Lobe separation angle

When is 108 not really 108?

Because in the engine.... its really (2x 108)= 216, and 112 in the engine is really 224,Lobe separation angle, Although its the physical easily visual "cut" of the cam, but in the engine its real definition is the degrees between peak lifts of the Exhaust and Intake lobes.

I was always vexed why are cams described by lobe separation angle, When the cam only turns 1/2 the crank speed.

Everything else on a cam is measured in terms of Crank duration.

This all came about when I was trying to describe how these "Thumper" , or "poser" cams actually work. They have a physically "tight" peak to peak Separation angle...but that specific exhaust ramp opens early off the seat but slowly, or not really enough to impact part throttle drivability.

Also what I think a lot of people might not visualize fully is that with a roller cam...the actual measured "PEAK" lift of the valve can 'Dwell' very close to peak for several degrees. So with asymmetrical ramp design the physical or Mechanical "peak to peak" is not nearly as critical to power and drivability as the intake closing and exhaust opening ramps leading to overlap. so the lobe spread and it's effect on idle and drivability is really more about the flanks of the lobes rather than the nose.

Anyway....I just thought it was interesting to ponder, I for one find it much easier to visualize the intake ramp opening and closing (and it's effect on power) than I do the exhaust, I often have to calculate just to visualize just how far off the seats the valves are during the full duration of overlap. And since overlap is the Closing ramp of the exhaust, I'm intrigued to know more about determining how and when you begin to open the exhaust valve impacts power and cylinder evacuation.

I was expecting you to open (continue?) the debate on whether LSA is a camshaft design parameter or simply the result of desired intake & exhaust open/close events...

Brad....you know me.....I don't finish debates, I only Start them and encourage them by offering my

Something else to add to the pondering, I replaced my 269/275@ 50 flat tappet with a 269/275@50 roller. Just looking at the cam sure looks like more duration to me in the roller, round nose vs a pointed nose. Something else, picking the exhaust opening for a 511 wedge should be much different for a 572 Hemi i would think, just to get a handle on the escaping input charge. Mine is not much different from my 511 wedge, 3*later on the Hemi cam.

This is from something I posted here years ago, but seems appropriate given your comments above

My previous solid flat-tappet came came from COMP's XX .875"-lifter family and is an older NASCAR-type lobe. It isn't as aggressive as the later .904"-type lobes, but has still proven to make good power and RPM.

COMP XX
@ .020" - 297.5 (adv 298)
@ .050" - 266
@ .100" - 232
@ .200" - 178.5 (adv 180)
@ .300" - 121.5
@ .400" - N/A

Lobe lift .3975 (adv .400)
Gross lift .596 (1.5)
LSA 108
Lash range .016" - .024"

My current COMP RX roller series is a moderately aggressive endurance lobe which Dwayne has said has been very successful for his customers looking for decent valve train life when running a solid roller.

COMP RX
@ .020" - 300 (adv 298)
@ .050" - 266 (adv 265)
@ .100" - 233
@ .200" - 184 (adv 183)
@ .300" - 134
@ .400" - 68

Lobe lift .434
Gross lift .651 (1.5)
LSA 108
Lash range .018" - .026"

From a seat duration & overlap perspective, both cams are very close, so I'm expecting drivability "should" be similar. The roller has a big high-lift duration advantage which suits the change in cylinder heads with the last rebuild.

I hear ya'... and apologies in advance if anything I post here steers the discussion (too far) off topic

I figured out that the only people who really understand lobe centerlines are the guys who work on twin overhead cam engines. They have the ability to easily change the lobe separation so they are able to dial in exactly what the engine wants. The rest of us guess at it a few times until we run out of money.

What i see is advertized durations don't match reality well. A cam with a lash setting of .028 and a 1.5 rocker will have different amount of true duration at the valve versus a cam with a 1.7 rocker and .019 lash. Then add in the intensity of the lobe design, and the difference becomes greater yet. I have two cams with supposedly similar specs, bud the differences i showed above. One cam used a 1.5 rocker and big lash, the other an inverted flank cut lobe, tight lash, and 1.7 rocker ratio. The tight lash cam with 1.7 rockers gave better average hp installed at 108, rather than 110, the recommended install.

Draw no conclusions w/r/t lobe shape of roller vs flat tappet.

https://board.moparts.org/ubbthread...ue/valve-lash-tech-talk.html#Post1277696

Try comparing the lobe timing on both types of lobes with the same duration, LSA and lobe on both at .050,.100 and then .200 and .300 see how much actual timing differences there are between both cam types at those lobe lifts

So I kind of went a little "ape [censored]" on this stuff a little while ago when I was trying to compare in very hard-terms the differences between:

1) hydraulic flat tappet cam: 238/244@050", 286/290 adv., 108 LSA, .536/.540 lift
2) hydraulic roller cam: 240/248@0.050", 290/300 adv., 112 LSA, .547/.544" lift

I've attached the PDF, which you need to rotate but also zoom in as far as you want. The attached pic is probably too sized-down to be usable.

Anyways, in that chart I overlayed the lobe lift with the actual piston travel in a bore (3.58 v 4) and piston speed. Was trying to understand how the bigger stroke translated to the engine seeing the same "fuel charge intake" at different points.

In the end my conclusion was: this is FAR too complex for a mere mortal like myself to understand!

And on something like a Coyote, all it takes is a few keystrokes and you can move both the intake and exhaust cams independently. I think most of the 1000+ hp coyote builds either lock the cams out completely or limit the phaser travel though.

I would imagine on a high power build they would find the sweet spot and just lock the cams. It would be interesting to know what the "sweet spot" is though. I do know on OEM engines they change the LSA between idle and WOT in order to maximize power and MPG. Widening the lobe centers can help with the idle quality, reduce emissions and improve fuel economy. Then they tighten up the LSA to improve pull on the intake charge to boost torque. Not so sure what they do at WOT. They might widen the LSA back out a bit.