Compression Ratio

[1QWK96GT]

Can someone help me figure out my compression ratio. I want to run a D1 around 20psi or so. I already have a B headed 4.6 4v that I plan to build. I got some forged H beam rods that are stock length. I stumbled across a really good deal on some stock bore forged ross racing -16cc dish forged pistons. My questions what would my compression ratio be with -16cc pistons and would that yield me with a good compression ratio for a D1 on 91 pump gas. I was thinking that if I do not get the compression ratio I want maybe I could compensate with a thicker or thinner head gasket. Anyway what are your thoughts? Anyone else running a similar setup?

Thanks in advance,

 

[Helomech74]

Stock bore, stock stroke and -16cc pistons assuming all other numbers are generic mod motor numbers, then your compression would be in the area of 8.6:1 which is horrible for a centri blown car. The car will be sluggish while out of boost, I'd seriously consider getting your compression up around stock or a little higher. I run 10.3:1 cr, F1A Procharger and 17psi of boost on 93 octane with no issues at all. I run a J&S Vampire, so I'm able to see if the car is knocking and I check my plugs constantly.

 

[1QWK96GT]

Thanks for the information.

Just curious, -11cc Pistons would be ideal?

Also how do you figure the compression ratio? I found a calculator online but I wasn't sure of all the different dimensions it was asking for.

http://www.csgnetwork.com/compcalc.html


Enter Bore/Stroke Designation Type
1 = Inches 2 = Millimeters
Enter Cylinder Bore Size
Enter Piston Stroke Length
Enter Head Gasket Bore Diameter
Enter Compressed Head Gasket Thickness
Enter Combustion Chamber Volume In CCs
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-')
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') :

 

[1QWK96GT]

Enter Bore/Stroke Designation Type
1 = Inches 2 = Millimeters= 1
Enter Cylinder Bore Size = 3.552in
Enter Piston Stroke Length = 3.543in
Enter Head Gasket Bore Diameter = 3.552in
Enter Compressed Head Gasket Thickness .030in
Enter Combustion Chamber Volume In CCs = 52cc
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') = -16cc
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : = .003in

Would these be the correct numbers assuming a stock engine?

Thanks,

 

[98cobra#1222]

I run -11cc pistons on my turbo car which lands me around 9.3:1 compression ratio which feels just fine to me off boost. Not sure how different it would be compared to a centri car though. Honestly if I was doing it over again I would probably elect to bump compression up a bit, say 9.5 or 10.0:1. 9.3 is as low as I would ever consider going.

 

[Helomech74]

Enter Bore/Stroke Designation Type
1 = Inches 2 = Millimeters= 1
Enter Cylinder Bore Size = 3.552in
Enter Piston Stroke Length = 3.543in
Enter Head Gasket Bore Diameter = 3.552in
Enter Compressed Head Gasket Thickness .030in
Enter Combustion Chamber Volume In CCs = 52cc
Enter Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') = -16cc
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') : = .003in

Would these be the correct numbers assuming a stock engine?

Thanks,

Piston deck clearance on a 4.6 is supposed to be .008, but it can change based on piston choices and how much the block is decked, etc. Also, Head gasket bore diameter is 3.65 and gasket thickness can vary according to which ones you use, I usually use .036" gaskets.