Cam Overlap: Finding the "Sweet Spot"
11-12-2007, 09:13 PM
#21
Patrick-Do you think the diminishing returns past 15* is due to insufficient exhaust? I see a lot of people running the huge cams but only running the hooker/jethott/flowtech/pacesetter design, with a horrible y-pipe. In comparison to Kooks/QTP/ARH and a better y-pipe and i-pipe.
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2013 Corvette Grand Sport A6 LME forged 416, Greg Good ported TFS 255 LS3 heads, 222/242 .629"/.604" 121LSA Pat G blower cam, ARH 1 7/8" headers, ESC Novi 1500 Supercharger w/8 rib direct drive conversion, 747rwhp/709rwtq on 93 octane, 801rwhp/735rwtq on race fuel, 10.1 @ 147.25mph 1/4 mile, 174.7mph Half Mile.
2016 Corvette Z51 M7 Magnuson Heartbeat 2300 supercharger, TSP LT headers, Pat G tuned, 667rwhp, 662rwtq, 191mph TX Mile.
2009.5 Pontiac G8 GT 6.0L, A6, AFR 230v2 heads. 506rwhp/442rwtq. 11.413 @ 121.29mph 1/4 mile, 168.7mph TX Mile
2000 Pewter Ram Air Trans Am M6 heads/cam 508 rwhp/445 rwtq SAE, 183.092 TX Mile
2018 Cadillac Escalade 6.2L A10 Pat G tuned.
LS1,LS2,LS3,LS7,LT1 Custom Camshaft Specialist For custom camshaft help press here.
Custom LSX tuning in person or via email press here.
2013 Corvette Grand Sport A6 LME forged 416, Greg Good ported TFS 255 LS3 heads, 222/242 .629"/.604" 121LSA Pat G blower cam, ARH 1 7/8" headers, ESC Novi 1500 Supercharger w/8 rib direct drive conversion, 747rwhp/709rwtq on 93 octane, 801rwhp/735rwtq on race fuel, 10.1 @ 147.25mph 1/4 mile, 174.7mph Half Mile.
2016 Corvette Z51 M7 Magnuson Heartbeat 2300 supercharger, TSP LT headers, Pat G tuned, 667rwhp, 662rwtq, 191mph TX Mile.
2009.5 Pontiac G8 GT 6.0L, A6, AFR 230v2 heads. 506rwhp/442rwtq. 11.413 @ 121.29mph 1/4 mile, 168.7mph TX Mile
2000 Pewter Ram Air Trans Am M6 heads/cam 508 rwhp/445 rwtq SAE, 183.092 TX Mile
2018 Cadillac Escalade 6.2L A10 Pat G tuned.
LS1,LS2,LS3,LS7,LT1 Custom Camshaft Specialist For custom camshaft help press here.
Custom LSX tuning in person or via email press here.
11-13-2007, 03:28 PM
#22
TECH Senior Member
It has some to do with exhaust design, but even more to do with our induction. With a large plenum EFI intake manifold with long runners, it's hard to make the most of a cam with lots of overlap, hence the diminishing returns after a certain overlap point. If you were to run an individual runner intake manifold, you could run 20-30 degrees of overlap at .050" with a 346 and still have decent street manners. Without a common plenum transfering reversion from cylinder to cylinder, you get a much cleaner idle and crisper throttle response.
11-13-2007, 03:31 PM
#23
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Thank you all for the information.
I appreciate the expertise. Good to get some actual numbers to compare and where the point of diminishing returns of overlap come into play.
I'll bookmark the thread and replies. You don't find this kind of information anywhere else.
Greatly appreciated..WeathermanShawn.
I appreciate the expertise. Good to get some actual numbers to compare and where the point of diminishing returns of overlap come into play.
I'll bookmark the thread and replies. You don't find this kind of information anywhere else.
Greatly appreciated..WeathermanShawn.
12-18-2017, 01:49 PM
#25
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It has some to do with exhaust design, but even more to do with our induction. With a large plenum EFI intake manifold with long runners, it's hard to make the most of a cam with lots of overlap, hence the diminishing returns after a certain overlap point. If you were to run an individual runner intake manifold, you could run 20-30 degrees of overlap at .050" with a 346 and still have decent street manners. Without a common plenum transfering reversion from cylinder to cylinder, you get a much cleaner idle and crisper throttle response.
Also if one is building a stock bottom end 346 track only car with a single plane intake with at least 20° overlap, would the IVC increase up 49° and 7500rpm shift points assuming good rod bolts are used as an added security mechanism?
12-18-2017, 04:36 PM
#26
TECH Apprentice
Actually, advancing the cam will bias the overlap toward the exhaust side of TDC. For high rpm use, you want to have your overlap centered over TDC or biased to the intake side of TDC. This is why Pro Stock motors run such big exhaust duration compared to the intakes...they bias the overlap to the intake side of TDC and enhance breathing at an extended rpm range.
For overlap in a 346, I've found diminishing returns past 15 degrees of overlap at .050". Once you exceed that, your gain in power does not keep up with the loss in street manners. Sure you'll make more power with 25 degrees of overlap, but the 10 rwhp gain (over a similar cam with 15 degrees of overlap) is not usually worth the loss of street manners. For a race car it's worth it. For a street car, usually not. In fact, for drivers who want decent street manners, I like to see overlap around 6 degrees at .050". That's a point that's still pretty easy to tune, yet will make a solid gain over a 0 overlap cam.
For overlap in a 346, I've found diminishing returns past 15 degrees of overlap at .050". Once you exceed that, your gain in power does not keep up with the loss in street manners. Sure you'll make more power with 25 degrees of overlap, but the 10 rwhp gain (over a similar cam with 15 degrees of overlap) is not usually worth the loss of street manners. For a race car it's worth it. For a street car, usually not. In fact, for drivers who want decent street manners, I like to see overlap around 6 degrees at .050". That's a point that's still pretty easy to tune, yet will make a solid gain over a 0 overlap cam.
I run a LQ9 408 with a FAST 102 and NW 102, AFR 230 heads, 11:1 compression, and my cam was custom cut for nitrous so I'm told, from my cam card specs are 243/251 with .624/.624 111LSA +1. So if my math is correct I'm running 25 degrees of overlap. As mentioned tuning is a bear and seems to be very sensitive to weather and elevation changes. But after lots of time have tune going pretty fair.
Has a level 5 FLT built 4l60e with a 3600 Yank, 12-bolt 3.73
Is this too much cam? I have never tracked the car or sprayed nitrous as it seems I'm constantly chancing the tune around.
Would backing off to a less overlap cam hurt me or actually help in performance. I'm guessing if I was to spray 100 or 150 with my current cam with 25 degrees of overlap it'd really run hard. I think that my car runs hard now and a few that have went for rides that have LSA supercharged cars with bolt ons have commented on how hard it pulls especially up top over theres.
Thought I'd add in on this so others can read and learn from others combos and what they are wanting to achieve.
Last edited by neblackshirts; 12-18-2017 at 04:39 PM. Reason: wording
