Originally Posted by GunnerTHB
I have a 5.3 Vortec....Cold Air Intake, Exhaust, and a Computer Tune. I am pretty sure this engine come stock with 1.7:1 Rockers. I have been thinking about putting 1.8:1 Rockers on it to get a little bit more out of it. I know I will have to retune the PCM, but my question is: Will I loose a lot of low end torque with the additional valve lift? I have calculated the valve lift with both rockers and the 1.7's are .466 intake/.456 exhaust, while the 1.8's would give it .493 intake/.482 exhaust. I couldn't find much information other than the lobe lift on the stock cam, and it is .274 intake and .268 exhaust. Any ideas? Thanks
You'll give up some bottom end and get it back on the top end. Where bigger ratio rockers get you is that the amount of lift per degree of cam rotation increases, not just a change to max lift. This makes the engine act as if a bit more cam is in it. At low RPMs the incoming charge doesn't enough velocity to over come reversion pressures during overlap and reverse pumping of the piston as it starts coming up after BDC. So some incoming charge gets pumped back into the intake, this reduces the density of the charge to be compressed in the cylinder and is seen as a power loss. This continues till the velocity of the gases in the ports is sufficient to overcome these reversal effects on the gas dynamics. Usually takes about 4000 RPM to do this.
Porting has the same affect as well for the same reasons. Porting, more cam, more lift, bigger ratio rockers all are additive effects the more of these you do the more power is traded from the bottom end to the top RPMs.