I had been ignoring geordi's thread, thinking it was the traditional bolts vs studs (ARP) argument seen on every forum - after reading thru, I think I may have some input that may be of some interest on this naturally-aspirated Diesel engine with tacked-on turbocharger
This epiphany resulted from only recently seeing a removed cylinder head (cylinder-side view) and gasket
For your consideration -
Most Diesel engines with cast-iron heads\blocks get composite gaskets with copper or steel edging around the cylinders, water passages, and bolt-holes -
this construction is very stable with cast-iron head and block assemblies - modern composite gaskets are OE-coated with appropriate sealer to meet spec'ed application
Hi-performance hi-output engines can get all copper gaskets, way better but very expensive
They also usually have 6-bolt pattern around the cylinders, with adjacent cylinders sharing one or two bolts between them, pattern-dependent
Some, usually indirect injection type, have 5-bolt pattern, adjacent cylinders sharing one bolt - the pre-cups can eliminate one bolt per cylinder
These patterns are very stable thermally and are suitable for n\a and turbocharger service with little change in assembly components, usually
only bolt tension and head gasket change for the resultant turbocharged higher cylinder pressures and temperatures
The VM 2.8L engine was obviously designed for normally-aspirated service, having only a 4-bolt pattern without between-cylinder bolts
This is not
even suitable for turbocharged service, and is further aggravated by having an aluminum head - aluminum absorbs heat and gives
up that heat easier than cast iron, with result of being much less stable than cast-iron in high heat service, as in turbocharged
A look at the head gasket reveals large areas of cylinder circumference not directly clamped by head bolt tension
- likely they calculated that the small-diameter cylinder could be effectively clamped with only four bolts for n\a service
- maybe they assumed thermal stability for turbocharging by increasing coolant flow thru the block and head
- likely the ECM was programmed specifically to limit turbocharged power output
- the layered\sandwiched steel gasket construction stabilizes installation between aluminum head\block
assemblies, preventing head movement in the horizontal plane
- the steel further allows thermal conduction head to block
The head cylinder-surfaces are evenly exposed to combustion chamber temperatures - some heat is evenly removed by engine coolant in the water
jacket across the cylinder area and surrounding the intake and exhaust valves
The aluminum head on the intake side is further cooled by intake air-flow, with less expansion\contraction than the exhaust side, and the bolts are
exposed to heat at normal engine coolant levels
The aluminum head on the exhaust side is further exposed to turbocharged exhaust temperatures up to 1800*F and higher, resulting in extreme
expansion\contraction of the aluminum across the exhaust ports, with conduction of that higher heat into the head bolts on that side
- exhaust-gas temperature is much higher at the exhaust-valve inside the port than at the exh manifold outlet to the turbocharger, which is higher
than exh gasses entering the turbine, even considering fully-combusted exhaust gasses which quickly lose combustion heat
- black smoke results in even higher temperatures thru-out resulted by increased mass of the unburned fuel, which is soot, which retains
combustion temperature far longer than fully-combusted exhaust gas
This should result in some loosening of the exhaust-side bolts directly adjacent the cylinders, not so much in the outer exhaust-side bolts,
being further from combustion-chamber heat conduction
I would suspect the head gaskets would be oem-coated with a hi-temp sealer with a heat-activated adhesive cement component because of the
aluminum material with only a 4-bolt pattern, but whadoIkno, eh
- or, maybe gasket-coating is to be done at assemby-level with an oem-supplied sealer
I would think that ARP studs, rather than bolts, with non-oem gasket(if oem is not cement-stabilized)would be one preventive solution at tear-down
- a composite gasket would likely be unsuitable because of some insulative quality, limiting additional thermal conduction into cylinder
block surface, with it's larger coolant-jacket
When studs are fully seated to specs in the block, stud and block threads are fully engaged, resulting in reduced upper-cylinder deformation under tension
- spec'ed head-compression tension is fully achieved by torquing the nut on the unmoving stud threads at the top end, rather than twisting the entire
bolt\threads into the block to spec'ed tension while torquing
- this becomes highly critical in this engine with only four (count'em: 4) bolts surrounding the cylinder
- a precision deck girdle is machined for racing engines, which is bolted to the block deck and all head bolts torqued to spec - only then
will the cylinders be final bored, honed, and fitted to the pistons and rings, thus guaranteeing perfect-circle seal and reduced blow-by for maximum
power output
IMO this is one turbocharged engine where BOOST pressures, EGT's and ECT's should be critically monitored when towing, also when enhancing
performance levels for whatever purpose
The OE ECM programming and TCM shift-patterns serve to protect the engine from critical abuse -
I further suspect that pilot-injection is a major factor that has improved this normally-aspirated but turbocharged Diesel engine's survivability quotient