Building Engines For Forced Induction
Pressurizing a cylinder through either supercharging or turbocharging is an excellent method of increasing horsepower, but it could very quickly create problems if certain issues are not addressed. A stock engine that was designed for normal aspiration can successfully accept forced induction if the desired power gains are not too lofty. But boost seems to fall into the category that if a little is good, a lot must be better. A pulley change or a turn of a screw by an exuberant owner is all that is usually needed to step up the boost level, with the power output following in lock-step. That is until you reach the limit of the engine components and disaster strikes. There have been volumes written on installing and tuning forced-induction systems concerning fuel flow and ignition control, but little if any information is available on what needs to be done internally to an engine to survive the rigors of boost. The art of balancing engine longevity and high specific output with forced induction is rooted in theory and is not application-specific to any particular make or design.
Recognizing this, the following information will lay the required foundation to integrate forced induction to any internal combustion engine. Unlike other traditional modifications such as camshafts and cylinder-head changes, which tend to respond differently in each case, pressurized induction has almost no inherent limitations on power production. Interestingly, compromises that are normally made between rpm and power levels are nearly eliminated with a properly designed and matched blower.
Recognizing the needs
Boost pressure is defined as the amount of intake manifold pressure above the accepted value assigned for atmospheric pressure of 14.70 psi. Thus, an engine operating with 10 psi of boost pressure is actually using 24.7 psi of pressure to fill the bore on the intake stroke. Well-established mathematical equations have confirmed that an engine's performance is proportional to the mass of the air inducted per cycle. This in turn depends primarily on the inlet air density. The output of an engine of a given displacement can then be increased by compressing the inlet air prior to entry into the cylinders. Methods for achieving higher inlet air density in the performance industry are limited to the use of either a turbo or supercharger.
Though it is common for horsepower to be referenced as a qualifier of an engine's output, cylinder pressure is what creates power. As the measured output of an engine is increased, so is the amount of cylinder pressure. Forced induction is very effective at raising cylinder pressure through increases to air density, pressurization above atmospheric conditions, and elevating the cubic feet of air throughput of the engine. In addition, horsepower is heat, so as the output of an engine is increased, so is the amount of heat generated.
Due to this, aspects of the engine that need attention are rooted in its ability to withstand the higher cylinder pressure while maintaining the seal between the block and the cylinder head. An OE design is not usually engineered to accept the doubling or tripling of the specific output that forced induction can easily create, but in many instances it can he easily modified to survive under these conditions.
Since each component of the engine shares similar but unique concerns In regard to being adapted for forced induction, they will he discussed separately.
The engine block is required to hold all of the power produced, so it becomes a key element to success. If possible, earlier manufactured blocks are better candidates than new models when dealing with domestic V-8 engines. The latest versions were lightened as a means of increasing fuel economy to meet government standards. They have a tendency toward increased flex and bore distortion over their older brethren. With either block, resist the temptation of excessive overboring, the theory being that it is better to retain mass to limit bore distortion than add a few cubic inches. A forced-induction system can very easily make up for the smaller displacement with a minute amount of additional airflow and boost. If a larger displacement is desired, a better approach would be to increase the stroke.
The deck surface should be milled for flatness and O-ringed for increased cylinder gasket sealing. There are two schools of thought for O-ring placement: the block or the cylinder head deck. Some argue that an O-ringed block is the proper approach, since this region is more stable and does not flex and lift as the cylinder head does. In contrast, advocates of cylinder-head O-ringing believe that the increased sealing needs to be located there, since the head does move, and this measure will help keep the head gasket in place. Head gasket failure usually occurs from either excessive boost pressure, detonation, or a combination of the two.
This does not usually occur until boost levels approach 10 to 12 psi. O-ringing requires cutting a receiver groove in the block deck around the perimeter of each bore. The depth of the groove is critical since it will be required to hold a soft copper wire. The wire needs to lie in the groove, with a certain portion extending above the deck surface. When the cylinder head is installed with the head gasket, an increase in clamping force is experienced around the combustion chamber due to installation of the wire.
Fel-Pro markets a receiver-groove head gasket (not for BMWs sadly) that uses the same theory but requires the groove be cut into the deck of the cylinder head. The wire instead is integrated into the manufacture of the head gasket. These gaskets are only offered for small- and big-block Chevrolet, small-block Ford, and Buick V-6s.No Bimmers are catered for as far as I am awarea but stock gaskets can be modified to do the job....
