Overlap and LSA?
Thread Starter
On The Tree
Joined: Jun 2022
Posts: 111
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From: Las Vegas, NV
Overlap and LSA? Looking for some clarification... have been reading on cams and finding some conflicting information regarding calculating overlap.
In this post LSA and Overlap overlap is calculated by adding intake and exhaust duration and dividing that by 2, then multiply LSA by 2 and subtract that from I/E duration/2.
On this site Calculating Overlap they state you calculate overlap by adding the exhaust closing point to the intake closing point.
I recently had a LG GX71 cam reground by Delta cams and they provided a cam card with this info...
Events rated at .05" Tappet Lift
Intake: Cline:112.0 Dur:220.9 Open:-1.3 Close:42.2 Lb Area:25.35 Lb Lift:.332 Vlv Lift:.531 RAR:1.6 Lash:Hyd LbSep:116.5 Overlap:-7.3
Exhaust: Cline:120.9 Dur:232.8 Open:58.8 Close:-6.1 Lb Area:27.14 Lb Lift:.337 Vlv Lift:.539 RAR:1.6 Lash:Hyd Adv:4.5
If I use the first method I get -6.15 overlap, If I use the second method I get -7.3 overlap which matches the cam card.
I'm trying to decide if I want to use the reground cam or go with the Cam Motion LS3 stage 3 high lift however I only have the duration 220/230 and LSA 117 which when calculated using method 1 I get -9 overlap
Before reading the method 1 article I was under the impression that a larger LSA would get a smoother idle (desirable for me) however won't a larger overlap result in a more choppy/loping idle?
In this post LSA and Overlap overlap is calculated by adding intake and exhaust duration and dividing that by 2, then multiply LSA by 2 and subtract that from I/E duration/2.
On this site Calculating Overlap they state you calculate overlap by adding the exhaust closing point to the intake closing point.
I recently had a LG GX71 cam reground by Delta cams and they provided a cam card with this info...
Events rated at .05" Tappet Lift
Intake: Cline:112.0 Dur:220.9 Open:-1.3 Close:42.2 Lb Area:25.35 Lb Lift:.332 Vlv Lift:.531 RAR:1.6 Lash:Hyd LbSep:116.5 Overlap:-7.3
Exhaust: Cline:120.9 Dur:232.8 Open:58.8 Close:-6.1 Lb Area:27.14 Lb Lift:.337 Vlv Lift:.539 RAR:1.6 Lash:Hyd Adv:4.5
If I use the first method I get -6.15 overlap, If I use the second method I get -7.3 overlap which matches the cam card.
I'm trying to decide if I want to use the reground cam or go with the Cam Motion LS3 stage 3 high lift however I only have the duration 220/230 and LSA 117 which when calculated using method 1 I get -9 overlap
Before reading the method 1 article I was under the impression that a larger LSA would get a smoother idle (desirable for me) however won't a larger overlap result in a more choppy/loping idle?
Last edited by vegas_ss; Dec 20, 2022 at 11:38 AM.
I use the Wallace racing calculator. Cam motion also has a calc you can download if you have a windows machine.
as far as lsa vs overlap, overlap has more effect than the lsa.
yes, a wider lsa typically means a smother idle, but if it’s a donkey dick cam it’ll still lope like crazy. Example: any of the stage 4 plus turbo/supercharger cams.
going crazy wide on the lsa will also flatten your powerband if your n/a
as far as lsa vs overlap, overlap has more effect than the lsa.
yes, a wider lsa typically means a smother idle, but if it’s a donkey dick cam it’ll still lope like crazy. Example: any of the stage 4 plus turbo/supercharger cams.
going crazy wide on the lsa will also flatten your powerband if your n/a
Last edited by bthomas; Dec 20, 2022 at 08:28 AM.
Thread Starter
On The Tree
Joined: Jun 2022
Posts: 111
Likes: 35
From: Las Vegas, NV
Thanks... When I use the Cam Motion calculator I get the same overlap results of -6.15 and -9.0 respectively for the reground LG G7X1and CM LS3 stage 3 (using method 1 above). I did have a typo of -6.5 rather than -6.15 and edited that.
This was the response from cam motion from using their basic form and inquiring about the titan-2 cam so it looks like I would have a smoother idle with the stage 3 ls3 than the current reground which is/was a LG G7X1.
"The Titan 2 is a great camshaft, but it will have a bit more idle sound than what you currently have. It will likely make a bit more power though. If you want a smooth idle try our Stage 3 LS3 High Lift"
This was the response from cam motion from using their basic form and inquiring about the titan-2 cam so it looks like I would have a smoother idle with the stage 3 ls3 than the current reground which is/was a LG G7X1.
"The Titan 2 is a great camshaft, but it will have a bit more idle sound than what you currently have. It will likely make a bit more power though. If you want a smooth idle try our Stage 3 LS3 High Lift"
Just use the Wallace tool. It is very good. The tighter the LSA, the more overlap. Exaggerate it. The following cam wouldn't work, but use it as an example only: Grind an imaginary cam, one with a 3° LSA. In this example, both valves would nearly always be open together! Totally, always on overlap. 
TECH Resident
Joined: Jun 2011
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This one is also good for visualizing events and overlap differences in addition to doing the calculations:
https://mgispeedware.com/camshaft-calculator/
https://mgispeedware.com/camshaft-calculator/
To find overlap of a camshaft, there’s several ways, but here’s two easy ones I use myself…
1. Add exhaust duration and intake duration together. Divide the number by four. Subtract the LSA. Multiply this number by two.
2. Add the intake opening degree number and the exhaust closing degree number. You MUST use seat to seat numbers here. .050 won’t work. .006 numbers only.
1. Add exhaust duration and intake duration together. Divide the number by four. Subtract the LSA. Multiply this number by two.
2. Add the intake opening degree number and the exhaust closing degree number. You MUST use seat to seat numbers here. .050 won’t work. .006 numbers only.
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TECH Senior Member
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From: Central Cal.
#1 is close to what I do- average the durations, double the LSA, then subtract that from the duration average. Should be negative for milder cams....
This is the quickest way when eyeballing cam specs without any electronics....
This is the quickest way when eyeballing cam specs without any electronics....
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To help get a better feel for how the cam will behave probably best to look at overlap at .006 & at .050
Also a good idea, in my opinion to subtract the .050 duration from the .006 duration to get a rough idea of how fast the lobe ramp is if the cam lobe is symmetrical.
Also a good idea, in my opinion to subtract the .050 duration from the .006 duration to get a rough idea of how fast the lobe ramp is if the cam lobe is symmetrical.
Joined: Apr 2006
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From: Grand Rapids, Michigan
That's why I like to know the @.006 overlap figures. You could have two cams with similar @.050 overlap figures but significantly different @.006 overlap figures due to varying lobe intensity. I swapped cams on my Formula years ago, and the new cam had 4 degrees more duration int./exh. @.050, but 2 degrees less @.006. With an LSA change from 113 to 111, drivability was unchanged, but the new cam picked up a lot of mid range torque.
