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 High lift lobe or rocker ratio
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Miles
Sitting Bull

Canada
154 Posts

Posted - 22 Nov 2017 :  07:50:16 AM  Show Profile  Reply with Quote
Which is easier on the valve train to get the same lift at valve, a high lift lobe or a lower lift and a higher ratio rocker? How would both differ as to pressure on the studs, ease of lifter following the cam, etc.?
For a solid roller camshaft.
Thanks for any thoughts.

Steve C.
Crazy Horse

1691 Posts

Posted - 22 Nov 2017 :  08:21:14 AM  Show Profile  Reply with Quote
Some cam tech articles will suggest to get the lift thru the lobe lift and less rocker arm ration. But there is base circle, roller diameter, etc to consider.

Then there is this comment.....

The goal with a properly sorted valve train is to get the action required at the valve, while maintaining control of the velocities present. You want the valve to do exactly as it is told by the cam. The problem with running a big lobe and a low rocker ratio is that you are increasing the velocity of the heaviest parts of the valve train (Lifters, push rods ect.). This requires a raise in spring pressure, which in turn requires stiffer (heavier) push rods, and a more robust follower.
By going to the highest ratio the rocker body will handle without creating design issue's, you can keep the velocity of the heavy side to a minimum, while maintaining high valve velocities. That way the spring is working more to control the valve, instead of the whole system.


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Edited by - Steve C. on 22 Nov 2017 08:31:27 AM
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Steve C.
Crazy Horse

1691 Posts

Posted - 22 Nov 2017 :  08:49:38 AM  Show Profile  Reply with Quote
Again fishing I dug up some additional reading material. I found this statement.....

"Equal lift, for equal lift, you need less spring with a higher ratio rocker than you do for an equal valve lift with a low ratio rocker and a more aggressive cam lobe. With the high ratio rocker, your moving valve only... with the cam lobe you have moved the entire valve train assembly. High ratio rockers are a bit of the secret to the high RPM hydraulic roller combinations out there.Fast valve movement, reduced cam lobe requirements, and you don't have to have the spring control all the mass through the range and speed of motion"


And another ......


"So now the math (.750/1.6= 0.470) tells us that the pushrod centerline is roughly 0.030 inch closer to the fulcrum pivot. This arrangement does more than just multiply cam lift; it also multiplies the loads on the pushrod and rocker arms, making the proper ratio critical. Too much ratio will open the valves too quickly and can cause valve float at high rpm. It also multiplies the spring pressure seen on the cam lobes, so running too much ratio can wipe out a flat-tappet cam in no time. Thankfully, the cam manufacturers have studied these problems, and most won't even sell you too much ratio unless they feel you've really got your act together. Since an increase in ratio also increases the loads on the rocker and its mounting stud, you should stiffen the whole assembly up in order to keep the rockers from wobbling all over the place."

Source:
http://www.superchevy.com/how-to/engines-drivetrain/sucp-0202-rocker-arm-ratio/

"Rockers, and their mounts, flex under valve spring loads. Increasing spring pressure and higher rocker ratios can increase the flexing, especially in combination with production components."

Source for that and with other fine information:
http://engineprofessional.com/articles/EPQ409_58-62.pdf


"A second effect of larger rocker arms ratios is not as widely known. The effective valve open duration is extended 3 to 4 degrees, depending on the cam type, as measured at .050"

"Does higher ratio rockers cause increased wear/ breakage? In general, the side loading on the studs will increase. However, as the 1.65 rockers must be set at a different height to maintain proper valve-train geometry, it is necessary to replace the stock bottleneck studs with straight studs. Most after-market studs are more than adequate for strength. We strongly recommend that big block Chevy 7/16? studs be used along with the polylock style lock nuts. A noted Pontiac Engineer advised that higher ratio rockers were chosen over a higher lift cam, because overall, less stress and strain on the complete valve train was required to obtain the same lift."

Read more: http://forums.superchevy.com/chevy-high-performance/70/8514490/general-chevy-technical-discussion/rocker-ratio-effect-on-lift-and-duration/page6.html

Another here....
"Increasing the lift ratio adds horsepower with little or no loss in low rpm torque, idle quality or vacuum. By opening and closing the valves at a faster rate, the engine flows more air for the same number of degrees of valve duration. High lift rocker arms also reduce the amount of lifter travel needed to open the valves, which reduces friction and the inertia of the lifters and pushrods that must be overcome by the valve springs to close the valves.

On the other hand, increasing the rocker ratio also increases the effort required to open the valves because of the leverage effect. The higher the rocker arm ratio, the greater the force the camshaft, lifters and pushrods have to exert to push the valves open, and the stronger the pushrods have to be to keep from bending. That?s why everyone is clamoring for stronger, stiffer pushrods today."

