Cam Rocker Ratio Choice

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67autocross

67autocross

A new iron curtain drawn across the 49th parallel
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Say I’m building a SBM (which is always the case) with a solid lifter cam and want around 600 lift at the valve…what are the pros/cons of using a smaller cam with a higher rocker arm ratio say 1.6 or 1.7 VS using a cam with more lift and a 1.5 rocker arm ratio…what is the consensus on the best way to get the lift….again solid lifter cam rollers need not apply
 
First, most motor builds do not achieve the lift of the cam due to loss in poor geometry. For example, if you buy a .500 lift cam and use the factory rockers/push rods, you'll most likely never get .500 lift. So, just keep that in mind. Second, I wouldn't try to add the lift I want through the rockers. I'd pick the cam I wish to run then if anything, try to correct the geometry so that I actually got the lift I hoped to get.

P.S. - my dirt road builds run poor geometry... lol. Just a FYI.
 
I would simply buy the camshaft with the lift you want. Higher ratio rockers are more for a fix for an already existing camshaft you may simply want more lift for.
 
It's my understanding that higher ratio rocker arms not only increase lift, but the rate of opening & closing as well. That tends to require an increase in spring pressures to keep from valve float.

When simply swapping a higher ratio rocker arm onto an existing combo, there is also a concern about coil bind and /or guide-to-retainer clearance with the existing springs.
 
67autocross said:
Say I’m building a SBM (which is always the case) with a solid lifter cam and want around 600 lift at the valve…what are the pros/cons of using a smaller cam with a higher rocker arm ratio say 1.6 or 1.7 VS using a cam with more lift and a 1.5 rocker arm ratio…what is the consensus on the best way to get the lift….again solid lifter cam rollers need not apply
Click to expand...

Here's David Vizards take on choosing a cam has some about rocker ratio.

Engine Camshaft Basics - Popular Hot Rodding Magazine

It really depends on the situation for most builds 1.5-1.6 are fine, Bore cid valves (head flow) has a lot to do with it eg. take 360 4" bore with 2.02 valves vs 402 4" bore with 2.02 valves, 360 has a better valve to cid ratio and is gonna a easier time filling the cylinders 402 is under valved look what most big blocks run for valves 2.02 is extremely small Higher ratios will help. Other things make sure cam will work the higher ratio you'll need more valve spring to control the valvetrain etc..
 
You are lift limited by the rocker gear. I would never use a 1.7 ratio rocker that has Chrysler shafts. The shafts are too big and the adjuster is too close to the shaft. If you use an aftermarket shaft rocker set up I think you can go reasonably to a 1.75 ratio. The issue with a 1.5 verses 1.6 ratio with the same lift is the 1.5 ratio will be a much more aggressive lobe to get the same lift with the same duration. If want want say to net .600 lift and you don’t need 260 degrees at .050 you’ll need a 1.6 rocker or higher to get it done. Otherwise you end up with more duration than you need to keep the lobe reasonable. Always use the highest rocker ratio you can fit in there correctly.
 
There is a thread on Yellow Bullet on this very subject. There are pros & cons, but with 600 lift, I think they are about equal. With very high lifts, the higher ratio allows for a larger cam barrel [ base circle ] which will resist deflection better & less loss of valve lift.
Also agree with Scamp's comments about high ratio rockers & the closeness of the adjuster/prod to the shaft. Would think that recessing an alum rocker for p/rod clearance would weaken it...
 
Most of what is out there seems to center on adding higher ratio rockers to an existing cam…I thinking about if starting from scratch which would be better…also what ratio has the best geometry
 
67autocross said:
Say I’m building a SBM (which is always the case) with a solid lifter cam and want around 600 lift at the valve…what are the pros/cons of using a smaller cam with a higher rocker arm ratio say 1.6 or 1.7 VS using a cam with more lift and a 1.5 rocker arm ratio…what is the consensus on the best way to get the lift….again solid lifter cam rollers need not apply
Click to expand...
la or magnum heads ??
 
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