Lobe Separation Angle Explained?
 

I could use some help understanding lobe separation angle. Not what it is. But why, for example, a cam with a tight LSA creates more low-end torque while a cam with a larger LSA makes power across a broader RPM band. I've looked for a simple, intuitive, explanation of this but haven't found one. The Pontiac engine books I have only go so far as to characterize the differences, not explain what's happening in the engine.

Is there a reference book, online resource, etc. that explains this? This has proven to be the biggest mystery to me in my effort to better understand camshafts.

I read that one. One of the best articles I found. still really only scratches the surface on what's happening in the engine. Here is the passage I noted:

Cylinder-head flow and LSA are directly related: as a rule of thumb, the better the head flows, the wider the LSA can be. A wider LSA moves intake- and exhaust-valve events farther apart and creates less overlap, which hurts intake-wave tuning.

"When you've properly tuned a performance motor, you create high- and low-pressure waves in the intake and exhaust that help cylinder filling," said Comp Cams' Billy Godbold. "When the low-pressure wave in the exhaust reflects into the intake, the pressure delta actually helps pull air and fuel into the cylinder. When you don't have any overlap, you can't have any wave tuning. A tighter LSA is always going to have more overlap, which allows the exhaust and intake system—if they're tuned well—to work together."

This wave tuning is what helps performance engines achieve volumetric efficiencies of more than 100 percent. To overcome this inherent problem in wide LSA camshafts, you simply need a higher-flowing intake port. This is why the LS series, and many other modern engines, can make great power on top and bottom with wide LSA camshafts and why a plethora of factory cylinder heads are flowing near (or above) 300 cfm.

Effect Of Changes In Cam Timing And Lobe Separation Angle

https://www.compcams.com/cam-timing-...paration-angle


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Thanks Steve. What I'm lacking is an understanding of why different LSAs result in different power bands. What is it about a tighter LSA that makes it more powerful at the low end? Is it the better cylinder pressure it builds? Similarly, why do wider LSAs produce more power at much higher RPMs?

The timing of the intake valve closing, and the exhaust valve opening are what determines the power band. The Wider LSA closes the intake later, increasing the time the engine has to pull air/fuel into the cylinder, providing more RPMs. The wider LSA usually opens the exhaust valve sooner also, keeping the air/fuel mix from getting diluted by the exhaust gases, and has more time to escape the cylinder for more power at higher RPMs.

Taking a cam and narrowing up the lobe separation narrows the powerband from the earlier intake closing , and later exhaust opening, but now has more overlap for cylinder scavenging. The increase scavenging causes reversion and dilution at the low rpms, moving the power band higher from low idle. But the intake closing and exhaust opening are doing the opposite, lowering the power band. In effect that is narrowing up the power band, and at the same time making more HP&TQ in a narrower power band because the increase in scavenging is making the engine more efficient in the now narrower power band.

Its why my old 455 with iron heads ran great with a 106. New engine with High Ports is getting a 112.
Poor flowing heads have to get it all done in a lower, more compact RPM range.

That is not really an apples to apples comparison since you will not be using the same lobes..

Stan

And only have half the results

LSA has no meaning to me until the Intake Lobe Center is selected/set. Then, the exhaust opening event is defined. Then you have the causality for the engine personality.

What gets me is the cam LSA is considered the be-all, end-all for the engine personality. Well maybe if the Durations are maxed-out then yes. However, i have enjoyed "over-camming" my Street engines for Street/Strip to attain Ping-free 87 octane work and good ETs in the heavy car.

Well, there are folks more experienced then me that can cite better pump gas results. Uhm 93 Octane is not my bag. The 80s-90s taught me that high compression is only cool with 110 octane leaded for Race, and 93 on light duty Street. It works.

But the shift to 9.0:1 and 87 Octane, with an OD Automatic is good sense. Smarter folks will advise higher compression like 9.5:1 (and oh I agree).

Back to cams: advertised performance is traditionally stoopid. Cams still today are described for 326-455! Bulls dancing on mud. The schmardt catalogs tell ya Duration at. 050" checking lift and an LSA, but not always provide an "advertized Duration (0.003", 0.005" checking lift). Well, the dutiful person will map the valve events on a polar spiral plot to see the cam events, then ponder the engine personality for a 326, un up to a 455 or 535, and decide yea, if i could only jerk the exhaust lobe earlier, or later, AFTER having the Intake Opening event set, the closing (defines the Int Lobe Center), and defining the EXH opening event and Closing (defines the EXH LC, making the LSA a byproduct of the desired cam profile.

