Rocker Arm Ratio - Power Multipliers

Rocker Arm Ratios Can Add Power

Mike Petralia Feb 1, 2002 0 Comment(s)
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The debate over the optimum rocker arm ratio has dragged on since the invention of the pushrod V-8. Even though Chevrolet made the decision easy for us when it engineered the 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 mean there's power lost with them. By simply equalizing all rocker arms to a consistent 1.52:1 ratio we've found there's much power to be gained. And by increasing, or more precisely optimizing, rocker ratios to alter the opening and closing events of the cam you can build even more power. SUPER CHEVY wanted to find out what the best ratio really is, 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 just finding 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's 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. So we strapped a small-block to Westech's Superflow 901 dyno in hopes of learning all we could from a set of rockers.

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Westech Performance Group loaned us its 383-cid Mule for the abusing. It has Trick Flow heads, a Comp Cams 292H Performer RPM intake, MSD ignition, and a 750 Demon drinking 92-octane Unocal 76 pump gas.

Since there's no point in only testing stuff that is way above most readers' budgets, we chose a mild 383-cid stroker for our pulls. The engine is one of Westech Performance Group's test mules, and they were willing to lend it to us for a day. The engine consists of a cast crank, stock 5.7 rods, and Speed Pro 9.5:1 forged pistons. Trick Flow aluminum cylinder heads with large 1.46-inch diameter springs and an Edelbrock Performer RPM intake, wearing a 750-cfm Road Demon double pumper, round out the breathing package. Westech 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 rockers arms first. But Comp advised us that stock rockers are so bad, we'd probably never get a consistent pull with the relatively strong valve springs 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. Last, 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 swap the intake and exhaust ratios to see what effect that would have. You'll have to read on 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 under-$150 price tag worth it. The full roller 1.5s did an excellent job of pumping even more horsepower, and the reduction in oil temp that comes 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, yet obviously can't move the valve or rocker arm stud, you have to move the pushrod cup closer to the pivot centerline.




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