Lobe separation angle
#3184782
10/18/23 10:52 AM
10/18/23 10:52 AM
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Joined: Jan 2003
Posts: 9,875 Weddington, N.C.
Streetwize
OP
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OP
master
Joined: Jan 2003
Posts: 9,875
Weddington, N.C.
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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.
Last edited by Streetwize; 10/18/23 10:57 AM.
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Re: Lobe separation angle
[Re: Streetwize]
#3184785
10/18/23 11:00 AM
10/18/23 11:00 AM
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Joined: Dec 2021
Posts: 1,127 Loudoun County, VA
Brad_Haak
super stock
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super stock
Joined: Dec 2021
Posts: 1,127
Loudoun County, VA
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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...
2021 Challenger 6.4L Scat Pack 1320 100% stock: 1.680, 11.894 at 113.75 (DA 175 ft) weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)
1973 Challenger 452 ci street/strip [2008] pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)
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Re: Lobe separation angle
[Re: mopar dave]
#3184812
10/18/23 12:38 PM
10/18/23 12:38 PM
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Joined: Dec 2021
Posts: 1,127 Loudoun County, VA
Brad_Haak
super stock
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super stock
Joined: Dec 2021
Posts: 1,127
Loudoun County, VA
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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.
2021 Challenger 6.4L Scat Pack 1320 100% stock: 1.680, 11.894 at 113.75 (DA 175 ft) weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)
1973 Challenger 452 ci street/strip [2008] pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)
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Re: Lobe separation angle
[Re: Streetwize]
#3184814
10/18/23 12:40 PM
10/18/23 12:40 PM
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Joined: Dec 2021
Posts: 1,127 Loudoun County, VA
Brad_Haak
super stock
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super stock
Joined: Dec 2021
Posts: 1,127
Loudoun County, VA
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Brad....you know me.....I don't finish debates, I only Start them and encourage them by offering my I hear ya'... and apologies in advance if anything I post here steers the discussion (too far) off topic
2021 Challenger 6.4L Scat Pack 1320 100% stock: 1.680, 11.894 at 113.75 (DA 175 ft) weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)
1973 Challenger 452 ci street/strip [2008] pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)
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Re: Lobe separation angle
[Re: Brad_Haak]
#3184827
10/18/23 01:06 PM
10/18/23 01:06 PM
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Joined: Jan 2003
Posts: 9,999 Frostbitefalls MN (Rocky&Bullw...
gregsdart
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Frostbitefalls MN (Rocky&Bullw...
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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.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: Lobe separation angle
[Re: Streetwize]
#3184833
10/18/23 01:33 PM
10/18/23 01:33 PM
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Joined: Oct 2007
Posts: 4,206 New York
polyspheric
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master
Joined: Oct 2007
Posts: 4,206
New York
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Draw no conclusions w/r/t lobe shape of roller vs flat tappet.
Boffin Emeritus
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Re: Lobe separation angle
[Re: gregsdart]
#3184837
10/18/23 01:42 PM
10/18/23 01:42 PM
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Joined: Dec 2021
Posts: 1,127 Loudoun County, VA
Brad_Haak
super stock
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super stock
Joined: Dec 2021
Posts: 1,127
Loudoun County, VA
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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. https://board.moparts.org/ubbthread...ue/valve-lash-tech-talk.html#Post1277696
2021 Challenger 6.4L Scat Pack 1320 100% stock: 1.680, 11.894 at 113.75 (DA 175 ft) weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)
1973 Challenger 452 ci street/strip [2008] pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)
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Re: Lobe separation angle
[Re: Cab_Burge]
#3184880
10/18/23 04:28 PM
10/18/23 04:28 PM
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Joined: Jan 2003
Posts: 1,760 Windsor, ON, Canada
Diplomat360
top fuel
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top fuel
Joined: Jan 2003
Posts: 1,760
Windsor, ON, Canada
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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!
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Re: Lobe separation angle
[Re: AndyF]
#3184894
10/18/23 06:03 PM
10/18/23 06:03 PM
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Joined: Jan 2003
Posts: 8,165 Plymouth, MI
Blusmbl
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master
Joined: Jan 2003
Posts: 8,165
Plymouth, MI
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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. 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.
'18 Ford Raptor, random motorcycles, 1968 Plymouth Fury III - 11.37 @ 118
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Re: Lobe separation angle
[Re: Blusmbl]
#3184898
10/18/23 06:16 PM
10/18/23 06:16 PM
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Joined: Jan 2003
Posts: 31,036 Oregon
AndyF
I Win
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I Win
Joined: Jan 2003
Posts: 31,036
Oregon
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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. 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.
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