Few turbochargers meet my criteria given the amount of CFM this engine will produce under the desired boost target (15 PSI), particularly as it relates to boost threshold below 4000 RPM.  That is, many high horsepower turbos do not flow well when mated to a stock port rotary and this combination may result in compressor surge or very poor boost response below 4000 RPM.  Conversely, many smaller turbos that flow well at lower RPM simply don’t offer enough flow above 7000 RPMs.  Units that meet these criteria are the Garrett T04E (only with certain trim configurations), Garrett T04S units (various trims and A/Rs make this a difficult comparison [The HKS T04S is basically a Turbonetics 60-1, 60 trim with T04S compressor cover and P trim turbine]) and the 62-1 such as that offered by Turbonetics.  Newer units, such as the Garrett GT turbos (GT30 - a ball bearing T04S [in various trims] compressor mated to a smaller T3 exhaust housing), GT35, and GT3040 (or GT3540R, GT35R) and the Apexi units prove suitable as well and appear to be efficient across the RPM range.  

*Note: a ball bearing turbo will offer a slight reduction in lag (typically 11%), but will offer no advantage re: compressor surge as boost threshold and lag are not the same thing.  That is, if the compressor is too large, it doesn't matter if it is connected to a ball bearing shaft. Yes, you can blow on some BB compressors and they will spin with little effort.  However, that's irrelevant if it is not efficient at, say, 20 lbs/min and 12 PSI. 

Below is a spreadsheet I've used to select a suitable single turbo with the appropriate calculations. 

Spreadsheet for calculations (click "Do not Update")

After numerous calculations, I've found two or three ideal turbos for my application- the Garrett GT30R, GT35R, and T04S. Compressor maps for the two newer GT turbos are displayed below.

GT30R and GT35R

Looking at the maps from Garrett above, note that I have added three horizontal lines at differing boost levels. I then used the calculations from the spreadsheet to plot the max flow at lower RPM points (strive for 50% or max RPM).  We can see that the 30R and 35R will boost to 15 PSI by 3500 RPM.  I've also plotted the boost threshold line from 20% of max CFM at a pressure ratio of 1 as per Corky Bell.  From 3500 RPM, we see that the 30R is in no danger of compressor surge at lower RPMs while the 35R is close.  Remember, these graphs are approximate.  Real data indicates that no compressor surge exists on the 35R while trying to get it to boost at 15PSI by 3500 RPMs.  In terms of efficiency, the 35R runs in the sweet spot at higher flow compared to the 30R. The 30R will make a little more boost at lower RPMs compared to the 35R- approximately 400-500 RPM lower based upon my calculations and these graphs.  At higher boost levels, the 30R runs out of steam compared to the 35R.  I don't plan on cranking up the boost to 25 PSI (look at the graphs), but...well... you never know.  Again, this appears to be a difficult decision as both turbos are very efficient and the 30R appears to have a tad less lag at lower RPMs. However, another factor to consider is that the VE calculations are borrowed form a NA 13 B engine. Third generation RX-7 motors are thought to be a bit more efficient.  Allowing for porting, a modification that drastically enhances VE, and the 30R will run out of its efficiency range at a mere 15 PSI (using 100 % VE at 7K RPMs- a reasonable figure on a ported rotary) as the air flow approaches 52 lbs/min.     

Is 15 PSI at 3500 RPMs okay? Well, maybe not for highway cruising...particularly if you still have your stock 5th gear.  On the track it's more than adequate. Most areas of the track where you get into lag issues are in low speed turns.  At 3500 RPMs the car will be traveling 31 MPH in second gear.  I can't remember when I've gone that slow in second gear on a track. For autocrossing I'd say get the 30R or perhaps a slightly smaller turbo still (like the Apexi). If you still have your stock 5th gear ratio (.719) and are too lazy to shift, the 35R is going to be a problem too as you will have to get to 87 MPH before you hit the 3500 RPM point. With the new ratio (an .827) it's only 76 MPH.

T04S

The Turbonetics 60-1 turbo and T04S in 60 trim has a surge line that lies just slightly to the right of the GT35R (above- you can find Garrett/Turbonetics 60-1 maps all over the web). It's a very popular turbo that, when mated with the correct turbine wheel and housing, works very well.  Garrett indicates that the GT30 utilizes a T04S compressor housing and 56 trim wheel, but compared to the 60 trim T04S, the GT30R is in fact more efficient than the older T04S at boost level above 20 PSI.  OTOH, the 60 trim T04S flows to 60-65 lbs. air/minute or so. At boost pressures under 20 PSI,  I've seen excellent dyno results for the T04S that rival or even best the 35R's (look at the S-PLUS/Java graphlet above). The problem may lie with the GTxxR's turbine housing and wheel size.  Garrett indicates that the newer GT turbos are more efficient, but there is not enough data to confirm that these newer turbos are "vastly" better at moderate to high boost rotary applications at this point, particularly given the exhaust housing issue and high EGTs generated by a rotary engine.  For example, Garrett's published improvements in compressor efficiency isn't going to net a huge improvement in HP.  This effect is further reduced with when using a decent intercooler. That's the key- a good intercooler.  For example, on a 13BREW at 15PSI, 69 degrees ambient, and 75% compressor efficiency, 90% VE, and 90% efficient intercooler, a figure of  452 HP is obtained.  With a 70% compressor efficiency and everything else held the same (ceteris paribus), this drops the horsepower to 449. Don't believe me? Do the calculations yourself :)

