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Have you ever wondered by all the Bi-turbo V6 engines are always less impressive than you expect? Specific Output always seem to be worse compared to 4-cylinder counterparts. Maximum Torque seems to always arrive later and the engine always seem a tad lethargic. In short, they always seem to SUCK relative to the 4-cylinder turbo engines the same manufacturer puts out. They really SUCK compared to Inline-6 legends like the Nissan RB26 or the contemporary BMW N55. For example:- GM made a 2.0T (LTG) with 272 hp @ 5,500 rpm with 260 lb-ft at 1,700 rpm. That is 136 hp/liter and 130 lb–ft per liter. This same engine is later tuned to permit more mid-range boost with torque rising to 295 lb-ft @ 3,000 rpm (147.5 lb-ft/liter). This engine operates on 18 to 22 psi of boost and is able to get to full torque at 1,700 rpm or 3,000 rpm. At the same time, GM made the 3.6 Turbo (LF3) with 420 hp @ 5,750 rpm with 430 lb-ft @ 3,500 rpm. That is 116.7 hp/liter and 119.4 lb-ft per liter. This engine operates on a paltry 12 psi of boost but can’t seem to get to full torque until 3,500 rpm. Throwing really fancy and expensive stuff like Titanium Connecting rods and turbine wheels at the engine helps a bit. But, even the uprated 3.6TT (LF4) still took 3,500 rpm to reach 18 psi. A boost level the ubiquitous Malibu engine has no problem hitting at 1,700 rpm. Why? REASON The reason is very simple. And, it is not something you can conquer with expensive turbos or fancy materials. Just look at the diagrams below... In a 4-stroke engine, the exhaust valve opens once every two rotations of the crank for about half a rotation of the crank. In a 3-cylinder engine, or any bank of 3-cylinders, exhaust flow to the turbo is interrupted for significant periods with the exhaust valves closed on ALL cylinders. A V6 Bi-turbo engine is essentially two Inline-3s with each turbo is feeding off one bank of 3-cylinders. In an Inline-4, one exhaust period is always beginning while another is ending. Hence, while exhaust flow diminishes, it never really falls flat completely. In an Inline-6 the periods overlap generously ensuring a continuous exhaust flow to the turbine that does not diminish appreciably throughout the entire cycle of the engine. SOLUTION The solution is to use ONE Turbo for both banks of cylinders in an V6. But that is problematic. You either have to route the exhaust from one side to another -- which is both a packaging nightmare, a source of leaks and a measure which causes it to lose a lot of energy (heat) getting to the turbo. Or, you can use a Hot Vee design with the exhaust exiting in the valley of the Vee. A Hot Vee is problematic in the V6 because most of them are 60 degree engines which has little to no room in the Vee for a turbocharger. In addition, a single large turbo will stick through the hood in a Hot Vee. Hence, a Hot Vee is seldom used. When it is used, it is employed in a 90 degree V6 with two turbos -- which defeats still results in interrupted flow to the turbines. Hence, V6es continues to suck in turbocharged applications -- then, now and (probably) into the future!
