Question: why do folks not add the 1.6 rockers to existing motors to increase cam lift? I read in the above mentioned thread that you have to "subtract" the "gain" from the exhaust side. So if this is the case, here are some hair brained thoughts on the idea. What would happen if you used the 1.5 rockers on the exhaust and the 1.6 on the intake? Answer Courtsey of our own moderator Marv Davis There a lot of things involved in intake and exhaust flow, but the magic number is around 78-80% (depending on which engine builder you takl to). You want the exhaust to flow 80% of the CFM number the intake will flow. The exhaust can be less because the exhaust is forced from the chamber by the piston rising in the cylinder. Just keep in mind the intake charge is drawn into the clinder by the pressure drop (vacuum) created as the piston decends the bore. There is a BIG difference between the vacuum intake side, and the forced exhaust stroke. That's why forced induction like blowers and turbos make such a DRAMATIC increase of power with only 5 and 6psi of boost. Just for simplistic numbers, let's say you have a intake port that flows 100cfm @ .500" lift. Good performance could be expected if the exhaust port flowed 80cfm. Let's say your exhaust port is lazy and only flows 65cfm at .500" lift. You can do one of two things to bandaid things. You can increase lift on the exhaust side in hopes to bring the exhaust flow up to 80cfm, or you can increase duration of the exhaust valve opening. If the exhaust port is a big restriction, opening the exhaust valve wider generally doesn't help much, some, but not much. So we increase the duration the valve is open, AND increase lift with the hopes it will flow the magic number. Then too you have what we call a 'knee' in the flow. The port, no matter how big of a valve, or how much it is open, can just lay over and simply won't flow more. Increasing lift on a very restrictive port like this can cause crazy pressure drops and pulses in the exhaust that can actually impeed flow even more. So the trick is to find a lift that takes advantage of the port design and flow capabilities, and match it with a duration that compliments the cylinder pressure and rpm range for the particular engine and it's use. This crap gets totally out of hand when you start theorizing scavanging effects of 'overlap' (being when the exhaust valve is open at the same time the intake valve starts to open). This will scavage the chamber of most all of the spent fuel, and even draw some of the fresh air/fuel mixture through the chamber into the exhaust stream. Cams with no duration and nearly no overlap will always leave some of the spent gasses in the chamber. You make power by burning fuel, if your not purging ALL of the spent gasses, and replacing it with fresh a/f mixture, your not making full potential of the motor. Cam designers have been looking for tricks to accomplish this efficiently since,, well since forever! There is also the vacuum peaks created as the pistom travels down the cylinder. It's not when the piston forst starts down the bore, because piston speed near TDC and BCD are at their slowest. And because the peak piston acceleration happens something like 60 or 70 degrees after TDC, you need to get the valve open the 'right' amount to take advantage of that greatest pressure drop during the stroke. Cam design turely IS rocket science. The cam companies do countless hours of R&D then live testing on the dyno, tweeking every grind to fit the 'best average' of what their customers are wanting to wo, and what equipment they 'typically' have. Sooooooo, in what you asked about switching from 1.5 to 1.6 rockers,,, are YOU smart enough to outthink all the design work that goes into the cams they offer for our cars???? Actually yes you are. The one thing you need to know is flow numbers for the heads your running. Not just peak cfm, but flow numbers at lifts from .200 to .600". Then you look at the specs for the cam you want to run and see if it looks like that extra .03" of lift of higher ratio rockers would really help that much,,, or not. Then when you think you have it all figured out,,, either go to the dyno or to the track and see if you really accomplished anything, or just wasted a bunch of time and $'s. Now I'm not saying 1.6 rockers are useless, and I'm not saying they will be this miricle power adder. For the most part, you will see very little gains, unless the added lift compliments the package you stuff them in to.