compression ratio advice needed

[nawlins-tim]

I have a chevy 350 .030 over with a basically stock bottom end. Flat top pistons with valve reliefs. Got a descent deal on a port matched set of L-98 alum heads and a Vic Jr. intake. I decided to use a bigger cam than I first planned on useing because of the 58cc combustion chambers on the 113 heads. Cam is an isky 284* cam .510 lift 106lsa. All has been well, I expected to possibly have deonation problems but none so far.
Question is what compression do you think I wound up with? And I have been planning trying a head change in the near future and was a little worried what kind of compression change I might get with 62cc or 64cc aftermarket heads? If AFR had a set of 58cc 180cc intake runners I'd get 'em tomorrow-since I know for sure how my motor feels with what I have now.
Any suggestions??!!!!!what head gasket would get me where I would have a strong compression ratio?

 

[megaladon6]

plug you numbers into the CR calc here http://kb-silvolite.com/calc.php?action=comp
just in case--if you happen to switch to iron heads you'll probably need to drop your compression. aluminum heads need about a full point more CR.

 

[megaladon6]

assuming a 0piston volume and deck height with a .045 head gasket for decent quench i get 11.8 static compression. going to a 62cc head gives you 11.2. you also need to find your cars intake closing point to get the real (dynamic) compression.

 

[nawlins-tim]

I would NEVER have figured 11.8 to 1 . I'm not complaining but I'm just surprised to have such good combustion without the least Bit of valve ping. YOU figured 0(zero) piston volume. Did you take into account the valve reliefs may bring it down a little.
Anyway I'd be just fine with a 62cc or 64 cc iron head you think? I kinda like the aluminum though. weight's less and seems to run cooler than any other high performance motor I've ran around town in. However, I'm starting to look for that extra 50 to 100 horsepower i might get with some really flowin' heads. You kno' how that goes. what you got is never enough for long anyway............

 

[86Cutlass383SR]

I, too, doubt it's that high. I'm not doubting megaladon6 math as I use the exact same page for my engine cr figuring.

Okay, you say "basically stock bottom end". If the block hasn't been decked, then the piston is roughly .025" - .035" "in the hole". (My machinist said my block was taller than even the standard 9.025" SBC deck height.) Is the piston a true flat top, or a stock type with the small factory dish? Even a true flat top piston will have 5 - 7 cc valve reliefs. The head gasket thickness can vary from .015" to over .050". The gasket bore diameter can vary alot, too, even if it's for a 4.030 engine.

I used 4.030" bore, .041" x 4.100" gasket, piston in the hole .025", 6cc valve reliefs, and a 58cc head, with a 3.48 stroke, and came up with with a more liveable 10.318 comp ratio.

I aslo plugged in some more numbers. Are those heads a true 58cc or more likely (if actually measured) a 60cc... and with an 8cc valve relief, a stock type .050" thick gasket with a 4.166 bore diameter, and the piston deck at .030" we come up with a 9.51 c.r.

BTW, AFR can/will mill the heads to any cc you want. If you want a 58cc L98 style head, they'll just mill the head to whatever cc you want it at. So don't let that stop you from getting some AFR's.

I figured some more for you. 62 cc (AFR) heads, .025" deck, .018" x 4.100" thick gasket (to keep a good quench of .043"), with a 7cc flat top piston, gives you a real nice 10.31 c.r.

 

[nawlins-tim]

Yeah that is more reasonable. I too figured in the valve reliefs , and wasn't sure of the actual deck volumn. I thought a factory block leaves the pistons down a tiny bit , but was not sure how much. Thanks for the figuring . You know basically I wasn't sure just how much I would loose going from a 58cc head to 64cc or 65cc. And even though it aint much I didn't want to go in the wrong direction you know. I'm lookin' for MORE power.
And as far as milling goes, I know it can be done but changing intake surface geometry scares me. Then who knows what else would be thrown off. Then I'll be forced to check Valve to piston clearances which I have never done. How easy is THAT? Do you use a caliper to mike the clay? Or do you use a dial indicator? seems that it is not easy to check thickness of clay. Or am I thinking out-of-the-box. AFR has a 65cc 180 head which might work. But actually I have been leaning toward an Edelbrock product the performer heads in a 180cc has I think a 60cc combustion chamber. Which is real close to what I want. I want something real close to what I have now (GM 113 castings w/58cc) But in no bigger than a 185cc intake runner and a quality high flowing Aluminum head.. Even used would be ok

 

[86Cutlass383SR]

If you're block hasn't been decked, then it'll be close to .025 in the hole. When you pull the heads, lay a straight edge across the deck and use a feeler gauge to see how far down the piston is. Measure it a few times rotating engine a few degrees at a time, starting about 20* BTDC. The smallest measurment is your deck height. It's not completely accuract but close enough for a street engine. If you have access to a dial indicator, that's even better and quicker.

After you get that measurement, find a compatable head gasket and shoot for a quench distance of .040 - .050. The quench is the distance between the top of the piston and the flat part of the head.

If you have the heads milled, the machinist can tell you if enough was removed to warrant milling the intake. You may also need to run a small spacer between the distributor gasket and the intake. A shorter pushrod may also be needed to keep valvetrain geometry correct.

Piston to valve clearance can be checked by clay before you install the heads. Put clay on piston top, set head on block without the gasket and snug in a few bolts to keep the head in place. Rotate engine, remove head, cut clay and measure. Add in gasket thickness. But, cutting the clay to get a good measurement can be difficult sometimes. Use a really sharp razor blade. Or, you can install head with the gasket and torque bolts. Use valve checking springs in place of some of the regular valve springs. Using a dial indicator, check the total distance the valve moves down to top of pistons at several degrees before and after TDC.