Do Cam Designers Expect a Consistent Rocker Ratio across Lift Curve?

[BradH]

I'm curious whether cam designers have a working assumption that the rocker ratios being used in their models have consistent ratios across the lift curve. I would think the fact that some rocker arm brands & systems can have regressive ratio curves (higher ratio off the seat, then the ratio drops off as the lift increases) would have an impact on the overlap triangle and area under the curve calculations, as well as modeling flow at different points in the induction & exhaust cycles. Thanks - Brad

[hoffman900]

The ones that design lobes with OHC rocker arms and finger followers absolutely do as the ratio varies by quite a bit. That’s why the lobes look so much different than say a pushrod or bucket lifter type lobe.

On a pushrod, there is too much variance for off the shelf parts to do it. If you wanted to pay for it, I’m sure they could design lobes for your particular engine with your specific rocker arm. For the most part, what they recommend is close enough and the applications for 99% of their customers don’t need that level of sophistication.

[Stan Weiss]

Where was your lifter on the cam lobe, when you decided what push rod length to use?

Stan

[BradH]

Where was your lifter on the cam lobe, when you decided what push rod length to use?

Stan

It's simpler if I post a link to a thread on another forum that "inspired" my question here: http://board.moparts.org/ubbthreads/ubb ... ost2448178

I think you'll be able to see the images, even if you're not a forum member. Thanks - Brad

[volodkovich]

A good cam designer does..

Here's the geometry setup on the design program I use.

[hoffman900]

Volodkovich, what program are you using?


Here is the change in rocker ratio on a SOHC rocker set-up I'm dealing with:

[Geoff2]

David Vizard has a rocker arm 'chart' in his latest BBC book that shows some large variations in initial ratio & overall ratio for various brands/styles of rocker arms. How does 0.614" valve lift compared to 0.680" valve for the same lobe lift grab ya! Both rockers were supposed to be 1.8 ratio, overall ratios were 1.744 & 1.932

[Stan Weiss]

As you have moved the rocker arm around and calculated the different ratio changes have you factored in the change in push rod angle?

Stan

[SchmidtMotorWorks]

A good cam designer does..

Very few customers have the input data required.
Close to none outside of professional competitors.

[CamKing]

If we are designing proprietary profiles for a customer, we design for the valvetrain geometry.
Our std pushrod type profiles are designed around the lifter motion.

[BradH]

As you have moved the rocker arm around and calculated the different ratio changes have you factored in the change in push rod angle?

Stan

Naaah... I was simply playing around with some rocker images in Visio and seeing what changes resulted with different pushrod lengths & adjuster contact geometry.

[BradH]

David Vizard has a rocker arm 'chart' in his latest BBC book that shows some large variations in initial ratio & overall ratio for various brands/styles of rocker arms. How does 0.614" valve lift compared to 0.680" valve for the same lobe lift grab ya! Both rockers were supposed to be 1.8 ratio, overall ratios were 1.744 & 1.932

I bought a copy of that book recently, but haven't done a "deep dive" into it to have found that info.

[midnightbluS10]

David Vizard has a rocker arm 'chart' in his latest BBC book that shows some large variations in initial ratio & overall ratio for various brands/styles of rocker arms. How does 0.614" valve lift compared to 0.680" valve for the same lobe lift grab ya! Both rockers were supposed to be 1.8 ratio, overall ratios were 1.744 & 1.932

On the Y block, Ted Eaton posted an article showing an increase in rocker ratio of .142 just by altering position of the lash adjuster from all the way in to all the way out. And that closest spec was found with the adjuster at about half travel.


http://www.eatonbalancing.com/2007/12/1 ... -pushrods/

[David Vizard]

I'm curious whether cam designers have a working assumption that the rocker ratios being used in their models have consistent ratios across the lift curve. I would think the fact that some rocker arm brands & systems can have regressive ratio curves (higher ratio off the seat, then the ratio drops off as the lift increases) would have an impact on the overlap triangle and area under the curve calculations, as well as modeling flow at different points in the induction & exhaust cycles. Thanks - Brad

Exactly right Brad! Even some experienced engine builders are surprised at how much difference can be made by utilizing rockers with a faster opening rate althoug it is not as cut and dried as just swapping one for another.

DV

[hoffman900]

If it's fast off the seat, it's fast coming back on the seat too. From a dynamics standpoint, why would I want that?