Originally Posted by ap72
it's actually 100% wrong... a 5cfm gain on the intake is worth more than a 5cfm gain on the exhaust, even though the % difference is smaller.
Also, the Vortec intake tract is better than previous sbc heads, but the exhaust is NOT worse, its actaully a hair better too, just not as much as the intake. People get caught up in the intake/exhaust ratio and fail to see what they're really looking at on the exhaust side. Also, there's NO crossover, which is a pretty big deal compared to most other stock sbc heads.
Can you please explain why you think vinnie is wrong since i know he is right and explained it clearly. Can you do the same for your side.
How does this 5cfm reference actually make a difference in the disscussion. Its just jibberish. It doesnt even relate to the real world since the ports cfm rating come from a flow bench that does not scavenge the exhuast or flow at the speed that is even close to the actual exhuast speed at redline. Flow benches push air thru the ports around 25-50 mph the exhuast will come out of the exhust port well past 300 mph.
If the ratio is not correct the engine will not be effecient the VE will drop and will never hit its true power potential.
During overlap the exhuast scavaging will actually pull the a/f mix from the intake port while both valves are open and even let it run out the exhuast port then the pressure wave kicks back at the collector and pushes the a/f mix back into the cylinder and possibly over 100 percent ve with proper cam and tuning.