Aww, you guys! Turbocharging is one of my very favorite topics
ever.
I'm still learning about forced induction and this is way down the road anyway, but tell me why a remote mount turbo is a bad idea.
A turbocharger's turbine is driven by
heat energy of the exhaust. Contrary to what some believe, a turbine is
not simply a fan that 'catches' exhaust airflow. The further you place the turbine from the engine's exhaust outlet, the cooler that exhaust air becomes, and thus the less heat energy it contains. If you take a turbine which is properly-sized for mounting near the engine, then mount it twelve feet away from the engine, it will perform poorly. To compensate, a savvy installer who is hell bent on mounting the turbo at the rear bumper will re-size the turbine accordingly.
what are the other drawbacks? Would lag time be considerably worse than an under-hood setup?
As has been mentioned, turbo centersections require a constant supply of oil. This means you'll need to run oil from the engine to the rear of the car, allow it to run through the turbo, then collect it and send it back forward to the oil pan. On top of that, if you choose to run a water-cooled turbo (which I strongly suggest both for durability as well as reducing any chance for oil coking) then you'll need to run engine coolant from the engine bay back to the turbo and then back forward to the engine. So now we have four lines running front-to-rear under the car, and--oh, yeah--we haven't yet discussed getting the charge air to the throttle body.
Speaking of, the answer to your second question is yes. It will take your charge air considerably longer to travel through twelve feet of pipe than it would to travel through two or three feet of pipe. Having said that, there is an important consideration: how will your car control its fuel injection? On a speed density setup, this additional lag time will cause no significant drawback. On a mass flow system, there will be a considerable delay between the airflow meter (located before the turbo) and the combustion chamber, which will throw off your air/fuel ratio any time there is a sharp/sudden change in the turbo's boost output.
I can't help to think that a hot turbo housing cooling down rapidly wouldn't cause some kind of bad outcome.
Turbine housings are typically made of cast iron. However, their nickel content can vary. Higher nickel content is desirable, as it increases the housing's resistance to thermal cracking. On my (former) FWD Chryslers, the factory added more nickel to the later models for this very reason. Driving through puddles could splash water up onto the turbo, which could allow for cracks to form on the early low-nickel turbines. I would
presume modern turbos have a decent amount of nickel, though that's just a guess on my part.
I've heard that a remote mount turbo with all the additional piping acts like a built in intercooler, which is very good.
Rick Squires used to tell everybody this when promoting his rear-mount systems, but all the engineers I've consulted (either in person or by reading their books on the matter) tend to disagree. Even a 12-foot piece of exhaust pipe makes a very poor intercooler, as it contains no turbulators or fins to "catch" heat and transfer it outside the pipe. Without these, the charge air has little opportunity to give up its heat... meaning you'll want to install an intercooler all the same.
All told, is there really a reason a turbo mounted to the exhaust manifold is out of the question?
This really is the important question. A turbocharger will never perform as well as when it is mounted close to the engine's exhaust outlet.
I don't know enough to know what yet, supercharger vs. turbo, still learning and looking at options.
Forced induction can be something special--almost magical, even. To maximize your satisfaction, you need to make a number of decisions up front. You should not simply ask yourself "turbo verses supercharger." Instead, you should be asking "turbo verses centrifugal supercharger verses positive displacement supercharger." Each of these has advantages over the other two, and each has drawbacks compared to the other two.
One thing I was looking into when I was considering adding a turbo to a FZJ80 was using methanol instead of an intercooler. You can buy kits that inject the methanol under boost, negating the need for an intercooler. The methanol helps control detonation and makes it safer to run under boost.
While this is true, it is looked down upon by a number of engineers and is often seen as a band-aid for a proper setup. By comparison, an air-to-air intercooler will never run out of methanol and cause you to melt pistons unexpectedly. While I do know some people who have run water injection on their street-driven turbo cars, personally I would never suggest such a setup to anyone.
