Help with my 351W Build

[GrillMaster]

Building a mild OBDII motor

'85 Block was machined 10 thousandths from deck and cylinder bores, running ross forged flat top pistons. What would my compression look like with some GT40 heads from a (’91 Lightning). Been having some trouble with the compression calculator.

Bore used 4.010
Stroke used 3.5
dome/dish used 4 (gustimate for valve reliefs)
CC 63-66 (used 64)
gasket .039-.041 (used .031 to compensate for .01 off the deck)
deck 0???? 9.503" decked .01 (used 0)

http://www.summitracing.com/expertadviceandnews/calcsandtools/compression-calculator

Bore: (diameter)
4.010

Stroke:
3.5

Cylinder Head Volume:
64

Effective Dome Volume:
Use (-) for Dome and (+) for Dish.
4

Deck Clearance:
0

Compressed Gasket Thickness:
.031

Number of Cylinders:
Compression Ratio : 10.73 : 1
Total Displacement (in.3) : 353.62
Total Displacement cc's : 5797.05

Looks like if I ran a .062 copper gasket it would get me to 10.2:1 which I believe would be ok on the street on pump gas.

 

[Viperwolf1]

With those numbers and a .062" gasket I get a 9.6:1 ratio. Piston height and rod length are variables though.

 

[GrillMaster]

Looks like you made the head gasket bore 4.1, should be 4.01 right? Stock length rods and pistons should be stock height flat tops.

 

[GrillMaster]

your calculator looks different than mine because it uses the actual deck height not the. Did some looking and I show the GT40 CCs at 65.5. Plugged in with a standard gasket i get this.

Bore: (diameter)
4.010

Stroke:
3.5

Cylinder Head Volume:
65.5

Effective Dome Volume:
Use (-) for Dome and (+) for Dish.
4

Deck Clearance:
0

Compressed Gasket Thickness:
.031 (.041 -.010 for the missing deck height)

Number of Cylinders:
Compression Ratio : 10.54 : 1
Total Displacement (in.3) : 353.62
Total Displacement cc's : 5797.05

 

[Viperwolf1]

I put in 9.493" for the deck height. (9.503-.010=9.493).

Head gasket bore is typically 4.1". It is bigger than the cylinder bore. You can't compensate for decking by subtracting from the gasket thickness due to the differences in bore.

 

[GrillMaster]

Ok, thought it may have been a typo. I see on summit you can get gaskets 4.01, 4.03, 4.1 etc. I have always bought the same size gasket as the bore. Maybe it does not matter, I'm new to motor building. What compression would you run on a mild build with iron heads and pump gas... think CA is 91 (premium). Was going to shoot for low 10s.

 

[Joe473]

With those numbers and a .062" gasket I get a 9.6:1 ratio. Piston height and rod length are variables though.

Viperwolf is that a spreadsheet you would be willing to share.? Or is that an online calculator?

 

[GrillMaster]

X2... seems like a good one. Better than the one I was using.

 

[Broncobowsher]

Trying to lower the compression ratio by going fatter on the head gasket kills quench height and generally makes an engine MORE prone to detonation while killing cylinder pressures and that kills torque, thus power. Keep the quench tight. If the compression is going to be too high you need to look at bigger chamber heads or more valve relief/dish.

 

[Viperwolf1]

Viperwolf is that a spreadsheet you would be willing to share.? Or is that an online calculator?

Spreadsheet written by Jason Fletcher. I can't seem to find a link to it now. I've had it for probably more than 10 years.

 

[Broncobowsher]

I made an excel spreadsheet based on https://fordperformanceracingparts.com/download/tipsPDF/EngineBuildingTipsSpecs.pdf

I could go in and change little things like head gasket bore and see that it did make a difference. You really need to know all the piston specs, guessing the valve reliefs and guessing the head chambers turns good math into just a fuzzy guess.

 

[GrillMaster]

New heads are not an option and these seem to be about right where I want them. Don't think I will need the thicker gasket. Just need to confirm the relief volume, deck height and chamber volume. From what I have ready they are 64-65 cc.