gemellocattivo.com

I figured I'd throw this in here as there doesn't seem to be a tech section... stop slacking off Mark....

So let's put on our pocket protectors and safety goggles and warm up the calculator....


There is loads of data on the web about cams, loads of data on valves, springs etc.. But there is not much when it comes to the actual engineering and design. For that you need to dig and dig. A few fortunate ones have managed to take Harvey Crane's Cam school course. The majority of engine builders don't actually spend much if any time on cam lobe design and spring rate requirements. Reason? it's complex and very time consuming. The lack of available clear information and affordable programs to run the more complex computations also make this difficult.

To avoid spending more free time then I have available I'll try and keep this thread going in bits at a time. I hope.

First a link that covers cam lobes and the technical aspect of what is going on, it's a very good site that walks you thru nomenclature and the history of cam lobe design.
http://www.tildentechnologies.com/index.html

I sat down and used the stock profiles on some of the Ferrari cams and calculated the basic physics of acceleration, velocity, and kinetic energy imparted to the tappet. A simple spread sheet will handle this just fine. The point? well stock springs are usually regarded as not having enough spring rate in force to properly do the job. By checking the math we can get a good approximation on if that's true or not. For the QV's it is, but only above 7800 rpm. While the spring has enough force overall the kinetic force of the valve train weight actually gets to within 64% of the spring rate on the closing flank at 7800rpm and by 8k rpm it's 68% and at 9k it jumps to 85%. What does that mean? valve float is a real possibility past 7800rpm.

Ok useful information but not a complete picture as those calcs don't help with valve loft, or to say it better. If the ramp rate is excessive enough the kinetic energy of the valve under acceleration will be such that the valve will lift off the cam lobe beyond the nose. That's an interesting effect and a dangerous one as the risk of interference or breaking parts goes really high. The impact velocity and point on the cam lobe is unknown and risky as well. Then you have spring harmonics, valve harmonics and a host of other things going on. So how is this calculated or figured out? well for the most part it's done by the spintron. However that's not to say that computational power isn't being utilized. With the advent of FEA software and star-CCm+ things get a bit easier to test virtually before real-world punishment comes into play.

I've been able to run the 308 QV valve geometry and cam lobe thru this motion analysis, checking harmonics, deformation, etc.. It's a time consuming process though and requires that I run a liquid chilled processor! An interesting thing I noticed was that at near 8k rpm the tappet bucket exhibited a harmonic distortion on the bottom skirt, for me that answers why I've found some of them to have cracks in them like a bell. Over revving the engine runs the very real risk of a tappet bucket to harmonically crack at the bottom skirt.

More later and some pics...

I was afraid or making too many sections thinking they'd mostly sit empty and unused....I'll add Tech to the topic name for now then pull out a section when it's clear we need it.

Just teasing you Mark You've got lots more important things to do... imagine if Lanna knew you had free time to play on the web!

'course I'm really hoping that I remembered her name correctly right now.. Doh!


OK some pretty pics of the stress and deformation plots..

Scott, three questions, which are somewhat germane to your topic:

What do cam lobes designed for a solid lifter look like vs those designed for a hydraulic? Pics appreciated

Most modern Ferraris are now using hydraulic tappets for their zero lash benefits, however what happens to the timing for that cylinder or cylinders when the lifters collapse or are bleeding down in use? Obviously those valves won't get the lift or duration that they should.

How can you accurately time an engine with a hydraulic lifter that is bleeding down? Domestic engine builders substitute a solid lifter for the hydraulic when setting the timing, in order to avoid errors introduced from the hydraulic lifter. Obviously that's not possible with an OHC engine having cams that directly actuate the lifters.