Turbos are a tricky subject with a lot of math involved behind the power. PSI doesn't mean anything! Get it out of your head. PSI is just a quick reference to a known flow rate for a given turbo at said PSI. The real power number is MFR, or Mass Flow Rate, measured in lbs/min. Like I said, there's quite a bit behind the chaos. I'll try to simplify and still get the point across.
Terms to be familiar with:
VE - Volumetric Efficiency - amount of clean air entering the combustion chamber per cycle, measure in %
MFR - Mass Flow Rate - air flow measured in lbs/min
PSI - Phucking Stupid Intiger
Turbine - Exhaust, or hot, side of turbo
Impeller - Intake, cold, or compressor side of turbo
A/R - Area \ Radius - Area of the outlet of a given side of a turbo (hot or cold side) divided by the radius from the center of the wheel (turbine or impeller) to the center of the outlet port. Confused? I'll try to post a reference pic.
Trim - Trim is the ratio of the inducer and exducer of a given wheel. Turbine side, the exducer is the small outer portion of the wheel while the inducer is the larger half inside the turbine. The compressor side is opposite with the inducer being the smaller half and the exducer being the bigger half inside the compressor housing.
PR - Pressure Ratio - Difference between intake pressure and output pressure.
That's about it for that.
First you need to find your power goal. Say 400 whp. Now add ~12% for driveline loss to get block hp (bhp), ~450bhp. Now to determine the MFR. Quick rule of thumb is every lb/min = 10hp. So 10lbs/min = 100hp. That said, we need a turbo to flow ~45lbs/min. I like headroom so we'll use 50lbs/min. We have wastegates afterall. Well, which turbo? Turbobygarrett.com provides charts to help you along, unlike trial and error at turbonetics. Nothing against them. Use can use these charts to compair similiar trim, A/R, and wheel size to Turbonetics turbos. (all three have to be the same though.) Anyways, using the GTX35R for reference.
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The bottom shows the MFR, the left shows PR. Looking at 50lbs/min, trace up to find the most efficient area (inner most circle) which is at a PR of 2.5. Now these charts are measured at a given ambient temp and barometric pressure, etc. Basically things that can make a difference but we won't dive into and it only makes a difference of a few HP, 10-20 tops. Anyways, we now have an idea of what PSI (that nasty thing) we need to run. But we need to find something else, if this thing will lag or spool like crazy. That's where turbine A/R comes in. Smaller A/R will spool quicker but choke at higher RPM, larger will lag but spin forever (so to speak). We need to know the max RPM we want to run for this, so say 6800 to be safe. The max air flow our engines can do on it's own at 6800 rpm is 25.6lbs/min, or roughly 255bhp with 100% VE. At 90% VE it's roughly 230bhp, which is 210ish with a manual trans to the wheels. Sad! Anyways, you usually want your max torque at about 65% of the max rpm. This is ~4400 RPM. (This is also the area of max VE and the reason cam timing is so important. You can adjust this RPM, by roughly +-10-15%, by adjusting your cam timing, and is also a reason why the largest duration isn't always the best!) Also, it's a good idea to have the max HP fall at about 90% of the max RPM, or roughly 6000 RPM. And you should shoot for full boost by half the max RPM, or 3400 RPM (this will go down once it's spoolled up so after a gear change it will spool faster!). In a racing application this will give you 2400 RPM of usable power per gear without having to peg your motor.
So now we know some key RPMs, 3400, 4400, and 6000. To get full boost by the 3400 mark you need to look at the turbine, as said above. Looking at the charts on Turbobygarrett,
you'll see three lines. We know full boost falls at a PR of 2.5. Now this is where it gets tricky. Something that I've found is you can simply use half the max MFR for this part, so 25lbs/min. Using 25lbs/min and 2.5PR, the best fit would seem to be the .63A/R. You will need a large wastegate for this to bypass the other 27lbs/min or over half, not a good idea. So we pick the .82A/R. Fuel and ignition timing will effect this also and is an important factor in why a good tune is needed, but now we have an idea of what we need.
Quick rundown, for an efficent 400whp (capable of much more!) running this turbo will give full boost at ~3400 RPM (quicker after reached once! This is from idle BTW), max torque around 4400 RPM (adjusted by cam timing and will vary depending on cams used) and a max HP at 6000 RPM of 500 at the flywheel. The IC will contribute to this as well. There are also other factors to consider such as pressure drop, humidity, altitude, etc. This is why I like the headroom. This turbo will easily get you to 650whp with higher boost and more fuel. But we are looking for efficiency for daily use right? BTW, if you must know, 400 is at ~21PSI.
Now this is by no means everything involved. There are a LOT of equations behind all of this and a LOT of them are left out just to have a quick reference. There is a LOT more to these charts that I left out just to make this as simple as possible and to make this a somewhat easy read. This is very basic and dumbed down just to give an idea. Hope this helps someone get a very rough understanding on how to pick a turbo.
I know I've talked about this before but I plan to make a more detailed post about this. Just no time at all.
