Turbocharger - Boost Basics

Charge Cooling

Any discussion on turbocharging would be incomplete with touching on the subject of charge cooling, or intercooling as it's more commonly referred to. The one downside to adding boost to any motor is heat, more specifically, elevated charge temperatures. Heat is a natural byproduct of compression and any time you add boost to an engine, the intake charge temperature will increase. Running 6-7 psi of boost pressure can usually be accomplished without running an intercooler, but every turbo engine (regardless of the boost level) will benefit from intercooling. Since the heated air temperature increases the likelihood of harmful detonation, any effort made to reduce the charge temperature reduces the chance of engine damage. Every effort should be made to reduce the intake charge temperature including a dedicated cold air intake. It makes no sense to go to the trouble of installing an intercooler in the system if you allow the turbo to draw heated engine- compartment air from an open-element air filter under the hood.

When it comes to intercooling, the three most popular methods are air-to-air, air-to-water, and water/methanol injection. There is no ultimate form of intercooling, just the best form for a given application. From a maximum heat rejection standpoint, the air-to-water system will easily surpass the typical air-to-air. The increased potential comes from a combination of the density and temperature of the cooling medium. Water is considerably denser than air and as such, offers greater thermal transfer. This is combined with the ability to dramatically lower the charge temperature using ice water. It should be noted that air-to-air systems can be run with nitrous oxide (or CO2) as an additional cooling medium to further improve their effectiveness. Drag racers (and land speed record attempts) often employ ice water to keep the charge temperatures down on their turbocharged (high-boost) race engines. Road race (and most street) engines usually rely on air-to-air intercooling, as the weight (and transfer from sloshing water) associated with the air-to-water system are both difficult to package and detrimental to performance. Plus, road race cars usually have a steady stream of airflow since they are continually running at speed. For short spurts (like drag racing), or where weight isn't a major factor (like Bonneville), air-to-water seems to be the intercooler of choice. For most street and road-race applications, air-to-air is most common. The most important thing for any street/strip application is to have efficient intercooling, and not get caught up in which form is the ultimate.

Another form of intercooling is water/methanol injection. The injection system can be used to supply either water or methanol, but the most common is a combination of both. The introduction of water and/or methanol into the air stream to act as a cooling agent to suppress detonation has been around nearly since the introduction of the internal combustion engine. The most recent systems, like the Boost Cooler pictured from Snow Performance, are significantly more sophisticated than the simplistic windshield washer bottles and pumps employed not long ago. The Boost Cooler featured a pushbutton, digital variable mapping controller, an LCD screen, and ultra high-output discharge pump. The mixture supplied can be varied using different nozzles sizes combined with changes in the mapping provided by the pump settings. Tailoring the supply of water/methanol will suppress harmful detonation without killing power. In many cases, enthusiasts have relied on water/methanol in place of a traditional intercooler, while some have combined the two in an effort to maximize the amount of ignition timing and boost that can be run on 91-octane pump gas.

One final form of liquid intercooling comes from, of all places, the carburetor. Used either in blow-through or draw-through applications, the atomized fuel supplied by the carburetor has a dramatic charge cooling effect on the pressurized inlet air. Testing on typical blow-through carbureted applications has shown a drop in charge temperature of more than 100 degrees. This is one area where carburetors actually out perform electronic fuel injection, as the position of the injectors down near the intake valve (either in the head or intake manifold) limit the time for charge cooling. In many cases, the carburetor functions as an intercooler on low-boost, street/strip application. For the ultimate in charge cooling on carbureted applications, cooling from the vaporized fuel can be combined with traditional intercooling (air-to-air or air-to-water) or even water/methanol. We have even employed nitrous oxide as chemical intercooling, though the gains offered by the additional oxygen molecules are considerably more significant than those offered through charge cooling.

The Hot and Cold Sides

Thanks to the 5.0L Mustang, turbo kits abound for small-block and Windsor Ford V-8s. Though designed for injected engines, the majority of the components can be used on a carbureted powerplant, especially the intake and exhaust tubing. For the do-it-yourselfer out there, a simple turbo system can be produced for significantly less than $1,000. Forget everything you have read about equal-length, mandrel-bent tubing, and concentrate on getting all of the exhaust to the turbo. You can build a very effective turbo kit using factory cast-iron exhaust manifolds or opt for inexpensive shorty headers. From there, you (or a local muffler shop) can route the exhaust system to suit the turbo placement. Simply merge the exhaust from each manifold into a common Y-pipe and weld on a T3 or T4 turbo flange. The larger T4 is probably preferred for a single-turbo, V-8 application, but don't concern yourself with the size of, or crimps in the tubing, especially for a street application. Make sure the turbo position provides adequate room to run a suitable down pipe (the part that runs from the exhaust side of the turbo to your actual exhaust system) of sufficient size--we've run 850 hp through a single 3.0-inch exhaust). From the compressor, route boost from the discharge (pressure) side of the turbo to the carburetor bonnet or (alternately) through the intercooler and bonnet. All that is left is to provide pressurized oil to the turbo and drill a hole in the pan to serve as an oil drain back.

Heat Management

One area that needs to be addressed with any turbo system is heat management. Heat energy is used to spin the turbo, but the radiant heat from the hot side of the turbo must be taken into account when positioning the turbo. You'll want to keep the exhaust (turbine) side of the turbo and associated plumbing (including running the crossunder pipe) away from components or paint that may be damaged by the heat. It's also possible to insulate the hot side with thermal barrier coatings, header wrap, or turbo shields, but that actually diminishes the life of exhaust tubing. Heat management is another reason why individuals choose two turbos over singles. Sometimes it's easier to package a pair of small turbos than one large(r) turbo. For some, having twin turbochargers improves the visual statement by offering better symmetry. There is an argument in favor of running two small turbos (over a single larger unit) to improve boost response, but bear in mind that only half of the exhaust energy is provided to each small turbo in the twin set up. In reality, both the single and twin set can offer identical boost response and power with proper sizing.

Turbocharging Your Vintage Ford

By now you've probably figured out that turbocharging your automobile has tremendous benefits. Doing so can double or triple your power output without changing your driveability--once you experience the power of boost, it is hard to go back to a normally aspirated combination. A turbocharger system would also provide the basis for some interesting bench racing at the next cruise-in or car show, where everyone else will be sporting the standard single four-barrel or the occasional EFI engine.

While there currently doesn't seem to be any aftermarket turbo systems available for vintage muscle cars, many of the Fox-body Mustang systems can be adapted to fit with minimal modifications. There's also the custom route, and you'll find that there are numerous shops that specialize in building custom turbocharger systems. Then there is the DIY enthusiast who just sits down with a reciprocating saw and a welder and builds his/her own. On websites like www.theturboforums.com and www.turboford.org, you'll find numerous turbo systems being crafted in the garage, and plenty of like-minded enthusiasts looking to share information on the subject.