Building an engine for turbocharged duty is, for many, uncharted territory. Boost is a familiar cornucopia of manifold-topping superchargers, but the notion of having its source plumbed inline with the exhaust is foreign. While the end result of both—positive intake pressure and a smile-inducing horsepower surge—is the same, turbocharging an engine does have some unique requirements that should be factored into a build. It’s easy to think that boost affects parts just from the pistons down, but nothing could be further from the truth. An engine is an ecosystem, and changing any one part affects the food chain from top to bottom. Here are 10 quick tips to make your next boosted build, whether small-block, big-block, LS, or LT, a fruitful one.
Connecting Rods and Crankshaft
It could be said that any power-adder application could benefit from a bolstered bottom end, and that would be true. But, with the power potential of even the most cost-effective turbochargers, a connecting rod upgrade is a wise move. When boost comes into the equation, cylinder pressures spike exponentially and most stock connecting rods, especially those with high mileage, are not up to the task. Often, it is not the connecting rod itself that fails but the fasteners, which were never designed to deal with the cylinder pressure or rpm a turbocharged application can offer.
For low boost applications (6-8 psi), stock crankshafts—especially in the LS family—have proven more than adequate. Turbochargers are by nature progressive and typically don’t have the power and torque spikes associated with nitrous and superchargers. This trait spares the crankshaft shock-loading that could be potentially catastrophic. That said, if your power goals are over 500 horsepower, or competition use is planned, investing in a forged crankshaft for your application should be considered a must.
Boost is the great equalizer. It can take a small port, lousy valve angle, and low-lift cam and blow gale-force winds into the cylinder. While a better flowing cylinder head will always move more air—boost or no boost—it is important to consider your power goals and budget. For example, Edelbrock’s Performer RPM head flows a solid 253 cfm at 0.500-inch lift and is a performance bargain at around $730 each (assembled from Summit Racing). There are more expensive, better-flowing heads on the market, but with a few psi of boost force-feeding the runners, budget heads can generate serious power. Rather than ultimate airflow potential, which is far more crucial in a naturally aspirated application, consider price, material, and deck thickness, which is critical to sealing high cylinder pressure.
Head Bolts, Studs, and MLS Head Gaskets
Boost is no good if you can’t keep it in the cylinder where it belongs. Performance head bolts, such as those from ARP, are a good start; head studs are better. Studs are not subjected to as much torsional (twisting) force as head bolts. Metal is extremely strong in tension, which allows head studs to generate improved clamping force over bolts. As boost goes up, studs become the preferred option.
Modern aftermarket head gaskets are light-years ahead of the gaskets of the muscle car era. But when it comes to boosted applications there is no replacement for a good MLS (multi-layer steel) piece. MLS gaskets use multiple layers of embossed steel to seal combustion in the cylinder. Their design, due to the spring rate of the compressed layers, can actually compensate for a small amount of cylinder head lift as combustion attempting escape pushes the head away from the deck surface. One consideration with MLS gaskets is surface finish. For proper sealing, both the block and the cylinder head deck surfaces must be machined very smooth. Most machine shops are capable of this, but is absolutely worth a dialogue with the machinist to make sure.
Rockers, Pushrods, and Valvesprings
It’s easy to think of boost as only affecting the pistons down. In fact, the valvetrain is equally affected. When the intake valve opens and pressurized air rushes into the cylinder, the backside of the valve is also pressurized. As the valve begins to close, boost pushes against it, making the work of the valvespring more difficult. For this reason, it is often necessary to install a stiffer valvespring capable of effectively closing the valve on schedule.
The exhaust valve also sees additional load. When the spark plug fires, pressure is created in the cylinder, which drives the piston down. But, before the piston reaches bottom dead center, the exhaust valve opens. For example, a popular LS cam grind has an exhaust opening point of 83 degrees before bottom dead center (BBDC). That means the exhaust valve is actually opening against combustion pressure, which acts on its face, attempting to hold it closed.
That force travels up the valve stem, through the rocker arm, and into the pushrod. While this occurs in all engines, the higher cylinder pressure associated with turbocharged engines exerts additional pressure on the valvetrain that needs to be accounted for. A thicker pushrod is a good start and Billy Godbold of Comp Cams likes steel rockers for these applications. Steel rocker arms are more fatigue resistant than aluminum rockers and, for the price, are typically stronger.
Pistons and Rings
Cast pistons are the equivalent of time bombs when it comes to boost. It’s not that they don’t have the strength, as many factory pistons are surprisingly strong, but their inability to tolerate detonation—something that will inevitably occur in an aftermarket turbocharged engine. Late-model engines have knock sensors that are precision-tuned from the factory. When knock/detonation is detected, the ECU can retard timing to lower cylinder pressure, eliminate the knock, and protect the rotating assembly. Few aftermarket engines have that luxury. Instead, moving to a forged piston, which is significantly stronger and more resistant to detonation, should be considered a must.
