Join Date: Oct 2009
Location: Gloucester, MA
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What is the engine that you are looking at turbocharging? Gas or diesel?
I will start with a caution about putting a turbo on engines that were not originally designed for them. If you don't change anything, you will increase cylinder pressure from compressing more air which can have adverse problems such as a blown headgasket. If the engine came with a turbo option, then this should not be a problem. You can lower the cylinder pressures to reasonable levels with a turbo by dropping the compression ratio (changing pistons or a thicker headgasket) but that is undesirable as the engine will not start as well and it will be less efficient. The other option is to try to make the cylinder hold up to higher pressure by putting in stronger head studs, heavier valve spring, and an o-ring in the headgasket. The ultimate diesel to modify is the 6bt cummins (5.9ltr offered in dodge pickups) and if you look at turbodieselregister.com or dieseltruckresource.com, you will see a ton of information on how to keep your head gasket intact.
To answer your original question, it really depends on your setup and whether the engine is gas or diesel. Gas engines are air throttled and stoichiometry (air/fuel ratio) really matters so you have to be a lot more careful with turbocharging. Diesels are fuel throttled and stoichiometry doesn't matter very much except at very low air/fuel ratios which is why we add turbos. Since a turbo is a thermodynamic device, it is a function of the amount of heat energy exiting your exhaust. This is directly proportional to flow rate (rpm) and temperature (egt).
Talking diesels only, the size of your turbo will really determine how it affects your hp/torque curve. The idea is that by forced air induction, you can inject more fuel into the engine. Really small turbochargers will have a large effect at lower rpms and will not provide much air at high rpms because they are out of their efficiency range and are injecting hot air instead of cold air which is not as efficient. This leads to a large increase in torque (lower rpm) and not a large increase in horsepower (higher rpm). A very large turbocharger will not spool at lower rpms but will provide a lot of good cool air at high rpms which results in not a large increase in torque and a large increase in horsepower. Because neither turbo is good at both, you will often see them in series referred to as "twins" which provides good airflow throughout your entire rpm range. If you compare a turbo and a supercharger, the supercharger is really good at low rpm where the turbo is not spooled yet and the charger is really good at high rpm. If you look at a naturally aspirated engine in general, the airflow is proportional to the rpm so the higher the rpm, the more fuel you can burn and the more power you can get.
I should add that what I said above made a few simplifications. I ignored different turbo housing sizes, turbo wheels, etc. Airflow is king in the modern diesel engine as we try to get more power for a given displacement and a turbo plays a large role in that. Boost is simply a measure of resistance in your intake so actually one of the best things that you can do for improved airflow is to port the head on the engine (this is for racing only, not cruising boats). In modern diesel racing, the vast majority of the research is in turbos these days although there is a lot of research in fuel injection and drugs as well.
If your car is actually gas, then the above explanation is true in very general terms but some of the specifics are different. The relative boost range for even racing engines is totally different, ~20psi for gassers versus ~150psi for diesels.
If you give me an engine model number, I can let you know if I know anything about that engine. You can read a turbo map to get in the ballpark but being able to learn from someone else who has already hotrodded a specific engine is much better. Like anything, selecting a turbo is about compromises.