Some OE engines, such as the Datsun 280ZX Turbo, used gas-filled O-rings to seal in cylinder pressure. The ring would drop into steps at the top of the cylinder bores and stand higher than the head gasket surface of the block. When the head was torqued, the gasket and seal were compressed, making for a leak-proof installation.
Regardless of the method chosen, if higher boost levels are desired, wire or gas-filled O-rings need to be installed. Align-honing is often a neglected procedure during the building of a normally aspirated engine, even though it should be performed. This technique guarantees that all the main caps are in line and that the crankshaft will run true to the block. In every engine, the crankshaft flexes as it turns from the uneven firing impulse's of the cylinders and the pressure created; these motions are amplified with forced induction. By align-honing the main caps, the crankshaft will be under less stress during its gyrations.
When any part that is made from metal is forged, cast, machined, drilled, or welded, a stress is induced from the procedure. When stressed, the molecular structure of the metal is rearranged and either weakens or distorts the piece. An engine block is no exception and is harboring residual thermal stresses from the manufacturing process. If left unchecked, this stress will either cause the bores to lose their concentricity or the block to crack.
To eliminate these concerns, the block should be cryogenic stress-relieved by a competent specialist company. This is accomplished through a controlled process that freezes the part in nitrogen gas to -300°F and then slowly warms it to room temperature. During this time, the molecular structure will realign and neutralize any internal stress. This is a relatively inexpensive procedure, with the cost determined by the weight of the block. Not only can it save a block from destruction, but, in most instances, it allows the successful use of a production block on a high-output forced-induction engine.
It possible, the use of a cast-iron crankshaft should be avoided; a cast-steel crank is the least acceptable type. A forged crankshaft is the material of choice, and on a very high-output (1,000 horsepower or over) race engine a billet crankshaft would be required. If a used crankshaft is considered, then it should be magnafluxed to check for cracking and, if in good shape, nitrided, hardened, and shot-peened. These procedures will add strength and serve a boosted engine's needs well. It your budget allows and one is available, a good after-market forged crankshaft from a reputable manufacturer should be considered. Be wary of low cost aftermarket crankshafts that seem too good to be true. The market is being flooded with inferior-quality components from China which suffer from poor metallurgy and are made from recycled materials. Do not be afraid to ask where a component is forged, cast, and machined. Often these suppliers hide behind the fact that they do finish machining in America, but the part is brought in from overseas. A poorly made crankshaft can cost you a complete engine if it fails under high-boost pressures.
Regardless of whether a new or used crankshaft is being considered, it should be able to withstand the power level anticipated by the choice of blower and, in addition, be cryogenically processed for stress relief.
Lower-horsepower applications can retain stock connecting rods if a small amount of work is invested in them. The big end of the rod should be resized and the small end bushed so that a full-floating pin can be installed instead of a press-fit pin. This will free up a little horsepower, but more important, it will reduce heat in the pin bore region. By polishing the beams (sides) of the rod, stress risers are removed and the part is strengthened. Aftermarket rod bolts such its those made by ARP need to be installed. The rod bolt is the most critical part of the assembly and absorbs most of the force as the crankshaft rotates.
Aftermarket connecting rods of high quality are a good alternative to reworking used OE pieces. The same concern for quality that was established for the crankshaft also pertains to the connecting rods. Either new or used connecting rods should he exposed to cryogenic processing for stress relief.
Most stock replacement pistons for earlier applications are cast aluminum, while late-model engines use hypereutectic designs (not manufacturer specific info). Cast pistons should not be used with forced induction since they lack the strength needed to withstand the additional cylinder pressures and the possibility of detonation. OE hypereutectic pistons do not have as much silicon as the aftermarket designs, and as a result they are brittle and break easily if detonation occurs. They should be replaced with forged units.
The need to use a lower compression ratio with forced induction is usually met with the piston crown design. Dished pistons that mimic the shape of the combustion chamber are the current trend. Full-floating pins should be used, and the pin bore should be placed as far down from the crown as possible. Applications that have the pin bore migrate into the oil ring area should be avoided with boost pressures above 10 psi.
