You’ve got questions. We’ve got the answers—the Summit Racing tech department tackles your automotive-related conundrums. This week, we’re examining how changing rocker arm ratio can help performance.
T.Y. Thornton, IL
Q: I am in the market for roller rockers. I own a 1966 SS Chevelle with a 396. The engine is bored .030 over, has 10.25:1 compression forged pistons, oval port heads, 290-degree advertised duration/.560-inch lift hydraulic cam, and an Edelbrock Torker intake with Holley 750 cfm Holley double pumper. The engine currently has 1.7-ratio rocker arms.
The car is quick now, but a friend says Harland Sharp 1.8-ratio roller rockers will improved my engine’s performance. Is there any advantage to running the 1.8 rockers versus the 1.7s?
A: Switching to the 1.8-ratio rocker arms will increase your actual valve lift to .590-inch, giving you increased high rpm performance. The downside is that you will lose some low-end torque. If you can live with that compromise, then by all means bolt on the 1.8 Harland Sharp rockers.
For more information on how rocker arm ratio relates to power and performance, take a look at this video from an earlier OnAllCylinders post.
[…] about weight? The pushrod moves a short distance compared to the valve, spring, and retainer due to rocker ratio. The weight is much less in comparison too. Because of this, the added strength more than offsets […]
Well is my 1.5 rockers going to work ok with a 510 lift ? And show do I get ahold of those push rod alignment guids that are bolted on to the rocker stud?
Great job.getting a lot of useful information. thanks
Not only valve lift, but think, valve action.
A increased ratio moves the valve off the seat faster and holds the valve open longer. That’s great for a slightly undersized cam or one that is close to stock. Things to consider are: idle quality, vacuum at idle speed. Additional timing may be needed if going from a healthy size cam for your cubes and compression, since you will lose some vacuum and cylinders may not be pumping as much air at idle and low speeds – why you may lose a bit of torque. Another thing to consider is the parts of your valvtrain. Are your springs and pushrods up to the task? Will increasing the rocker ratio cause valve float 4 or 500 rpm earlier in the curve? I went with tool steel retainers and 0.080″ wall pushrods when I decided to go from 1.5:1 to 1.6:1 rockers. Another thing is : are you happy with your rear end gears? Will losing any low end torque necessitate a gear swap to leave the line as hard or better than before? How wild are the lobe profiles of your camshaft? If they are already fast ramp profiles, do you need faster valve action ?