Engine pushrod /rocker geometry question

[Clem Z.]

Re: Engine pushrod /rocker geometry question

Duke et al

I also collected data on rocker flex and found that on the valve opening ramp, the stud flexes towards the valve .003, and on the closing ramp flexes away from the valve .001 using the correct length pushrod. With the longest pushrod (+.110) the outward flex increased to .004.The flex motion changes direction as the lifter traverses the nose of the cam. With 300# crane springs and same OE rocker, the flex is .0035 and .002 respectively. Changing to 1.6 Proform rockers using the OE spring the numbers are .003 and .002 respectively. The 1.6 rockers and #300 pound spring generated .005 and .002. This is all in a static measurement state. In a dynamic operating environment I can only imagine that the effect is amplified. It indicates to me that changing to 1.6 rockers and stock springs introduces some level of additional stress on the valve train; but less so than increasing spring pressure - as found within the parameters of my testing. I'm hoping the additional lift of the 1.6s will get me me at least as much power as increasing spring pressure to get a few more revs. I did a lot of this testing to help me decide whether to install 1.6 rockers on the intake side. It seems I can't stop the search for more power on a basically stock GM components 327 (exception headers and bigger carb). Yes, I'm still running ignition points. Hoping to break into the 12 second range this summer. Got to 13.16 at 108+ last year and feels good to match or beat some stock c5's - just for fun.

i have found that going to 1.6 rockers on some cams caused the valves to float earlier. the stud flexing causes a valve timing change and that is why you should run a stud girdle. using strobe light and see thru covers you will see all kinds if things happening at speed

[Don C.]

Re: Engine pushrod /rocker geometry question

Clem

Did you find the valve float problem occuring using 1.6's with stock GM cams or was it with more aggressive aftermarket profiles? Theoretically a higher ratio rocker should increase the RPM limit, all other things being equal, according to David Vizard.

[Clem Z.]

Re: Engine pushrod /rocker geometry question

Clem

Did you find the valve float problem occuring using 1.6's with stock GM cams or was it with more aggressive aftermarket profiles? Theoretically a higher ratio rocker should increase the RPM limit, all other things being equal, according to David Vizard.

after market cams

[Jerry G.]

Re: Engine pushrod /rocker geometry question

It's a road race motor so I am using a more aggressive cam profile. i think Clem may have solved the problem. He related how they used to solve this kind of problem by cutting the push rod guide plate in half and then aligning the contact point to center on the valve stem and then weld it back up. Another buddy suggested use of an adjustable push rod guide plate sold by ISKY. Using this approach.the rocker contact will not be perpendicular to the longitudinal axis of the block but it will then be centered and track in a straight line. I love having these great people to help me through what for me is a first time problem. Thank you all. I'll order a set of adjustable guide plates in the morning.

[Duke W.]

Re: Engine pushrod /rocker geometry question

Clem

Did you find the valve float problem occuring using 1.6's with stock GM cams or was it with more aggressive aftermarket profiles? Theoretically a higher ratio rocker should increase the RPM limit, all other things being equal, according to David Vizard.

Increasing rocker ratio for any given lobe will decrease valvetrain limiting speed because you are increasing dynamic loading at the valve at any given RPM, so you run into valvetrain limiting speed earlier.

Duke

[Jerry G.]

Re: Engine pushrod /rocker geometry question

So an interesting tradeoff analysis would be to see if the increase in RPM would offset the increased air flow capability of the 1:6 rockers. My induction system was not at the limit at the end of the test.

[Michael H.]

Re: Engine pushrod /rocker geometry question

Increasing rocker ratio for any given lobe will decrease valvetrain limiting speed because you are increasing dynamic loading at the valve at any given RPM, so you run into valvetrain limiting speed earlier.

Duke

Yup, I was just going to post and mention that. The lift/acceleration rate and velocity becomes more aggressive with higher ratio rocker arms. That requires more valve spring.

[Michael H.]

Re: Engine pushrod /rocker geometry question

So an interesting tradeoff analysis would be to see if the increase in RPM would offset the increased air flow capability of the 1:6 rockers. My induction system was not at the limit at the end of the test.

The big advantage with higher lift isn't actually the higher lift. (huh?) In order to have aggressive opening ramps, the lift must be higher to give the valve more degrees/time to slow down and reverse direction.
Most older small block intake ports become the restriction factor in overall port flow, not the valve itself. Once past about .5" valve lift, the valve is no longer the restriction.
If we increase the opening rate/positive acceleration, and velocity, which is exactly what happens with higher ratio rocker arms, the negative acceleration must also increase. (thus, the need for more valve spring pressure)

Many GM dyno tests showed that the difference between 1.5 and 1.6 rocker arms was minimal, even with the correct valve lash required for the new ratio. Several combinations of both ratios were tested with only minor changes in torque/HP.

I think I sent some of the dyno sheets to Duke a few years ago. Maybe he can post one or two? If not, I can try to dig out a few and post.

[Don C.]

Re: Engine pushrod /rocker geometry question

Michael/Duke
Thanks for the educational repsonse. You guys sure do know your stuff. I would be interested in seeing any dyno graphs you have of this experimentation. So the increased leverage of the 1.6's acting on the pushrod side of the valve train does not offset the effects of the changes to the lift profile. Very interesting.
many thanks

[John H.]

Re: Engine pushrod /rocker geometry question

There was a feature article in HOT ROD magazine about four years ago to settle the questions about 1.5-vs-1.6 rockers. They did dyno runs on a small-block engine that was built up with four different camshafts, each of which was run with 1.5 and 1.6 rockers (8 runs), and they never gained more than 3hp by changing from the 1.5 to the 1.6 rockers. The summary of the article was "don't bother - it's a waste of time and money".

[Duke W.]

Re: Engine pushrod /rocker geometry question

I think I sent some of the dyno sheets to Duke a few years ago. Maybe he can post one or two? If not, I can try to dig out a few and post.

None of the examples you sent me compared different rocker arm ratios.

Duke