Let's say you are designing an SBC motor or any other motor for that matter. You have decided that you want .500" lift at the intake valve. Do you design the cam to achieve .500" lift with the stock 1.5 ratio rocker? Or do you design the cam to achieve .500" lift with a 1.6 ratio rocker? Obviously, the cam lobe for the 1.6 rocker would have less lift to still achieve the .500" lift. I know it is very popular to use the 1.6 rockers. But why? Why not just use a bigger cam to start with. I'm not sure, but it seems to me that the 1.6 rocker would introduce more stress in the system. I can see choosing the 1.6 if you already have a cam in and are not quite happy, so you try to crutch the cam with more rocker ratio. But if you are buying a cam, why not get the lift you want with a standard 1.5 rocker? Is there an advantage either way? I'm not planning any engines currently, but I have been curious about this for a long time. What do you engine gurus say?
For a small block dodge guy (LA) the 1.6 ratio can bring the pushrod too close to the hole, which gets worse if you have fatter pushrods. The LA head can be ground a bit for clearance there, to a point, but it's an issue that needs to be accounted for. This is one example, where the cam lift vs rocker ratio issue can have outside effects on other parts (pushrod slot interference for me), Other engines could have similar or even different "outside" effects. But if you're just wanting to know about stress in the cam-rocker interface, then I think you have to look at the ramp profile and pushrod stiffness to get your answer. -rick
He’s asking about a small block Chevy. I would go for a 1.5 rocker ratio unless there was a reason not to do so like clearance or something.
I was responding to the second half of his first sentence, where he mentioned "or any other motor". just offering up other aspects to the situation. -rick
Yes, I have had to get out the die grinder to correct some push rod interferences on several sets of SBC heads, but I'm just asking if there is an advantage with 1.5 or 1.6 rockers in the absence of any physical limitations. Purely hypothetical. Is there more horsepower or durability in one or the other?
On the same cam, you can get about 20 degrees of additional valve duration with the 1.6 ratio vs 1.5. Common to do on exhaust valve on SBC. But as mentioned , it comes at a cost with clearance issues that will need machine work in most cases. Cam choice involves more than just lift.
I think the higher ratio works on paper if you can understand it . Just like a longer rod is supposed to help the fill at bottom,,,. I think the ratio helps by getting the valve to move faster compared to a regular rocker . Which in turn helps the flow,,,it might be in milliseconds,,,but I’m sure it helps . Do y’all remember back in the 90’s when Earnhardt was able to pull out of the draft and p*** 4 or 5 cars at a time ? He wasn’t cheating,,,they just had a new valve angle in the heads and used a much longer rod . They started using a 6.125 rod,,,,and a new head design ! By the way,,,,that valve angle was the same one Mopar had used all the time,,,,and Mopars factory small block rod length was 6.123 all that time . Wasn’t Chevy 23 degree valve angle ? And I think Mopar was 17-18,,,,I’m not sure . I guess old Mopar wasn’t so dumb after all !,,,,lol . Anyway,,,,yes,,,I think the rocker ratio helps,,,,maybe not in ordinary cases,,,,but in racing when the rpm goes up and stays up most of the time . Tommy
Yes, I know that the higher ratio will give a gain in duration as well as lift, but again why not just design the extra lift and duration into the cam to start with.
I hadn't thought about the speed of the valve movement. I can see where that would be an advantage to flow. In this instance the valve would have to travel farther in the same amount of time. The valve has to be moving faster, higher and longer with a 1.6 compared to a 1.5 rocker. I can see that as a big advantage. Thanks for that thought.
A larger ratio will increase the speed the valve opens without increasing the actual ramp angle on the cam
Yes. Now that I have seen the light, the speed of the valve movement is the answer to my original question. Thanks guys! I have been curious about that for a long time.
There’s also some engines where the cam journal isn’t big enough to get as larger lobe through the hole as you’d like so changing rocker ratio is a simpler way to overcome the problem rather than over boring the cam journals.
Increasing rocker ratio doesn't add seat to seat duration. It is going to open the valve more while the lifter is on the actual lobe.
I just read something about why guys would use 1.6 RAs. This was just regarding a SBC. Seems who wrote said that allowing more exhaust flow really woke them up in the upper RPMs. The article went on about that’s why “they “ sell 1.5/1.6 sets of RAs. I’ve no experience using 1.6 RAs, so take it for what it’s worth
F From what I see, it is kinda common practice on SBC builds to use 1.6 on the intake and 1.5 on the exhaust
Increasing the valve speed could produce damaging effects on valve springs , I've always liked the videos of valve springs dancing ( & destroying themselves ) @ high rpm .
My experience with increasing rocker ratios has been with Fords and Mopars but valves and valve springs don’t know what head they’re in. All I will say is have the cam grinder grind the cam for the rockers you want to use. Have the cam ground for the lift and duration needed and don’t try to cheat more out of it. If you’re cheating on the cam you have to wrong cam!
But what about the extra load on the push rods and rocker arms with the higher ratio's? There has to be some added load. Dad changed his stock Pontiac stamped steel 1.5's to a stock Pontiac Ram Air 1.65 ratio stamped steel arm and ran it for awhile. It did make more power, but in doing so it burnt the rockers. All 16 looked as though they had a torch on them, blue as could be. The only change was the ratio. He then went to a roller type rocker with no ill effects.
I think what you are getting at is this: If you want .500 lift from both cams, the ramp angle on the cam will have to be steeper to get there when using a 1.5 rocker than using a 1.6 rocker because the cam lobe will be taller. Reversing that idea, if you have a cam and 1.5 rockers, and it works just fine, simply changing to 1.6 rockers will increase the actual lift but did not change the angle needed to get there. This is usually not a problem in lower lift cams in mild engines as there is usually room to accommodate the extra lift. I think what you are saying about designing the cam to work with the longer rocker is accurate and that's what they do in high performance engines. I also think that using a longer rocker allows cam grinders more la***ude in lobe designs. By that I mean the top of the lobe does not have to be as high to get a certain desired lift if the rocker is longer. That simply means the ramp design does not have to be as aggressive.
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