What role does cam overlap play in idle?

[Rinkrat456]

In many idle tuning guides I've read I see a common trend to give cams with more overlap a lot of 'loving' within the tune.

See the following for examples.
http://www.silveradoss.com/forums/to...sing-hptuners/

http://ls1tuningguide.com/tuningmanu...ons%20rev3.pdf

http://www.hptuners.com/forum/showth...8776#post78776

Can anyone help me wrap my mind around what overlap does to idle? I ask because by most tuning guides, my cam with 38 degrees of overlap should be a wicked wild beast to tame, though it's an extremely timid 206/212 (259/265) 112lsa 110icl in an iron block LS1.

[jimmyblue]

Both valves open will let intake air transfer to exhaust, and/or vice versa,
depending on the pressure ratio in-the-moment.

Air shot through to the exhaust will fool the oxygen sensors, because they
work off residual oxygen and only produce an -estimate- of original AFR
based on that. So you may see an indicated-lean off the wideband and
narrowband even if stoich or richer.

On the other hand exhaust gas pulled back on the intake stroke dilutes the
cylinder charge (this is how the '01-'02 get by without EGR - cam difference)
and reduces your volumetric efficiency.

The more overlap, the more of this you see.

I don't see how you have 38 degrees of overlap. Maybe at the negligible-lift
point. Even my 224/224 on 116 cam required a fair bit of massaging to get
right, at any rate. On 114, 112, 110 it wants progressively more low RPM
VE fading in the table.

I've always wanted to get a decent rule of thumb for that but it's not
easy (or practical, even) to judge the quality of work on different cars,
different tuners in the repository.

[Rinkrat456]

38 degrees of overlap found here.
http://www.compcams.com/Company/CC/c...csid=1084&sb=2

[jimmyblue]

I would guess that this is the same as the difference between the
"duration" and "duration at 0.050" numbers - "any at all" and
"meaningful" gas flow. But I'm no cam specification expert. What
would probably be more useful is an animation of the valve lifts and
piston as the crank rotates through 720 degrees, that follows the
cam grind particulars.

[Rinkrat456]

Well now you're just getting picky lol.

If you've got a website or a program that will do the 720 example I'm game to try it out.

[wyochimneysweep]

the overlap that you are getting with them specs is at cracking of the vavle open for intake and the exhaust vavle almosted seated. airflow doesn't really start till .050 lift and up ussually so your not lumpy of a cam. kindof like when I brought my cam i seen 260/268 duration. thats at .005 lifter lift which is .0075 vavle lift. at .050 valve lift theres no overlap

[Rinkrat456]

For kicks, any idea what my 'realistic' overlap is then?

My engine seriously idles just about as smooth as factory. A lot of that has to do with the countless hours I've got into just the idle tune, but a 206/212 @ .050 bump stick in a 347 is a baby by all standards amongst us. I chose it for the near-400lbft of torque it makes right off idle.

If I knew the advertised duration of any of the stock truck cams I wonder how they'd compare to my XR259. All I know about the 5.3L cam that I took out of the 5.3L I built into the 347 was that it's rated as a 190/191 on a 114lsa.

[wyochimneysweep]

yeah I choose my cam for the 1500-5000 rpm range cuz of my tune port. more overlap the higher the power range moves up. not sure of the stock cam's specs. Tried to find it on mine also but no real luck. mine idles very well but like alot of advance at idle. but never stalls or hangs. very happy with it for that part but wanted more lopey sound. my machine shop guy thought the 260/268 with at .050 lift. boy was i surpise when i got the card and seen otherwise

[BlackZJ]

For kicks, any idea what my 'realistic' overlap is then?

The overlap should be at -7.5°. That's why it idles so mooth...
206/212 is quite small for a 346. I had a 210/210 (113 lsa) in my 3.8L and idle was almost like stock.

- Mat

[Rinkrat456]

The overlap should be at -7.5°. That's why it idles so mooth...
206/212 is quite small for a 346. I had a 210/210 (113 lsa) in my 3.8L and idle was almost like stock.

- Mat

I'm curious, how did you figure that out?

You're absolutely correct about the 206/212 @ 0.050 being quite small for an iron block "LS1". When I built the engine I swapped the 706 (5.3L castings) heads for 241 (01-03 LS1 castings) heads hoping to enhance where the trucks usually taper off in their power band after 4000rpms. My fuel cutoff is at 6200, though all shifts happen between 5900 and 6000 so to me the XR259 was a no-brainer. When cam shopping, I wanted something that used the LS6 "yellow" springs so everything remained quiet and cheap to replace so the .515/.522 lift of the cam seemed to fit the requirements well. I juggled the next size bigger cam (XR265 = 212/218 .522/.529 115lsa) but honestly most of what I use my truck for is all putt-putting around off road and daily driving 35-37" tires with heavy solid axles. The XR259 makes a helluva lot of cylinder pressure. At 10.3:1 (8.54DCR) I can't come down off of 91 octane without experiencing slight KR.

[BlackZJ]

(([Int.dur] + [Ex.dur]) - [lsa]) = overlap

means:

((206 + 212) / 4) - 112) = -7.5



It's not a bad choice in terms of low end torque. The 212/218 115lsa has the same overlap so Idle and emissions (if you need to pass the sniffer test) would be no problem too.
What you're saying is absolutey correct. When you're driving a lot offroad (especially with big tires) you better have a quite small cam.
But it also depends on other factors like bore and stroke.

[wyochimneysweep]

so may cam has a -6.5 overlap.

[Rinkrat456]

I'm bored...so allow me:

1998 - 2000 Fbody
202/210 int/exh @ 0.05" duration
0.496" / 0.496" int/exh lift
116 LSA
-13 degrees overlap

2001 - 2002 Fbody
197/207 int/exh @ 0.05" duration
0.467" / 0.479" int/exh lift
116 LSA
-15 degrees overlap

2001 LS6 cam
207/217 int/exh @ 0.05" duration
0.525" / 0.525" int/exh lift
116 LSA
-10 degrees overlap

2002+ LS6 cam
204/218 int/exh @ 0.05" duration
0.551" / 0.547" int/exh lift
117 LSA
-11.5 degrees overlap

6.0 LQ9
196/207 int/exh @ 0.05" duration
0.479" / 0.467" int/exh lift
116 LSA
-15.25 degrees of overlap

5.3 LM7
190/191 int/exh @ 0.05" duration
0.466" / 0.457" int/exh lift
114 LSA
-18.75 degrees of overlap


You can see the trend in factory spec'd cams which shows as the cam gets more aggressive leaning towards high end power, overlap moves from negative towards positive duration of overlapping valve events to take advantage of the exhaust pulling the intake charge into the cylinder.

Interesting.

[Red Heeler]

If you are interested in how overlap effects idle then you need to calculate the overlap area in inch-degrees, it is a better measure than just degrees. Timing of the overlap peak lift value is also important.