Changing the rocker arm ratio (RAR) on your engine parameters can make a substantial difference in the amount of power the engine is producing and will also have an impact on where in the rpm range the engine is making peak horsepower. However, this adjustment should not be considered without taking the proper precautions to determine that it is suitable for your combination.
Let's review what rocker arm ratio (RAR) is and how adjustments to it could benefit your combination. A rocker arm is essentially a lever and the RAR is the parameter that depicts the amount of leverage the rocker arm applies between the top of the pushrod and the top of the valve stem. The lever action acts such that when the pushrod rises, the valve stem is depressed a distance as defined by the ratio of the lever lengths which form the RAR. If the RAR were 1:1, then the lift the camshaft produces at the pushrod, would be the same lift that is measured at the valve stem tip for valve opening. This is theoretically true, but never actually happens, as RAR for a Pontiac is commonly 1.5:1 or 1.65:1, resulting in an amplification of the valve lift by this ratio. With a 1.5:1 RAR, a camshaft lobe lift of .350" will become .525" lift at the valve stem (.350 x 1.5). With no other changes except a 1.65:1 RAR, the same camshaft would deliver .578" lift at the valve stem (.350 x 1.65). This is an increase in lift of 10%, which makes sense because the RAR of the 1.65 is 10% larger than the 1.5 RAR.
Let's take a closer look at the lever itself. The RAR is determined by taking the distance from the roller tip to the centerline of the trunion (trunion length) and dividing by the distance from the centerline of the trunion to the pushrod cup (cup length).
Everyone loves math, so let's do some. A typical Pontiac rocker arm has a trunion length (X) of 1.5". If the cup length (Y) is measured as .905", then the RAR is X/Y or 1.657:1. To function properly, a Pontiac rocker arm needs a trunion length between 1.4" and 1.5". If your engine is assembled, an alternate method of determining the current RAR in use is to divide the maximum lift measured at the valve stem by the lobe lift shown on the cam card.
Let's assume we have an example engine that we want to assess an RAR adjustment for a potential hp gain. We can either remove a rocker arm and measure it or use the alternate "if assembled" method to determine the current RAR. Now that we know what RAR is currently on our example engine, let's determine what other RAR we could potentially change to. Measure potential rocker arm replacements to determine their RAR or alternatively, the advertised RAR could be used but that is nominal and does not always reflect the exact RAR. Measuring confirms the RAR and gives an exact value to work with.
Let's evaluate what this adjustment might do for us. If we determine the RAR on the engine is 1.5 and want to consider a change to 1.65, this is a 10% increase in the camshaft lift. If the camshaft flow was a limiting factor on the engines power level, this most likely will result in a power gain. However, if the port flow, carburetion, exhaust, or any other parameter was the limiting factor, this adjustment could result in no power gain at all or even a reduction of power. Additionally, the limiting factor could have been the camshaft flow at the exhaust valve only (or intake only), which would mean that the increased RAR would have only been needed on the exhaust (or intake) to create a power gain.
Let's determine our modified camshaft lift. If our example engine had a .525" valve lift at 1.5 RAR, we can divide the valve lift by 1.5, then multiply by 1.65 to arrive at our new lift. This is .578" which is a 10% increase and naturally comes with an expectation of a hp gain.
Changing the RAR for more lift without evaluating whether it results in a hp gain is not recommended. The curious will ask "Why not just replace the 1.5 RAR with 1.65 RAR...I mean more is better right"? Sometimes more is not better. The increase in the RAR does come with some negatives. The larger RAR applies more pressure at the pushrod tip stressing the pushrods (10% more pressure). This might result in accelerated wear, a failure or at the least, an expenditure to buy stouter pushrods. Also, in our example the valve is being opened farther by 10%. The valve spring will have a higher pressure at this lift, resulting in a larger friction hp component (i.e. less net hp). Additionally, there are a series of clearances that need to be checked to ensure that interference between components is not being created.
Important items needing clearance verification:
- The valve spring can compress this extra distance and not approach coil bind. Coil bind results in breakage. Even approaching coil bind creates additional heat which reduces valve spring life expectancy.
- The bottom of the retainer does not contact the top of the valve stem seal, which destroys the seal and allows oil to leak into the combustion chamber. Note that the measured distance will be reduced even more at high rpm where valve train components will float and close the clearance further.
- The valve head does not contact the top of the piston when opening the additional amount. While physical interference is obviously undesirable, the valve head should always be .200" clear of the piston top to ensure that the heat load of the piston top is not transferred to the thinner material sections at the perimeter edge of the valve destroying the valve seat. An overheated valve head can have accelerated seat wear, which will affect cylinder sealing.
- Valve train geometry should be verified to ensure that the higher lift does not make the rocker arm roller tip run off the valve stem. Excessive travel past the stem centerline will induce side loading on the valve guides and result in accelerated guide wear.
- Verify that the new RAR does not relocate the top of the pushrod to a point where it interferes with the top of the pushrod guide hole in the head.
Because of the potential negatives associated with higher lift, it is important that the adjustment is made only if it results in more hp. If the limiting factor in the combination was not the valve lift, then changing the RAR will only incur expense, increased stresses, and potentially result in accelerated wear. However, valve lift is commonly a limiting factor because the flow window around the valve head is smaller than the remainder of the port. It is important to evaluate your hp level beforehand and then confirm a hp gain afterwards by either track or dyno testing. Confirmation that the engine is making more power is the verification that the RAR adjustment was a positive change. This will require changing the RAR in stages to evaluate exhaust only, intake only and both, while verifying the hp gain/loss between each change.
An alternative to all this testing is to consult with professionals that are already familiar with the combination like the pros at Tin Indian Performance or other performance Pontiac engine builders. The ultimate goal is more power, because we all love FAST Pontiacs!
~ Matt Dalton (aka Triggerman) with consultation from Chris Hunt @ Hammer Time Racing