ZDEN

i'll be putting the radix on in jan. and was thinking about doing some heads to lower compression. are the 317's to big?

vanillagorilla

iTrader: (1)

No sir, especially if you're cammed and uping the boost. It'll drop your static CR to a boost friendly 8.5:1 too. I plan on doing this to my 4.8 when I find where 3rd and 4th gear went.

Even if you don't up the boost, you'll be able to run the pee-water cheap gas.

99Silver6.0

iTrader: (1)

It will drop the CR. But valve shrouding is gonna become a problem. Bore sizes are quite different.

ZDEN

i was wondering if valve shrouding would be a problem. i was thinking 317's because there is a set for sale on here, maybe i should just pick up some ls1 heads.

vanillagorilla

iTrader: (1)

The 317's have a 2.00" intake valve. You'll be fine with the 4.8 bore, as long as you don't go any bigger on the valves. So what's the valve size on an LS1 head....

natural1

The LS1/6 have a 2.00 int. / 1.55 exh. (same as 6.0L). The 6.0L/317 head is a lot like the LS1/6 heads only with a larger combustion chamber. I believe the 6.0L head has flow characteristics similar to the LS6 head due to the D shaped exhaust ports.

LS1 - 66.70cc 12559853 (to '01)/12564241 (after '01) casting #
LS6 - 61.15cc 12564243 casting #
6.0L - 71.00cc 12562317/12572035 casting #

vanillagorilla

iTrader: (1)

Exactly my point.

TurboGibbs

iTrader: (6)

The 317's I have in the shop have a combustion chamber width of 3.930. They are used on a 4.000 bore with the 6.0L of course. The 4.8 uses a 3.78 bore. You will have a stepof .075 on each side of the bore. That will cause some shrouding and some turbulance which may result in poor fuel mixture within the chamber. The extra low compression would probably counter act/reduce the signs of problems. Just my opinion. I would not run a larger pulley on the blower than a 3.0 with that low of compression due to the loss of low end torque you will have. I'd want the blower pushing boost as soon as possible. Although I am currently running a 3.0" pulley on my 4.8 with stock heads and am about to try a 2.75". The mods in my sig may help you out some.

vanillagorilla

iTrader: (1)

Yeah, there will be some shrouding, but the turbulence isn't as bad as you'd think. It probably wouldn't be too bad with the flat tops of the 4.8. 5.3 guys have run them for awhile now on thier engines and the 5.3 has the same bore as the 4.8. 5.7's run them too with awesome results and they only have a 3.898" bore. It's obviously not the most ideal way to drop compression, but its the cheapest and easiest means of doing so.

kbracing96

iTrader: (26)

I think 317's would work well, that's what F8L Z71 was running on his 5.3 STS truck running 14.5 lbs of boost and low 12's. (same bore as 4.8)