overlap - LSA - durations

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pastry_chef
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overlap - LSA - durations

Post by pastry_chef »

For fun I generated a simple excel file (attached) that lets people pick overlap, LSA and start duration. Then it provides you with intake and exhaust duration combinations that will match that overlap/ LSA target.

There are some who specialize in certain engine families that preach their BRAND engines prefer a set LSA, deciding on LSA before either intake or exhaust duration's are selected.
I believe valve events and overlap are the importance. :)
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Re: overlap - LSA - durations

Post by econo racer »

Why do some camshafts have 250 deg, at 50 and the advertised duration is 300? Then another cam have 258 deg. at 50 have the same advertised duration at 300? Would the 258 deg, make more power. I know that depends on other things. Many yrs. ago I remembered more about this subject #-o
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Re: overlap - LSA - durations

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econo racer wrote: Wed Nov 14, 2018 2:36 pm Why do some camshafts have 250 deg, at 50 and the advertised duration is 300? Then another cam have 258 deg. at 50 have the same advertised duration at 300? Would the 258 deg, make more power. I know that depends on other things. Many yrs. ago I remembered more about this subject #-o
The difference between the advertised and @ 0.050" durations shows you how aggressive the lobe is; a smaller difference between advertised and @ 0.050" means that the lobe is more aggressive and opens / closes the valve faster. -You want to check out the difference in duration up at 0.200" and look at the diff between advertised and 0.200" and 0.050" too as the accel rate can change -the valves are normally decelerated towards close from 0.050" to 0.000" so that the don't SLAM closed hard and cause damage or valve bounce or make that sewing machine sound like a flat tappet CompCams XE lobe...

A more aggressive lobe profile allows more power under the curve -you keep your seat to seat durations as low as possible to keep your Dynamic Compression Ratio / cylinder pressure higher (more torque) but then you want the durations at higher lifts to be higher to make more power / RPM -when you open the valve further you're spending more time with the valve at the higher CFM flow rates for more power, too. -Less time spent at lower lifts = less reversion and at lifts where the valve curtain area is the bottleneck. (More HP and more torque for more aggressive lobes but more $$$ and more wear.) -The benefits and outcomes of a more aggressive lobe are similar to going with a higher rocker ratio.

A more aggressive lobe requires more spring and a better valve train and makes for more wear if part / materials quality doesn't improve.

-Compare the durations at different lifts from some old-school "Classic" Flat tappet grinds, then a more modern Flat Tappet Grind, then some middle-of-the-road hydraulic roller cams and an extreme hydraulic roller lobe (See Mike Jones's EHR lobes or comp QXI (If I remember correctly)), then compare that to solid roller lobes and you'll get it quickly. -You'll also see that something like an XE Flattappet lobe REALLY does look like a standard hydraulic roller lobe by-the-numbers.


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Re: overlap - LSA - durations

Post by CamKing »

NewbVetteGuy wrote: Wed Nov 14, 2018 2:59 pm
econo racer wrote: Wed Nov 14, 2018 2:36 pm Why do some camshafts have 250 deg, at 50 and the advertised duration is 300? Then another cam have 258 deg. at 50 have the same advertised duration at 300? Would the 258 deg, make more power. I know that depends on other things. Many yrs. ago I remembered more about this subject #-o
The difference between the advertised and @ 0.050" durations shows you how aggressive the lobe is;
Not always.
First off, "Advertised" Duration can be measured at whatever lobe lift the cam company wants to measure it at. In many cases, it's at .020", but it can also be measured at .015" or .030", or .010" or anywhere in between.
The other issue is, even if two cams are measured at the same lift for "Advertised" duration, if the lobes are designed for different lash, they can't be compared at the same lift.
In short, "Advertised duration" is useless, and will tell you nothing about the cam.
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Re: overlap - LSA - durations

Post by NewbVetteGuy »

econo racer wrote: Wed Nov 14, 2018 2:36 pm Why do some camshafts have 250 deg, at 50 and the advertised duration is 300? Then another cam have 258 deg. at 50 have the same advertised duration at 300? Would the 258 deg, make more power.
Yes. The 300/258 deg cam would make more power than the 300/250 deg cam but at a slightly higher RPM. You'll often find that that's not the only change between the two cams and you'll get a little bit more lift out of the 300/258 deg cam.

