Click on a thumbnail to see the full-size image
Engine Sealing: High Performance Head Gaskets
Stock head gaskets hold up well enough under normal loads. But as compression ratios and combustion pressures go up, a stock head gasket may not be able to maintain a leak-free seal.
By Larry Carley
Page 1 of 2
One thing no engine builder wants to worry about is a head gasket failure, whether it is a NASCAR cup engine, a Pro Stock drag racing engine, a dirt track engine, a hot street engine or even a stock engine. Stock head gaskets hold up well enough under normal loads. But as compression ratios and combustion pressures go up, a stock head gasket may not be able to maintain a leak-free seal. And once hot combustion gasses start blowing past the gasket's combustion armor, bad things begin to happen. The combustion armor cracks or burns through, compression is lost and the gasket fails. Or, oil and coolant start finding new ways to circulate inside the engine, often with negative consequences.
Over the years, performance engine builders have used a variety of tricks to keep their engines sealed such as using wire O-rings around combustion chambers and running solid copper head gaskets. Various kinds of sealing solutions have also been developed by aftermarket gasket manufacturers for sealing high performance engines such as high temperature graphite and non-asbestos composition gaskets with special coatings and beefed up combustion armor, head gaskets with stainless steel or copper wire rings inside the combustion chamber armor for added reinforcement, and head gaskets with oversized wire rings and combustion chamber armor that function similar to O-rings.
One of the hottest products to emerge in recent years for high performance engines are Multi-Layer Steel (MLS) head gaskets. MLS gaskets first appeared on many Japanese engines in the late 1980s and early 1990s, and were later adopted by Ford for its 4.6L V8 family of engines, and Chrysler for its 3.5L V6 and newer Neon engines. General Motors also uses an MLS head gasket on its LS1/LS6 family of engines.
What may be news to some of our readers is that most of the top NASCAR teams are now using MLS head gaskets, as are most Pro Stock drag racers, many circle track racers and even many street performance machines (including turbocharged and nitrous-boosted sport compact cars). Performance MLS head gaskets are now available for most popular engines, including small block/big block Chevys and Fords (including factory racing block versions of these motors), also Chryslers, Pontiacs, Oldsmobiles, and imports such as Honda, Mitsubishi, Nissan, Toyota, VW, etc. Consequently, MLS head gaskets have become the gasket of choice for many forms of racing because they have what it takes to handle the pressure without leaking, blowing out or burning through. More on MLS head gaskets in a minute.
EVOLVING HEAD GASKET DESIGNS
Anyone who has shopped around for high performance head gaskets knows there are a lot of different styles from which to choose. Some performance head gaskets use a traditional composition construction with a soft graphite or non-asbestos facing material on both sides of a solid steel or perforated steel core. The soft outer layer provides conformability for a good cold seal while the steel core provides rigidity and strength. Most have some type of outer coating such as silicone to improve cold sealing, or an anti-stick agent to improve lubricity and make removal easier. Many head gaskets also have raised elastomer sealing beads printed on the surface to concentrate loading in key areas.
Other performance composition head gaskets use a somewhat different approach and sandwich a layer of graphite between two layers of steel. The outer surface is coated with silicone or a similar material to improve cold sealing.
The facing materials used in performance head gaskets may be the same as those in a stock head gasket, or they may use a higher temperature version of a similar non-asbestos material, or graphite. Graphite can handle heat well and has natural lubricity, but it is a soft material and must be handled with care.
Another characteristic that also distinguishes performance head gaskets from ordinary head gaskets is stronger combustion chamber armor. Most use a high grade stainless steel armor which may be stronger and/or thicker than the armor on a stock head gasket. For higher horsepower applications, a stainless steel or copper wire ring may be placed around the combustion chambers inside the combustion armor. This provides added support for the armor, concentrates loading around the cylinders, and helps the gasket maintain a tight seal under high combustion pressures.
