Hello! . I have a lq9 with the factory 317 heads. I’m putting a cam in the truck this weekend (truck Norris) valve springs, and long tube headers. The stock heads have already 2 broken exhaust manifold bolts and I thought to replace them. I was reading that 706 heads will give more low end torque which is were I’m at almost all the time. Buttttt it would put the compression ratio up to over 11 to 1! What is the limit of pump gas? I have e85 available so that could be an option. Should I keep the 317 heads and try to get the bolts out and port them? Any help is appreciated, thanks!

 

Are you sure your math is right?

I'd shoot for 10.5:1 but wouldn't be scared of 11:1

 

Usual go-to for that, is 243/799 heads. Small bump in compression, not near as much as the 706/862, but usually manageable. Pretty common in the buzzard nests on mid-late 00's trucks.

 

don't forget that SCR is only part of the equation.

i don't remember the specs (and ain't digging them up right now), but that cam is probably increasing valve overlap a bit... and that reduces your Dynamic Compression Ratio - which affects overall cylinder pressure.

off-hand assumption is that cam'd run great with 799s, premium octane (91+), and a good tune. if i remember right, my 10.6 SCR heads + big cam + 13psi of boost make ~8.8 DCR, and i don't knock tuned for 93 & no meth (and technically usually running 91 at 3000-7000ft elevation around here right now).

i'd guess the 799s would leave your DCR close to on-par with your stock setup while maintaining the top-end flow 317s are known for. 706s would bump CR up a notch, increasing responsiveness / efficiency, but have less top end due to the smaller ports & valves (unless you worked them up). the higher compression would also leave a little less tuning margin.

 

706's, 243s work great on a 6.0 and run reaaally freakin good. High compression and awesome torque it'll fly from idle through wherever you cam dies off at. built this combo more than once for poor boy performance parts out of the boneyard.
They also wake up 6.2's in trucks in the area where most daily driven trucks work at.
Dont care what the numbers are the above is great stuff it in and roll parts that the factory didn't really intend to be mixed and matched.
It's the modern day equivalent to the 70's when you could throw a set of 2 barrell 305 heads on top of a 350 and make a screamer out of a smog turd

 

if it was a LQ4 with dished pistons, i'd immediately jump to 706s (which have smaller combustion chambers than the 243/799 castings)... but he is already starting with LQ9 flat tops, so they already bump the compression some, and the 799s would flow better on the top end plus provide better tuning flexibility due to the slightly lower compression than the 706s.