WS6T3RROR

I thought I would put this here, maybe somebody is interested or has a different idea about it than I do. But I see plenty of misconceptions about it and I am throwing this out there.

I know people like to pretend that the lifter and pushrod mass etc does not matter. It does, its part of the system and a required input if you're modeling the reaction of the system to excitation.

Now if we look at the system under a steady state condition lets say 6000rpm, and compare two situations lets say they both have the same valve motion but one is a 1.5 ratio and the other is a 1.7 ratio. The only difference then is the acceleration required of the lifter and pushrod. Along with the greater acceleration of the lifter comes higher velocities. The velocity is the issue here since kinetic energy is 1/2 mass times velocity squared a small change invelocity causes a moderate change in kenetic energy, the spring has to soak up this kinetic energy to keep the lifter on the valve (although there will be some loft depending on mass, spring rate, and damping ratio). The higher rocker ratio wins here because its moving less (less kinetic energy) with the same frequency of excitation and should have an rpm advantage due to this.

Now a bit about hydraulic roller cams and why high ratio rockers win there and with flat tappets as well. With the flat tappets you are velocity limited there is a certain point which is geometricly defined that if you try to make the lifter move faster than that you will catch the edge of the lifter face and frag the whole works. With hydraulic rollers its slightly different, you are stuck with a maximum acceleration, because there is no edge to hang on. What there is with hyd lifters, is a point where the contact between the lobe and the roller wheel is so severe (perpindicular to the axis of the lifter bore) that it becomes too much of a bending moment and the lifter will either jam or fail or the hard surface of the cam will fail. With a lobe designed for high ratio rockers you can exceed the velocity and acceleration of the low ratio setups. Basicly you can get more area under the curve for lift. High ratio rockers should be used to improve the area under the curve and work around geometric constraints not to get more valve lift in general. The other side of this is the cam must be designed to suit the high ratio rockers and not be like most items and be hard to control even with 1.5 rockers.

Now we should all be running high ratio rockers because its a free lunch right? Unfortunately.. no. No such thing as a free lunch. The high ratio rockers result in a multiplication of comrpessive forces on the camshaft and lifters from the springs. Just as it multiplies the lift from the cam it does the same to the compressive force on the lifters, and is as a result harder on the studs. The other issue is that with a sbc valvetrain your only real option without going to a shaft rocker to improve rocker ratio is to move the pushrod cup in the rocker closer to the stud since the stud to valve length is set pretty much. This does not make the pushrods happy due to the resulting geometry (angularity). Also unfortunately with the high ratio rockers you have to be much better at grinding the camshaft accurately. I know everyone wants to believe cnc machines are magic but they arent they have bad days as well and they just like you have thier limits. Just as an example draw something on a notebook page, now put it on a matchbook in the same detail and scale. What I am getting at is the machine has its limits too and if we want to make money and move any volume at grinding cams we are not going to use space age slow process speed stuff that takes all day to cut one cam.

Now what should a high ratio rocker engine look like. Well since we have more compressive force on the pushrod from the high ratio rockers we'll have a higher bending moment on the cam. So we need to improve the moment of inertia to stop the deflection from bending as a side effect it will help with torsional deflection. So now we have a large diameter cam, and as another added bonus we have more room to get the lobe design ground correctly, man are we doing good now. But since we know the center of the cam doesnt do much work for the moment of inertia so lets gun drill it and keep the mass down. Too bad with the bigger core we have to raise the cam up to clear the connecting rods and crank now though.

Pushrods are also a misunderstood part of the equation. Most people think it is the cross section of the steel that is the issue alot of the times and pure compressive stress that is the problem. Stress is defined as force per unit area. The thing is that steel is very very tough, and while increasing the diamter of the pushrod increases the amount of steel it corrects a less intuitive problem. The problem is that pushrods are long and 'skinny' they are a column, columns often times to not fail just from pure compressive stress but from 'buckling'. I dont wish to get into it but lets just say that increasing the diamter increases the moment of inertia and increases the load it can handle before buckling becomes a problem. Also since the deflection is reduced the fidelity translation of the valve motion to the valve is improved which is the real concern.

In case you have not realized it at this point, what I have described above is basicly an ls1 or pretty much any raised cam sbc block (bbc core is usually standard). It may not apply to most people heres street lt1's etc but it is something to think about and maybe we can get something mildly interesting going here in advanced tech again.

rskrause

The most important of the many points you covered relates to the limits imposed on the lobe profile by the diameter of the cam and the geometry of the lifter/cam interface. A high RR is needed to get the desired valve motion (both lift and acceleration). I am not sure what is the highest RR in use, but I have read that about NASCAR engines builders use of 2.0:1 rockers. I agree, TINSTAAFL. But in many cases a higher RR is about as close to "free" HP as you can get.

Rich

WS6T3RROR

I have seen as high as 2.2 ratio rockers in use. I know Lingenfelter did some playing around with all of it in the mid 90's. I remember reading an article somewhere in there where he kept having cams cut to match and on slightly different profiles and upping the rocker ratio more and more custom making the shaft rockers himself at the time. The power kept going up and so did the rpm it was stable to. I almost want to say it was for a silver state challenge effort. I believe it was also using a big block chevy cam core in some sort of aluminum block.

1989TransAm

"Now if we look at the system under a steady state condition lets say 6000rpm, and compare two situations lets say they both have the same valve motion but one is a 1.5 ratio and the other is a 1.7 ratio. The only difference then is the acceleration required of the lifter and pushrod. Along with the greater acceleration of the lifter comes higher velocities."

IMHO the acceleration of the lifter and pushrod remains the same regardless of the rocker ratio. The cam lobes, lifter and pushrods remain a constant and do not change. If you change the rocker ratio the fulcrum of the rocker arm will change, the valve, springs and associated hardware will change velocity but that is on the other side of the pushrod.

WS6T3RROR

I am speaking of the same valve actuation, but doing it with a different rocker ratio and camshaft lobe profile. Under those conditions in order to complete the same amount of cycles the lifter for the lower rocker ratio has to travel farther to do the same job and therefore has a higher velocity and acceleration as a result. There is no opinion to it, just hard fact from ma nature.

I am sorry if it seems confusing as I did go from talking about conditions of the same valve actuation with different rocker ratios to optimizing things and beginning to take advantage of high rocker ratio systems.