I'm looking for an equation to calculate my valve overlap on my camshaft. Does anybody have an equation? I'm sure it can be figured out, but I'm not sure what units are used for duration. Thanks.

 

The basic valve overlap number is the degrees BTC that he inlet valve opens added to the degrees ATC that the exhaust closes, but this number has limited meaning. If the inlet valve opens at 30 degrees BTC and the exhaust closes at 26 ATC, the vavle overlap in degrees is 56.

You also have to consider how the timing points are specified. A current SAE standard calls for specifying the opening/closing points at .006" actual valve lift, and this is typically used for "advertised" duration numbers, but not always, and the .006" lift points are also effected by actual rocker geometry. Most manufacturers specify the .006" valve lift points assuming a 1.5:1 rocker ratio, but, for example, the rocker ratio of OEM SB rockers varies from about 1.37:1 at low lift to about 1.44:1 at .450 lift. Regardless of what rocker arms you are using and whatever their specified ratio, they are NOT constant through out the full range of lift.

EFFECTIVE vavle overlap is the actual flow area available during the overlap period, which is a function of vavle size and valve acceleration in addition to the basic opening point of the inlet valve and closing point of the exhaust valve.

As an example, with the same timing points, a four-valve head will have more EFFECTIVE overlap than a two valve head because of the four-valve head's larger total valve curtain area (the sum of lift times circumference for all four valves).

The Engine Analyser simulation program computes actual valve flow area in square inch-degrees, which makes it a much more useful tool for actual effective overlap comparisons.

Duke

 

Your first sentence, before the comma, is the WHOLE answer.
Timing points are based on tappet rise or tappet lift, not valve lift. Rocker ratio, or valve opening distances do not come in to play for the purpose of specifying valve timing events. The only camshaft specification that is affected by rocker ratios or valve opening, is the max lift number. Good luck, and...

RACE ON!!!

 

[QUOTETiming points are based on tappet rise or tappet lift, not valve lift. Rocker ratio, or valve opening distances do not come in to play for the purpose of specifying valve timing events. The only camshaft specification that is affected by rocker ratios or valve opening, is the max lift number. Good luck, and...

[/QUOTE]

Try reviewing SAE Standard J204D sometime!

If you want to understand valve events at a level of detail that supports accurate analysis, comparison, and digital simulation, there is a WHOLE lot more valve timing than the oversimplied specs that are quoted by cam manufacturers.

Duke

 

MN, are you trying to figure out if the ZZ9 cam is letting the boost out the exhuast valve?

I would like to hear what you find out, I have that same cam in my car.
I know someone ran it on a supercharged LT1 and made 545RWHP.

 

Bingo, you got it Bill. I was going to compare it to the stock cam, but from what people are saying here I don't think I can. The calculation seems to be beyond my scope of knowledge. I did find out from TPIS that the usable valve overlap is -3.xx degrees. I believe that this means the boost doesn't really blow out.

 

The question sounded fairly "basic".
Like "What time is it?". I merely was answering the question, rather than trying explain how to build a clock. I don't dispute anything you said.
However, outside of an engineering lab, that is about all, most of us, have to work with. I'm sorry if my clearification of tappet lift rather than valve lift, as being the basis of the timing events, offended you.

RACE OFF!!!