I am preliminarily spec'ing out a new sbc build. Thinking about cam lift and rocker arms. Let's say I have decided on .550" of lift. I can get there 2 different ways. Use a lobe lift of .367 with a 1.5 rocker or a lobe lift of .344" with a 1.6 rocker. Which is the most desirable method? Is there an advantage either way? What do you think?
Depends on the rest of the lobe profile. Taller lobes GENERALLY have more duration, so you'll get more top end performance with the larger lobe/smaller rocker ratio than by going with the smaller lift/larger ratio route: Even though the lift is the same, the larger cam has more duration (again, GENERALLY). Smaller lobes do tend to set the valve down easier which is nice at high rpm/spring pressures, but modern profiles have eliminated a lot of that on the closing side of the lobe. (Lunati's Voodoo cam profiles come to mind, and are what I have a cam made with when I get a custom hydraulic cam. They open the valve quick and set it down slowly).
I feel it’s not only the numbers but how it all fits with the geometry of the action on the spring/valve. Is this a day driver/weekend cruiser or for other use. The action of the cam ramp can be different on each depending on the cam grinder. The engine I use has a shaft rocker arm system and continuously wore out valve guides. No roller tip rockers were available so I designed some and had them made. I also changed the ratio to 1.6 and used the same flat tappet cam. The added lift could not be added any other way, but your not looking for added lift. Today if you’re looking at a roller tappet cam with roller rockers I doubt that there would be much difference. I believe with a flat tappet cam the action on the valve would be a might bit quicker in the first .100” of valve lift with 1.6 rocker arms. Just my opinion.
Deciding on a cam based on lift seems kind of strange to me. Duration has more effect on engine operation, so that's where I'd start.
Running a 1.6 ratio rocker vs 1.5 will give more lift, also should give slightly more duration at 0.050, and thus make more power in theory. The best course of action IMO is talking with the cam grinder on expected performance. They will give their opinion on what rocker ratio to use. The only way to know if a gain is made is to test each rocker ratio. Dyno or acceleration times. You can run. 1.5 on intake and 1.6 on exhaust or vice versa. Lots of articles on this subject. I know some who increase the ratio on stock cams, how much is gained I don’t know. Dan
I'm not deciding on a cam based on lift. I guess I didn't ask the question correctly. I just picked a random lift number to illustrate my question. My question really is why do 1.6 rockers even exist when you could have a cam designed with the same lift and duration to use the standard 1.5 rocker? There are a whole lot of 1.6 rockers out there, and all of them are not being used as a second thought to gain lift. There are engine builders out there designing their engines to run 1.6 rockers from the start. So, what is the reason to use 1.6 rockers?
I could be WAAAAAY off base here, but I've always been of the opinion that 1.6 rockers were a cheaper, easier way to alter the cam profile, vs. Replacing the cam and kit. Sure, you could replace the rockers with a higher ratio version, but if the spring rate and/or guide to retainer clearance is marginal, it's really a moot point. IMO a person would be money and aggravation ahead to just get the cam and kit that works best with standard ratio rockers.
I agree, a simple, cheap resolution to adding a little more lift and duration to a cam. I always build based on 1.5 rockers, this gave me a way to "add" a little more cam at a later date should I want to by going to 1.6 or 1.65 rockers. I never built a motor and wished I had went to a smaller cam, always the opposite. ...
It could be that by using higher ratio rockers that the lifter and pushrod accelerations aren’t increased at any given valve lift, maybe adding a little stability at high rpm’s, I also could be full of shit. Dan
https://www.enginebuildermag.com/2021/08/rocker-arm-tech-part-1/ https://www.enginebuildermag.com/2021/08/rocker-arm-tech-part-2/ https://www.enginebuildermag.com/2021/09/rocker-arm-tech-part-3/ https://www.enginebuildermag.com/20...ocker-arms-valve-springs-retainers-and-locks/
When people say I'm full of shit, I say, “ Are my eyes blue?” Yes they are. That means I’m a quart low...
I didn't go through all the links @In_The_Pink provided, but did look at the first one's first paragraph. They do look good. The OE design would take into account all the limits of metallurgy, acceleration, geometry and forces at the time and in a volume street production. The typical 1.6 aftermarket was a racer's way of being able to test a bigger profile quickly and getting more performance over a short life rather than getting custom cam grinds. Split lift/duration cams were out there before, but they really began showing up more when testing proved that some designs responded to more exhaust opening. That was decades ago, and porting and cams kind of made the 1.6 unnecessary for most unlimited cases. If you are limited by rules or funds, they may still be viable.
The Jalopy Journal
