Let's start back at the original request:..

Vehicle/Drivetrain

• It's a roughly 3600lb truck, 4L80E (that means 2.48 1st gear), 3.42 rearend ratio, and 3000-3200 stall. Tire size isn't stated, but let's assume (unless he says otherwise) he's not going for 33s since it's 2WD anyway. Likely 26-28" tall. Not a heavy vehicle., stall is decent, however 1st gear x rearend is a little shallow..
• Engine is a 6.0L (larger side of the truck LS, long tubes to help torque, no convertors to breath through. Head choices are either 317s (not low, but mild compression), or 706s (high compression).

Use/Manners

• Street cruising only
• No track use
• No towing
• Wants responsiveness to begin around 2500rpm
• Pull to 6000/6200rpm
• Pump gas
• Two proposed cams indicate that a "lope" is desired.

Concern

• Previous cam in a smaller displacement (5.3L) was "too big", OP dissatisfied with performance.

Put all this information together, IIRC Summit nailed it. If the OP uses the original 317s either stock or slightly milled then the 8720R1 would meet all the above criteria. With the 706 heads, the 8706 would be the best choice. For the OP's setup, their desired purpose and characteristics, those are the match-up the closest. It all plays a role. Change a couple of the variables (or maybe even one critical one) then yes maybe a more or less aggressive camshaft fits better.

Unless you've invented the Ping-Master 2000 by not having a low enough DCR to run your intended fuel octane, and can't back off timing sufficiently; undersizing the camshaft is only disappointing, but oversizing (as the OP discovered) is frustrating. Can't fault them for wanted to verify the details, especially considering previous experience.

 

Here's a spoon. If you put 706 heads on your LQ9 bottom end. And everybody agrees you have 11.44:1 compression with that combo and all you need is a camshaft with an IVC event of 72.5° @0.006" You will have exactly 8.52 in your dynamic compression calculator. Does everyone agree This is what is normally recommended for a street engine 8.5 dynamic compression calculation?

 

Aluminum heads and 8.5:1 DCR could run 87 octane. 91 octane you can run ~9.5:1 DCR.

 

A slight drop to 67 IVC, increases it closer to 9 with 706 heads. However, where the concern would be, is the combined area of very low rpm with high load operation. Since the OP has a 3000-3200 stall with moderate weight and 4L80E/3.42 gears it won't see that zone. By the time the driver commands heavy load, it'll push past the rpm range where the potential problem might have been. The overall setup factors in, not just the engine (and tuning for the octane of course). Drop the stall to <2500rpm, require towing a boat, etc., etc., story changes. Part throttle though, the 67* IVC and 2* overlap will feel more responsive.

 

Correct 28.5/29" tall tire.

@stockA4 that seems to be the consensus among everything i've read. I'm not a proficient enough to say otherwise as I've not had experience in running high compression motors.

A higher cam duration will bleed off some compression which allows to have a lower dcr. But, the p-2-v is also an issue with the IVO. @Summitracing states around 5* IVO is where issues come into play.

 

Is this on your personal combo? I've read running higher than 8.5 dcr on 93 on some folks are having to back off timing. I suppose every combo is different. But, it looks like 91 octane is going to be more feasible for me. So, this makes me think 8-8.5 dcr is what I need to shoot for.

And from what I've read, scr doesnt mean much. Pretty much dcr for octane.

 

DCR is part of it, cylinder pressure, chamber design, quench and other factors all play into whether it will detonate or not.

Personal combo in a 6,200 lbs brick that tows a 6,000 lbs travel trailer. 4L85E with a converter that stalls 2,800 rpm at heavy throttle and it has 3.73 gears with 31.7" tall tires.

With the Rhoads V-Max lifters in a state of maximum bleed off it has a dynamic compression ratio of 9.57:1. With the Rhoads lifters fully pumped up, which happens around 4K rpm, it is at 8.97:1. Static is 10.97:1. I run 31* total advance on 91 octane with it. At cranking speed without an intake manifold on the engine, I recorded ~225 psi cranking compression in every cylinder.

Timing is what timing is, not sure why you see "backing off the timing" as a bad thing. Higher compression packs the air/fuel charge in more densely and the flame moves more quickly, resulting in the need for less timing advance. Less timing advance means less negative work on the crankshaft as the piston is on its way up to TDC with the flame is already lit. More timing is not a good thing when it comes to efficiency. A large open chamber with a low compression ratio may tolerate 42* BTDC timing advance. A LS3 chamber makes power at ~20-22* BTDC. Both are reaching peak cylinder pressure at perhaps 15* ATDC. The LS3s quicker burning chamber results in less negative work on the crank. I will run higher compression with less timing over low compression and a load of timing anyday.

 

This is completely true I can't disagree with any of this nor would I.

But I do drive an engine dyno with the same combo being discussed, I have a heavier fbody with 2.73 years and the stock converter. I can lock the converter at 1100 RPM in third and fourth gear.

I tried three different shelf cams in this combo before ordering a custom grind.

I've never said I've cared about raw numbers, at least for me and what I'm doing with my engine, how it drives is more important to me than what kind of number is it makes.

