Lobe intensity vs rocker ratio

[KnightEngines]

Say you have a target valve lift, you can achieve it with either a fairly agressive roller lobe & "normal" rocker ratio, or you can use a softer lobe & more ratio.
Ignoring lobe design specifics which option will result in the most stable valve train & best lifter life?

[twl]

IMO, the softer lobe gives less acceleration and velocity on the pushrod side, which has the most mass and also the longest length which can be deflected. The higher rocker ratio concentrates the higher forces on the lighter and shorter parts of the valve train on the valve side of the rocker.

So IMO, the softer lobe with higher rocker ratio would give more stability in the valve train because the big heavy parts see lower speeds.

[RevTheory]

I've been contemplating this very topic for a couple of months now and I seem to be able to make an equal argument for both. One week it's a softer lobe with more rocker and the next week it's a more-aggressive lobe and less rocker so it'll be interesting to hear what Mr. Jones has to say about the matter.

[MadBill]

For a roller application, an 'aggresive' lobe translates into a higher maximum pressure angle, which has major durability and loading implications, so beyond some PA in the mid to high twenties, there is little choice but going with a higher rocker ratio, other than a complete re-engineering of the block to accept much larger journals and lobes.

[900HP]

To me it's cam core dependent. If you already have a small base camshaft, why make it smaller by trying to get more lift out of it?

[F1Fever]

I would lean towards a higher ratio rocker.
The valve spring will have more leverage on closing keeping the lifter in contact with the lobe better. On open the lifter will exert less side load on the bore.
As noted the pushrod will also have less total travel (but will be exposed to the same total load).
I also *think the rocker tip will follow a straighter path *if it's the valve side of the rocker that is increased in length although it's much better to shorten the pushrod side but this would accentuate side loading of the pushrod increasing the chances of flex.

On a total blank sheet of paper, IMO, higher ratio rockers are the easy answer. In practice with set constraints like rocker stud location, lifter bore location it's likely a tradeoff no matter which way you go. I still lean to higher ratio rockers

[CamKing]

Ignoring lobe design specifics which option will result in the most stable valve train & best lifter life?

As long as you can keep the rocker geometry in check with the higher ratio, you will have better valvetrain stability and longevity.
In both cases, if done correctly, the acceleration, velocity, and travel of the valve, retainer and spring will be the same.
With low lobe lift, and higher ratio, you reduce the acceleration, velocity, and travel of the lifter and pushrod. The lower lobe lift allows for a bigger base circle. The lower lifter velocity and larger base circle cause less side loading on the roller lifters.
The problem with the higher ratio is that the spring pressure is multiplied by that higher ratio, and can cause more pushrod flex, and lifter wear. It can also cause increased bleed-down with hydr lifters. The good thing is, since the acceleration of the lifter and pushrod isn't as high, you don't need as much spring pressure to control it, so you can reduce your spring rate.

[Walter R. Malik]

IMO, the softer lobe gives less acceleration and velocity on the pushrod side, which has the most mass and also the longest length which can be deflected. The higher rocker ratio concentrates the higher forces on the lighter and shorter parts of the valve train on the valve side of the rocker.

So IMO, the softer lobe with higher rocker ratio would give more stability in the valve train because the big heavy parts see lower speeds.

However, that will relay more actual force at the lifter / camshaft junction; (not a good thing for a street engine which idles or is at low speeds a lot).
SO, how this engine is to be used is paramount in what to do.

Mr. Jones has pointed this stuff out in very easily understood vernacular.

[BOOT]

The good thing is, since the acceleration of the lifter and pushrod isn't as high, you don't need as much spring pressure to control it, so you can reduce your spring rate.

Good to know I always thought you needed more spring pressure with higher ratio rockers.

[PackardV8]

So IMO, the softer lobe with higher rocker ratio would give more stability in the valve train because the big heavy parts see lower speeds.

Going back nearly fifty years, after I'd spent a week lightening and polishing a set of Packard V8 rocker arms, a pretty good cam grinder told me, "Well, you didn't hurt them too much, but it's the valve side of the rocker which makes the difference. Spend your time and money making the valve and retainer lighter. On the cam side, make the pushrods and rockers stronger. Flex will kill you there, so don't sweat the weight."

Who'd agree or disagree?

[KnightEngines]

Thanks for the input guys, I was leaning towards increased rocker ratio being overall the better option.

It was just something I was thinking about, couldn't decide.

[groberts101]

The good thing is, since the acceleration of the lifter and pushrod isn't as high, you don't need as much spring pressure to control it, so you can reduce your spring rate.

Good to know I always thought you needed more spring pressure with higher ratio rockers.

Unfortunately there are limitations to that ideology. When the rocker ratio increases to greater degree it increases the valves acceleration rate (which is of course what makes the cam become bigger at the valve throughout the lift curve/more area). At the very least you'd want to consider increasing the seat pressure to help reduce lofting or bounce created from the faster valve action. The greater the valve train speed.. the more important it becomes to balance things back out with added spring pressure.

Also consider that running high rocker ratios(1.8 or higher) would be akin to a simple lever in design in that shortening the fulcrum length(achieved by moving the pushrod cup closer to the fulcrum) will increase the loads on the pushrod(or lever) used to actuate said fulcrum.

[PackardV8]

Also consider that running high rocker ratios(1.8 or higher) would be akin to a simple lever in design in that shortening the fulcrum length(achieved by moving the pushrod cup closer to the fulcrum) will increase the loads on the pushrod(or lever) used to actuate said fulcrum.

FWIW, the current Mopar hemi is an incredible design success; seems to be the current production pushrod champion. The tradeoff is the radical valve and pushrod angles are wearing the pushrod cup in the rocker arm. My machinist is rebuilding many sets of rockers for these engines for the local large rebuilder. He has to toss way more rockers than usual in the older designs. These are often from lower mileage engines which have lost an intake seat. Had the head not dropped a seat, it probably would before long have been done in by a failed rocker.

[427dart]

Also consider that running high rocker ratios(1.8 or higher) would be akin to a simple lever in design in that shortening the fulcrum length(achieved by moving the pushrod cup closer to the fulcrum) will increase the loads on the pushrod(or lever) used to actuate said fulcrum.

FWIW, the current Mopar hemi is an incredible design success; seems to be the current production pushrod champion. The tradeoff is the radical valve and pushrod angles are wearing the pushrod cup in the rocker arm. My machinist is rebuilding many sets of rockers for these engines for the local large rebuilder. He has to toss way more rockers than usual in the older designs. These are often from lower mileage engines which have lost an intake seat. Had the head not dropped a seat, it probably would before long have been done in by a failed rocker.

Do I need to worry about this in my stock 2014 Ram pickup with 5.7 Hemi??

[CamKing]

When the rocker ratio increases to greater degree it increases the valves acceleration rate.

Only if you don't change the cam along with the rocker ratio. If the lobe is designed for the higher ratio, it won't have any more valve acceleration then a lobe designed for a lower ratio with the same duration and valve lift.