Re: 290 vs. 820 Heads
IMHO 290 is the true correct head for an LS5 .
Here's what Mortec one of the chevy motor/blocks/heads/casting id info websites states:
3964290....69-70...oval..CLOSED..396, 402, 427, 454, 101cc chamber
Large or small hex spark
3993820....71......oval...OPEN...402, 113cc chamber, 255/114 ports
================================================== ===========Now ack
OK,now back to my take on this.
In 1970 bbc's l34 396 & ls5 454's came with 290 small chamber lrg oval port heads & approx 10.25.1 compression.
The larger chambered 820 lrg oval port heads were used on 71 bbc's due to emmissions laws for 71 which dropped compression from 10.25.1 to approx 9.0 comp.
The only big difference bewteen the the 290 & 820 heads is mainly the combustion chamber size being larger and that its now also an open design that may breathe a bit better. But both the 290 & 820 heads are roughly comprable/= perf wise given same compression & all else like cam/intake/carb etc being = too. So if you use the proper larger domed pistons to get the compression back up in a 71 bbc motor the 820 heads will perf just as well as the 290s do on a 1970 or older bbc l34 or ls5 motor with stock pistons.
Or if in your case you 70 ls5 moor still has stock pistons with stock dome design/size you can simply remove the 820's and install a set of 290's and your good to go.
The reverse of that would be for example if you had a 71 bbc that came with 820 heads you can simply install the smaller chambered 290 or 063 or 215 heads all being small comb chamber lrg oval port head designs with closed chambers to get the compression back up to the factory rated 10.25.1. Thats because those 71 motors still had the same dome size & design pistons that were used in the 1970 l34 & ls5 bbc motors . It was just the larger chambered open chamber design of the 820 heads that lowered the comp & helped the 71 bbc's meet the newer emmissions regs for those motors in that timeframe.
Now onto the supposed 820 head 2/70 casting date,maybe the prior owner could have mistaken a 3 or an 8 for a 2 because 2-Feb 70 seems kinda early for the larger 820 heads that were not supposed to neever be on any 1970 l34 or ls5 pass car motor to be going onto them.
I have been working on thses motors for just shy of 40 yrs and have never seen 820 heads on a known/verified 70 bbc. But i have obviously not seen every bbc motor built in 1970 either so i guess you never know what may have happend at the factory esp with respct to a very late built 1970 bbc pass car motor.
But possibly if the date on your 820's is correct maybe gm was already mfg the 820 heads in 2/70 to meet truck emmissions regs that can be stricter/comming in a yr ealier then for the pass cars and thats why you have 820 heads dated 2/70. Or who knows,maybe GM was simply casting the 820 bbc heads months ahead in 1970 to get a jump on the 1971 builds which could be possible,crazier things have happend over the yrs.
But normally the earliest you should be seeing 820 heads on the chevelles/pass cars would be for the new 1971 pass car's bbc motors for early production of new 1971 cars built in mie-late july or early august 1970 for early built 71 pass cars to hit the sales floor.
You really need to pull the valve covers and very that head casting id & date code for yourself and if its truely 2/70 cast date look into if either trucks got those heads in 70 or if gm was casting the heads months earlier then normal to get a jump on things which i have never heard of happening but again,you never know.
1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
2002 MAXIMA (DAILY DRIVER/1 owner,GOT 3/2013 w-44k miles)
2009 HD ELECTRAGLIDE CLASSIC ULTRA (GOT 11/14 W-9,700 miles)
Last edited by SWHEATON; Mar 25th, 10 at 11:18 AM.