Read more: http://www.enginebuildermag.com/2007/09/push-rods-lifters/

Valve lift : Rocker ratio vs. lobe size
To achieve lift @ the valve is it better to use a higher ratio rocker or is it better to use a larger lobe on the camshaft.

http://www.yellowbullet.com/forum/showthread.php?t=379802


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Edited by - Steve C. on 22 Nov 2017 08:51:37 AM
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BearGFR
Bear

USA
581 Posts

Posted - 24 Nov 2017 :  12:34:53 PM  Show Profile  Visit BearGFR's Homepage  Reply with Quote
Personal opinion, (your mileage may vary, void where prohibited by law, kids don't try this at home, etc)

When it comes to spring pressures and what the "lifter side" (lifter, pushrod, and cam lobe) "sees" it all works out the same. Probably true that a higher ratio rocker doesn't need as much RAW spring pressure (pressure at the actual valve spring) because that pressure is going to be MULTIPLIED more by the higher rocker ratio --- with the end result being that the pressure exerted through the pushrod, to the lifter, and onto the cam lobe will be the same as what would result from a stronger spring working through a lower ratio rocker. For any given combination of cam lobe, lifter mass, and pushrod mass --- it's going to take a specific amount of pressure on that side of things to prevent all those parts from being "thrown off" the cam lobe throughout the engine's RPM operating range. Rocker ratio on the lifter side of things doesn't really matter as long as the spring force multiplied by the rocker that is counteracting the inertia effects from the movement of these parts is sufficient to keep everything happy.

On the valve side, there's also inertia effect from the valve movement and the 'long' arm of the rocker, but I'm assuming that this side doesn't need as much force to keep things under control as the lifter side does. Even though the valve is moving both farther and faster than the lifter, there's not as much total mass involved - so less inertia.

What IS different is the amount of stress put on both the rocker itself and its mounting stud/nut. All those parts have to be stronger for 1.65:1 rockers than they do for 1.5:1 rockers. Also it makes sense that with 1.65's there's going to be more of a tendency for things to move around - deflection/movement in the rocker stud - from those stresses, so maybe more important to also run a stud girdle or convert to shaft mounted rockers.

I'm in the process of re-building my 461 after having experienced a rocker failure on the 2016 Power Tour. I was running aluminum 1.65:1 rockers and a solid roller valve train. The engine had been together for more than 5 years when this happened, and had already completed one previous "long haul" Power Tour. I drove it probably on average, 100 or so miles a month. To be totally honest, now with the benefit of hindsight there were some warning signs of the imminent failure that were present when I checked and adjusted the valve lash prior to the Tour where it failed, but I probably didn't fully understand the implications of what I was seeing at the time so I chose to ignore them. On a few of the rockers I saw some 'witness marks' at the rear of some of the rocker slots where they'd been kissing the backs of the 'fat' poly-locks that the stud girdle mounts to. I decided it was a self-correcting problem and had created some clearance, without stopping to think about what a hammering those impacts had imparted to the whole rocker, including the trunion roller bearings (where the failure happened later). Lesson learned.

Still, on this rebuild I'm choosing to make some changes. I'm "dropping back" to 1.5:1 rockers, Crower stainless steel this time. I'm also changing cams. The new one is from Bullet and will actually be hotter than the previous one with about 10 degrees more duration on both sides and just a taste more valve lift - even with 1.5:1 rockers.
LSA and intake centerline will be the same as previous. Because of the rocker ratio change and more aggressive lobes, it'll also need 'more' spring. Tim Goolsby at Bullet helped me spec it all out.

Another side effect: Getting this lift with 1.5:1 rockers meant a slightly smaller cam base circle - otherwise the cam lobes wouldn't have through the cam bearings an I wouldn't have been able to install the cam :) A side effect of the smaller base circle is that the lifters sit lower in their bores - low enough that their oil bands don't "see" the lifter bore oil feed holes when they're all the way down. So, I've got two choices there. Cut some oiling grooves lower into the lifter bores in the block, or cut some oiling passages up from the oil bands "up" on the sides of the lifters that will be next to the oil feed holes. I'm going to modify the lifters, because although they are pretty pricey and there's always a chance I could mess one up and have to replace it, at least they are replaceable. My block, on the other hand, is the original numbers-matching block the car was born with and is therefore non-replaceable at any price.

My new cam specs:
Duration at 0.050: 251 intake / 257 exhaust
Lobe lift : .4300
Gross valve lift : .645 / .645
Lash (hot) : .020 / .022
Net valve lift : .625 / .623
LSA : 110
Intake Centerline: 106
Seat Pressure : 235
Open Pressure : 573
Installed Ht : .910

Bear
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Steve C.
Crazy Horse

1691 Posts

Posted - 24 Nov 2017 :  7:57:32 PM  Show Profile  Reply with Quote
"On the valve side, there's also inertia effect from the valve movement and the 'long' arm of the rocker, but I'm assuming that this side doesn't need as much force to keep things under control as the lifter side does."

I'm a bit confused at that statement.


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