All the tin is important, but the tin at the rear of the engine is REALLY important. After the air has cooled your engine it exits out the lower rear of the engine bay . . . but with the rear tin missing, the hot air will be pulled back into the top side of the engine and recirculate. After a few trips around it's way to hot to absorb any more heat and you engine will keep getting hotter and hotter . . . at some point, something will snap, crack, pop, break or lock down. So, like Big D and Bugnut said . . . Get the tin!!!