How are they calculated?

Ryan

 

I've been wondering that also. I have a 290 advertised duration Comp cam in my LX. Wanted to know exactly what it equates to. TTT.

 

Try this tutorial from Comp.

http://compcams.com/information/tutorials/

Enlarging the diagram helps.

Check catalog cam listings also. pg. 86-87

http://compcams.com/catalog/086_087.html

 

Keep in mind that different cam manufacturers use different ways to measure advertised duration. Some measure advertised duration at .006 lift, and others will pick .045 (mayber there are others), which changes the number by a few degrees. Make sure you are comparing apples to apples when comparing advertised durations.

 

Thanks for the link, oldsstroker.

I'm still a little confused about the significance of .050". What is special at that point?

Guess1: That is where the intake/exhaust over lap is.
Guess2: That is the point where the valve is opening fullspeed.

Ryan

 

[quote]
Guess1: That is where the intake/exhaust over lap is.
Guess2: That is the point where the valve is opening fullspeed.
[quote]



That 'point' is when the valve is significantly raised enough for pressure to bleed off.

Overlap is purely a function OF duration that takes in to consideration lobe seperation angle.

The intake lobe's center and the exhaust lobe's center will be a certain number of degrees apart, this is what "lobe seperation angle" is (aka LCA, such as "112").

Also in the middle of that 112 degrees will be TDC if the cam is put in straight up. So, 106 to the left and to the right are the lobe centers. If you have more than 106*2 (in this case) of duration on both lobes, then you would be at overlap.

Follow this logic with some indepth thought and you'll be able to tell when each valve opens and closes in relation to before/after top/bottom dead center.

Ben T.

 

Derrick,

What does 290 mean you ask? It means that starting at .005" of valve lift (from when the valve opens off of the seat) it will be open for a *duration* of 290 degress, though peak lift, then back down to .005" from the valve seat as it begins to close. This is commonly called "advertised duration".

You will be getting enough flow at such low lift to hurt some things if you're not systematically designed to operate there. Following 'this' logic now, the 234* that you have at .050" lift is the duration/amount of time the valve is raised off of the seat and flow is allowed at the specified lift rather until the valve is lowered past it.

Have you heard people talk about "the new" extrem grinds from Comp? How the CC306 is being out-dated by newer grinds? That's because the new grinds have faster ramp rates. They have less "advertised" duration, but more duration at .050" lift- which is more representative of how the cam will operate.

This means when the valve is open they'll flow more ("more area under the curve/larger integral) without letting pressure be bled off by holding the valve open too long. If you have too much advertised duration, idle quality, emissions, vacuum, etc. is affected.

Now, with this said. Don't think the CC306 won't make power because it will, but they have newer grinds what are more powerful than it is BUT the faster ramp rates are harder on your valvetrain. Just a few thoughts ...

Ben T.