Here's some calculations. . . .
Assuming: 1) Boost pressure of 14.7 psi, 2) Turbocharger turbine and compressor -- 60% effciency, 3) exhaust gas temperature at the manifold -- 1700 deg F, 4) exhaust gas temp at turbocharger in back of car: 1200 deg F (the website said the turbocharger ran 500 degrees cooler in the back).
This figures come from using the standard air tables.
Compressor work: 47.25 BTU/min/lb air
Turbine energy: 78.75 BTU/min/lb air
exhaust back pressure with turbocharger mounted to the exhaust manifold: 1.77 atms absolute, or 11.28 psi relative pressure.
Exhaust back pressure with turbocharger mounted behind the gas tank: 2.14 atms absolute, or 16.92 psi relative.
The net result will be that you will have considerably more turbo lag then you would otherwise. If the turbo was used all the time (as with a diesel engine), your fuel efficiency would take a hit. For practical purposes, a spark-ignition (gasoline) engine really only uses the turbocharger with the throttle wide open. Therefore, you should not notice that much a difference with fuel efficiency unless you are running the the throttle wide open all the time.
