Well this is my whole quim in a nutshell with this system. Whenever you tune an intake manifold you tune the cylinder as whole. Whenever you "tune" an intake manifold you tune the runners by volumn length and crossection. Basically volumn because the other 2 equal V.
Whenever the valve opens Thats when the air starts moving in the ports. Whenever it starts to move this starts the pressure waves that help tune the engine to supercharge it. Whenever you have 2 intake runner lengths like that you only get half the ramcharging effect. But thats not the only disadvantage. Because the volumns are diffrent for each runner the pulse is also weakened this way as well. Because whenever you tune for low rpm you want a long small cross section runner. Whenever you go for high rpm you want a thick short crossection runner. If you try to go half and half the pressures waves arnt nearly as strong for either. It gives an incredibly flat curve. BUT its an all out effort for drivability. You sacrifice alot at the top and in the midrange for your thickness at 2000 rpms.
THats just the intake manifold part. THe cam timing part is a mystery to me. Because the entire time you have a cylinder open you want the maximium amount of air flowing into it. Everyone goes after rocker arm crushing ramp rates that take 400 lb springs for a reason. If the valves are going to be open in a cylinder they want them open damnit. By having a cylinder open past bdc you are loosing power across the board if your intake manifold isnt ram charging the cylinder. Its simply going to push the air back out. It doesnt help the tuning of the intake manifold to any degree. A slightly larger lobe requires a slightly longer runner and vice versa for a smaller lobe to tune to the same rpm.
The diffrence inflow between 8 degrees of seat duration just isnt anything though. 8 Degrees of seat duration is probably only 2 or 3 degrees of .050 duration. Just not enough diffrence eto make a large impact.
On the zetec engines our first cam upgrade to street/strip is usaully about 40 degrees of seat duration and 28 degrees of .050 duration. And the ramp rates of the cams are MUCH more aggresive so that change is even more radical then 40 degrees of oem seat increase.
Every penny counts but on a larger scale the diffrence of 1 valve opening at a diffrent point is just not much to get excited about. Im pretty convinced that it causes a swirl effect in the cylinder like others have stated for a slightly better burn and a better sniffer test.
Last edited by Travis; 04/21/04 12:20 AM.
