just wondering how far i can go on the lobe sep angle reguarding vaccum needs. i know that the biggest aint the best and thats not what im planning. thanks

 

It depends on the cam, but a lot of factory cams do 116-118, for performance use I'd stick in the 112-114 range for good vacuum. Of course duration is gonna affect this as well, if vacuum's a major concern try and stay under 215 degrees @ .05 lift.

 

ok but i have a cam right now thats 224 dur. at .050 and it does just fine...dont mean to question you but ...maybe theres something im not understanding.

 

Isn't the other way around? Higher lobe seperation improves vacuum and overall driveability. Lobe seperation under 112 will cause problems.

 

Im running a 218/224 @ .50 w/ 110 LSA


And I have problems going from WOT to Brakes, loss of power assisted brakes.

 

Ok, let me clarify. I'd stick from 112-114, go under you're gonna have vacuum problems, go over you will may lose some performance. BUT REMEMBER LSA is not the only determining factor. You also have duration, lift, cam profile, and intake centerline, not to mention engine displacement, intake design, cylinder heads, ignition advance, heck even your torque converter can affect your idle quality, which is why I say it depends on the cam. Hell yes you can have 120 degree's lsa, I think that's what the ls1's are running, or get a decent idle vacuum with 224 duration, but all that other stuff factors in, as well as what one person considers a decent idle may not be ok to another. So I played it safe and went middle of the road to conservative in my reccomendations.

 

Really it all comes down to valve overlap and cylinder filling and scavenging. The LSA affects this as well as the amount of valve overlap, the amount of duration and the centerlines that the cam is ground on for both LSA, Intake and Exhaust. It's impossible to go into all the different combos. The best thing to do is to stick with a known cam that works or call the cam maker and ask, except for Ultradyne, they hosed Vaper by selling him a cam that was way to big for a TPI motor.

 

I kind of think that overlap is more critical than LSA, LCA or other semi-meaningless numbers in determining the suitability of the cam to the application. Unfortunately, overlap is rarely released in the published spec's for a cam. Instead we get LSA's. Yet you can take two very different cams with identical LSA's: one would be perfect for the motor, one would be a mess. For instance, let's compare two sticks out of CompCams catalog: CS-328ER-14 and CS260AH-14. Both have identical 114° LSA's. But the first has lift numbers of 0.714"/0.660"(int/exh) and 0.050" dur. of 288/300°. Wanna try and make that puppy work in your street car? The second, milder cam has lift of .444/.444" and 0.050" dur. of 212/218°. Perfect for a stock or lightly modded TPI motor. In this case(and most others) the identical LSA's are meaningless.

 

Overlap is a product of LSA and Duration.

The closer the Lobes are to each other, and the Bigger Duration of the cam, Obviously the more overlap you will have.

 

Not necessarily. If you take two cams with identical LSA and Duration(use advertised or 0.050" duration), but gave them different profiles, the overlap could be dramatically different. Even if the max. lift, another factor to consider, were identical. One cam could have fairly mild lobe attack and departure angles, resulting in very little time spent near max. lift. The other cam could have more severe angles which would allow more time near max lift. Now of course, the more severe angles will magnify nose-over float and valve bounce. But the point was to illustrate how little LSA has to do with overlap. And the overlap is critical to cylinder filling. Exhaust scavenging, which would not be possible without overlap, can exert 25+ times more pull on the incoming air than the downward movement of the piston.

LSA seems to be one of those comparison benchmarks people use, without understanding what(if any) relevance it has to what they are trying to accomplish. Kind of like flowbench numbers for cylinder heads.

 

Touche!

Good point, wich I failed to take into consideration.

I wonder how much Lobe attack Varies between the Various grinds we have avaliable to use.

One would think that the Cam grinder would use the most agressive attack angle possible to get to max lift as quickly as possible, and to maintain max lift for the largest part of the duration as possible.

If only we could have rectangular Lobes

However, I still argue that LSA has alot to do with Overlap, and resulting Idle quality and idle vaccum.

If you take Too Cam's with Identical Lift's, Idetical Durations, and Identical Attack angles, yet one has a LSA of 108, and the other 116, Then the 108* cam Is going to have alot More overlap than the 116.

Well, Like I stated Before, It seems that LSA is a convient way to Compare the potential low rpm vaccum qualities of a few camshafts w/ similar profiles.

Like Cyl head flow #'s, Of course it dosent tell the entire Story, but when some one sticks a Comp XE262HR on a 110* LSA, and a Comp XE262HR on a 114* in front of me, Im going to be more inclined to belive the 114* will behave better with my MAP sensor

 

If you take Too Cam's with Identical Lift's, Idetical Durations, and Identical Attack angles, yet one has a LSA of 108, and the other 116, Then the 108* cam Is going to have alot More overlap than the 116.

I'd agree that if the lift, dur. and lobe angles are the same - effectively the same lobes on both cams - then a difference in LSA will change the overlap period. But saying you can determine a cam's idle, vacuum and suitability for an application by specifying a particular LSA misses all of the more relevant info you need to make that determination. All of the above characteristics are controlled by overlap(which again can change even when LSA doesn't).

One would think that the Cam grinder would use the most agressive attack angle possible to get to max lift as quickly as possible, and to maintain max lift for the largest part of the duration as possible.

There are two problems every single cam suffers from, which limit the maximum attack and departure angles.
1. "Nose-over lifter float"(there's probably a simpler term for this) occurs when the lifter goes past the max lift point of the lobe(the nose) and momentarily loses contact with the lobe face. The steeper the attack angle, the faster the lifter is accelerating, the more severe the lifter float. This is an unavoidable consequence of the non-circular nature of the lobe face.
2. When the valve comes speeding back towards the seat, it is unavoidable that the contact will result in the valve bouncing off of the seat. Because we use a high-pressure spring to close the valve, the valve actually bounces off of the seat several times.

So while a higher pressure spring would help the lifter follow the lobe past it's nose, it would also increase the valve's tendency to bounce at it's closing event. Thus, a lobe face has to be designed with attack/departure angles that limit the float and bounce to an acceptable degree for the application.

If you want to get a feel for the difference between two cam's lobe angles, look at the lobe's opening and closing points(adv. dur. is better than 0.050" dur. for this) and it's lobe center angle. Because the LCA denotes the lobe's max. lift point(which is not necessarily the center of the lobe), the cam that has less duration from the base circle to the LCA, will have the steeper face. At that point the cam's LSA would be relevant to me, because, having a feel for the attack/departure angles and knowing how far apart the max. lift point of the two lobes are, lets me get a sense of how much overlap duration is and how much each valve is off the seat during the overlap period. Still it would nicer and easier, if all of the cam manufacturer's got together to promote a standardized measure by which overlap could be described. Then folks would be asking: "how much overlap is too much for my motor" not "how much lift/duration/LSA is too much".

If only we could have rectangular Lobes
While some of the really 'lumpy' circle track cams out there are getting very close to perpendicular lobe faces, a square cam would be a nightmare. Can you imagine trying to control lifter and valve motion with this sucker?

Now my wee little brain is hurting and I'm going to get some sleep.