Adding a pre-flattened ring around the combustion chamber raises the sealing force to roughly three times that of a standard head gasket, and allows the gasket to withstand combustion pressures from 1,500 to 3,000 psi in engines generating over 1,000 horsepower.
Some performance head gaskets have oversized wire rings around the combustion chambers that act like O-rings to improve sealing. These require machining receiver grooves in the head or block to accommodate the wire.
The point at which an upgrade to some type of high performance head gasket usually becomes necessary is when an engine's power output approaches two horsepower per cubic inch.
COPPER HEAD GASKETS
For many years, serious racers have considered copper head gaskets to be the hot setup for high horsepower engines. Most top fuel dragsters and funny cars run copper head gaskets because nothing else works. But that doesn't mean copper head gaskets are the best sealing solution for all types of racing. They aren't for a variety of reasons. But they do offer certain advantages over conventional head gaskets:
Copper head gaskets are great for extremely high compression ratios (over 14:1), turbocharged or supercharged engines that are running lots of boost pressure (over 15 psi), or engines with nitrous oxide kits that add an extra 150 to 200 or more horsepower.
Copper conducts heat much better than most other metals, which helps disperse and stabilize head and block temperatures. This helps prevent hot spots that may cause detonation, preignition and/or head warpage, and it helps make engine tuning easier. It also reduces the risk of the gasket blowing out or burning through.
Copper has a 25 percent coefficient of elasticity which allows it to stretch before it fails. So if an engine goes into detonation because the fuel mixture leans out, there's too much spark advance, or there's too much compression and not enough fuel octane, copper provides an extra margin of safety.
Copper is also strong. The alloys used for copper head gaskets may have a tensile strength of up to 32,000 psi, which is many times that of the facing materials used in conventional performance head gaskets.
And copper gaskets are reusable - at least for a limited number of times. This is a plus in situations where the heads are on and off the block between races, or frequent tear downs are required.
One of the drawbacks of plain copper head gaskets, though, is that they do not seal oil and coolant very well. If the engine does not contain any coolant, coolant leaks are not an issue. But for engines that do run coolant, getting a copper head gasket to cold seal and maintain a seal can be a challenge. The gasket must be coated with some type of sealer, and both mating surfaces must be absolutely smooth, flat and clean.
Though many people think copper is a relatively soft metal, it does not provide much conformability. On one hand, this is a good thing because the gasket doesn't crush when the head bolts are torqued down. Consequently, the thickness of the gasket remains the same and does not change. On the other hand, the gasket may not conform very well to small indentations and surface irregularities in the head or block. So some type of sealer coating must be applied to both sides of the gasket - unless it comes pre-coated with sealer or has raised elastomer sealing beads printed on its surface. Several companies make coated copper gaskets that reportedly seal as well as any other gasket, and maintain their seal. The gaskets are also reusable provided the sealer isn't damaged when the gasket is removed.
Copper can be annealed (softened) by heating it in an oven or with a propane torch, and some racers do this to improve conformability. But heating also causes a certain amount of oxidation which may weaken the gasket and increase the risk of it cracking. Some gasket manufacturers do not recommend annealing copper head gaskets. Others say it is okay provided the gasket is not reheated more than three times.
The best way to anneal a copper head gasket is in a controlled environment such as an air-tight vacuum oven. The gasket should only be heated until it is a dark red (about 900° F) and no more. After the gasket has air cooled, the surface needs to be cleaned with a brush or abrasive pad to remove oxide from the surface. The gasket should then be cleaned with brake cleaner or a similar product and allowed to dry before it is recoated with sealer. Also, the sealer must be allowed to dry before the gasket is installed. This may range from 20 minutes to overnight depending on the type of sealer product used. Some aerosol sealers may require multiple coats for best results. RTV silicone also works, and may be applied around coolant openings in the gasket, block or head. But only a thin coating should be used, and it must be allowed to set before the gasket is installed.
Page 1 of 2