How Do You Choose A Lobe Seperation ?

[427HISS]

Where does the lobe seperation come into play, when you're figuring out other specs of the intended use of the engine, valve train, etc,
when building a camshaft ?

What makes the differences between a solid flat tappet, hyd roller and solid roller ?

[chilly460]

You can read volumes just on LSA.  It impacts overlap and intake closing point so it does quite a bit to cam behavior. 

You can't even get a consensus from the well known cam grinders.  One guy will say he plots the lobe timing to optimize blowdown and still trap compression and LSA is what it is once he's plotted the lobes (a bit of a generalization there).  Another guy will take intake to exhaust ratio, rod length, head runner volume to cubic inch ratio, and head flow to cubic inches all into play and use that info to influence LSA.  Vizard has his 128 "rule" that he uses to calculate it, but that's geared more from experience on one specific platform (the SBC) than a proven formula, IMO. 

I think at the macro level, you'd want to consider need for power brakes as the LSA influences overlap and therefore vacuum.  Then you get into idle quality and "snottiness" if that's a concern, along with playing nice with a tight convertor if a guy were running an auto. 

speedtalk.com forums have some good info, but you'll wade through a lot of bickering amongst builders....and like I said you'll end up with viewpoints 180* from each other, both coming from what seem to be fairly well respected guys so it's hard to distill what's what.  But you will definitely learn from reading. 

Further muddying the waters....trend seems to be to tighter LSAs for carbed combos and power keeps increasing as obviously cylinder head tech gets better.  On the flipside, the latemodel stuff making huge power on EFI generally has wide LSA. 

[blykins]

As chilly pointed out, it (along with duration) controls the amount of overlap. 

There will be infinite iterations for how much overlap an engine should have, depending on the cylinder heads, rpm, displacement, application, and about 87000 other parameters. 

I would watch about drawing a line between carburetors and EFI though.  My last pulling truck engine had two 1450 Dominator carbs on it and used a 119 LSA.  It sees 8500-9000 rpm on a pull.  Contrast to that, you could have a small engine with a very low rpm goal, with not much overlap at all.....it could be that you could run a 107-108 LSA, even with EFI on it, because the overlap would be so low. 

When I sit down and figure out what a cam should be, I look at duration first.  Duration controls where the horsepower peaks and where the hp/torque curves lie.  That is based on displacement, cylinder head flow (head flow is BIG component), and a few other things.  Advertised duration vs. .050" duration (along with the .200" duration) will tell you how aggressive/lazy of a lobe it is, which will determine whether or not the cam will be a spring eater, whether it will be easy on parts, whether it will be noisy, quiet, etc.   Lift is based on where the heads flow the best, along with a few other tricks.  The intake centerline is based on how much cylinder pressure you need and how efficient the engine will be.  Lots of guys put a big emphasis on an advanced cam not pulling high, or a retarded cam not having bottom end, but I haven't seen a lot of direct relationships like that.  I do know that advanced cam timing on a street engine makes them pull very hard on the carburetor.  LSA focuses on overlap, how much vacuum you need, what rpms you're turning, and how you want the hp/torque curves to play out. 

As chilly also said, you could write books on this topic.   There are a couple of generalities that you can make, but they are few and far between.  To me, there are no universal cams and each engine will demand a specific recipe.  My biggest gripe is looking through a cam catalog and seeing the EXACT same cam specs for EVERY SINGLE ENGINE FAMILY in the catalog.  I know it's easy for a cam grinder to make a nice lobe and then duplicate it for different engines, but if they think a Tunnel Port FE wants the same cam as a SBC, they're not doing their due diligence as a grinder.

[chilly460]

Brent, thanks, I got a bit too "general" in there.   This may be old information by now but I know Pro Stock stuff at one point had gone wide LSA, explained to me is that they'd gotten to a point with duration being so long they needed to reduce overlap.  Regardless of exact reason, it just goes to show that LSA should always be tuned to the combo. 

I'm trying to understand cam design beyond the simpler concepts of knowing effects of duration/lift/overlap.  Seems LSA is a pretty hot topic and is treated as one of the "black boxes" where some power cam really be made if done right.   Problem is for normal dudes, it's fairly complex and definitely expensive to go playing with different cam grinds to test theories.  That's why the small cost involved in having a builder, that at least has some directional trends and experience to call on, have a cam ground is the most well spent $$$ that can be spent IMO. 

[My427stang]

I like Brent's summary and as he shows, all the valve events matter and they are based on many components and desired use

I especially don't like LSA as a stand alone number.  If I was forced to pick specific numbers, I would say intake lobe open/close/lift first and exhaust lobe and overlap second.  It would end up with an LSA, but without those, the LSA doesn't define anything

That being said, my EFI 489 is at 110 LSA my carbed 445 was at 112 and my carbed 390 is at 112. 

My 462 for the F100 may even go 114, we'll see, but man I like the sound of a little overlap

[427HISS]

 If one engine and the specs are all exactly the same,  how can the LSA affect the engine, say from 106 to 115 ?

[Joe-JDC]

Hypothetical question:  Trying to find the most torque and horsepower over a 3000 rpm band from 3500-6500 rpm, what do you think the camshaft specifications should be?

