I would thing is because of less friction at the valve tip,

 

Are you talking about stiffness, or "specific stiffness?"

The latter I would believe.

There is more to a rocker than stiffness or friction. Dan Jesel has made his empire on such concepts.

 

As stated less friction at the valve tip and to add to that proper wipe pattern is achievable. The one drawback of the stock rocker is the wipe pattern the pad provides will never match that of a properly setup roller tip. This helps with sideloading and wear characteristics on the guides. I learned this mostly from reading Aaron's (405HP ZO6) posts. On my new setup I will run stock rockers that have the updated HS trunion modification as I am using a stock GM head and they are said to have very hard guides.

 

So that's the only reason?

Your basically saying the only reason to run roller rockers is for aftermarket heads with "softer" valveguides, and for adjustability? Other than that, there is no benefit on stock heads with a stock 1.7 setup?

 

My many thoughts on this: [Skip to the Cliff Notes at the bottom for a quick summary. ]

1. You need a hard surface on the tip of the rocker arm that touches the valve. That's probably obvious. The hardness is needed because of the high unit loads (psi). You need to keep from squashing (mushrooming) the tip of the rocker. There is also some relative motion (sliding) between the rocker tip and the valve tip so the hardness imparts wear and galling resistance.

2. Aluminum doesn't get anywhere near hard enough, and even if you coated the aluminum with a very hard coating like DLC, the aluminum underneath the very thin coating would still deform under load and things would fail.

3. Aluminum is cheap and easy to make into rocker arms. Most aluminum RAs are sliced from a rocker-shaped aluminum extrusion and relatively minor machining is then done. The entire body of the aluminum arm is NOT fully machined. This is very economical.

4. The hard steel roller provides the strong wear surface and does roll a good share of the time perhaps 10°-20° (of 360°) depending on how good your geometry is. The rest of the roller does nothing except transfer the load from the valve tip into the body of the rocker arm and go along for the ride. This load all goes thru an axle which is usually about .250 (1/4) inch in diameter. If there are needle rollers in the roller tip, they are about 1 mm (.040 inch) diameter and the loads are transferred through just a few of the bottom ones. This is not only weak and often the initial point of failure after many cycles, but not very stiff. IOW, the roller mounting actually weakens the rocker arm "spring rate".

5. While the steel roller itself is very stiff so it doesn't deflect under loads, the aluminum RA is much less stiff, and a good share of it is carved out and drilled to allow the roller to be mounted. That makes the tip of the aluminum RA even less stiff.

6. A well designed STEEL rocker arm (LS1 OEM for example) is much stiffer than an aluminum roller arm, and even stiffer overall than a steel arm with a roller. Typical hardness of a steel RA might be around Rc50, so no roller is really necessary.

7. Everything in the valve train is a spring, with some being stronger than others. From weakest to strongest they are valve spring, pushrod, rocker, lifter, spring retainer, valvestem, valve head, rocker mounting, camshaft and cylinder head material. The last 5 or 6 might get shuffled around in order depending on the particular setup. The range of stiffness (spring rates) might be from about 300-450 lbs/inch (valve spring) to over 60,000 lbs/inch, a range of 200 times or there abouts. All of these "springs" are basically in series with each having a different natural or resonant frequency. That means they can each cause the valvetrain to resonate (or go unstable) at many different rpms. The stiffer the component the higher their resonant frequency is. So if you had all stiff parts, the resonant frequencies could be so high that you would not reach them at any engine rpm, with the exception of the weakest spring, the valve spring itself.

8. Good steel RAs (like the OEM LS ones) are actually lighter than aluminum roller RAs, and more importantly they have less mass near the valve tip, which moves (accelerates) the most and therefore generates the major loads that must be controlled. In other words. the non-roller steel arms have less Mass Moment Of Inertia (MMOI). In simple terms, that means it take less force to accelerate them from rest to their maximum velocity and perhaps more importantly, less force to decelerate them back to zero velocity. The RA are at zero velocity (stopped) when the lifter is on the base circle (valve closed) and at the very high point of the cam lobe lift (max. valve opening) so they accelerate to max velocity and back to zero twice during each valve opening/closing.

9 More mass over the RA tip (more MMOI) requires more force to accelerate/decelerate. That means there are more compressive loads in the lifter, pushrod and rocker arm so these parts deflect/compress/bend because they are effectively "Springs". When the load is released they give back some of the energy that deflected them. The valve spring decelerates them as they approach the nose of the lifter and then accelerates them down the closing side of the cam lobe. The more loads there are due to more mass and more "springs" storing energy, the more Force it takes to start and stop the parts.

10. In the days of the SBC with it's stamped steel ball-mounted rocker arms, aluminum roller arms running on short shafts with bearings offered a lot less friction, and often equivalent/better stiffness, as well as varied ratios. If you add in shaft mounts, the overall valvetrain stiffness can be considerably better than 3/8 inch stud mounts as found in the SBC.

11. Now this doesn't necessarily carry over to the LS engine. In the LS the ball is gone, the trunnion (rocker shaft) bolts directly to the head and doesn't try to bend a stud. The rocker bolt just resists tensile loads, not bending. The cast steel rocker has material only where it needs to be to carry the loads, and has almost infinite lifespan, unlike aluminum. No roller is needed.

