High Lift Rocker Arm Discussion

The stock rocker arms in the 3800 V6 engine have a leverage ratio approximately 1.6 to 1. This means that the distance the valve opens up is 1.6 times more than the actual lift at the camshaft lobe. One well known high performance engine modification technique is upgrading the stock rocker arms with aftermarket rocker arms that have a more aggressive ratio. This higher rocker ratio allows the same camshaft to produce higher valve lift. Not only do the valves open higher with higher ratio rocker arms, but the effective duration of the valve is also increased. This is because the valve now achieves higher lifts earlier during the opening and closing phases of the valve timing. This higher lift allows greater air flow rates and allows the engine to breath better. And the better an engine breaths, the more horsepower it can potentially produce.

Higher ratio rockers arms typically will increase the horsepower output of the engine. But like most everything else, too much of a good thing can be bad. This philosophy applies to rocker arm ratios. If 1.8 to 1 rocker arms produce significant performance gains doesn't necessarily mean increasing to 2.0 to 1 or 2.5 to 1 will be even better. Besides the mechanical limitations to increasing rocker arm ratios, there are flow dynamics of the engine that must be considered as well. When faced with this situation where increasing rocker arm ratios will improve performance to a point and beyond that point performance drops off begs the question, "What is the optimal rocker arm ratio that will optimize the performance of the stock camshaft?" A further probing question that should be asked is, "What is the optimal rocker arm ratio for the Intake Valve and Exhaust Valve independently that will optimize the performance of the stock camshaft?" This last question acknowledges the fact that due to combustion chamber design and port design, the Intake Valve and Exhaust Valve do not typically become optimized at the exact same valve lift. The best rocker arm ratios would be one ratio that bring the intake to its optimal lift and another ratio that brings the exhaust valves to its optimal lift.

ZZP ER Rocker Arm Kit

 

Optimizing High Ratio Rocker Arm Design

ZZPerformance as well as other aftermarket performance companies had developed high lift rocker arms for the GM 3800 V6 engine. Typically 3800 V6 performance enthusiasts would use 1.90 to 1 rocker arms for both the intake and exhaust valves. This has traditionally been accepted as the best rocker arms to use with the stock GM camshaft on either naturally aspirated or supercharged 3800 V6 engines. However, ZZPerformance had the notion that this high lift was not the best for the naturally aspirated version of this engine. Also, they felt the intake needed to be favored slightly more than the exhaust. With this rational, they developed their Split Ratio designed high ratio rocker arms for the naturally aspirated engines. Specifically, they choose 1.85 to 1 for the Intake Valve and 1.80 to 1 for the Exhaust Valve. These ratios were chosen by more of an educated guess than by scientific experimentation. From this standpoint, what was needed was an in depth investigation to scientifically determine the optimal valve lifts for both the intake and exhaust valves independently for the naturally aspirated application.

To accomplish this goal, Easy Performance Products (EPP) teamed up with ZZPerformance to conduct the necessary testing and data analysis. ZZPerformance agreed to supply the materials and what ever rocker arms were required to conduct the experimentation. EPP agreed to design the experiment, conduct all data acquisition and perform all the data analysis as well as supply the test vehicle. Because the goal was to treat the intake valve lift separate from the exhaust valve lift, we knew a multi-factor experimentation scheme was necessary. To accomplish this, a fractional factorial design of experiments was used. This type of experimental design allows for the most amount of information to be gathered with running the fewest number of experimental runs as well as not have any of the experimental factors confounded with each other. The experimentation would allow for independent analysis of each valve type's lift as well as investigate any interaction between the two valve lifts.

Another concern was determining the response variables to be used with determining the optimal performance. The notion of performance can mean any number of things. Even though the highest horsepower output is what is most desirable, this does not specify exactly what measurement of horsepower should be used. For example, is it most important to: Optimize peak horsepower? Optimize horsepower at a specific RPM? Optimize the horsepower over a certain RPM range? If so, what RPM range should be used? Another performance measurement could be volumetric flow of the engine. A third could be the volumetric efficiency of the engine. Because the horsepower of the engine is dependent on other factors beyond the engine's ability intake air/fuel mixture and expel exhaust gases, looking at volumetric data can more directly measure the affects of changing valve lifts than horsepower values can sometimes offer.

 

Experimentation Setup

The test vehicle used for this experimentation was a Pontiac Grand Prix GT with an L36 3800 V6 engine.º Modifications to the car prior to the testing were U-Bend removal from exhaust system and installation of a 180ºF drilled Thermostat. These were the only modifications on the vehicle. The vehicle was absolutely stock in every other aspect. 93 Octane fuel was also used during the testing. The PCM was never reset during any of the testing period. During the experimentation, the test conditions held fairly consistent with the temperature varying between 77 ºF to 80ºF, Humidity of between 48% to 71% and a barometer of 30.02“of Hg to 30.18"of Hg. The vehicle had ~6,000 miles on the engine.

Testing Procedure

Using a fractional factorial experimental design specified exactly the number of experimental runs to be performed as well as the exact rocker ratio to be used on the intake valves and the exhaust valves. The order in which the various rocker ratios combination were used was randomized to reduce timeline affects. A minimum of 6 data collection runs were performed with each combination of rocker ratios. Test conditions were monitored and recorded for each test run. Dynamic data acquisition was performed to collect all engine parameters required to calculate engine horsepower, torque, volumetric flow, volumetric efficiency and spark timing parameters. Specifically, an OBD-II Scan Tool was incorporated to monitor and record various vehicle parameters during the test runs. All of the runs were performed on the street with all acceleration runs conducted in first gear to just past the point where the transmission shifts into second gear. The engine temperatures were held close to 182ºF throughout the experimentation.

