. That might keep the cost down. Another important point is that regardless of the carb you use, you should seal those pressed-in brass cups Holley uses on cast metering blocks with epoxy. Under boost pressure, it's possible for these cups to push out, which would spray pressurized fuel all over the engine compartment and would instantly cause a fire. These plugs were never intended to have pressure applied to them and could push out under boost pressure. A boost-referencing plate to prevent fuel from pushing out past the throttle-shaft seals is also a good idea. A diagram of this plate can be found in the Hugh MacInnes book, Turbochargers, from HP Books. It's available on amazon.com for $12.82. We'd like to thank Ken Crocie for bringing this to our attention. It's an excellent point.