Originally Posted by Advevo
Last thing bmw wants is a high power 4 cil turbo with a lot of lag. If you want a lot of horse power out of a 4 cil engine you need bigger turbo s and that means more lag. A new M2 needs at least 350hp to make sales over a M135i.
Why is the 1M that good because it has almost the throttle response of a normal aspirated engine. A lot of base power is already coming from the 3.0 6 cil. The boost is not that high. Then you a get a nice throttle response.
I have driven/owned evo s and subaru s with 350hp and that is a lot throttle lag compared to the 1M.
I'm pretty sure what your talking about is a non-linear torque curve due to turbo spool. This has very little to do with the throttle response
of the car. You can make the gas pedal as twitchy as you want but it won't make any difference until the RPMs are high enough and the exhaust gas is creating enough pressure over the impeller blades to spin the turbo and force air into the throttle body. The 1M is actually a great example of this. M mode mainly remaps the throttle response to make the car "feel" faster many dyno's show there is little to no change in actual horsepower with M engaged.
Also, one can build a 350+ HP four cylinder turbo charged engine with a linear torque curve. Conceptually its fairly simple, you take a 2.0 to 2.5 liter cylinder motor tweak the compression ratio and install two turbos. One that has a low RPM operating efficiency and one that has a high RPM operating efficiency. You then tune them to roll off / on in sequence such that your torque curve remains mostly constant. A good engineer could even channel exhaust gas based on RPM to control spool and reduce lag and control heat and wastegate duty in the smaller turbo at high RPM.
In practicality this is more expensive, technically complex and difficult to maintain than traditional parallel turbo setups. In the past it has not made much sense to go this route due to these factors. In short, its easier to make those HP numbers with a V6 and two small turbos spinning at the same time. However, emissions standard and gas prices are forcing car builders to get creative with engine design and dual stage turbos is one option.
All that said, it can and has been done one example of a sequential twin turbo production car is the Mark IV Toyota supra.