eLTwerker

Had to start a new thread for this so as not to thread jack. Patrick G just explained that idle quality and lope are a result of overlap. More overlap, choppier idle. I thought idle quality was a result of EVO. I.E. an earlier EVO would result in a choppier idle b/c the sound of combustion was reverberated past the open exhaust valve and less muffling of its sound. How does overlap effect idle quality? Explanations appreciated. Thank you.

eLTwerker

To The Top

LS1Formulation

iTrader: (13)

Overlap effects idle quality because on the exhaust upstroke, both valves are open slightly which allows a certain amount of exhaust to enter the intake tract. At low RPM when there's not a lot of exhaust speed and volume, nothing is keeping the exhaust from entering the intake. This causes the typical rough idle we associate with cams having a lot of overlap. At higher RPM, the column of gas in the exhaust pipe is moving fast enough that it imparts a suction in the combustion chamber that draws out all of the exhaust in the chamber and can actually start pulling in fresh intake charge, which is why a certain amount of overlap is beneficial to high RPM power. Hope this helps.

hammertime

iTrader: (1)

Case in point, the '97 - 00' LS1s had a wider LSA than '01+ engines. As a result, LS6 intake manifolds did away with EGR. Idle quality on later LS1 engines is a little rougher than the early engines, despite the truly modest durations.

Keep in mind that a one degree change of LSA nets two degrees change in overlap.

eLTwerker

Good example..you get your gold star for the day.