I am thinking about putting 1.7 rockers on my stock cammed 351w. Any input on the proform ones? I know scorpions would be better but they are pricey and also sold out everywhere.

 

When changing rocker arm ratios you never know if it'll help or not until you give it a try. This is best done on the dyno because many time more ratio doesn't = more power especially on a hydraulic cam where you can't move the lash around to fine tune the combination.

 

OOOORRRR! if your heads run out of air at 0.500 lift, and your cam is 0.512, your already past their, usable lift, adding 1.7 to get to 0.544, is just empting your pockets, for unneeded lift, that the heads still cant flow.

 

Cams and cylinder heads don't work like that. The point where the head "stops flowing" has no real bearing on how much lift you can use in the engine.

What really happens when you change the amount of rocker arm ratio is you change the amount of duration that you have at the valve. The engine doesn't always react positively to increasing or decreasing the duration. It is a similar effect to changing the amount of lash on a solid lifter valvetrain.

 

Why would it not, if your running E7 heads, and at 0.512, why would you spend $250-300, to up the lift, to 0.544, when the head has stop flowing at 0.500, and they prolly done way before that.

 

Why would you? Because it can make more horsepower. If the head stops increasing in flow with lift at .500 but you lift the valve to .550 or .600 lift it spends more time at or above the point of maximum flow.

 

What cam do you have?
When I got my cam, I knew it was for 1.6 ratio. If I went to 1.7 rockers, I'd be at .580 lift!!

That's impossible.
I think you mean to say is "amount of lift is changed at specified duration when rocker arm ratio changes".
Duration is fixed in the cam lobes, and is measured in degrees. The moment a valve opens to the moment it closes. Does not matter if I got 1.1 ratio or 1.72, the duration will always be the same.
Here is a graph of cam lobe lift, and two different rocker ratios, duration is fixed across the x-axis:

 

I just looked up some proform reviews, and not good.
Get the scorpions, compare the warranty's.

 

There's nothing impossible about what I'm saying. Duration is defined as degrees of opening at a given amount of lift and that lift can be at the cam lobe or at the valve. Since you're using lift and that lift is multiplied by the rocker ratio the effective duration at the valve is changed by the rocker ratio. This is one of the reasons why cams for flathead engines seem to have such radical lobes. This because they don't have any rocker arms to multiply the lift.

So if you change the rockers to ones with a higher ratio the duration at any lift above zero becomes larger simply because those lifts are reached more quickly. When I get to the shop I can run a lobe through Cam Analyzer and give you some data but you're changing a lot more than the gross lift when you change the rocker ratio,

 

This is a plot of the cam lobe that is in the 460 in my truck. It was designed by Mike Ingram. Here it is with a rocker ratio of 1 as if it were run in a flathead engine. Cam timing events are the same as the events at the valve. It has a lobe lift of .279 which gives .482 lift in my 460 engine.

Here is the same lobe but now running with a 1.5 ratio rocker arm as used on a small block Chevy. Notice how the duration at all lifts is now greater. It even has .100 degrees of duration at .300 lift vs zero without the rocker arm.

Now here's the lobe with a 1.6 ratio rocker arm as it would have in a Ford small block. We've gained about 10 degrees of duration at .300 and now more than 50 degrees at .400 lift where no duration existed before.

Here's the same lobe one more time but with a 1.73 ratio rocker arm as it has in my 460 engine.

What is interesting about the rocker ratio change is how since it is multiplying the lift as the lift gets greater the changes in the ratio make a greater difference. These durations are what the engine actually "sees" and while the durations at the cam are fixed the duration at the valve certainly is not and can be changed for better or for worse.

 

Dave, what type of lifters were used to obtain the #'s on your plots?

I would tend to agree that a different ratio rocker arm would change the duration #'s, when measuring from .020 or .050, but not from seat to seat duration.

 

That plot of the cam profile was made using my cam test stand and a flat follower.

Duration at any lift will change at the valve when the rocker arm ratio is changed and depending on what you use for your "seat to seat" duration it will change more or less. It would not change if you used zero lift because you'd be multiplying your rocker ratio X zero. But zero lift could never be used because the profile would never start or stop. I used .006 which is pretty typical but not universal for a hydraulic camshaft profile but if you used a different number it would be a little bit different.

Changing the rocker ratio is even more interesting if you're using a solid lifter cam. The ramp height of the profile stays the same but since it's height is multiplied by the rocker ratio the higher the ratio the greater amount of lash that needs to be used in order to maintain the same seat timing. Let's say you've got a profile with a ramp that's designed to run with .015 lash with a 1.5 ratio rocker arm. If the rockers are changed to a 1.73 the lash required to keep the same seat timing is now .010 X 1.73 or .017 lash. If the lash is kept the same at .015 then the pick up points are no longer the same and the duration is increased because the cam profile will pick up the valvetrain lower on the ramp.

 

How does it effect valve opening and closing points ?

 

if the lash is tighter the opening event happens earlier; the closing event happens later and vice versa.