The quick-and-dirty description of an engine’s internal combustion process is suck, squeeze, bang, blow. But that’s not entirely accurate. In reality, it’s suck, squeeze, bang, lift, blow.
The lift in the second sequence is when the cylinder head actually lifts off the deck of the engine block during the combustion phase. It’s subtle, measuring from .001” on the low end and as much as .003” on the high limit. Multiple factors go into head lift, and there are a number of ways to deal with it. To find out more, we caught up with Ron Rotunno, product manager for performance products with Fel-Pro gaskets in Skokie, Ill.
Rotunno explained that head lift can stem from a number of factors. Combustion pressures, more specifically high-compression engines, power adders such as nitrous, turbos and superchargers, engine architecture, proximity and number of head bolts, size and torque values and the internal structure of the head all can play a role in head lift. Detonation and preignition also exacerbate head lift.
“Cylinder head lift occurs in any internal combustion engine, be it a gas or diesel engine,” Rotunno said. “When the cylinder pressure builds in a particular cylinder, the force wants to push the cylinder head up off the deck of the engine.”
That is, of course, less than optimal in a racing application where you’re trying to contain combustion pressures as much as possible to maximize power output. Typically, we see it in lightweight engines that produce a lot of power for their displacement. You know, racing engines.
The phenomenon became more prevalent in the early 1990s when OEMs began using lightweight materials and hardware to save weight. Lower head bolt torque values also play a part because increased torque specs on head bolts can distort the cylinders and create piston-ring sealing issues.
Head lift is normal, and you can’t eliminate it, Rotunno said. Using cylinder head studs is superior to head bolts, and racers have been adding them to their engines for decades, but it’s rare that a car comes from the factory with head studs. Japanese OEMs began addressing head lift and pioneered the use of multilayer steel head gaskets because they have a spring-like quality to them. Today an estimated 98 percent of new engines are designed with MLS gaskets as standard equipment.
“If you look at an MLS head gasket and think of it as a spring, given the design of the gasket, you’ve got multiple layers of embossed steel that have a rubber coating on them. Those embossed layers stack up to form a spring,” Rotunno said. “As that head lifts, the spring is expanding and it’s maintaining the contact pressure between the head and the block that maintains the seal.”
We tend to think of the process in slow motion, but think about what it’s like at 5,000 to 7,000 rpm, which is where most racing engines operate for 25, 35, 45 minutes at a time — or in the case of endurance racing, hours at a time. Sustained high rpm creates an almost constant rhythm of head lift across the mating surfaces. When you look at it that way, it seems almost violent.
“Don’t think of head lift as cylinder one, two, three, four. It’s 10 different places on cylinder one, 10 different places on cylinder two,” Rotunno said. “So it’s not like as that cylinder fires everything lifts on that cylinder and then slams back down. You’ve got varying amounts of head lift per cylinder based on where it’s at in relationship to the head bolts, for instance. Or if it is a cylinder that is on the side where it adjoins another cylinder, that could be another key to where there is more head lift.”
Fel-Pro engineers begin the process of creating MLS gaskets by examining each engine and identifying problem areas. Once they know where head lift occurs in a given engine, they can plan for that in the design of the replacement part. The company uses softer elastomeric coatings on the steel layers for stock replacement parts, and a little harder compound such as Viton for racing applications.
Racing gaskets require a lower RA number on the deck and head than stock replacement parts. RA stands for roughness average, which is represented by a 1-to-100 scale. Lower numbers, such as a 30 are smoother than, say, a 60. Generally speaking, a 50 RA surface on the head and block will suffice.
The task of sealing combustion falls on the embossed bore bead, which encircles each bore opening and the stopper layer if so equipped. Head gaskets featuring laser-weld technology take on a “belt and suspenders” approach where the stopper acts as the primary combustion seal and the bore bead acts as the secondary. The stopper also prevents embossed layers from being over-compressed. Fel-Pro laser-welds its stopper layer to the central or “shim” layer of the gasket.
Assembly technique also is important to combating head lift, Rotunno said. Fel-Pro recommends installing the cylinder heads, torqueing them and letting the engine sit overnight. In the morning, break them loose one at a time in the correct sequence, then torque them back down. If you’re working with V8 or V6 engines, torqueing the heads with the intake attached and torqued also helps mitigate some of the causes of head lift.
“There’s not a lot you can do about some of the factors,” Rotunno said. “But luckily head gaskets are a pretty darn good way of compensating for some of that stuff. You’re going to have some level of head lift, and a head gasket is all that’s really separating you from having trouble or not.”