Rocker Arm Ratio - Power Multipliers

Rocker Arm Ratios Can Add Power

Mike Petralia Feb 1, 2002 0 Comment(s)
Sucp_0202_08_z Rocker_arm_ratio Stamped_steel_rockers 1/8

Since the factory stamped-steel rocker arms are notoriously out of whack, we installed Comp Cams High Energy stamped rockers for our baseline because they're built much closer to a true 1.52:1 ratio. These rockers surprised us by making over 465 hp.

So now the math (.750/1.6= 0.470) tells us that the pushrod centerline is roughly 0.030 inch closer to the fulcrum pivot. This arrangement does more than just multiply cam lift; it also multiplies the loads on the pushrod and rocker arms, making the proper ratio critical. Too much ratio will open the valves too quickly and can cause valve float at high rpm. It also multiplies the spring pressure seen on the cam lobes, so running too much ratio can wipe out a flat-tappet cam in no time. Thankfully, the cam manufacturers have studied these problems, and most won't even sell you too much ratio unless they feel you've really got your act together. Since an increase in ratio also increases the loads on the rocker and its mounting stud, you should stiffen the whole assembly up in order to keep the rockers from wobbling all over the place.

That's what stud girdles are for and why shaft rockers are so much better, even yet. A stud girdle ties all rocker arms together, distributing the loads from any one to all eight. Shaft rockers transmit the loads directly to the cylinder head without using any rocker studs at all. That's why companies like Comp, Crane, Jesel, Crower, and T&D can offer shaft rocker ratios up to 2:1 but won't go any bigger than about 1.8:1 with normal stud-mounted roller rockers.

Makes Your Cam Bigger, Too!
An increase in rocker arm ratio nets more than additional lift. It will also change the cam's duration characteristics. Because the increased ratio effectively speeds up valve movement, that means the valve will reach any opening height sooner than it would with a lower-ratio rocker arm. Higher ratios open the valves quicker and close the valves a little later. Since the increase is symmetrical on either side of the cam lobe centerline, a higher ratio will lengthen the overall valve timing, making your cam act bigger. The higher ratio also causes valve timing to increase proportionally as the valve opens farther (see chart above).

Sucp_0202_09_z Rocker_arm_ratio Roller_tip_rockers_test 2/8

The next logical test was Comp Cams Magnum roller-tip rockers in a 1.52:1 ratio. They add power by eliminating the friction at the valve tip, and since they're made from chrome-moly steel, they're much stronger than stamped rockers, so flex is no longer a power limiter.

1.6 Trouble
We know from experience that a higher-ratio rocker makes more power in engines that would normally need a bigger cam. But we weren't able to prove it this time. After installing the 1.6s, we were shocked to find the engine dropped more than 40 hp! As we explained earlier, rocker arms increase the ratio by moving the pushrod cup closer to the rocker fulcrum pivot point. That's where our problem was. The pushrod was contacting the top of the clearance hole in the cylinder heads with the 1.6 rockers. So, you can see that swapping rocker arms involves more than just deciding what ratio to run. In fact, this problem is very hard to spot because it's typically hidden beneath the pushrod guideplate, which is why it took us a while to find it.

Westech came to our rescue, however, and dug up some old dyno tests comparing 1.5s to 1.6s on other small-blocks. Those results found the 1.6 rockers making more than 20 hp over the stock 1.5 rockers. But, that engine was equipped with a smaller cam than this one, and it seemed to really need the additional lift and duration afforded by the higher ratio.

One cool thing we learned from this is that smaller cams really do make more low-end power. The pushrod binding caused by the 1.6 rockers bled the hydraulic lifters down and didn't offer full lift or duration of the cam. So, in effect, we were running a much smaller cam. How small? We don't know, but torque at 3,500 rpm with the binding pushrods jumped by almost 30 lb-ft! How can you duplicate this low-end power increase, you ask? If you're running a very mild engine and are not worried about power above 4,500 rpm, you can reduce the rocker arm ratio instead. That may not sound right to most of you, but it helps low-end power.

You won't get the same peak power you would with a higher-ratio rocker, but if your engine never sees that rpm, then why bother? This is a great idea for tow vehicles, 4x4s, and boats that never rev very high and need all the bottom end they can get. We know of at least one company making reduced-ratio rockers that we might try. Crower Cams has 1.2:1-small-block and 1.5:1-big-block rockers that they use to "break-in" flat-tappet cams with stiff racing springs. Whichever rocker arms you choose, do a little research before you buy. Maybe you can borrow a higher-ratio set from a friend and try them out to see if they'll fit and, more importantly, if more ratio will make more power for you.

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