Piston coatings, though not new to the motorsports industry, are finally being seen as worthy additions to a forced-induction engine. Swain Tech Coatings apply a ceramic thermal barrier to the crown of a piston to reflect the heat of combustion back into the expansion event while keeping the piston and ring package cooler. The coating not only allows the engine to produce more power, but limits the possibility of detonation and decreases brake-specific fuel consumption. In addition, a dry film lubricant should be applied to the piston skirt to help protect it against scuff from the high temperatures experienced during boost.
A forced-induction engine does not require any special attention to ring material, and there are many reputable sources that can supply your needs. A concern does arise in regard to ring end gaps with high-boost engines, and manufacturers should always be queried on their recommendations.
Choosing the wrong cam profile is the most common mistake when building a forced-induction engine. Often a grind with an excessive amount of overlap is used by the novice and results in disappointing performance. The valve events are extremely critical with either a turbocharger or supercharger; if they are timed incorrectly, boost pressure will not build up but will instead blow out all open exhaust valve. Lobe separation angles of a minimum of 116 degrees should be used, and often the best results are produced with values of 120 degrees or greater.
Since a forced-induction system pressurizes the intake track, there's little problem filling the cylinders with charge. The issue comes when the bore needs to be evacuated on the exhaust cycle. For this reason, the camshaft should have at least 10 degrees more duration on the exhaust lobe than the intake side to allow the increased cylinder fill to leave the bore. Lift profiles should be determined through flow bench test results, as would be with a normally aspirated engine.
The cylinder head selected can either make or break your forced-induction program; the added cylinder pressure that a boosted engine experiences makes quenching detonation a number-one priority. Aluminum's heat-dissipating ability is a major factor in adding octane tolerance and is the material of choice. A modern combustion chamber design that swirls the incoming charge and then uses a squish pad to establish internal charge acceleration will limit abnormal combustion. Likewise, the ideal cylinder head will have the spark plug oriented as near the center of the bore as the design allows, with the electrode facing the exhaust valve.
Both the intake and exhaust ports need to be efficient but, as stated previously, the exhaust port is of greater concern than the intake side. For this reason, install the largest exhaust valve possible and concentrate on low-lift flow efficiencies to help evacuate the spent gas. The higher the velocity of the exhaust during blowdown, the quicker a turbocharger will spool up. An efficient blowdown cycle will also limit the pumping losses of the engine with a reduced amount of work performed by the piston on the exhaust stroke. The thickness of the cylinder-head deck needs to be considered since it will add rigidity to the casting and in turn seal the cylinder bore better. This was well established by the popularity of supercharging the early Ford 5.0 EFI Mustang engines. These castings got to be known as "the flexible flyer," since their light weight allowed the head to twist and lift and blow the head gasket during high boost.
Just like the pistons, any cylinder head can benefit greatly by the use of Swain Tech or similiar Coatings. A common practice is to use a ceramic thermal barrier on the combustion chamber, valve face, exhaust port, and quench region to increase flame travel, retain heat, and reduce the possibility of detonation. The exhaust-port coating is especially interesting. By limiting the amount of heat that transfers into the water jacket, the cooling system does not work as hard. An additional benefit is that it also increases the exhaust velocity, since a hot gas looks to travel to a cooler region.
Valve springs somtimes need attention because a forced-induction engine usually requires a higher spring rate since they are working against boost pressure. High boost pressures tend to blow the valve open with symptoms that would be similar to valve float, bucking, popping, and backfiring. Lower hp targets are normally fine with stock BMW springs, certainly on M3's anyway.
High-strength performance cylinder-head bolts or studs need to be used to help contain the increased cylinder pressures. ARP is an excellent source for these parts and markets quality engine fasteners for almost any horsepower level.
Often the most neglected part of a blower engine buildup, some thought needs to be given to the cooling system for a successful project. Thinking out of the box then becomes necessary. Conventional coolants, even when modified by additives, still leave much to be desired. An interesting product does stand alone in the industry: Evans NPG coolant. Designed not to boil until 369°F at zero pressure, it does a superior job of cooling the combustion chamber and allowing high boost levels by limiting detonation. Working as a stand-alone product, the cooling system needs to be filled with the Evans product alone - no water is required.
Building a forced-induction engine that can survive on the street or during the rigors of competition is not necessarily complex, but does require a certain level of attention to detail that most overlook in this hobby. The paradox of a boosted engine is that it has two distinct personalities and has different requirements for normally aspirated and boosted conditions. Using the tips provided here, the joys of forced induction can be applied to any engine.
Published in Hemmings Rods & Performance. June 2001