VAC Forced Induction Headgasket

[mleveroni]

Anyone know anything about these headgaskets?

According to them the stock gasket is .03in thick and they offer a .08in thick gasket. Was curious to see if anyone had an idea of how much this would drop compression.

https://store.vacmotorsports.com/vac...s54-p2215.aspx

[heinzboehmer]

Thought it would be fun to quickly calculate this:

For a stock S54 the bore is 87mm = 8.7cm, stroke is 91mm = 9.1cm, and compression ratio is 11.5:1.

Compression ratio is defined as the ratio of the total volume in the cylinder chamber at BDC to the total volume in the cylinder chamber at TDC. In other words: compression ratio = total volume at BDC / total volume at TDC. Since we know the bore and stroke, we can find the (approximate because pistons tops aren't totally flat) volume below the top of (or in) the block pretty easily. volume in block = pi * (bore / 2)² * stroke = 540.692cm³.

Now, the total volume at BDC is going to be equal to the sum of the volume at TDC and the volume in block. Combine that with compression ratio formula and you get this:

compression ratio = (volume in block + total volume at TDC) / total volume at TDC = volume in block / total volume at TDC + total volume at TDC / total volume at TDC = volume in block / total volume at TDC + 1.

Now, we know that the total volume at TDC is 51.494cm³. Assuming those head gasket thickness numbers are when torqued, the 0.08in head gasket will be 0.127cm thicker than the stock gasket. This results in an increase in volume at TDC equal to pi * (bore / 2)² * head gasket thickness difference = 7.546cm³. With the thicker head gasket the total volume at TDC will be 51.494cm³ + 7.546cm³ = 59.040cm³.

New compression ratio = volume in block / new total volume at TDC + 1 = 540.692cm³ / 59.040cm³ + 1 = 10.158.

So without changing anything else, this head gasket should drop the compression ratio to roughly 10.2:1

However, this will also be affected by the shape of the tops of the pistons (which is why you can change your compression ratio with different pistons), so keep in mind that this is just a rough calculation.

[mleveroni]

Thought it would be fun to quickly calculate this:

For a stock S54 the bore is 87mm = 8.7cm, stroke is 91mm = 9.1cm, and compression ratio is 11.5:1.

Compression ratio is defined as the ratio of the total volume in the cylinder chamber at BDC to the total volume in the cylinder chamber at TDC. In other words: compression ratio = total volume at BDC / total volume at TDC. Since we know the bore and stroke, we can find the (approximate because pistons tops aren't totally flat) volume below the top of (or in) the block pretty easily. volume in block = pi * (bore / 2)² * stroke = 540.692cm³.

Now, the total volume at BDC is going to be equal to the sum of the volume at TDC and the volume in block. Combine that with compression ratio formula and you get this:

compression ratio = (volume in block + total volume at TDC) / total volume at TDC = volume in block / total volume at TDC + total volume at TDC / total volume at TDC = volume in block / total volume at TDC + 1.

Now, we know that the total volume at TDC is 51.494cm³. Assuming those head gasket thickness numbers are when torqued, the 0.08in head gasket will be 0.127cm thicker than the stock gasket. This results in an increase in volume at TDC equal to pi * (bore / 2)² * head gasket thickness difference = 7.546cm³. With the thicker head gasket the total volume at TDC will be 51.494cm³ + 7.546cm³ = 59.040cm³.

New compression ratio = volume in block / new total volume at TDC + 1 = 540.692cm³ / 59.040cm³ + 1 = 10.158.

So without changing anything else, this head gasket should drop the compression ratio to roughly 10.2:1

However, this will also be affected by the shape of the tops of the pistons (which is why you can change your compression ratio with different pistons), so keep in mind that this is just a rough calculation.

Wow, thank you for the explanation! I tried messing with some calculator online but did not know all the inputs. I understand its not an exact number but I'm surprised that it would be that significant of a change. I am eventually planning on low compression pistons but I was considering this as a cheaper alternative for the time being. Now the question is whether this is a reliable part or not, I know some prefer to stay away from VAC.

[eastcoastM3]

Good one cheers!

[Rudbari]

Good one , now is head gasket good for rise up 2 to 4 psi if your FI

[digger]

Wow, thank you for the explanation! I tried messing with some calculator online but did not know all the inputs. I understand its not an exact number but I'm surprised that it would be that significant of a change. I am eventually planning on low compression pistons but I was considering this as a cheaper alternative for the time being. Now the question is whether this is a reliable part or not, I know some prefer to stay away from VAC.

VAC dont make them they get someone like cometic to do so and then charge you a markup ($175 gasket $220 for arp = $395)

https://www.cometic.com/applications...2--198ci32l-i6

[Mspir3d]

I put a JE athena gasket into my engine. There wasn't a need for me to drop compression but you can get them with a copper spacer.