[Ecky]

Reading "How do you get your car to warm up quickly?" I was struck with an idea. I haven't fully thought it through yet, and would like some feedback. It might be completely idiotic for reasons I haven't considered yet.

My premise:
Having too large an exhaust pipe can rob an engine of low-end torque because it disrupts scavenging which is carefully tuned by the manufacture to assist the engine in removing exhaust gases, right?

If that's correct, I'd surmise that it increases engine efficiency. What happens then, if there is decreased air pressure on the exhaust side? Would it function something like a reverse-turbo, giving "free" power, so long as you don't actually suck any unburnt fuel out of the cylinder during the period of valve overlap that many engines have?

Of course, something would have to create the vacuum, but you don't necessarily need a belt-driven turbine, which would almost certainly defeat the purpose. What about using something like the venturi effect? Use restriction someplace like behind the radiator (where you can't help but have turbulent airflow anyway) to create vacuum, and run a small hose to somewhere on the tailpipe.

If the added vacuum-assistance is non-negligible, would this do anything for engine efficiency?

Thoughts please! I may go out tomorrow with a shop-vac and some duct tape and experiment with different positive and pressures on my exhaust pipe. If I pull the IACV, I should be able to tell whether I'm generating power by the idle RPMs.