12. OK, so why are aftermarket aluminum roller rockers for the LS so popular? IMO, many people think a roller must be better just because it rolls. That's easy to get one's mind around. What is not as easy is the concept of stiffness in a rocker arm, mass, MMOI and their effects on valvetrain stabililty. So, the market fills a preceived need. I find it strange that ony one rocker manufacturer (Harland Sharp) offers to upgrade the weakest part of the LS rockerarm, the trunnion shaft and needle bearings. It's a bargain (in the rocker arm business) @$260.

IOW, build it (aluminum roller rocker arms) and they will come!

13. Finally (!) can steel, non-roller arms be made that have the low friction at the valve tip of rollers, allow very high valve lifts without any problems, have a selection of ratios and have the advantages of LOTS more stiffness, less mass and especially less MMOI than aluminum roller rockers? Sure, but they are VERY expensive.


Jon

 

Thank you for the comprehensive answer. I will soon be changing the cam and springs in my LS6 (stock heads) which will increase the lift from roughly 0.550 to 0.600 and increase the spring pressure to 375 lbs. Based on my reading of you explanation I should keep the stock rockers with the HS upgrade, which is probably what I will do. Is there a lift or spring pressure threshold at which you advice would change from the stock rocker to something else?

 

moral of the story...

stock gm heads use the stock rockers with the Harland Sharp modification.

aftermarket heads use harland sharp RR's


Thank you everyone who has added some good info on this thread.

 

If it were my engine, I would check with my valvetrain designer on that. Different folks have different experiences. What many forget is that the valvetrain is a SYSTEM and all of the parts need to be working together.

To answer your question...it depends. That's not a cop out. When I see stock lifter, stock rocker HR drag engines turning 78-7900 every run with valve lift over the "magic" number, I wonder about the gurus who say it can't be done. Some of them write for magazines.

Jon

 

In my case my valvetrain designer recommended stock rocker arms. The only reason I was even researching rocker arms is that I wanted to address the needle bearing issue particular to the stockers. I happend to be price indifferent between certain alternative remedies for the needle bearing issue, those being: (a) the HS conversion; (b) the lowest price HS aluminum rockers; and (c) Yella Terra Ultralites, so I was interested in your discussion of why aftermarket rockers can actually be mechanically inferior despite their higher price. Thanks again.

 

Hmm...If I were trying to guess who your valvetrain guy is, you just narrowed the field considerably.

Jon

 

I know for a fact that with the right acceleration on the lobe, a larger dia. pushrod, the caddy lifters, LS7 stock rockers and LS7 valves, the valvetrain would not see valve float until 8700RPM!!!!!!! This was a 0.630 lift cam with , wait for it,...... beehive springs!!!!

Take that!!!!

 

"a larger dia. pushrod,"

Not to be overlooked in the above post.

 

Yep, that's generally the second weakest "spring" in the valvetrain. It's hard for some folks to believe that a 120 gm pushrod will make a valvetrain more staple at high rpm than a 70 gm pushrod, and require less spring to control it. It seems counterintuitive unless you understand the physics involved.

Not every valvetrain needs a 120 gm pushrod, of course, but some do.

Jon

 

What are the Roller Rockers you would recommend and go with?

Harland Sharp and Yella Terra ?

 

c. None of the above.

I don't think that I have suggested using roller rocker arms on LS engines.

IMO, the step above stock rockers are rollerless, bearingless, steel rocker arms. They are rare and very expensive. The one in the picture is a high ratio LS7 exhaust. Note that the adjuster screw is 3/8, not 7/16 and the ball is 5/16. That will help you with the scale. It's very light, even with the adjuster.

 

If only someone was producing these in larger quantities. The Jesel J3S is a nice looking steel rocker, but the price is out of sight. I just couldn't get my stockers to provide a good wipe pattern and finally gave up and went with Yella Terra but if a rocker like this were available that would provide a good geometry and longer life of steel I would have used them instead. Right now I am thinking that when the valve springs get swapped I may need to replace the Yella Terra's just to avoid an unpredictable fatigue failure.

 

Note thet the J3S still has a roller with needles which is less stiff than a rollerless arm. It also still uses needle roller bearings on the trunnion shaft which is also less stiff than a "bearingless" design. Steel on steel? How can that work?

I actually like the "available adjusterless design". Anyone want to explain how you use this with solid roller (or solid flat) lifters? It does reduce the rocker mass by eliminating the 12 or so gms of the smallest adjuster. When you are talking sub 100 gm steel rockers that is significant. The large bore in the bearingless black arm is the same size as the trunnion shaft on the J3S.

The J3S probably is priced close to the black one shown in previous posts. If you have to ask...you probably can't afford it. I think that came from an old Ferrari ad. Actually I don't know the price of a J3S system, but my understanding is they are custom order. You might not get much change from a $5000 bill on these type of high-end systems. T&D has a nice steel (Cup-style) system (roller tipped) that they quoted ~$5500 a year ago.

Jon