Performance Head Gaskets - Pressure Science

A Look Inside High-Performance Head Gaskets

Bob Mehlhoff Jan 23, 2007 0 Comment(s)
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Whether you're making some stunning numbers with the help of a big-inch small-block, nitrous, or forced induction, higher cylinder pressure demands that the head gaskets are up to the task, or else all that power can be lost in an instant. Fel-Pro and SCE Gaskets have spent years engineering their gasket lines, and without a doubt they tend to work exceptionally well in a variety of applications.

Like all aspects of building a high-performance engine there are choices that, if made properly, will help your engine perform at its best. However, to do this you'll have to be armed with solid information. Yes, it sounds cool to say that you have high-performance head gaskets made from copper, or the latest MLS set on your engine, but before you build your next hard-running bullet, let's explore some head-gasket science and the options available.

Gaskets 101
Imagine standing in a room at atmo-spheric pressure and the pressure on the opposite side of the door is as much as 100 times greater. The door would be on the verge of blowing in. Now add lots of temperature, say about 200 degrees F, and start applying heavy forces to the walls around the room. Seems pretty tough, right? Now add some high wind from a supercharger, maybe a little nitrous, or worse yet detonation. Get the idea? Yes, on a high-performance engine the head gaskets must withstand some tremen-dous forces, so the gasket must be installed properly.

Engines with lots of power may use either copper or MLS styles, but they must be selected specifically for your engine. Full-race aluminum engines with nitrous and superchargers may work well with one gasket and LS-series engines with another. But what we can show here are many of the differences in their construction and application. If you didn't already know, some gaskets are offered in varying thicknesses to change the com-pression ratio too.

Copper Time
Copper head gaskets offer a malleable metal-to-metal combustion seal that easily conforms to the surface. Addi-tionally, a copper head gasket has much higher tensile strength than its composite rival, and copper has a 25-percent coefficient of elasticity, so it's less likely to rupture. Today so much technology is built into an SCE Gasket, especially the ICS Titan copper head gasket. Unlike conventional copper head gaskets, the ICS does not require an O-ring to be grooved into the head or the block. These gaskets use a built-in combustion seal, which blocks lateral flow of combustion pressure. The special coolant seals applied to the copper head gasket are slightly offset from the top to the bottom to trip the load to the most important combustion seal and provide a stronger bond.

Fel-Pro's MLS
On any engine, when each cylinder fires, the head will invariably want to take off and fly away from the engine. Of course this never happens (at least we hope not) because the cylinder head is secured with a series of head bolts (or studs) properly torqued to the engine. Average cylinder pressures are over 1,000 psi on high-performance engines and can be as high as 2,200 psi. Incidentally, detonation can raise that pressure to 3,500 psi and higher. Of course the higher the pressure, the more separation, and the head gasket is unloaded. MLS gaskets are designed to maintain gasket loading under severe cases. The embossed bead on the gasket acts as a valvespring that will expand and recover. The multiple layers and polymer coatings are purposefully located to surround the combustion and coolant passages.

Clamp And Load
Critical to any head gasket's perfor-mance is that an engine will only have a certain amount of clamp load available. This load is determined by the amount, size, and thread type of the head bolts or studs divided across the surface area to be sealed. By using beads, combustion armor, or wire inserts the sealing force can be increased. This improvement in sealing force may counteract another portion of the surface, so like all other aspects of engine building, there must be a balance of engineering to effectively seal the entire head gasket from the forces the engine exerts during operation.

Before You Install
Installing a new head gasket may be easy-simply position it over the dowel pins and press into position-but before you get that far you'll want to be certain that you've prepared and checked the surface correctly. This means you'll want to first clean the surfaces of both the head and the block and check them for straightness. For a high-performance engine the maximum out-of-float, as measured with a straightedge and feeler gauge, should not exceed 0.0025 inch.

With traditional performance head gaskets the surfaces should be very clean and in the 60-100Ra (rough average) range. Fel-Pro's PermaTorqueMLS typically requires a surface of 30 Ra or smoother. Before installing any performance head gasket it's best to check with the manu-facturer for the suggested surface finish as well as aluminum and iron applications.

Follow Procedures
The wrong torque value or improper torque method can cause head-gasket problems. Obviously if the head gasket is not torqued properly between the head and the block, combustion gases and engine fluids can leak past the gasket. To achieve the correct clamp-load, the head bolts or studs must be stretched properly. This will not occur if the fasteners have too much friction during installation. Before installing and torquing any head bolt, coat the threads with the manu-facturer-recommended lubricant.

Remember to:
*Use hardened washers
*Tighten head bolts with a smooth action.
*Follow the torque pattern.
*Use the bolt manufacturer's lubricant.
*Torque aftermarket fasteners to themanufacturer's recommendations.

Retorque Time
In many high-performance applications it's highly recommended to retorque the head bolts or studs after the first engine run and complete cooldown. To do this, break each bolt to the just loose position (about 11/48-11/44 turn back) and retorque the bolt slowly. SCE Gaskets recommends using a traditional beam-type torque wrench to monitor the fastener strength closely.

Figuring Your Compression Ratio
An engine's compression ratio is defined as the ratio between the total volume above the piston at BDC, and the total volume above the piston at TDC. In equation form it looks like this:CR = (A+B) B


SCE Gaskets
Lancaster, CA 93534


How To
A Look Inside High-Performance Head Gaskets
Bob Mehlhoff Jan 23, 2007


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