Compression Ratio Calculation
#1
Compression Ratio Calculation
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”
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”
#3
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.
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.
#4
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
#5
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 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.
#6
cumbustion chamber size
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!
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