Engine: 375 cubic inches
stroke 4.000
bore 3.860"
rod 6.300"
Compresion 13:1
head flow 290/205
1 5/8" step to 1 3/4" four into one into 3" collector headers.
wet sump
tunnel ram/2-650s

Going to dyno very soon to verify my camshaft selection, but interested in ideas of those who consider themselves gurus.  Joe-JDC

[chilly460]

I’m no guru but I’ll throw in ideas. 

For power in that specific range, I’d say a 6000rpm hp peak and 4500tq peak would be about right, on a 375 with decent heads I’d say that’s a around a 238* intake lobe with a solid roller.  Engine has an intake bias with tunnel ram and intake port flow, plus headers that may be a bit small on the topend but probably very good for overall curve.  The high compression should help the exhaust side as the faster expansion should get the exhaust side moving when the valve cracks open.  I’d say 242* exhaust.  6 or 8* split could be argued but that’s what every shelf cam has so I can’t do it.

LSA I’d say 108*, the good intake doesn’t need much overlap to get it moving, and with 13:1 it’s not essential to trap cylinder pressure.  I could easily be talked into 106 though

Lift, no idea with head flow figures but typical small block heads with those flow numbers I’d say .650 gross intake, .625 on exhaust.

Interesting to see how far off I am...or if reasoning is sound.  Guessing this could be a 550hp/525tq combo pretty easily

[blykins]

Hypothetical question:  Trying to find the most torque and horsepower over a 3000 rpm band from 3500-6500 rpm, what do you think the camshaft specifications should be?

Engine: 375 cubic inches
stroke 4.000
bore 3.860"
rod 6.300"
Compresion 13:1
head flow 290/205
1 5/8" step to 1 3/4" four into one into 3" collector headers.
wet sump
tunnel ram/2-650s

Going to dyno very soon to verify my camshaft selection, but interested in ideas of those who consider themselves gurus.  Joe-JDC

Need to know peak flow lift  and also port volume.  Also need to know type of cam you need....hyd roller, solid flat tappet, etc.

[blykins]

Lift, no idea with head flow figures but typical small block heads with those flow numbers I’d say .650 gross intake, .625 on exhaust.

My guess is that it's not a small block....is it a Y-block, Joe?

[Barry_R]

The more I study this stuff the more I become a stab and try sort of guy.  And I have stabbed a lot of cams into reasonably similar combinations....sometimes walking away feeling like I know even less than I did going into the effort.  Damn motors cannot read and sometimes they just like what they like, no matter how hard we try to force them to like "something new"...

That noted, I bet JDC will have damn near no duration, a startlingly tight lobe separation, and it will be installed so advanced it'd scare most anybody.  Seems I have gone that direction until it makes so much running cylinder pressure through torque peak that you think it'll rip the heads off the top and push the crank through the pan at the same time...  Of course I had been running limited octane and was always listening for the sounds of impending doom, which never happened.  Means I was not trying hard enough...

Of course he will probably win with a cam that looks like it would work in an 812 inch Pro Mod and make everything I said look silly....cuz motors cannot read....

[chilly460]

Lift, no idea with head flow figures but typical small block heads with those flow numbers I’d say .650 gross intake, .625 on exhaust.

My guess is that it's not a small block....is it a Y-block, Joe?

Jesus, just saw 375ci and tunnel ram and assumed....didn't even look at the bore/stroke that early in the AM.  Fail. 

[My427stang]

No guru here, especially with a combo like that

Don't know end use but thinking its an EMC with the unreasonably wide desired range for a small cid motor

My WAG for a solid roller, mid 250s intake lobe, a real fast ramp, a little more on the exhaust side,  lift in the mid 600s or more if the heads support, 106-ish LSA.  Sort of short oval track kind of motor, but I am with Barry though, that would be a stab and try

EDITED for bore and stroke foolishness.....beans above the franks had me crossways didn't change my recommendation

[blykins]

No guru here, especially with a combo like that, BTW, just to be anal, it's 388 cubes no?  or was the bore and stroke a typo?

Don't know end use but thinking its an EMC with the unreasonably wide desired range for a small cid motor

My WAG for a solid roller, mid 250s intake lobe, a real fast ramp, a little more on the exhaust side,  lift in the mid 600s or more if the heads support, 106-ish LSA.  Sort of short oval track kind of motor, but I am with Barry though, that would be a stab and try

I'd say it's not a typo.  Could be a completely wrong guess, but with Joe's history, I'd say it's a stroked 312. 

If it is an EMC engine, that's not a lot of rpm for a tunnel ram, and I'm along the same lines as Barry......it wouldn't surprise me to see this engine needing something like a 104 LSA, installed on a 98-100° ICL, with as much lift as you can reasonably fit with the valvetrain. 

Curious to see the answers to my questions.....I'm eager to take a stab at it and may even float Joe a camshaft to try. 

[chilly460]

No guru here, especially with a combo like that, BTW, just to be anal, it's 388 cubes no?  or was the bore and stroke a typo?

I think you flip flopped the bore/stroke...