So, based upon the compressor maps, it's down to three choices: GT30R, GT35R, and T04S.  Now we need to look at the turbine side at the desired pressure ratios (download or view the spreadsheet from above), it is clear that the 30R cannot flow in an efficient range given the rotary's high exhaust temperature at 15+ PSI boost.

So we're down to two: the GT35R and the T04S.

Now we must have the proper amount of fuel delivery.  Max Cooper's site has an excellent fuel calculator used to find proper injector sizing and fuel pump(s).  I'm shooting for at least 380-400 RWHP, so 1680cc secondaries and a stout fuel pump such as the Bosch 0 580 254 044 (220 LPH @ 72 PSI)  should provide enough fuel for up to 430 RWHP.  

Go to this page for fuel system details.

Additionally, to accomplish all of the above you must have a programmable engine/fuel computer.  I'm using the APEX’i Power FC and am very happy with this unit's performance and flexibility.  

Finally, you must mate the turbo with an appropriate intake and exhaust. Many folks use either too small or too large exhaust and intake.  Most use air intakes that draw from an underhood air source (i.e., do not utilize outside ambient air).  Not good.  More on these later. 

How much does this cost?

So you want to buy a single turbo, eh?  You must be thinking that, since the RX-7 is turbocharged already (in fact, has two turbos) that this will not be very expensive.  And, many of those kits advertised on the internet are roughly $3,250, so that’s about it, right?  Wrong.  In case you’ve been sleeping, this car’s upgrades are expensive.  A single turbo is not the only item you have to upgrade (unless you want to blow your motor the day of your install).  This is what you need and the approximate cost (assumes you’ve already upgraded your exhaust):

The above assumes that you’re buying a turbo kit that allows you to keep the air pump like that offered from HKS.  If you don’t, then you’ll need a pulley kit that allows deletion of the air pump ($150), a belt, ($12.00), and block off plates for the air pump ($60).  This comes to a whopping $7,571.  If you want your car to be emissions legal, then you need to find a way to incorporate an air pump.  I’ll be doing this.

The installation

I purchased a single turbo kit from M2 Performance.  This was a limited edition run of just a few pieces and I was the test mule. 

First, let's pay homage to the 62 pounds of sequential twins...

After some testing, it was discovered that the wastegate runners on the M2 kit were insufficient to properly vent exhaust gas, thereby limiting the amount of exhaust energy diverted to the wastegate.

Due to this issue, I decided to purchase a GT35R kit from A-Spec tuning.  This is a very nice kit that utilizes a TiAl 46mm gate as well.  I used the TiAL blow off valve from the previous installation and had the downpipe and wastegate dump tubes ceramic coated from High Performance Coatings.  

Also used much adhesive backed heat barrier on the firewall, strut area, heat shield, and sides/rear of the M2 airbox (also customized for the single turbo). Another advantage of this kit, is the weight reduction- roughly 25 lbs!

Upon installation, boost control was very manageable, but a bit higher than the wastegate spring boost (12 versus 10.3).  I believe some of this has to do with the location of the boost source going to the side of the wastegate. I moved the boost source to the compressor outlet location which seems to be working very well.  

Now, the good stuff:  Click for bigger.

I took the car to Excessive Motorsports in Portland, OR for dyno tuning.  After a couple of non-boost runs, we did two pulls: one to 6K RPMS and another to 7K.  Boost was set at the spring level, which was 12 PSI at the time.  

The car made 362 RWHP, but the clutch began to slip very badly. Not a bad first run!   Note the dip in the HP and torque curves in the image above.  Ambient temperature was about 67 degrees and max injector duty cycles were 68% (550 cc primaries and 1600 cc secondaries).  We noted a possible issue with the fuel pump as the fuel pressure line deviated somewhat with boost around 5000 RPMs.  However, I'm not sure if this was due to pump limitations, a slipping clutch, or voltage drop to the fuel pump due to insufficient wiring.  I will try to re-wire the fuel pump for the next series of dyno runs.

Unfortunately, I couldn't do any more high boost pulls due to the slippage.  I've ordered a 6-puck RPS clutch and heavy duty pressure plate, which should take care of the slippage.  More dyno runs later, I guess.