Imagine, that i loath identfying a series of cams by an LSA, when their Durations could be 210* through 263* lobe duration. Nutz.

For a given PMD CID, say a 470 CID, start with a 210* intake Duration ate 0.050", then every 10* Duration increase on either lobe should cause the optimal LSA to walk a certain path, for best "performance". Well you would "get this" if the goal was the best ping-free performance. I have settled on the optimals for high-comp & low compr in my .
signatures.

Surely, ther is a schosh improvement by others' cam profiles. Yet the maximum RPM becomes the next onion layer. Guess what; my 12.0 ET High-Compr peaked at 4950 RPM, and the 12.2 crested 5050 RPM. So the max RPM become a figure-of-merit. Ping-free low RPM Street-Strip performance is tough.

Corolarry: A fella can buy a cam based on Duration and LSA, once the Post 10 activitiies were understood.

Otherwise the cam purchase is based on some tribal knowledge/experience advice saying "Ya gonna need a 108 LSA with 245/255 duration for that combo".

Oh, but why? " we got some track data that is this much good." That is excellent, but what is it about the cam profile and how sensitive, or how much can each parameter vary and stay on the bubble of excellent?

i thought we were only allowed to run 068, 041, or old faithful in the street section anyway ...

Jay this is great. Thank you. Exactly the sort of concise explanation I've been looking for and until now unable to find. In an effort to see if I understand this, let's see if I can explain it back. I'd be thrilled to know where I'm wrong.

Comparing two cams that are identical except for their LSA:

Wider LSA @ High RPM - Benefits: Later intake valve closing leads to more air/fuel in and increased efficiency of current power stroke; earlier exhaust valve opening helps exhaust escape, reducing dilution/increasing efficiency of next power stroke.

Wider LSA @ Low RPM - Benefit: Less overlap leads to less dilution/reversion, more efficient combustion.

Tighter LSA @ Midband RPM - Benefits: Greater overlap allows for greater scavenging and more efficiency/power as RPMs come up. (More specifically I think what the greater overlap really accomplishes is that it allows the negative exhaust pressure to better reach through the cylinder and increase intake flow /volumetric efficiency, right? Or in other words, scavenging isn't just about getting spent gases out and avoiding dilution, but getting fresh mixture in and increasing VE.)

At the highest RPMs, it sounds like the ability to get air in and out via a wider LSA becomes relatively more important than the scavenging benefits of a tighter LSA. In essence the tighter LSA is beginning to starve the cylinder, relatively speaking.

Three notes:

1) I'm guessing that although all engines benefit from a better scavenging exhaust system, such a system is what makes a tighter LSA cam really hum. And that a poor exhaust leaves you with only the downsides of such a cam.
2) Even the best exhaust system can't overcome the at-idle issues (e.g. low vacuum, inefficient combustion) caused by tight LSA cams.
3) The high-RPM power benefits of a wider LSA are often pretty minor on lower RPM street engines.

This is exactly why I'm asking questions. I'm not gonna become an overnight expert here. But I want to be at least conversant with engine builders. I have a project coming up. Three months til I can pull the engine. Lots other stuff to get through. In the meantime I want to learn something. I've got two cars in my past - a 1970 stingray and a 1970 challenger. Both a long time ago. And I didn't dive too deeply into the inner workings on the motor in either case.

There's lots of good information throughout, but at least read posts 1 , 26, 44, 45 https://ls1tech.com/forums/generatio...-matter-2.html

182,you forgot Stump puller!Tom

I'm afraid to venture beyond an 041 cam, who knows what would happen.

i am quite happy with OF 1.0 on a 112 and my rev limiter set to 5500. but sometimes i think a 290b6 could use the same valve springs and be a lot more fun with the rev limiter set to 6500 ... even though it is on a 106.

Can you say 112 LSA is a happy medium? Not too far one way or the other? Yes thats a general statement.

how much street time does the cam in your signature get?