Exhaust exiting out the back

[Shrek]

The beast woke up today (and it was angry ); just before i put in the exhaust system in, what would happen if you would of exit them out the back into that low pressure there and the underair sucking out some of the gases too to get more horsepower, what would i get in terms of FE, and i was also thinking(probably not doing though), what if you had them exiting through the bumpers (cut out a hole just big enough and route them through that with custom pipes) out where it could also get into even more of a low pressure zone?

[maximilian]

An automotive version of base bleed as well (marginally anyway).