Originally Posted by 0trbo4myCHEVUICK
I get that your trying to push me towards bigger heads but I didnt think the cam was over kill... Which brings me to another question, do you subtract 8 degrees from overlap total to account for hydraulic lifter travel?
I'll admit to being stymied over your question here concerning subtracting 8 degrees until I went over to Speedtalk and read up on where you got the idea. Subtracting 8 degrees is not to account for hydraulic lifter travel, it's for comparing a solid cam to a hydraulic cam, subtracting 8 degrees from the duration of a hydraulic cam in order to compute the same approximate duration as a solid cam, less valve lash. While I did learn something by reading the attached thread from Speedtalk, I think it's MUCH ADO ABOUT NOTHING
in my personal opinion. I don't choose a cam by determining the idle quality and could not care less about it, as long as the cam makes power where I want it, based on compatibility with the static compression ratio of the motor and revs to the point that I need it to based on gears and and tire size.
Here's the link.......knock yourself out........
Why does overlap make a rough idle? • Speed Talk
Further, I am of the opinion that it is not so much overlap that gives you that erratic idle quality, as much as it is the piston rising in the bore with the intake valve still open and shoving mixture back up the intake tract. The venturi sees mixture going both ways and adds fuel appropriately to what it sees. This makes a very rich condition and even forms a ball of fuel mixture hovering over the airhorn of the carb that you can see with a strong light on a dark night.
Here's what Iskenderian has to say about it.......
Tech Tip - 2002
What Causes Intake Reversion? Once and for all, let us have the TRUTH!
With the proliferation of the Motorsports Industry over the years, many new faces have come on the scene. In the cam grinding business today, there are many younger, less experienced companies struggling for recognition of their talents and a few have turned to postulating new theories in order to attract attention. However, they are I believe unfortunately, too often guilty of shooting from the hip.
Two in particular are responsible for perpetuating the "myth" that an earlier opening of the intake valve (even by a mere 2 or 3 degrees) causes the phenomenon known as "reversion". Nothing could be further from the truth! This misconception not only defies common sense, it also establishes a false premise from which other, incorrect conclusions can be drawn. Simply put, those who focus on overlap are on the wrong end of the cam-timing diagram!
Reversion, carburetor/Injector "stand-off" or the general effect of the backing up of the intake Fuel/Air charge normally associated with longer duration high-performance camshafts is actually caused by a Later Intake Closing! How do we know this to be true? The answer lies in the basic principles of physics. For as with geometry and trigonometry, these sacred truths do not change simply because someone chooses to ignore them in an attempt to garner a reputation.
Specifically, when the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little (virtually no) exhaust gases remain in the cylinder. The piston is in the vicinity of T.D.C. (only .425" down the hole @40o BTDC - on a typical 350" Chevy with 5.700" rods) and no appreciable threat is posed to the forthcoming intake charge. The "False Reversion Hypothesis" taken to an extreme would lead one to the equally false conclusion that any overlapping of the intake and exhaust valves is totally undesirable. Automotive engineers of the late 1800's and early 1900's used to think this way. They were deathly afraid of overlap, so much so they actually employed "Negative" overlap (minus 5 or 10 degrees) to be absolutely sure none would occur. What was the result? These engines were severely "throttled back" or limited to low speeds and mediocre output. [ Reference: Iskenderian's Tech Article "Cam Degreeing is Simple"] But, more progressive engineers of the early 1920's who performed "brazen experiments" with longer duration cams proved these overlap fears to be only so much "stuff and nonsense", as both power, rpm and performance were actually improved. These engineers demonstrated that overlap did not cause engines to quiver, backfire or lock-up on the spot! Although, the ignorance displayed by their predecessors is easily explained by their lack of experience, (internal combustion engine design being in it's infancy) it was none the less the result of an incorrect hypothesis.
Should you need further persuasion that reversion is not caused by earlier intake opening and the resulting extension of valve overlap, consider this: What happens when you advance any camshaft? The intake as well as the exhaust valves open earlier. Does this advancing of the cam cause more reversion? Of course not. Throttle response and torque are enhanced. Yet, if these theories were correct wouldn't the engine run more poorly, especially at lower RPM? The answer is obviously yes, and because so, these theories are invalid. A brief look at what's happening on the other end of the valve-timing diagram will tell you why.
For when a camshaft is advanced, not only do both valves open earlier but they of course also close earlier - and here in lies the key to reducing Intake Reversion. Close your intake valves earlier and any tendency for the occurrence of Reversion or the backing up of the intake charge as the piston rises on the compression stroke will be reduced. It's not complex, nor is it a mystery. And the circumstances surrounding it's occurrence have not changed. In fact any experienced mechanic could tell you as much, for, as Ed's good friend the legendary Smokey Yunick might say, "Only country smarts are required to solve the problem."