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Re: overlap - LSA - durations

Post by NewbVetteGuy »

CamKing wrote: Wed Nov 14, 2018 3:05 pm
NewbVetteGuy wrote: Wed Nov 14, 2018 2:59 pm
econo racer wrote: Wed Nov 14, 2018 2:36 pm Why do some camshafts have 250 deg, at 50 and the advertised duration is 300? Then another cam have 258 deg. at 50 have the same advertised duration at 300? Would the 258 deg, make more power. I know that depends on other things. Many yrs. ago I remembered more about this subject #-o
The difference between the advertised and @ 0.050" durations shows you how aggressive the lobe is;
Not always.
First off, "Advertised" Duration can be measured at whatever lobe lift the cam company wants to measure it at. In many cases, it's at .020", but it can also be measured at .015" or .030", or .010" or anywhere in between.
The other issue is, even if two cams are measured at the same lift for "Advertised" duration, if the lobes are designed for different lash, they can't be compared at the same lift.
In short, "Advertised duration" is useless, and will tell you nothing about the cam.

Thanks for the clarification Mike.

-If I would have replaced every instance of "Advertised Duration" with "0.006" duration or some other low-lift value, the rest of what I said would still be roughly correct, right??

(I thought these days at least with modern cams "Advertised" == "0.006" duration; apparently not.)

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Re: overlap - LSA - durations

Post by F-BIRD'88 »

You need to know the lift checking height spec of the "advertized" duration of both cams. Must be the same to compare.

More relevant is knowing the real lash point duration.
"Seat to seat" @ lash point.

Advertized duration can be @ any check height.
Often chosen (and unspecified) , for marketing purpose.

You'd want to measure any 2 cams on say a Cam DR. To make real comparisions.
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Re: overlap - LSA - durations

Post by Bazman »

pastry_chef wrote: Sat Sep 08, 2018 12:27 pm For fun I generated a simple excel file (attached) that lets people pick overlap, LSA and start duration. Then it provides you with intake and exhaust duration combinations that will match that overlap/ LSA target.

There are some who specialize in certain engine families that preach their BRAND engines prefer a set LSA, deciding on LSA before either intake or exhaust duration's are selected.
I believe valve events and overlap are the importance. :)
Thank you, that is helpful to see the correlation at a glance. I'm going to run a Mike Jones hyd roller cam when the motor comes out next year and want to bore/align/bush for 0.904 lifters on my LS3 so that i can run fast ramps without the wear. Block will be put on a 4 axis cnc machine anyway so I might as well do that.
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Re: overlap - LSA - durations

Post by digger »

On the ohc stuff I always plot out valve lift curve to compare cams, it's about the closest you can reasonably do to sift through the irrelevant info and compare directly
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Re: overlap - LSA - durations

Post by CamKing »

NewbVetteGuy wrote: Wed Nov 14, 2018 3:09 pm (I thought these days at least with modern cams "Advertised" == "0.006" duration; apparently not.)
.006" lobe lift is only for hydr cams, and not all companies use .006" for hydraulics.Some use .004". Some use .006" valve lift.
Mechanical cams could be .010".015",.020",.025",.030", or any place the cam company decides to call "Advertised". Even if two cams are checked at the same lift, if they're designed for different valve lash points, you still can't compare them. Say you take 2 cams that "Advertised is checked at .020". One runs .030" lash with 1.5 rockers, and the other runs .018" lash with 1.5 rockers. For the one with .030" lash, the .020" advertised duration, is the actual opening and closing points. For the one with .018" lash, the .020" advertised duration is .008" lobe lift(8-16 degrees) away from actual opening and closing points.
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