GARY C wrote: ↑Sat Jul 27, 2019 9:23 am
David Vizard wrote: ↑Sat Jul 27, 2019 9:02 am
SchmidtMotorWorks wrote: ↑Mon Feb 19, 2018 6:27 pm
For a cam design that is intended for common builds, he must use the nominal ratio to design around unless there is a particular geometry that is specific and reliable for that engine.
Assuming the cam is well designed, using a geometry that changes the motion defeats the design intent of the cam.
John,
The intent is to get the greatest output from the engine.If it works reliably then the goal is achieved. AS I have said many times all the stuff I put in my books is the result of hundreds of hours of dyno testing. That is not even close to an opinion. If you had taken the time to read my BBC book you would have realized I pointed out both the pros and cons of the situation and more to the point, told how to make the best of the situation concerning any negatives.
I did a test of two 1.8 rockers and the one with the characteristics I advise to use made 18 hp and about 10 lbs-ft more.
DV
John, How well can one design a cam if they don't know what the rocker is doing with the motion provided by the lobe? Seems like they should be looked at as one.
Right, in Utopia, the geometry is known, but in reality (as we often see here), people can't agree even on what the desired geometry is or determine what it actually is.
I examined a wide selection of rocker arms for a head I designed at Edelebrock to optimize the dimension for rocker pivot point.
The biggest variation I found between all of the rockers was on the pushrod side.
Let's say that if everyone agreed to use mid-lift for pivot point, they would still have a wide variation caused by the differences in the position of the ball/socket on the pushrod side. One of my colleagues there pointed out how difficult of a problem this is for a hobbyist that just wants to buy some parts and assemble them, so he started making a table to help sort it out but it became very complex before all of the issues were resolved.
The other problem was consistency and access to data, he had a vast collection of heads to take measurements from but there was no upside to publishing the data (which was inconsistent anyhow, QC in the performance aftermarket is very loose).
I think the best you can do is to try to find a pivot point that makes the lifter to valve motion ratio as consistent as possible, then choose a cam that is designed for lifter motion. That way, if you choose another cam, you don't need to reconsider the other variables.