Taken from HTT website guidelines
Turbo Upgrade Guidelines
Turbochargers are matched to an engine size and its most used RPM range.
Putting on a bigger turbo doesn’t mean the engine will perform better – it will likely perform worse (unless the turbo was mismatched from the beginning).
Increasing boost doesn’t necessarily equate to more horsepower. You can pinch off your tailpipe to increase your boost pressure but at some point the backpressure will hold back performance. There are tradeoffs to consider.
There are two BASIC changes you can make to a turbocharger depending on your ENGINE’S requirements … Turbine housing A/R and compressor size.
TURBINE CHANGES
One basic change that can be made to alter performance on any turbocharged engine is to change the turbine housing size (A/R ratio).
Generally, the smaller the housing (A/R ratio), the earlier (in RPM) the turbo will begin to boost. A smaller housing will often provide more power in the "torque range" and also equate to less "turbo lag". ) The smaller the housing, the greater boost potential to a point though there is always a concern about excessive exhaust restriction.
Larger turbine housings will allow increased high RPM engine breathing due to decreased exhaust restriction. Ultimately larger housings will allow for greater horsepower though the boost will come in later in the engine’s RPM range. Larger housings often provide lower exhaust temps.
COMPRESSOR CHANGES
By itself, changing the compressor side often won’t help performance unless the ENGINE has been modified to require more air. These modifications can include anything that might, 1) increase the RPM potential, 2) increase fuel need and delivery, 3) major timing changes, 4) camshaft change, 5) performance computer chips, 6) exhaust changes, air intake changes, head porting or any other modifications that can also benefit from increased compressor flow.
Any engine that requires increased fuel delivery for any reason also needs more air. Most often these changes to the engine require a compressor size increase to increase the flow volume. Very minor changes to compressor characteristics might be accomplished by changing to a different compressor wheel style (as on Ford Powerstroke diesels) but most often you must change the compressor wheel and housing in order to increase flow.
Engines that require more compressor volume will often exhibit a surging feeling and some associated noise.
Just as adding more fuel or advancing timing further won’t offer performance gains at some point, installing too large a compressor won’t you offer any performance gain either.
Potential turbo modifications require you to take into account current and future engine modifications as well as where you want most of your power for the way you drive.
Upgrade Guidelines Part 2
Common concerns and questions:
How much boost will this upgrade give me?
Changing to smaller or larger turbine housings will likely increase or decrease boost pressure, but many things determine boost including: timing, fuel delivery, inlet restriction, exhaust restriction, and many other factors other than the turbo itself. So nobody can really make accurate boost claims.
Changing compressor housings may or may not change boost pressure. If you have too large or small a compressor for your engine’s needs, the increase in efficiency of a properly sized compressor and MIGHT allow for increased boost.
Boost isn’t directly related to horsepower. A well-matched turbo for your driving conditions is what you’re looking for. Consider where you want your power. For low-end response you want a smaller housing but smaller housings will choke-off top-end power. If you don’t care about low-end response but wants maximum horsepower then a larger housing will be what is needed.
How much boost do I need?
Once again, every engine is different.
Dodge Cummins engines can handle 50 plus pounds of boost but boost pressure over 30 pounds is detrimental to actual horsepower increase. Boost pressures exceeding 30 pounds probably points toward the need for a larger turbine housing unless you want a lot of bottom-end response and is unconcerned about top-end power. Well-matched Dodge Cummins turbos should produce 25-35 pounds boost.
*Another detrimental factor regarding excessive boost is that the turbine will overspeed and wheel explosion becomes a serious concern.
Automotive applications usually run between 5 to 15 pounds boost. More boost is often of more potential harm to the engine (blown head gaskets, detonation or worse) than benefits gained. If you have built your engine with a very strong bottom end, uses low compression forged pistons and wants to o-ring his heads, and use aviation fuel then you can get away with 20 pounds of boost or more without trouble.
I want the biggest turbo available !!!
Too often consumers expect a larger turbo to make more power by itself. But if you install an HX40W (for example) on a stock 215 horsepower HX35W equipped engine, this engine will perform poorly. HX40W turbos are used on engines pushing over 350 hp. Now, if you have changed to a free-flowing intake system and installed a 4" exhaust system, installed 370 HP injectors, and changed the injection pump or added a 350+ HP performance chip … then, YES. You need an HX35/40 hybrid or an HX40W … depending on the amount of low-end response you expect.
*The advantage to a hybrid turbo is that the turbine wheel has less mass so it spools up quicker.
Stage I with a 12cm2 turbine is best for most "daily driver", low-end performance trucks - 245-300 hp.
Stage II upgrades with a 14cm2 "quickspool" housing match up well for approx. 300-350 hp engines.
A stage II upgrade is a pretty good compromise for low-end performance along with horespower potential.
Stage III upgrades or HX40W turbos meet the performance potential of 350-430 hp engines.
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