Before I start swapping pistons and rods, I wanted to see if there would be any benefit to using a smaller pulley on a centrifugal setup, and adding a wastegate between the supercharger discharge and throttle body to control boost levels. The engineer in me doesn’t like the idea of it because it’s an inefficient way of doing things. Regardless, I considered doing it to get max boost from ~6k rpm to redline, with the tradeoff of higher discharge temperatures and more power consumed by the supercharger.
In other words, I’d get more low end and midrange, at the expense of peak power and efficiency.
My tuner says he can tune around it. I think part throttle would be hard to tune around, because you have two competing discharges. The intake wastegate and bypass valve are both dumping boost (unless the bypass opening dropped boost pressure enough that the wastegate closed). The ECU is using the info given to it by the MAF sensor for fuel calculations, so the car would need to be tuned to estimate how much air is being dumped through the wastegate, and how much is being dumped through the bypass. At full throttle or off throttle, things are better defined.
For the estimations, I used the same boost curve that I used previously for the 14.5 psig twin screw and 15.5 psig centrifugal (henceforth, I’ll drop the “g” from psig). I multiplied each 100 rpm increment by a fraction related to the new peak boost (so in this case, I multiplied each data point by 8.5/14.5 for the twin screw and [8.5, 12, 15, 18]/15.5 for the centrifugal). I did not go back and recalculate the compressor efficiency at every single data point. I did glance at the compressor maps, and the twin screw operates just above the “sweet spot” at 14.5 psi, and just below it at 8.5 psi; the centrifugal starts out in an area not covered on the compressor map (so data would need to be extrapolated), then moves towards the “sweet spot” of 73% efficiency. For the purposes of what I’m showing, a 5% change in compressor efficiency is irrelevant, since it only equates to a few degrees of discharge temperature and tenths of a horsepower.
The 15 psi and 18 psi pulleys would probably require a whole new pulley set to avoid belt slip, since the supercharger pulley would be too small with the standard crankshaft pulley, and the power required to turn it is higher. A Vortech SCi trim compressor would need to be spun to about 53k rpm (2.36” pulley using the stock 5.11” crank pulley… and that won’t happen without serious belt slip :lol).
First is boost pressure. Peak boost is reach at:
6800 rpm, 8.5 psi pulley
5900 rpm, 12 psi pulley
5300 rpm, 15 psi pulley
4900 rpm, 18 psi pulley
Even with maxing out the centrifugal supercharger, the twin screw has a huge advantage at low rpm.
Next we have the volume flow rate (cfm) of air dumped by the wastegate, and air passing through the throttle body. Ideally, only the air that is passing through the throttle body would have passed through the intercooler. The air that is not going to be used by the engine would be dumped before it passed through an intercooler. All things being equal, a lower volumetric flow rate means cooler air will exit the intercooler and into the engine, because there is less hot air to cool.
If the boost pressure graph shows the advantages of smaller pulleys, the discharge temperature graph is one of the areas where the downsides appear. I used an ambient temperature of 70 deg F, like with my previous high boost analysis (by the way, “standard day” is actually 59 deg F).
Obviously, some sort of aftercooling is necessary. Max discharge temps for each setup:
174 deg F, 8.5 psi
209 deg F, 12 psi
236 deg F, 15 psi
261 deg F, 18 psi
196 deg F, twin screw
When I lived in the California desert, ambient temperatures regularly got up to 110 deg F.
222 deg F, 8.5 psi
259 deg F, 12 psi
289 deg F, 15 psi
316 deg F, 18 psi
246 deg F, twin screw
I think we can agree that a well-developed means of cooling is necessary.
A smaller pulley is a compromise. It provides the midrange benefits of a high boost setup without the stress of high boost pressures, which is great for a stock engine. Because the volumetric flow rate of air entering the intercooler is lower, less cooling capacity is required, even if discharge temperatures are the same.
Then again, you deal with many of the downsides of a high boost setup, without enjoying the significant power advantages that high boost provides. The power required to spin the supercharger is the same whether you use the boost or dump it, so there’s a lot of waste:
Using the formulas I used previously, we can estimate wheel horsepower. Again, this doesn’t take into account power adders such as headers, FI-spec camshafts, or head work. It also doesn’t include losses due to intercoolers, heat of the intake air, or conservative tuning.
IMHO, a 12 psi pulley, with an 8.5 psi wastegate dump, appears to be the best compromise for a centrifugal setup. Max boost comes in before 6k rpm. Discharge temperature and horsepower consumed by the supercharger are mildly greater than a twin screw at high rpm, and it will run cooler and be more economical everywhere else. However, for maximum average horsepower at identical peak boost levels, you can’t beat a twin screw.
Overall average horsepower, 3k-6.8k rpm:
226 whp, 8.5 psi pulley
240 whp, 12 psi pulley
247 whp, 15 psi pulley
250 whp, 18 psi pulley
264 whp, 8.5 psi twin screw
Average horsepower, 4k-5.5k rpm:
216 whp, 8.5 psi pulley
233 whp, 12 psi pulley
247 whp, 15 psi pulley
255 whp, 18 psi pulley
263 whp, 8.5 psi twin screw
Average horsepower, >5.5k rpm:
281 whp, 8.5 psi pulley
297 whp, 12 psi pulley
295 whp, 15 psi pulley
291 whp, 18 psi pulley
298 whp, 8.5 psi twin screw
If you’ve got a centrifugal supercharger, it may be worth pursuing this mod, along with aggressive gearing to minimize your time spent outside of boost.