Since the pushrod lengths are being measured for exact length, what is the difference between using 1.7 vs 1.8 rockers? I assume it has something to do with over all valvetrain travel, or some other valvetrain physics?

 

Eric, the 1.7 is a ratio number, same as a 1.8. It’s a multiplier. Say cam lobe lift is .300. Multiply that times 1.7 and you get .510 lift at the valve. Use the same lobe with a 1.8 rocker and you get .540 lift at the valve….as long as we are talking hydraulic cams. A solid cam will have a little less because of lash.
A greater or bigger ratio opens the valve quicker and will also add a tad of duration because it opened the valve a little quicker.

 

Hey, Scott. I understand the rocker arm ratio, but is there any other kind of physics that makes one better, like shorter travel or faster valvetrain? Just curious if there is any reason why people switch rockers other than to try to mimic a bigger cam.

 

Also, if the pushrods are being made to specific measured lengths, then wouldn't the pushrod length negate the difference between the 1.7 and 1.8 rockers, or am I thinking about the ratio on the wrong side of the rocker?

 

Naturally Eric, the higher ratio rocker will open the valve faster. If the ratio gets crazy…2.0-2.1 etc, it’s gets VERY hard on valve springs.

The ratio only affects the valve side of the rocker, but pushrod length can vary due to rocker design and actual cup placement distance from the fulcrum.

 

No the push rod length doesn't "negate" anything to do with the rocker arm.

PR length and rocker ratio are not related in any manner way shape form or fashion. With any given set of rockers and heads and valves, all that changing the PR Length does, is change how far down the lifter plunger is held when the lift is at zero. If they're too long, the valves can't close; if they're too short, there's slop in the valve train. That's all.

All that the ratio changes, is how far the PR is tilted toward the valve. Without relocating the trunnion or SIGNIFICANTLY shortening the valve, the only way to change the ratio is to move the PR cup in the rocker, since the distance from the valve to the pivot is fixed. Butt that doesn't affect lifter preload, in and of itself.

THIMK about a rocker for a minute. If one set of rockers has some particular geometric relationship among the pivot, the push rod cup, and whatever pushes on the valve stem, then if another set of rockers has the same relationships, they'll want the same length PRs, regardless of their ratio. However we have NO WAY IN HELL of knowing that, especially when the rockers are too different to just eyeball. Is the PR cup a little deeper or shallower? The tip a little thicker? The trunnion a little higher or lower in the body? It's just too hard to tell about all those things, even if you think of and try to observe every single possible factor.

Which is why EVERY TIME that ANY part in the valve train of one of these motors is changed, the push rods MUST BE checked for proper lifter preload, unless you're feeling lucky today. (I'm never lucky, unless you count bad luck, anyway) There's just too many ways for things to be different to "assume" anything. A set of rockers that's "different" in ANY way might have additional differences that are independent of whatever specific detail you think you're buying.

 

UGH. I should have bought the CTSV.

 

So if two identical engines are being built, one with 1.7 rockers, and one with 1.8 rockers both with independently measured pushrods.... would the engines end up being identical?

 

No. With the same cam, the 1.8 rockers would give extra lift and duration to the valve opening. They would also put much more stress on the valve springs because of the faster opening and closing rates.

 

The 1.8 ratio rockers would also put more compressive load on the push rods.

 

No. If they were, what would be the point? Why even have "ratio" at all?

What gametech and pannetron both said.

My point is, changing the ratio does NOT, in and of itself, alter the push rod length requirements. Which is NOT to say that changing the rockers, regardless of the ratio, doesn't affect the PR length needs; only, that the ratio itself isn't a factor. It would be entirely possible to make 1.8 ratio rockers that will want EXACTLY the same PR length as some random 1.7 ratio rockers; just, they can't be guaranteed to, and very well may not, any more than one set of 1.7s will always want the same PR length as some other set of 1.7s. Too many variables affect that, butt ratio isn't one of them.

 

Eric, aren’t you using a shop in AZ to assemble this? If not, PM me.

 

Yeah I am. I was just being a whiney baby.

 

Made me smile here. Those guys will figure all of the details out. I realize that you are simply trying to learn here. Honestly the best way to learn is to get over there on top-end assembly day and watch. Seeing how it all goes together is the best way to understand. Tell them your simply wanting to learn and you won’t get in the way…they hopefully will oblige.

 

That would for sure be the best approach. Some people just can't visualize geometry in their head based on reading numbers. Actually seeing it would make it obvious. Something about "a picture is worth a thousand words" sort of a thing.

 

I like to look at the duration factor like this: Using Scott's figures, with the .510" increasing to .540". When you go from the 1.7 to 1.8, you had zero duration above .510" before, because lift ended@.510". When changing to 1.8s, you now have duration from .510" to .540" and back to .510", where you had none before. It was easier on my end to visualize it like that. As he also stated, it will also open and close the valves quicker. Worked for me....

 

I would also like to add one other thing. While this difference may not be a huge difference, a 1.7 rocker has greater leverage on the valve spring than a 1.8. Put another way, the 1.8 rocker puts slightly more pressure on the pushrod and lifter. Now, if I'm incorrect, I'm sure someone will chime in.....Quickly!!

 

The change in duration isn't due to what happens at max lift (the .510" vs .540" deal); it's about what happens near the seat. More like, the duration the valve sees, changes from the cam's "rated" .050" duration, to something like what its "rated" .045" duration would be if it had that, since with the higher ratio, the valve reaches .050" at the point where the cam would have raised it .045" (or whatever) with the original duration. It's not a "yyyyyyyuuuuuuuujjjjjjjjjjje" effect or anything, usually just acoupla degrees, butt it can be noticeable sometimes. Depends on how steep the particular cam's ramps are around the seat, and different cams can be VASTLY different in that regard. Also different on the same lobe between what happens at opening vs at closing. Hard to predict the end result with any precision unless the cam's ENTIRE profile is known. All you can be sure of, is that it'll be "more".

And yes, the higher ratio causes the valve spring to have more leverage on the PR, so it does increase the pressure on the lifter. Again, probably not enough to make any kind of real-world difference to anything, like probably less than putting new springs on vs running ones w many miles on them, butt it definitely occurs.

 

I agree on the quicker opening, but also still believe that duration now happens at a lift that didn't exist previously, too...