Originally Posted by Corbic

Considering a stock NA 2JZ can't even do 25mpg I'm calling bullshit.

People build 550whp LS engines for pocket change*.

Originally Posted by Kingtal0n

engine size/manufacturer has little to do with fuel economy. It mostly comes down to vehicle weight, rolling resistance, and rotating mass (assuming we are talking about sequential fuel injection and proper tuning). The 2j has a fair amount of rotating mass AND engine weight, so usually swaps in a 240sx end up around 3200lbs and with an automatic trans net about 25mpg. with a six speed it gets a little bit better. and with some lightweight rotating components such as alumnium rods and lightweight flywheel/driveshaft etc... it gets better and better.

But the same applies to say... an LSx engine. You can also get 25mpg out of that in the 240sx chassis... mainly for the same reason- final vehicle weight is around 3200lbs.

It all adds up, Look at an SR20DET that nets 30mpg such as the s15 six speed swap. The low rotating mass (compared to the automatic transmission) and numerically low gearing (3.4x at the rear differential) help keep the energy lost to turning the internals of the sr20det low. In other words, it takes X horsepower to push a 3000lb car the same distance no matter what (forget wind resistance). Now as you reduce engine RPM, you reduce energy lost due to turning components. So as we lower and lower the cruise RPM of that 3000lb vehicle we use less and less fuel to go the same distance. Thats because you have to first calculate the HP required to move that 3000lb vehicle- and THEN add the HP required to turn every single drivetrain component at a given RPM. That cost decreases as RPM decreases, generally. but the cost to push the car remains the same.