To enable quick progression through the experiment in the least amount of time, once one combination of rockers was tested, the car was brought back to the shop and immediately changed over to the next combination of rockers. Therefore, all work was performed on a hot engine. Typical turn-around time from one test run to the next was about 30 minutes. All experimentation was conducted within two days.

After all data was collected from all the rocker combination runs, the data was analyzes using multi-variable analysis of variation techniques. We decided to use use the following response variables (performance measurement parameters) to independently determine the optimal rocker ratios for the intake and exhaust valves:

•  Peak Horsepower
•  Peak Mass Air Flow
•  Wide Band Horsepower - calculated by integrating the Horsepower curve between 3,100 RPM to 5,800 RPM.
•  Wide Band Mass Air Flow - calculated by integrating the MAF curve between 3,100 RPM and 5,800 RPM.
•  Optimal Mass Air Flow Band - calculated by integrating the MAF curve between 3,800 RPM and 5,800 RPM.

It should be noted that the Horsepower data and Mass Air Flow data is completely independent of each other. How well the results determined by these two independent response variables can be used to judge the accuracy of the test methods and data integrity.

Once the optimal intake rocker ratios and optimal intake rocker ratios were calculated, a set of optimized rocker arms were manufactured. When these came available several weeks later, a confirmation run was performed with this set of rockers. A day for this successive experimentation was chosen that nearly matched the test conditions of the original experimentation. The purpose of the confirmation testing was to verify the improvements predicted by the mathematical model developed by the optimization experimentation. The accuracy of the predictions is substantiated when subsequent experimentation confirms the predicted outcome.

Results

Analyzing the experimental data produced a variety of surface response plots that indicated where the optimal performance would exist relative to rocker arm ratios for the intake and exhaust valve. While the actual response curves for this experiment are held proprietary, the following chart give an example of what these response curves look like.

The experiment's math model was used to not only to determine the optimized rocker ratios for both the intake and exhaust valves, but was also used to predict the performance this rocker ratio combination should yield. The model predicted these optimized rockers should produce the widest power range as well as produce ~184 WHP when tested. The rockers were installed on the test vehicle and subsequent performance data was collected and analyzed. The dyno chart obtained from this testing agreed extremely well with what the experimental math model predicted. As seen by the dyno chart in the Power Measurement section below, the optimized rockers came in at 183.3 WHP. This was within 0.4% of the horsepower predicted by the experimental model. Not too shabby.

 

Air Flow Measurement Results ¹:

Stock 1.60 Rocker Arms Volumetric Chart

 

High Ratio 1.90 Rockers Volumetric Chart

 

ZZP ER (Optimized) Rockers Volumetric Chart

 

Power Measurement Results:

Stock Rocker Arm Horsepower & Torque Chart

 

High Ratio Rockers Horsepower & Torque Chart

 

ZZP ER Rockers Horsepower & Torque Chart

 

Conclusions

In the final analysis, all of the response variable results, data analysis and performance charts indicated nearly the exact same conclusions. Some of these conclusions were, but not limited to, the following:

The experimentation indicated a strong interaction between the intake and exhaust valve lifts.

The optimal performance, regardless of which response variable was used, was obtained with the intake and exhaust rocker ratios NOT being the same.

The engine did NOT produce the best performance curves when both intake and exhaust rocker ratios were using high rocker ratios of 1.90 to 1.

The optimal rocker ratio combination predicted by the experimental model for the intake and exhaust was not exactly one of the combinations tested. The model recommended the best intake rocker ratio lies between two intake rocker ratios tested. The same situation occurred with the exhaust rockers tested.

Confirmation testing of the model's recommended intake and exhaust rocker ratio combination verified the performance and power curve predicted by the experimental math model. The math model was within 0.4% of the actual performance obtained with the optimized rocker arms.

As a result of this extensive research, testing and data analysis, ZZPerformance discontinued the manufacturing of the previous Split Ratio Rocker Arms in favor of the optimized rocker arms developed. Because these optimized rocker arms not only produce higher peak horsepower, they also improve power production over the entire RPM range. This fact has prompted ZZPerformance to call these high ratio rocker arms "Extended Range" Rocker Arms or ZZP ER Rocker Arms as they are commonly referred to now.

º All testing performed on a GM 3800 Series II (L36) Naturally Aspirated Engine in a late model Grand Prix. The engine was brought up to stable operating temperature and conditions for all test data collected. Engine coolant temperatures were held within a few degrees of 182°F.

¹ Mass Air Flow measurement data was obtained using an OBD-II Scan Tool. Mass Air Flow values were read from the MAF sensor via the Powertrain Control Module (PCM).

² Volumetric Efficiency is a quantitative measurement of the engine's ability to breath (pull air into the combustion cylinders through the induction system and push it out through the exhaust system). Volumetric efficiency measures the standard volume of air entering the engine and divides this value by the theoretical volume of air the engine should be displacing based on its total, physical displacement. Volumetric efficiency is closely correlated to the torque an engine can produce and thus the horsepower than can be made.