[xntrik]

I have gotten several PMs about this, so to be lazy I am going to address this here. Should I copy and paste your PMs?


We've discussed the tighter than usual LSA on several threads. Allow me to expound.

K I S S

2 cams. Identical symetrical lobe shapes, durations, lift, etc.

One ground on 108* LSA one ground on 112* LSA.

What is the difference when installed in an engine?

Think carefully. Analyze it. Post if you want to.

This is a learning exercise.

[CptKaos]

All else being equal and assuming enough lift and duration to flow properly the lower LSA would create less engine vac at idle, need a higher idle setting and a higher stall converter but would capable of more torque and horsepower in the mid to upper RPM range.

Larry

[RippinRon]

What I understand from lots of reading is that a wider LSA will have more vacuum like KAOS said. That is why fuel injected cams have a LSA of at least 112. Higher vacuum is need for the MAP sensor to operate properly. I also have heard that wider LSA have a broader power band and tighter LSA have more peaky power band.

[406 ss monte]

valve overlap

[Hippie]

Originally Posted by 406 ss monte

valve overlap

That's keeping it simple.

The shorter the overlap the better the idle and vacuum. Short duration cams can maintain decent idle quality and vacuum with tighter LSA's while boosting low and mid-range torque because they still have relatively short overlap. As duration increases the difference in overlap between otherwise identical cams increases with the tight LSA cam having more degreees of overlap which has a more pronounced affect on idle quality and vacuum and the RPM at which peak TQ occurs. To maintain good idle quality and vacuum you need to increase the LSA as duration increases.

A good example is the GM Marine roller cam which is also the cam they use in the 350 Ramjet crate engine. Usually EFI cams have 114-116 LSA with 112 being considered the lower limit but the Ramjet/Marine cam is ground on 109 LSA. It is a very short duration grind which keeps valve overlap short enough that it still works well with EFI and gives decent vacuum and a "sewing machine smooth" idle in a carb application. Start adding duration to that cam and the idle would start to get pretty choppy with 109 LSA.

[xntrik]

Good,
Ok let's not get off into "what if" land.
We're not changing durations, etc.

Sure "overlap", but how? In which manner is it affecting the engine? What is actually happening besides having both valves open at the same time for a longer period of time?

These 2 chunks of bumpy cast iron are lying on the table..... what's the difference between them?

What is the difference between these two cams, timing wise,

and then let's talk how it affects the engine.

Hippie brought it up, Why do fuelies have a larger overlap? What you said is sort of correct..... they do, but why? Why does the "almighty computer" have to have a wider LSA..... can't they just tune it differently? mmmmm?

It is as obvious as the nose on your face. Sometimes it is hard to see the forest for the trees !

[curtis73]

single plane intake on most efi's reduces the vacuum buffer. The plenum sees more pulses instead of a steady vacuum source. More LSA = steadier vacuum.

A little birdie told me.

Other things: reducing LSA closes the intake valve sooner shifting the tq peak down a bit. It causes exhaust tuning to be more crucial since more time is spent in overlap. Too much scavenging pulls raw fuel out the exhaust and too little doesn't take advantage of all the time in overlap. EGTs tend to be lower with less overlap.

Did I win?

[xntrik]

Game ain't over yet......

[techinspector1]

Posted by curtis73:
"reducing LSA closes the intake valve sooner"

Not necessarily. What if you moved the exhaust centerline but left the intake centerline alone?

[xntrik]

Originally Posted by techinspector1

Posted by curtis73:
"reducing LSA closes the intake valve sooner"

Not necessarily. What if you moved the exhaust centerline but left the intake centerline alone?

What if you left cam straight up?

[CptKaos]

lower LSA = increased overlap, which improves scavenging which improves mid to top end performance at the price of lower idle vacuum, increased cylinder pressure and a narrower power band.

Larry

[454C10]

Cylinder filling is the key.

Poor flowing heads prefer a tighter LSA. Exhaust scavenging from overlap helps the intake pull in more cylinder charge. Therefore, helping the intake port flow more.

Carbs prefer a tighter LSA. A carb needs more signal from the engine to deliver the intake change and exhaust scavenging increase the signal. An efi system can deliver fuel no matter what.

Not all efi's use manifold pressure to determine the amount of fuel required. Therefore, using wider LSA to increase vacuum for a MAP sensor is not the only reason to do so. EFI just doesn't need a lot of signal (air flow) from the engine to deliver fuel.

Rod/stroke ratio also effects the amount of LSA required. The location of the piston in the bore relative to the crank angle and valve timing can make big changes. Typically higher ratios prefer tighter LSA.

[strummin67]

I haven't seen one word about cleaner running for pollution control yet. But then is this what you were looking for?

Strummin

[curtis73]

Originally Posted by techinspector1

Posted by curtis73:
"reducing LSA closes the intake valve sooner"

Not necessarily. What if you moved the exhaust centerline but left the intake centerline alone?

In an earlier thread someone was talking about all other factors being equal. I was just operating on that assumption.

[RippinRon]

Okay well we're on the topic of camshafts and LSA. I don't mean to hijack this thread but I'm just finishing up my 350 rebuild and can't decide what cam to use. My engine specs are as follows

350 .030 over
stock steel crank
scat 6" i-beam rods
speed pro hyper. pistons 5cc flat tops
pistons are .022" in the hole
bowtie phase 2 heads 2.05, 1.6 valves, ported, polished, valves unshrouded

I plan on using a .021 thick head gasket which will give me a static compression ratio of 10.3:1. Here are the two cams i'm deciding on.

comp cams nx274h
275, 292 adv. duration
230, 244 @ .050
.487, .501 lift
113 lsa

comp cams xm270h
270, 286 adv. duration
226, 236 @ .050
.480, .489 lift
112 lsa

I plan on using 1.6 rockers to get my lift over .500. Both of these cams have an intake closing point of 65 and my dynamic compression comes out to be 8.2:1. What do you guys think?? It will be on a steady diet of 94 octane fuel, and sequential fuel injected. Any comments or suggestions??