One larger Turbo is the exact opposite of what a good daily driver needs.
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It really doesn't matter if it's one or two turbochargers. There is a perception that two turbos will "spool faster" than a single turbo. That just not the reality. A single turbo-
sized appropriately will spool just as quickly. Now, I'm really not defending costs associated with moving to a single turbo setup- just the cost of the upgraded turbos vs the gains. As soon as ATP or some other manufacturer produces a bolt on kit with something like a pair of G25 series turbos, I'd be all over it.
Turbos are great, but the drawback is they are speed machines. They build boost through a feedback loop meaning that once enough energy (in the form of heat and pressure) begins to drive the turbine- which then drives the compressor, the speed increases as a square of the exhaust energy. That's why once the boost comes on it takes so little time/rpm to hit whatever peak boost has been commanded. If that peak boost is early in the rpm band, you get what people refer to as more "torque" which is really more hp at lower rpms. However, with fixed turbine geometry, as mass airflow increases with rpms what follows is very high back pressure and heat between the exhaust manifold and the turbine inlet choking off any additional boost gains since that back pressure will propagate back into the cylinder via valve overlap. This in turns produces what looks like higher boost pressure all the way back to the input manifold. A very nice way to make your exhaust valves glow in the dark followed by detonation and engine replacement. So, you can either have a turbine that allows mass airflow to produce gains at higher rpms making more horsepower, or one that produces more horsepower at lower rpms.
You can find all kinds of examples of identical engines with people looking for more "torque" and then showing 25+ psi of boost, when an engine with a properly sized turbine shows the same or higher peak horsepower at 15 psi of boost. That's because the "boost" pressure is actually turbine back pressure propagation. I'd see this all the time back in my Supra and STI days because people were looking to have maximum "torque" and less "lag" upset because they couldn't make peak hp numbers they wanted.
When I first look at turbo engine for mods, which I did on my ST, I look for base dyno results (thanks
@LokiWolf ) making peak boost (as shown by torque figure) at about 2800rpm. For the reasons discussed, that doesn't bode well for HP. I've even seen 5 star tunes making boost even lower. They are no doubt trying to tune for higher "torque" figures, but it shows the limitations of the turbine housing size which is why if I were shooting for a lot more HP, and adequate twins would not fit, I'd go the single turbo route. And that does NOT mean that it wouldn't work for a daily driver. Given a stock RPM limit of about 6500 on the stock valve springs (I'm curious as to whether these are the same as the Raptor since according to posts, those exact same valvesprings are also used on the GT with a 7500 rpm limit) I'd shoot for a turbo(s) full boost about 4000-4500 rpm which those upgrades are not supplying.
So a long winded post, but one I hope is informative.
