Rocker Arm Ratio - Power Multipliers

Rocker Arm Ratios Can Add Power

Mike Petralia Feb 1, 2002 0 Comment(s)
Sucp_0202_01_z Rocker_arm_ratio Rockers 1/8

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.

Sucp_0202_03_z Rocker_arm_ratio 383_test_mule 2/8

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.

Sucp_0202_08_z Rocker_arm_ratio Stamped_steel_rockers 3/8

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 4/8

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.

Sucp_0202_10_z Rocker_arm_ratio Comp_rockers 5/8

Time was short and we needed to test the full-roller rockers next. But instead of going with an aluminum rocker, we installed Comp's top-of-the-line Hi Tech stainless rockers with a 1.5 ratio.

More Ratio Also Means More Timing
As rocker ratio is increased, so, too, is valve timing. Note how changing from the stock ratio, which is actually closer to 1.46:1, to a true 1.60:1 will increase your total valve duration by almost 20 degrees! This geometric increase is true for any cam in any engine.

Rock On
The power our 383-cid Mouse gained just by bolting on a set of Comp Cams Magnum roller tip rocker arms made this whole thing worth the price of admission. The extra power and reduced heat and friction offered by the full roller Pro Magnum rockers in a 1.5:1 ratio also really makes them an attractive bolt-on.

Sucp_0202_11_z Rocker_arm_ratio Crower_rockers 6/8

Crower has developed a new rocker arm to cure many ailments, including the one we ran into. By relocating the hole in the rocker trunion (left), Crower eliminated the problem of pushrods hitting the heads. This also moves the rocker away from the retainer and can correct the problems associated with using longer valves and rotated valve angles.

Hot Rockers
The new kid on the block is definitely Hot Rockers from Everoc Industries. This unique system gives your engine the best of all rocker ratios automatically. By switching ratios as you drive, Hot Rockers gives great low-end grunt and fantastic economy along with a smooth transition to a high top-end ratio for more power. Hot Rockers have basically a full-roller, shaft-mounted design that offers stability and reduced friction at the valve tip, as well. We haven't had the chance to test Hot Rockers yet, but plan to do so soon. We'll let you know all about them in an upcoming story. Check out www.hotrocker.com to learn exactly how this ingenious system works.

Things To Check
Anytime you plan to increase the rocker arm ratio, there are several points to check.

Sucp_0202_16_z Rocker_arm_ratio Diagram 7/8

The Hot Rockers system works by shifting the fulcrum pivot of the rocker arms as you drive. All rockers will shift together, so you cannot run different ratios on the intake and exhaust. The four-position fulcrum provides ratios from 1.1:1 up to 1.75:1 (higher or lower ratios can also be specified). An ECM shifts the ratios at user-defined rpm points via an electric motor mounted on the valve covers.

Piston-To-Valve (Ptv) Clearance
Increasing the rocker ratio adds lift that could cause the valves to tap the pistons if they were close before. Since an increase from a 1.5- to a 1.6-ratio only nets about 0.030 inch total extra valve lift, and total valve lift is not what you're checking when you look for PTV clearance, this may not be an issue. A minimum of 0.100-inch-intake and 0.080-inch-exhaust clearance is recommended.

Retainer-To-Seal Clearance
With your rocker arms properly lashed, check to make sure there's about 0.030 inch clearance between the spring retainer and valve seal at max lift. This check is easiest to do with checking springs installed. You can also measure the total clearance with the valve closed and subtract your total lift to calculate the clearance. Remember to calculate lift using the new ratio.

Valve Spring Coil Bind
This only requires you to find your valve spring installed height and subtract the coil bind height to find out how much lift your springs can take. It's best to keep a minimum of 0.030 inch between a few coils if you want your springs to live.

Pushrod-To-Head Clearance
Increasing the ratio of a rocker arm could make the pushrods hit the head or guideplate. There's really no way to check this without installing the new rocker arms and cycling the engine by hand. Carefully check under and around the guideplate to make sure the pushrod doesn't hit.

Rocker Arm To Retainer Clearance
If you're installing big-diameter springs, make sure that your rocker arms don't hit the retainers. To cure this you can install lash caps on the valve stem tips. There are also many different-height valve locks to try, but that will affect your spring's installed height. Also, not all retainers are created equal, and you might be able to find a different set that will clear.

Sources

Crane Cams
Daytona Beach, FL 32117
866-388-5120
http://www.cranecams.com
Comp Cams
Memphis, TN 38118
800-999-0853
http://www.compcams.com
T&D Machine Products
Carson City, NV
775-884-2292
http://www.tdmach.com
Crower Cams
San Diego, CA 92154
619-661-6477
http://www.crower.com
Erson Cams
Carson City, NV 89706
800-753-2386
pbm-erson
Iskenderian Racing Cams
Gardena, CA 90247-9990
Jesel Inc.
Lakewood, NJ 08701
Everoc Industries (Hot Rockers)
Frisco, TX 75035
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