Power Multipliers

Rocker-Arm Ratios Can Add Power

By Mike Petralia

The debate over the optimum rocker-arm ratio has dragged on probably since the invention of the pushrod V-8. Even though Chevrolet made the decision easy for us when it engineered its small-block to run around a 1.5:1 ratio (COMP Cams says most stock rockers are actually about 1.46:1), it's well known that the inconsistencies of stock rockers and the friction and heat they create means there's power lost with them. By simply equalizing all rocker arms to a consistent 1.52:1 ratio we've found there's power to be gained. And by increasing, or more precisely, optimizing, rocker ratios to alter the opening and closing events of your cam you can build even more power. ENGINE MASTERS wanted to find out what really is the best ratio, but didn't have time to debate the issue. So we thought we'd just test it instead.

ROCKER DECISIONS
There's actually much more to determining which rocker arms are best for your engine besides finding just the right ratio. Stock rockers flex and make heat, but there are roller rockers made out of chrome-moly and stainless steel, as well as the familiar aluminum versions engineered to cure those problems, and then there are shaft rockers to consider, too. There are also some trick new rockers available with unique designs like Crane Cams' ingenious variable-ratio Radi-Arc rocker arms and Crower Cams has been working on rocker arms that fix many of the problems associated with increasing the ratio and/or lengthening your valves. There's even a trick new electronically controlled variable ratio shaft rocker conversion kit called Hot Rockers for street small-blocks that we'll tell you more about in a bit, but we wanted to cover the basics for this test.

Since there's no point in always testing stuff that is way above most readers' budgets, we chose a mild 383-cid stroker for our pulls. The engine consists of a cast crank, stock 5.7-inch rods, and Speed-Pro 9.5:1 forged pistons. Trick Flow aluminum cylinder heads with 1.46-inch-diameter springs and an Edelbrock Performer RPM intake wearing a 750-cfm Road Demon rounded out the breathing package. We installed a COMP Cams 292H Magnum hydraulic single-pattern camshaft to make sure that dual-pattern lobe profiles would not affect our results. Hooker 1 3/4-inch roadster headers were also used.

For the actual tests, we planned to baseline the engine with stock stamped-steel rocker arms first. But COMP advised us that stock rockers are so bad, we'd probably never get a consistent pull with the relatively strong valvesprings we were running, so we opted for COMP's High Energy stamped steel rockers for the baseline pulls. Then we switched to COMP Cams' Magnum roller-tip chrome-moly rocker arms with a true 1.52:1 ratio. Lastly, we tried COMP's full-roller Hi-Tech stainless steel rocker arms in both a 1.5:1 and 1.6:1 ratio. We even tried a 1.5 intake and 1.6 exhaust rockers combo and then planned to reverse the intake and exhaust ratios again to see what affect that would have. You'll have to read on a bit to find out about the trouble we ran into and what we learned there. The best power-per-dollar gains came from switching the stock rockers to the roller-tip Magnum rockers with the 1.52:1 ratio. The power we gained made their low-dollar price tag worth it. The full roller 1.5s did an excellent job of pumping up even more horsepower and the reduction in oil temp that came along with the reduced friction these rockers offer make them an easy choice.

HOW ROCKERS ADD POWER
The rocker arm mechanically multiplies the cam's lobe lift. It does this by moving the pushrod closer to the fulcrum pivot point than the valve stem tip is. A simple example would be: if the valve tip centerline is located 0.750-inch away from the rocker fulcrum pivot centerline, then a 1.5:1 ratio rocker would have the pushrod cup located 0.500-inch from the pivot centerline (.750/1.5=.500). When you increase the ratio to 1.6, you obviously can't move the valve or rocker arm stud, you have to move the pushrod cup closer to the pivot centerline. So, now the math (.750/1.6=.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, lifters, 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 much 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 causes valve timing to increase proportionally as the valve opens further (see chart).

1.6:1 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 when 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 rubbing 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 guide plate, which is why it took us a while to find it.

We dug up some old dyno tests comparing 1.5s to 1.6s on other small-blocks, which 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 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 3500 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 motor and are not worried about power above 4500 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. Several cam companies make 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.