In my opinion, turbocharging is a cost effective way to make power in a carbureted car. Turbos and other components required in a budget set-up are plentiful and cheap to obtain. Mind you, turbo setups can be really expensive if you go with high-tech, exotic, professional race-quality parts, but chances are if you had that much money to blow on a turbo setup, you wouldn't blow it on an old car with a carb. This site deals with budget turbocharging, thus focuses on setups using, where possible or recommended, second-hand and/or original equipment components from other cars that come turbo from factory. Although maximum power potential cannot be achieved by going this route, turbocharging your carbureted engine with OEM components can yield performance that is equivalent to high-power sports cars at a fraction of the cost. Maximum performance is not the objective of a budget system; I'm sure that anyone interested in budget turbo setups would not be willing to spend the big bucks on stronger engine internals and high tech peripherals to support extreme power outputs anyway, and thus understands that budget setups sacrifice peak performance potential for the sake of low cost. Those interested in turbocharging carbureted cars must also realize that carburetors are inferior fuel metering devices to fuel injection, especially in turbocharged applications, but with some patience and careful designing can be made to work well. For us backyarders, there are advantages to turbocharging carbureted cars; these mainly are that tuning the fuel curve does not require expensive electronic fuel injection components and their related expertise in tuning/installing them, and that vintage (thus carbureted) cars usually have low compression engines from factory, which helps prevent detonation when one wants to run higher levels of boost.
Blow-thru vs. Draw-thru setup
Carbureted turbo systems fall into two categories: Blow-through and draw-through. Blow-thru setups are where the turbo is upstream in the intake flow from the carb, thus blow pressurized charge though the carburetor (hence the name) to mix with fuel before entering the engine. Draw through systems, as you can guess by now, have the turbo downstream from the carburetor, thus have the turbo drawing air/fuel mixture from the carb to be pressurized, and then fed to the engine. My argument for blow-thru is that, unless your particular engine layout and/or limited engine compartment clearance dictates a draw-thru approach, the advantages to blow-thru far outweigh those of draw-thru. I believe that draw-thru's advantages are that no special preparation of the carb to withstand pressurized air are needed, that carb inlet temperatures are consistent (because there is no turbo in front of it heating up air as pressure rises) making fuel tuning a bit easier and that the turbo might serve as a fuel atomization device by swirling the air/fuel mixture (but fuel vaporizes less readily or even condenses out of the mixture under higher pressures). Some also claim that draw-thru setups yield higher horsepower than an equivalent blow-thru, but there is little evidence to support this. However, the setbacks of draw-thru systems more than defeat the advantages. Draw-thru setups have been known to develop carburetor icing under severe duty due to the amount of evaporating fuel (which absorbs heat) being drawn by the turbo, which could can lead to the dangerous situation of a throttle jammed open. Remedies for carb icing call for routing hot engine coolant around the carb body or locating it near the exhaust manifold, but this only works once the engine is up to operating temperature. These solutions, however, introduce the possibility of otherwise-unnecessary heating of the intake charge, and hot intake increases the risk of detonation and is less dense which reduces power. Draw-thru systems also dictate that the turbo not be at the lowest point (i.e. closest to the ground) in the intake routing for fear of fuel puddling in the compressor housing of the turbo. Also, turbos used in draw-thru must have a mechanical face oil seal on the shaft behind the compressor wheel. Since the throttle plate is upstream from the turbo, sudden closing of the throttle at high engine speeds will cause a strong vacuum in the compressor housing, sucking out mass amounts of oil from the turbo's center section into the intake in the absence of a proper seal. Most turbos used for gasoline engines have this seal, however most diesel engine turbos do not, so one cannot use a diesel's turbo for draw-thru setups. Lastly, it is claimed that draw-thru setups are inferior to blow-thru in terms of the engine's drivability and cold-starting ability; exactly why this is I am not sure, perhaps it is due to blow-thru systems having an almost-stagnant turbo serving as a choke during cold starts and better fuel atomization during regular operation.
Blow-thru systems overcome the weaknesses of draw-thru at the minor cost of having to prepare the carb for pressurization (the carb should be rebuilt for turbo duty anyway) and that a little extra attention is to be paid to some systems that operate on engine vacuum. Therefore, it is my recommendation (and the focus of this site) that a blow-thru system be the system of choice
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