Why do the valve events and overlap change using different lift points? Is it a "theoretical" closing point assuming some set ramp rate or something? I understand durations change at different lift points, but there is only one IVC, EVO etc..So why is it different at .050" and .006", for example. Same question for overlap. how is there 66* overlap @.006 and 12*@.050? Doesn't a cam have a quantity of overlap regardless of what lift point, as long as that lift point is not during overlap, of course?

 

Both numbers are somewhat arbitrary. But some people choose 0.050" because it's a bit easier to measure than 0.006" and they figure that no significant flow occurs until the valve is open at least that far anyway. The big thing is consistency - use one or the other.

Rich

 

Thanks for the reply, but that wasn't what I was really asking. Why are valve events and overlap different at different lift measurements? A cam will have its valve events set in stone(minus, of course, degreeing, different ratio RRs, preload/lash adjustments, etc..) Same with overlap. Why does a cam's valve events change when measuring from different lift points? Put cam specs @.050 and @.006 in a valve events calculator. You will get different(at least numerically) valve events and duration of overlap at the different lift points(.006,.050,.200, etc..) Why is this?

 

The cam has different durations at different lift points.

Rich

 

I understand that, buy why would the valve events change. Measuring duration from a higher lift point would result in lower duration-the valve's open for a shorter period of time. But that should cut off some of its opening duration points, not change when it closes.

 

"Closing" is also measured at a defined lift point. So, the duration may be expressed as number of dergrees between 0.006" open and 0.006" closed or as 0.050" open to 0.050" closed.

Rich

 

Oh..so its all a relative thing. Here's sort of a follow-up question: How can a valve events calculator know closing point just from duration at a certain lift point? Is it assuming some ramp rate?

 

Ramp rate has no effect on the opening/closing points - the duration set's those points.

Let me guess.....you are using desktop dyno?

 

Duration sets opening/closing points. I get that, but two cams can have the same durations @.006, but different durations @ .050 due to different ramp rates. So, my question was, how can a valve events calculator know cam's closing points using specs at a certain lift point? Duration @.050, or .006, or .200 for that matter, don't tell the entire story. Especially if it is a custom lobe design. So, a valve events calculator must be assuming some kind of ramp rate to be able to give closing points given specs at .050 or .006 or whatever.

That's a negative on the desktop dyno.

 

I think you were getting pretty close to understanding the answer to your question when you used the word "relative." If you attach the concept of "arbitrary" to the concept of "relative" you'll be that much closer.

Years ago, while trying to master geometry, I struggled very briefly with identifying the absolute value of "pi." How significant is it to use 3.14159 instead of 3.14159 26535 89793 23846 26433 83279 50288 41971 69399 37510? Obviously, extending the numeric value of pi to 50 decimal places gives you a more accurate value of the area of a circle but, it could be even more accurate if you extend pi to another 50 decimal places or another 100 decimal places and, in the final analysis, the line must be drawn somewhere. "Somewere" in comparing camshaft profiles is set at either .006. .050, or .200.

As an alternative, you could plot the profile of the camshaft on a graph by measuring the lift at every degree of duration throughout the cycle. You'd have a lot of information but not a lot of practical data.

 

I see what you're saying, but assuming some ramp rate to "guess" a valve closing point dosn't seem very accurate. DCR can vary half a point by just 2-3* difference. Given specs @.050, .006, and .200 would give a respectable representation of the shape of the lobe, but I don't see given specs @ only .006 or only .050 could do that. Maybe that's why Bret says to leave the cam designing to the professionals?

 

You need to understand this, once you do it will answer all of your questions.

Read This: http://www.compcams.com/Technical/TimingTutorial/

Good Luck,

Bret

 

link not working

 

What is the point of this post so everyone can help you a bit better?
1. Is it because you plan on giving someone specs and having it custom ground?
2. Are you trying to pick the best one from a company?
3. Are you just learning?
If 3, then we can give you a ton of links (or you can just google it, there are tons of acticles out there).

I really hope it isn't 1, because it takes a ton of experience with a given engine/combo (and how the customers want a car to feel/drive) to make an educated guess.

 

No, just confused on how a valve events calculator can give valve events given the little info it asks for. J-Rod's VE calculator over on ls1tech give valve events@.006 or @.050 given only duration, LSA, and ICL at those points. How accurate can this be?

The overall theme behind this thread was, how much trust can one put in a valve events calculator to accurately predict valve events and give compression? By simply entering in duration@.006,LSA, and ICL the calculator mentioned above will give you valve events, overlap and subsequently DCR.