Hey guys:
I'm getting my sh*t together for a SBO stroker build and looking for 10.5:1 compression ratio; but my calculations are coming up with a CR of 10.09:1 with a deck height of .004". This yields a squish of .042".

If I want a CR of 10.5:1, is it OK to reduce the squish or should I mill the heads for a smaller chamber size? The ProComps heads have already been milled to 70cc chambers, but I think they can be cut a little more. Here's my math on this, let me know if you see any errors. Thanks!

CR = V1 + V2 ÷ V2
V1 = Displacement of one cylinder
V2 = Compression Volume

V1 = 3.14 x (1/2 Bore) x (1/2 Bore) x Stroke
Bore = 4.125”
Stroke = 3.5”

V1 = 3.14 x (4.125÷2) x (4.125÷2) x 3.5

V1= 46.7741 cid

V2 = PistonDishVol + HeadVol + DeckVol+ GasketVol
PistonDish = 4.8 cc 0.2929 cid
Head Chamber = 70 cc 4.2717 cid
Cut Deck Height = 0.004” 0.0554 cid
Gasket Thickness = 0.038” 0.5265 cid
Gasket Diameter = 4.2”
*divide CCs by 16.387 to convert to cid

V2 = 0.2929 + 4.2717 + -.0554 + 0.5265

V2 = 5.1465

CR = (46.7741 + 5.1465) ÷ 5.1465

CR = 10.09 to 1

Squish = Gasket Thickness + Deck Height
Gasket Thickness = .038"
Deck Height = .004"

Squish = .042”

 

Have you already purchased the pistons? If not, use flat tops. Otherwise, you will need to mill the head. Forgetting the "squish", you still want to maintain a piston-to-head clearance of at least .035

 

Cut the heads, they can be milled a bunch before you run into problems.
Plus cutting the heads will lower the intake port, it'll be easier to port match that way.

Simple formula for figuring compression;

Bore x Bore x Stroke x 12.87 = cc.
Then Cylinder volume cc / head, dish and gasket cc +1. Like this;

4.125x4.125x3.5x12.87= 766.46
Chamber=70cc, gasket=9cc, deck and dish approx. 5cc =84

766.46/84=9.12+1=10.12.

Cut the heads about .020.

 

Jim:
I haven't bought anything yet, I'm working out all the details before getting out the credit card. I'm budgeting for some off-the-shelf Chevy pistons from Icon (recommended by Mark-CutlassEFI). They have 4.8cc valve reliefs and cost $600 w/1/16" ring pack. Custom flat-top pistons will cost over $1000 and put me over my budget. I don't think cutting the heads will cost $400 so that seems to be the better way to go.
Rodney

 

I agree.

 

I've read in some other threads with iron heads a .005" cut reduces the chamber size by 1cc. Is this the same with the ProComp heads? Mark, you suggested cutting an additional .020". Will that make the chambers 66cc? Thanks!

 

I've found that about .006-.007 = 1cc on Procomps. A tenth of a point here or there won't matter.