[enkeivette]

Duration or lift?

[Vettezuki]

I believe it's primarily a function of the overlap (LSA) and rpm. Lots of overlap at low RPMs basically means lope. LSAs of 114 (like I have are more agressive than most OEMs, but still very streetable, smooth idle, and emissions compliant). What you hear at the drags are a lot of 110s and 108s, maybe more. They are optimized for maximum breathing through the heads at high RPMs (5,500+).

Haven't read this article in a while, but as memory serves it's awesome. Cam technology, especially now with with all the different schemes for variable valve timing, is fascinating and very complex.


How Cam Shafts Work

There's still a lot of improvements that can be made with ICE. I just read an article about a scheme for variable compression on boosted engines that uses, are you ready for this, linkages for variable length rods! Hi compression at low boost for most getting araound (better thermal effeciencies) low compression higher boost for more power production under load.

[enkeivette]

Originally Posted by Vettezuki

I believe it's primarily a function of the overlap (LSA) and rpm. Lots of overlap at low RPMs basically means lope. LSAs of 114 (like I have are more agressive than most OEMs, but still very streetable, smooth idle, and emissions compliant). What you hear at the drags are a lot of 110s and 108s, maybe more.

Haven't read this article in a while, but as memory serves it's awesome. Cam technology, especially now with with all the different schemes for variable valve timing, is fascinating and very complex.


How Cam Shafts Work

I have a 110 LSA, my cam isn't that bad. Or is it?

[SeanPlunk]

I've always heard it's the LSA too. 110 qualifies as pretty damn lopey. You probably just can't tell because of the blower.

[enkeivette]

Found the answer, it's not LSA or lift. It's duration, specifically intake duration, very specifically on the closing end of the ramp. Lope is caused when the intake valve closes so late in the rotation that the piston actually begins to move air back up into the intact tract which messes up the metering of the carb... or the fancy shmancy FI.

Then again, having more intake lift could theoretically contribute to lope, as more lift will allow more air to revert.

"Another thing to consider is late IVC alowing the piston to push the mixture
back into the intake manifold at the beginning of the compression stroke."

"Lope is cause 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."

Source

[BRUTAL64]

Originally Posted by enkeivette

Found the answer, it's not LSA or lift. It's duration, specifically intake duration, very specifically on the closing end of the ramp. Lope is caused when the intake valve closes so late in the rotation that the piston actually begins to move air back up into the intact tract which messes up the metering of the carb... or the fancy shmancy FI.

Then again, having more intake lift could theoretically contribute to lope, as more lift will allow more air to revert.

"Another thing to consider is late IVC alowing the piston to push the mixture
back into the intake manifold at the beginning of the compression stroke."

"Lope is cause 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."

Source

Yep. That's correct.

LSA does have to do with the overlap though. 108 will lope more than a 114 -everything else being the same.

[enkeivette]

But I would think that the overlap would not effect the intake opening into the compression stroke.

Isn't the overlap just measured by how much the exhaust stays open into the intake stroke? As I remember, LSA is measured between lobe peak to lobe peak.

[BRUTAL64]

Originally Posted by enkeivette

But I would think that the overlap would not effect the intake opening into the compression stroke.

Isn't the overlap just measured by how much the exhaust stays open into the intake stroke? As I remember, LSA is measured between lobe peak to lobe peak.

Yea, but it still effects Vacuum and lope. The tighter the LSA the loper the cam. Just a fact of life here. Overlap is what effects idle/vac.

[enkeivette]

Originally Posted by glenn64vette

Yea, but it still effects Vacuum and lope. The tighter the LSA the loper the cam. Just a fact of life here. Overlap is what effects idle/vac.

I C. This is probably whey I can only pull 6 or 7" of vacuum at best at idle with my 110 LSA.

[94cobra69ss396]

I have a 110 on both my Cobra and my Chevelle. The Cobra has a .480 lift with 220 @ .050 on both intake and exhaust and it pulls 15" at idle. Like you I also have a blower only it's a centrifigul. I've never checked the vaccum on the Chevelle but it has more lope than the Cobra does. But it also have more lift at .622 and more duration at 246 @.050.