Bro.chill.
Turbos do add mpg. Several car makers agree. 80s oil crisis saw tons of turbo vehicles.
Sts turbo has empirical evidence that adding nothing but their turbo and programming yielded the most mpg increase over any after market setup at the sema show.
They help with scavenging. Increase air density and makes the motor breathe easier when off boost.
Throttle butterfly has very little to do with overall engine efficiency. Its there to retard airflow. The maf or map sensor will tell ecu to inject fuel based off airflow. It will enrichen based off load and temperatures to prevent knock. Valvetronic motors are wide open throttle when warmed and use valve timing to retard engine performance. Reduction in vacuum in the intake tract in front of the throttle will allow the engine to breathe easier.
Big reason you don't see mpg increases in the 335 vs 328 is that they were going for performance. I bet if you drove the 335 in 6th gear everywhere it would yield more mpg over the 328 which lacks the low end torque that the 335i's tiny 1500rpm turbo delivers.
In any respect.turbos get more efficient with load and complimentary gearing would propel it much better. And give higher mpg.
I've also never heard of a company bypassing the turbine for more mpg. In off boost situations the hot side aids in exhaust scavenging and why you'd chose to negate that is beyond me. They are basically operating off wasted glass energy until they need the drive pressure to add power (boost)
All the best mpg cars out there are turbod for a reason. Bit it doesn't go without saying its not automatic. It must be done in a complimentary fashion otherwise your lowering mpg.
And since engine friction is non linear, rather exponential in regards to rpm increase. Using less rpm generally means better mpg. Turbos help lower operating rpm. Too much load will obviously be enrichened .
I don't really think you quite understand how turbochargers work fully.
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Originally Posted by serialk11r
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I think you don't understand what I'm saying. A turbo does not usually increase the efficiency of the engine, if you're lucky under certain heavy load situations it will help a tiny bit. If you reduce the engine size and then turbo it, you are reducing engine efficiency most likely but the reduced engine size allows one to run the engine at higher load and thus better overall efficiency.
The biggest 2 distinctions that need to be made when talking about fuel efficiency and turbos are 1. throttle plate or fancy intake valve control system 2. electronic wastegate/bypass or not.
With a throttle plate, increasing part load efficiency is a lost cause. As long as the throttle is partially closed the turbo can't do anything because it can only blow off the pressure it builds. With a variable duration and lift system the turbo can build boost even under part load and increase efficiency because the turbo is able to "recycle" a little bit of energy from the exhaust stream (more precisely, it is the sound/shock waves/pulses that give you "free" energy).
Then there's the question of bypassing the turbine under part load, which should be done whenever the turbine efficiency drops past some point. The average turbo setup has a spring actuated wastegate that cannot do anything about this.
Air/fuel ratio and timing is going to depend on temperature and knock resistance. A relatively big cam along with turbo can potentially see higher efficiency at near maximum load, if everything is set up correctly.
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