In Memory of Harold
Join Date: Oct 2002
Location: Senatobia, MS, USA
Ahhhh---A subject worthy of a chapter in my up-coming book.
Thanks for the SAE .004" explanation. IRREGARDLESS of where a cam manufacturer rates his hydraulic cams, the SAE has very good scientific proof that .004" is the correct rating point for hydraulic cams. Don't throw up about Rhoades lifters, they are a special case.
You should always use SEAT timing when working the DCR formulas, or any 'Desktop-Dyno'-style computer simulation programs. When I get through here, I hope no one thinks that the .050" number is the one to use. You can even have cams with the same .050" duration number, but DIFFERENT timings at .050", and an engine will see them as 2 different cams.
I hope everyone will not think that I skip around too much, but here goes.....
We'll start with the intake opening, although the exhaust actually opens first. I suppose you've got to have charge in the cylinder in order to have something to exhaust out....
An intake opening point in itself does not tell what duration or LSA/ICL a cam is on. Cams range from Symmetrical to various degrees of UnSymmetrical-ness. Each of these various cams, all of the same overall seat duration, will be seen by the engine as different cams. I have even said that the most important degree in your cam is the one BEFORE the intake valve opens. If we could get an instantaneous reading of the exhaust gas volume and pressure right before the intake valve opens, we could predict how the cylinder would fill. Identical opening points, but different exhaust gas volume and pressure(REVERSION) will cause different rates of intake filling. When the intake valve starts off the seat, this higher-pressure exhaust gas flows partially into the intake runner. What keeps it from going too far is that the piston is slowing down for TDC, and pushing less and less on the exhaust gases. In fact, we can use where the intake valve opened as a relative guide to the amount of reversion.
Now what about the .050" number?
First off, a good part of the .050" duration is on the WRONG side of TDC. The piston isn't sucking charge in while it is moving upwards toward TDC, it is PUSHING stuff OUT of the cylinder. Airflow into the cylinder does not start in a N/A engine until after TDC, and AFTER the reversion that had entered before TDC has been cleaned out of the runner.
The SOONER airflow starts into the cylinder, the higher the port velocity(the Rate of Filling the Cylinder), and the longer inertia ram will continue to fill the cylinder after BDC.
To keep this post relatively short, I won't go into the effects of high-lift area.
When the intake valve closes, the port velocity governs how the cylinder is still filling, even 60 to 90 degrees ABDC. Low port-velocity, later closing means less cylinder pressure and less torque. High port-velocity will use late intake closing to pack even more charge in, and delivery high cylinder pressures.
I will close this with a small example that I have used many times in the past---my 288R lobe.
Imagine a symmetrical 288R on a 102 ICL---It opens at 42 BTDC and closes at 66 ABDC.
My April 1980 288R on a 102 ICL opens at 39 BTDC and closes at 69 ABDC.
This cam has 3° less reversion in the intake runner, so it starts airflow earlier(a little more than 3°, because of the way the back=pressure drops). This gives it a higher velocity in the runner, and a higher rate of cylinder-filling.
Now look at the closing points. The symmetrical cam, with a lower port-velocity and rate of cylinder filling, shuts the intake valve EARLIER, allowing LESS charge in the cylinder.
My 288R Unsymmetrical cam shuts the intake valve 3° later, with a higher port-velocity and a higher rate of cylinder filling. More charge in the cylinder, higher cylinder pressure, more torque.
Engines see the seat opening and closing points, and the .200" duration numbers, as a close guess as to the cam. They actually see the entire valve lift curve as the cam, not a number like .050". This number is for our convience, to have a commonly-understood point to talk about cams. Engines don't know what .050" durations are.
It's worked this way for almost 24 years now.
If you have other questions, just ask. I may have to answer some tomorrow....