If you put the right Band-Aid on this combo You'll be able to put a considerable amount of load on it at 1100 RPM and yet it will still pull past 6500 and at least flatline there for a bit without giving up.

Sure the throttle response is slightly better at part throttle with a cam with an earlier IVC, but it can't be loaded as much at lower RPM without bucking nor will it pull as high or as far past peak with the earlier IVC choking it.

This combo has a horrible mach index but it still wants to rev so we have to work around that. In Neanderthal terms that means that the air velocity inside the intake port increases with rpm at a higher rate than any other factory LS combo would

Compare this combo to any other factory LS combo in a mach index calculator and You will understand what I'm talking about. This combo becomes choked way earlier in the rev range than an LS3 does This is obvious. And LS3 cannot tolerate a comparatively late IVC but this combo certainly can and it will still be better below 3K RPM then the LS3 would yeah it will do it's very best to keep up past that because you're always going to have to use a considerable amount of intake duration to get the IVC and the IVO where they need to be despite what kind of exhaust you may think it needs

 

Ive thought about all of this. And I've come to terms with running 799 heads on this. Ive come across a good deal locally for a set. So LQ9, 799 heads. I'm still thinking of going the 8720r1 cam from @Summitracing .

SCR is around 11.2/3 and dcr is around 8.8/8.9 which I'm willing to run 91/93 if need be.

Some very good information here and Ive learned quite a bit simply posting questions and re-reading some replies. So, I thank all of you.

One final question. If my pistons are .008 out of the hole and proper quench is .035-0.40, I would assume having a cometic gasket of around .048 would net me right at the .040 magic number? I had to order a deck bridge and I'm guessing they are .007-.008 out of the hole. They certainly are definitely out by looking at them with the heads off.

 

GM 12589227 should work fine. It is a LS2 gasket, 4.020" x 0.051" compressed MLS. My LQ9 and LQ4 both had pistons above the deck. There will not be a noticeable difference between 0.040" and 0.045" range, IMO.

 

Your pistons will probably be a variety of distances out of the hole like ~.005 to .008

Engine builder should provide a build sheet with exact details for each piston if the builder is on top of the game in my experience.

 

Nothing wrong with what you're doing, You will have an iron LS2 and that is an excellent reliable combination.
I used the 706 heads on my combo because I had them lying around it was years ago and I hadn't heard of anyone putting them on a larger bore in a low RPM application so I was curious to see how they would perform.

Standard replacement head gaskets are usually around 0.048" That's a good place to be.

Sounds like this thread is about all but buttoned up. The only thing we need now is for some of the lurkers that have been reading along to post some crappy cell phone idle videos of their setups in the hopes of convincing you to buy a larger cam. I'd be happy to tee things off haha

 

You are correct an earlier IVC will not like being heavily loaded at very low rpm. I generally keep my 383 over 1,500 rpm under load. IVC is about 51.5°. Above 4,000 rpm my IVC is 61.5. With the weight it is lugging around there is little point to running it below 1,500. I let it upshift through 4th by about 40 mph at light throttle and then leave the converter unlocked until ~52 mph. It will run down the road at 1,500 rpm at 55 mph smoothly. If I push the pedal enough to unlock the converter (40% throttle @ 60) its at 2,800 rpm almost immediately and accelerates smoothly even in overdrive, push it even more (60% throttle @ 60) and it drops to 3rd and pulls quickly and effortlessly up to whatever speed you want to run (80-85 mph speed limit in places in TX) back off the throttle some, converter locks and just cruise. On a two lane road running 60 mph, smack the pedal to the floor to pass and it drops to 2nd and takes off like it was rear ended, shifts to 3rd @ 5,600 at 90 mph and keeps pulling. On flat land it cruises at 75 mph at ~2,200 rpm at 15% throttle with a ~60 KPA MAP reading.

 

Fast355 your poor little pinky toe must get sore putting around in that thing all day

 

That is why I have cruise control, lol. It wants to run much faster than is legal or I am even comfortable running it. At 6,200 lbs it will effortlessly run 120 mph if it weren't for the bend in the road or sudden stop at the end. I really find myself in no rush on road trips now. Often find myself in the far right lane rolling 65 in a 75 just taking the surrounding. More to a roadtrip then getting there as quickly as possible.

 

For those still questioning high dynamic compression ratio pulling a heavy vehicle around. This series of screenshots is data I recorded while tuning on my 383. I started at the bottom of a ~6% grade roughly 1 mile long and pulled up to 75 mph without entering PE and maintained 75 mph up the remainder of the grade. Even with the 4L85E shifting way earlier than it now does, ZERO detonation present on the pull which put a fairly sustained moderate load on the engine.

 

In my experience if a high compression setup is going to ping it is going to be near peak torque and while running near stoichiometric air/fuel where the cylinders are at the highest cylinder pressure and the mixture is burning the hottest. My torque peak is at 3,200, so running 2,800 its pretty close to maximum VE and the throttle is open enough that the MAP is near atmospheric for most of this pull.