First, I'm building a Chevy 355.  I have a Comp Cams XE268H cam (Dual pattern 268/280 @.006" tappet lift, .477/.480" @ .050').  I purchased Sealed Power pistons H345NP (Hypereutectic 4VR) on the basis of advertised compression ration of 9.5:1.  Once my head came out of the dark crevice of nature, I realized my compression ratio would be more at .030" over.  I intended to use the L31 Vortec heads on this engine, but after some math, I believe my compression would jump to approximately 9.6:1, which is too much. 

Now, I'm considering the use of AFR 180cc heads with the 68cc chamber.  This would drop my compression to approximately 9.2:1, which is ok with me.  BUT, now I'm concerned with the "overscavaging" of the exhaust with these heads.  My question is this:  If I increased the rocker ratio from 1.5 to 1.6 on the intake side only (from .477" to .509" lift), would this solve the scavaging problem, or create other problems?

Allright Mark, slow down and take a breath. There's a couple things you can do way short of new heads.

The 9.6 with the Vortec and 92/93 octane should be OK. Lets face it you were expecting 9.5; really an addition tenth ain't much and certainly won't be crazy compared to 9.5. Now if you computed 9.5 and got 10.6, then there's space to be concerned.

You can always kiss the Vortec chambers with a grinder and whip out a few ccs, just by smoothing the rough cast surface. The simplest and least expensive way is to mess around with head gasket thickness. You didn't say what gasket you were using. They come from the coated shim at .015 inch compressed to compositions at .051 compressed, this is for the ZZ4 but will work just fine sandwiched between slabs of cast iron. You can always use a copper gasket. They range from .031 to .062 and these can be stacked to dial the compression in right where you want it, they can even be combined with a convention steel shim or composite gasket.

So your not the owner of the disaster you think you have. Go back run the numbers with these dimensions and see where your at. Also, don't forget the clearance volume between the top of the piston and the head deck, often there's something there as it's unusual for a piston to be zero clearanced here.

Bogie

Hey, thanks for helping me pull my head out!  I guess I was so wrapped up in head selection, the gasket thickness NEVER crossed my mind.  My figures had been based on a .041" compressed thickness.  I found a copper head gasket made by SCE, which is "Reliable, Reusable, and Race Ready."  For $92 / pair, they better be.  The 4.060" bore and .062" thickness dropped the compression down to approximately 9.1:1, which should be perfect for the Vortec heads and my future gas expenses.  Speaking of expenses, since you helped me save about $600, where do I need to mail the check?

Hey MarkyB,just to let you know I built a 355 last summer and installed it in my '84 Trans Am.It ran fine on pump gas(ok on mid-grade,better on premium)with just a little tweaking of the base timing.I haven't got max performance out of it yet because the car was totalled with 1700 miles on the engine!It is now slated to go in the '72 Chevelle I bought to replace T/A and will probably benefit from different carb and distributor setup(had computer controlled carb and Quadrjet on T/A,will have true hei and edelbrock carb on performer rpm manifold)my compression ratio is approx 9.5/9.75 and have had no detonation problems ! Good luck!