Lope is caused by the intake charge being pushed back up the intake tract as a result of late intake valve closing. This charge backs up through the venturi and upsets the signal that the carb is using to meter fuel with the incoming air. With enough cam, the charge can actually be seen surrounding the air horn above the carb and looks like a white cloud. Lope is the sound of the motor being inefficient at low r's until it can get "up on the cam".
I don't have the equipment or good sense to figure this out, I have to lean on professionals who do this stuff for a living. Case in point is this article from Iskenderian Tech:
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 theexhaust
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."
Oh and this: Don't worship false idles.