I've seen lots of guys use 1.6 Roller Rockers for both intake and exhaust on their LT1 and LT4 motors. I have also seen some guys use different sizes for exhaust and intake. Usually, intake is larger with the combos being 1.7/1.6, 1.65/1.6, 1.6/1.5 and so on. What is the benefit of using different sizes? Is this related to the cam you choose? What combo would I want with a set of AFR LT4 210 heads on a 396 stroker that I'm planning? Thanks for the enlightenment.

 

It really just depends on the cylinder head flow at various valve lifts and the lift and duration of the cam you are using. If the heads offer more flow at higher lifts, then higher ratio rockers may provide a small benefit. But if the heads are aleady at maximum flow, adding more rocker ratio is not going to help as much, if any. Alot of times, you might actually see some performance gain by running more lift on the exhaust side, rather than the instake side, to make up for a less efficient exhaust port or restrictive exhaust system.

Say you are running a 1.5 ratio rocker and it yields a .500 valve lift, with the AFR LT4 210 competition ported heads, that would give you maximum of 276 cfm which could potentially support around 550hp, by going with a higher ratio rocker like a 1.65 on the intake that would yield a maximum of akmnost .550" valve lift and 282cfm. Theoretically, you could potentially see around a 10hp increase. However maximum lift and flow is just one benefit, another benefit is that the higher ratio rockers will open the valve faster since is is multiplying the lobes lift at a higher rate, so the the valve will spend more time at higher lift across the lift duration, increasing low and mid rpm torque and power as well. Although higher rocker ratio's do not change lobe duration, they do increase theoretical duration (when measured at 0.050" for example) at the valve. It's not a significant amount, a few degrees, but it is a small benefit as well,l allowing a little more time to either fill or evacuate the cylinder.

Alternatively, be aware of valve train geometry, coil spring binding, and interference between the rockers, retainers, guides etc. It's gerenally not an issue, but it's always wise to check.




[Modified by Monty, 11:32 AM 7/31/2002]

 

Thanks Monty. Great explanation. I will have to talk to my tuner to establish what sizes would work the best on my planned setup. :cheers:

 

Thats an awesome setup your working on. Some guys in texas in the XCCT club have strokers like that. You can expect 450+ rwhp.