spend $25 and have Pat G spec out the cam. he takes all that into account and knows his stuff. his cams always do well, especially with the LSL lobes. Comp on the other hand seems very hit and miss wether or not they have any idea what they are talking about.

 

I believe some of the early L92's found in the Escalade's had hollow intake valves, there was a small epidemic of them snapping the face of the intake valve off and obliterating a piston, cylinder, and head. I believe GM said the valve springs were too strong and was causing the problem, I replaced 2 of them that did this in '08, unfortunately GM made us DESTROY the engines beyond any possible use and recycle them, that really sucked, I had hopes of a free 6.2.

 

The reason the head breaks off is due to loss of control of the valve and it hitting the piston due to not enough valve spring or it bouncing off the seat due to not enough spring pressure.
OR
I bought a 07 take out that broke a intake valve at the keeper under 2000 rpm with 250 miles on the car.
This engine had a 150lb spring on it and we had a bunch of people look at the valve and it appears that it was just a bad valve.

I bought a new set of ls3 and installed the same springs on them and put 4000 miles on them with no issues.

GM had some bad valves out there. One failed in the keeper groove.

A 150 -400 spring should be no problem for a ls3 valve. If you have a problem it is just a bad valve.

Dads is a daily driver and mine goes 7500 with no issues same springs and same valves.

One thing we did do is put Ferrea exh valves in both engines due to the waffer hardened tip for the rocker arm starting to seperate on the stock gm exh valves. Have been told this is found in all ls valves that have miles on them. Would this cause problems? I was not willing to find out for $150.

Bottom line if you control the valve you should not have issues. I would rather have a little more spring pressure than not enough.

Tim

 

I thought I would add to this post to correct some information and maybe help out the next guy.

821 and 823 heads are NOT the same casting.

We just developed a CNC program based on the 821 head. We ran it on an 823 head to see how it went and this is a short list of the differences.

Exhaust port is larger on the 823. Our program that cut almost 80 percent of the surface on the 821, maybe cut 50 percent of the surface on the 823.

Stock height of the casting is bigger on the 823. The 823s I have right now, about 4 pair, all measure 4.755, while the 821 measures 4.732 which is the spec listed.

Area around guide on 823 is larger It seems that the space between the guide and the wall on both the intake and exhasut is larger on the 823 casting.

Some of these differences in size might be as little as a couple of thousanths, but the point is, is that they ARE different. We flowed both heads after they were ported and even though the 823s had less material taken out, they still flowed as well as the 821s.

I will try and get some pictures, but my 821 head I have has had so many test cuts done on it, I don't have a set of ports left to show the comparison.

 

I was lol'ing reading this thread. Sucks because it makes it confusing and difficult for people trying to learn.

Your right about Comp tech support. I called about my nitrous jettings question and I got one guy that seemed more like sales man reading stuff dropped the call and got another guy (after another 15 mins on hold) and he was helpful.

 

I thought the ls3 head had some sort of swirly do dad hump in the intake runner? ( that is a technical term) or am I thinking of lsa heads??I run the l92s with the heavy valves and it turns to 7k no problem with comp 918 springs and 230/238 575 585 Cam with an 88mm turbo.

 

LSAs have the bump / tab thing your thinking about. Ls3s and L92s have a bump in the runner, but it is the bottom of the valve pocket.

 

I weighed the L92 valves, mine were 109grams. Thats really NOT overly heavy,compare that to some aftermarket stainless stuff. However the 91gram hollow LS3 valves would be better for a 7000+rpm engine.
If you are running a plastic intake, you DONT need the hollow valves.

 

i think the swirly thing you guys are refering to is the "dorsil fin" found in the ls7 casting that helps induce swirl. i do think lsa heads have this as well as being roto casted.

as far as head choice on an lq4 being a dished piston, i would go with the cathedral heads and deck the crap out of them... gm rates the l92 heads at 292 in stock form on a 4 inch bore at .600 lift. only problem with l92s is valve shrouding. they both will make great power, but the ls3 intake is a better design than the 90mm ls2. the cathedral heads have greater port velocity than the l92s for a 4 inch bore. cubes isnt the only thing that affects these heads, but the cylinder bore size makes a big difference. hence why aftermarket castings give a cubic inch range, but notice how they call them large bore heads. gm rates the l92s at 313 at .600 for the ls3 (bore 4.065). thats over 20 cfm at .065. cam should be properly speced for the heads. ls3/l92 doesnt need as much intake duration as the 243 give that the intake valves are much larger 2.165 vs 2.00. match the heads with the cam and either combo is killer... the overall power wont be tremendously different