The engine compartment in Gordon Bishop’s Rat-powered AMC joins a pair of large aluminum t
Joe Wahrer; Lima, OH: Greetings, I am a vocational teacher at the Allen Correctional Institution in Lima, Ohio. A student saw your feature on Gordon Bishop's '65 AMC Rambler in the Nov. '10 issue ("AMC It Go," pg. 68). Can you tell us where we could learn more about the carb hat/turbo setup? You are talking about some serious plumbing with taking the compressor side clear to the back seat and then forward to the carburetor. What are the benefits? Tell Mike Yoksich he caused a stir here. Awesome job, Car Craft! Keep up the good work.
Jeff Smith: Turbocharging has finally come into its own for car crafters and we're seeing hair dryers show up in all kinds of different race and street cars. Gordon Bishop's AMC you mention is a wild 8-second drag car with twin 80mm turbochargers designed by Bullseye Power in Muskegon, Michigan. Creating 26 psi of boost (which is almost two atmospheres of pressure), these turbos make a massive amount of heat. Even on a drag car, they spike the inlet air temperature coming out of the compressor side of the turbocharger to somewhere between 220 and 250 degrees F. Hot air entering the engine is less dense than it would be at the same pressure if the air was cooler. The ideal situation to make more power is a combination of high pressure and low temperature. Achieving this ideal situation requires cooling the air after it exits the turbochargers. Many drag racers prefer air-to-water intercoolers because they are more efficient. This system is basically a large tank in which ice water is pumped through a radiator. A large container, which in Bishop's car is located right behind the driver, is built around the radiator and plumbed to the turbocharger outlets. The hot, pressurized air from the "cool" side of the turbos blows through the radiator, which reduces the temperature but also creates a slight restriction or pressure loss. Then the cool air is ducted back to the engine and directed through the carburetor hat and into the engine. While a pressure loss might seem counterproductive, there is still a major net power gain because of the dramatic reduction in charge temperature. Our pal Kurt Urban has tons of experience with drag race turbos and he says that with a good intercooler using ice water, at the starting line you can see discharge temperatures into the engine as low as 50 to 60 degrees! At the end of the run, the inlet air temperature will rise to maybe 120 degrees. While this temperature seems high, it is still a reduction of 125 degrees or more, which is a major increase in density. The key is to create the lowest discharge temperature across the intercooler for the least amount of pressure loss. Urban says a pressure drop of 3 psi or less is a good goal. He also says the key to these systems is ensuring the pump system can produce a high rate of water flow. Intercooler efficiency is based largely on how quickly the system can circulate the ice water.
Returning from the intercooler, the upper tube is connected to the carburetor hat. The duc
When designing a system like this, you also have to take into consideration the overall cost in terms of efficiency for performance. Besides building the entire system with ducting running between the engine compartment and the back seat–located intercooler, the designer also has to account for the added weight. That's why the intercooler is behind the driver and the intercooler tank is on the driver side in the trunk alongside the fuel tank. All this adds weight that the car must then accelerate down the track. The aluminum ducting, the intercooler, the pump, and the water (which weighs 8 pounds per gallon) could easily account for 50 to 75 pounds or more of added weight. This weight may not be worth the trouble if the boost level is perhaps 10 or 12 psi because the power gain from the cooler charge temperature is not enough to overcome the weight penalty. With lower boost levels, a less complicated system would be a stand-alone air-to-air intercooler that just uses incoming air to reduce the discharge temperature. While lighter and requiring less plumbing, air-to-air intercoolers are not as efficient in terms of reducing the discharge temperature. As is the case with most things in life, there are no free rides, except maybe running E85 or straight methanol, which do a great job of cooling the charge temperature based on their latent heat of vaporization—but that's another story.