Question about rocker stability at higher ratios

[OutCast]

If given a static number for valve lift, let's say .600 and all other parts being equal. Hardened pushrods of the same diameter, 7/16 rockers and studs, same valve spring setup.

If that amount of lift is accomplished using a smaller cam lobe/higher rocker ratio versus a bigger lobe/lower rocker ratio, which method will put less strain on the rocker studs and keep flex to a minimum?

[firebird_1995]

That's a great question and one I've always wondered myself. I don't know the answer, but I feel as though if you are having a cam custom ground, there shouldn't be the need for higher ratio rockers. However, on my last custom cam, the lift numbers advertised were WITH the use of 1.6 ratio rockers (and I had comp 1.52, effectively making my lift and duration slightly less). I know a guy with the answer though....
NightTrain66

[OutCast]

My assumption is the higher ratio rockers put more stress on the studs but it's nothing more than a guess.

[myltwon]

My assumption is the higher ratio rockers put more stress on the studs but it's nothing more than a guess.

I've read that running anything higher than 1.6 that isn't a shaft mount can cause valvetrain geometry issues. Not sure if there's any truth to it.

[noice]

The larger base circle the cam is, the better it is at RPM. The large side loading of a small base circle cam on the lifters is not ideal. For road racing a large diameter camshaft is better. I have an engine with a small base circle camshaft with .900+" of lift with 700+lbs open but it only runs for 1-2 minutes at a time and the valve lash is checked every couple of passes to see if anything is failing.

As for rocker ratio, the bigger the ratio is, the more of an arc the rocker tip makes. This will mean a larger sweep on the valve tip. It make make for a lot of work to get the geometry right.

[NightTrain66]

lots of variables and probably different answers for different types of cams BUT if we are talking about a hyd roller cam with 130-160 lbs on seat and 330-420 lbs open pressure, any good aftermarket stud should be fine and the amount of stress here is not gonna matter either way. All being equal as you mentioned, there is more mass and weight on the lifter and pushrod side of the rocker that can be slowed down with a 1.6 rocker and lower lift lobe compared to the same lift with a 1.5 rocker and higher lobe lift (both ending at same valve lift).

If you have stud problems on a set up mentioned above the quality of stud (stock??) or the rocker travel (bottoming out on stud) are causing the problems instead of the difference in 1.5/1.6 rocker ratio and slightly more/less lift at lobe.

A solid roller with 225-265 lbs on seat and 550-700 lbs open will see more stress at stud and as long as using 7/16 ARP studs it will be fine.

Not all lobes are the same and increasing rocker ratio will also speed up ramp speed making valve open faster and that can put slightly more load on the stud compared to a 1.5 rocker and same lift if both lobes have the same ramp speed. If the ramp speed is adjusted to a slower rate for the 1.6 rocker it would be a wash.

I guess if you want to know what works and what doesn't (as far as spring pressures, ramp speed, what lobes, what rockers, what qua;lity studs are needed, etc) I can help you out. If it is a physics debate and trying to see what % of stress increase is at the stud for each set up, I can not give you the exact #'s or formula.

Does that help?

Lloyd

[OutCast]

Thank you, it does help. I'm not bringing it up for debate as I don't have any in-depth knowledge.

My reason for asking is I'm starting to plan for my next build and thinking about the parts I have and what might be carried over into the new build. I'm still trying to figure out exactly what my goal is going to be so it's hard to talk about anything specific at this point.

[OutCast]

I have a unicorn that will ride on top, it's everything else that needs to be decided. It won't be an all out build, but the bottom end will be built strong. I'm going to take it as fast as it will go with LT1 head castings and a set of 7/16 1.7 rockers and ARP studs. The unicorn is setup to accept 3 stages but I'm only planning on using two. I'll start a new thread to start planning the build as a whole.

The unicorn, from Taner's old combo
http://ltxtech.com/forums/attachment.php?attachmentid=15577&d=1360817653