goodsy replied:

Have been told this many times also. When I asked why he told me its because the exhaust valve and intake valve are open together for a very short time and you need back pressure to keep the fuel air mix from going down the exhaust. This did not make any sense to me so I got the extractors and exhaust anyway. Well worth the money, motor was a lot more free reving propably got about 10% more power and I did not loose any torque down low.