kylerj133

Im building a turbo motor that will be going into a 72 chevy k10 with a 5 speed manual. and have a few questions about compression ratio goals and head selection.. I have 5.3l bored to ls6 spec, compstar 6.125 rods, stock crank, arp mainstuds and headstud, wiseco -11cc pistons, comp cams custom grind 230/232 duration at .050 on a 115 LSA, with .627/.627 lift..
piston comes out of hole .008"

My question is what compression ratio should I shoot for and what thickness of head gasket and head selection?? my 2 head options would be 317 heads which should be close to 9.01:1? with .040" thick gasket?? or 243 heads which I could run a thicker gasket (.075") to get it close to the same as the 317s... I would rather use the 243s if I could.. Im just not even sure what is a safe compression ratio and stay strictly on pump gas for the streets and possibly meth kit if i want to really turn it up

goal is a safe 600rwhp but able to push it if I wanted.. somewhere around 750-800?

thanks in advance this is my first post on this forum so I hope I dont sound retarded lol

svslow

Assuming you mean a 3.898" bore, a 243 head should put you around 9.4:1 with an LS9 head gasket. Would be 8.7:1 with the 317 head. I would go with the 243.

newschool72

iTrader: (12)

Im not one of the gurus on here, but I will give a little friendly advice based on what I did on my setup. Be careful of quench area on the 243s if you go that route. Although I couldnt find any real data when I made my decision to use a "fat" head gasket to control static compression, it seemed most agreed .050-.075 quench is the danger zone for detonation. With a .075 gasket on the 243s you would be around .067 quench. If the rumors are actually true, then you might have issues with that setup.
Just so you know what I wound up doing....I had a bone stock LS3 long block with 10.7 comp. I sold my LS3 heads, bought a set of 317s off here for a song, ported them myself to 220cc intake/89cc exhaust, cleaned up the combustion chambers to 73.5 cc, and I ordered a set of Cometic .097 MLS gaskets. With my pistons (stock flat tops) about .006-.008 out of the hole, I got outside the so called danger zone to about .090 quench and dropped compression from 10.7 to 9.12.
So far everything seems to be doing great. The engine has plenty of snap at low RPM cruise out of boost and although Ive only made just under 7lbs of boost so far, the plugs show no signs of detonation. I have the boost issues fixed now (I hope) and as soon as its back together , I am shooting for 12 lbs on straight 93 pump and a big A to A. My goal is somewhere in the 650-700 whp range.
Hope this helps a little. Good luck !

stevieturbo

Anywhere in the 9-9.5 range would be a very safe CR. Could you go higher ? of course, but I like safe.

With the pistons out of the hole like that, you're maybe into aftermarket gaskets, although a GM MLS at around 51thou would be preferred. Simply because they're cheap and they work.
That still gives you 43 thou clearance which should be fine though.

And use the best flowing heads you can get.

kylerj133

it ended up being a 3.903", Ls9 gasket?

kylerj133

according to one of the previous posts it should be around 9.4:1 I'm still going to play around with a compression calculator... So if that's in the "safe zone" of running 93 octane how much boost I can I through at it? I know the tune alone has a lot to do with it...

I also have the SS 2.020int. and 1.600ex I'm going to try and get the most out of these stock heads

stevieturbo

There are many variables....boost should never be a target anyway.

I'm 9.0:1 and can easily run 20+psi on pump alone. Although on the road I've no traction to do so so boost is usually lower. and when racing and there is some traction I usually use meth too ( although not always, just depends really )

kylerj133

Sounds like you got yours figured out even with using a thicker head gasket. I'm still thinking about using the 317 heads I have, I just don't want the compression to be too low and be a dog out of boost..

truckdoug

iTrader: (25)

go with whatever head & gasket combo gets you the correct quench. the combo stevie suggests is great. .040 is money.

.067 quench is bad news bears IME motors with too much clearance run like **** no matter how much timing or fuel you throw in (or take out) 'em

kylerj133

Okay cool, thanks guys for being such big help! after spending alot of money I just want to make sure to get it right the first time

looks like my plan is to go with the 243 heads and .051" stock mls head gasket. someone has mentioned a ls9 head gasket should I get that? or just one that fits my bore for ls6/ls1? I see cometic has them for a 3.910" bore

newschool72

iTrader: (12)

I think a tight quench is the end goal of any well built engine, but Ive yet to see an example of quench being a make or break on a good running engine. my 6.2 runs really strong at low throttle inputs. Out of boost, Im hard pressed to tell a difference in the power or response between this engines last NA set up and the way it is now.

NA: LS3 long block appox .043 quench, 10.7 comp, 220-230 at .050/629-629 108+4, GMPP dual plane , cranking compression 228psi

Boosted: LS3 short block, ported 317 heads, .090 quench, 9.12 comp, 225-229 at .050/585-585 114+2, Holley single plane, cranking compression 168psi