All about cams

jseng1 said:

Dances with Shrapnel said:

...In my mind the only way to understand the differences is to take direct measurements in valve lash from cold to hot for the various pushrod and barrel materials. With air cooled vintage engines, this measurement approach is not an easy task and there is a whole spectrum of how hot an engine is and how cool a pushrod is at any given point in time (hauling up a hill WOT in hot weather versus down hill at 1/8 throttle in the winter time).

The big difference I see from an applications standpoint are the relative stiffness of the different materials for pusrods...

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Yes. And from memory of what I heard you can have about .020" clearance with steel pushrods in alum cylinders when hot after setting them to 0 when cold. Confirmation is needed. So the question is - what are you loosing and gaining? If you are getting more clearance with steel pushrods then you are loosing lift. If alum pushrods are compressing then you are loosing lift (but clearances are tighter). So does it come out the same? No one seems to know for sure. Lots of bounce is happening at the valve at high RPMs but I'm not seeing any problem with the alum pushrods in Comstocks spintron (see page 4 of this thread). If I get a smooth ramp cam together for the spintron I will ask to have the pushrods filmed when its on the spintron at high RPM.

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Looking at the pushrods in a spintron will not tell you anything unless failure is pending due to buckling. Strain guages on the pushrods would be more telling. Better still, run a simulation on 4StHEAD.

What you have to gain with steel versus aluminum pushrods is less lag in the valve opening event. As I mentioned above, the Youngs' Moduls for steel is three times that of aluminum. Basically aluminum is three times "springier" than steel. Think of the pushrod as a spring. Ticking over a motor at idle or a few thousand rpm will yield little significant difference in behaviour between steel and aluminum but as the loads increase to the square of the speed, the valve opening event lags behind with aluminum when compared to steel.

I have run the steel pushrods and Steve Maney aluminum barrels and based on the engine sound, I highly doubt I had 0.020" lash when hot but I have not checked to confirm. As I mentioned above, the thermal strain of the barrel is probably a little more complex than that of the pushrods which are bathed in oil.

This is not saying everyone should be running out to their shed to change out their aluminum pushrods to steel.

Great thread.

The majority of the factors mentioned here in earlier posts on this topic are relevant considerations (modulus of pushrods and their attendant elastic/springy nature, lost lift due to excessive growth in lash, etc). However, in my mind, perhaps one of, if not the most important considerations would be the following. At full operating temperature the running lash must be maintained such that valve opening/closing occurs on the opening/closing ramp of the cam. Comnoz has clearly shown previously, especially at high engine speed, how finicky valve trains can be to seemingly subtle changes in ramp design. Thus the criticality of ramp design is well established and having lash grow to a point where opening and closing events are not occurring on the constant velocity ramp is a major issue.

Although a significant range of opening/closing ramp length and lift have been employed by designers, it is quite common to employ constant velocity designs exhibiting lifts of 0.012" occurring over 50 crankshaft degrees, with preferred hot lash settings of 0.008-0.010". A stock CDO cam provides a good example of this style ramp. The graph below shows the subject ramp, with a red line provided as a reference to allow the reader to readily discern the linear or so-called constant velocity region of the design where the lift curve remains parallel to the red reference line before transitioning to the flank of the cam. A notation was also included showing the stock intake lash. It is clear from this graph that if lash grows to greater than 0.012-0.014" the ramp will be completely eliminated from the design, no benefit whatsoever will be derived, and valve opening/closing events will be significantly harsher as compared to operating with the ramp.

As Dances mentioned, the valve train components/materials as well as all the engine components/materials must be considered when determining an appropriate cold lash setting. The graphic below suggests that valve lash on a stock CDO must maintain relatively close to the cold lash setting (within 0.005-0.006") or valve train contact would occur off the ramp.