Proper piston selection is more involved than simply lowering the compression ratio. A piston designed for boost will have more material in key areas. In boosted pistons, the top ring land is moved down on the piston crown, which helps protect it from the heat of combustion as well as creates more rigidity in the land itself. Also, piston material comes into play. Forged pistons are typically made from two alloys: 4032 and 2618. Forged 4032 pistons have more silicon in their makeup and do not expand as much as 2618 pistons, making them great for street engines, which experience a wider temperature variance and need to start cold. Forged 4032 pistons are perfect for mid-level builds but lack the ultimate strength offered by 2618 pistons. Forged 2618 pistons are ductile and forgiving in harsh, high-horsepower environments but are softer and wear quicker than 4032 units.
Piston rings in turbo engines will need more end gap than in a comparable naturally aspirated engine. Because forced induction puts more air, and subsequently more fuel, into the engine it will also generate more heat, which causes the rings to expand more. When choosing ring material, carbon-steel rings, as opposed to the gray iron often found in cheaper and older, stock ring sets, is a preferred option. Carbon-steel is much stronger, resists detonation better, and does not need to be made as thick, which reduces friction against the cylinder wall.
In a turbocharged engine, careful attention to camshaft selection can pay huge dividends in power, torque, and driveability. Because positive pressure in the intake manifold (boost) is force feeding air into the cylinders, a turbo cam can often be very mild in comparison to a naturally aspirated grind, needing less lift and duration to accomplish a similar horsepower goal. Also, because there is inevitably backpressure between the exhaust port and the turbo’s turbine wheel, careful attention needs to be paid to valve overlap. Too much overlap for the application can cause exhaust to backflow into the cylinder and heavily dilute the air charge.
“Honestly, the boost to backpressure is what we really need to know to pick the camshaft,” said Comp Cam’s Billy Godbold. “A cam in the 270’s (degrees duration) at 0.050 with a 110 LSA might be right on a system with very little restriction, and very little backpressure.”
Large turbo, low-backpressure systems found on, say, a high-horsepower race car will be much more tolerant of high-overlap camshafts. This is why many tuners have found success with nearly stock cams in high-backpressure street turbo applications as they offer very wide lobe separation angles and very minimal overlap. High backpressure may sound unappealing, but such a pressure ratio can be useful in creating a turbo setup with excellent throttle response and minimal turbo lag—perfect for a street car.
Fueling a turbocharged engine always requires more octane than a comparable naturally aspirated engine. There is a multitude of ways to accomplish this. Premium pump fuel when boost, ignition timing, and intake air temperature are kept in safe ranges is the most convenient—but probably the most power-limited. E85 (ethanol-based) fuel, which is oftentimes cheaper than gasoline, though less readily available, is another great alternative.
E85 has a higher latent heat of vaporization than gasoline, meaning it can help pull heat out of the air charge and has a 100-plus octane rating—though that can fluctuate slightly depending on the mix, which is rarely 85 percent ethanol, 15 percent gasoline as claimed. E85 has a stoichiometric ratio of 9.75:1, which is lower than gasoline (14.7:1) and means it will take a larger volume to achieve the same horsepower level as gasoline. E85 does have some cooling benefits that gasoline does not. Also, whenever boost is employed, a variable-rate fuel pressure regulator will be required to keep fuel pressure equal to boost pressure and avoid leaning out the tune as boost rises.
Boost, whether delivered from a blower or turbo will inevitably heat the intake air as a byproduct of compression. Hot air is less dense, which means less power, and is more prone to detonating. In order to quell the risk of detonation and improve power, it is ideal to remove the heat. This can be accomplished a few ways. Water/meth injection, such as kits supplied by Snow Performance, spray a fine mist of water and methanol mix into the intake air stream. As the particles of water and methanol shift from liquid to gas (known in physics as a phase change) they absorb energy. This sucks heat out of the surrounding air particles and can radically cool the intake charge. More conventional forms of intercooling, such as air-to-air intercoolers rely on airflow over a bar-and-plate heat exchanger to pull heat away from the air charge.
Air-to-water intercoolers are similar to air-to-air except they employ a liquid medium. In some cases, this is an ice bath, which is incredibly effective at removing heat but is impractical for a street car due to space requirements and a constant need to replenish the rapidly melting ice.
In carbureted and aftermarket fuel-injected applications especially, ignition timing is a major consideration. Distributors are a great means of transferring spark energy to the cylinder but they are quite dumb. No offense intended, but distributors don’t receive any feedback from the engine—nor would they be equipped to deal with it if it did—and are ignorant of any knock occurring. For this reason, it is paramount to have an intelligent ignition device feeding a signal to the distributor that can detect boost and retard the ignition advance respectively. MSD’s programmable 6AL, when coupled with a MAP sensor, does a great job of this. Most aftermarket ECUs can accomplish the same feat, and factory ECUs, when paired with a MAP sensor capable of reading boost (2 bar and up) are also able to keep timing in check.
Fuel Injection vs. Blow-Through Carb
This is the big-ticket item, and the one that frightens a lot of old-school carburetor aficionados: to inject or not to inject. It all comes down to control. Blow-through carbs are not the black magic they once were. They work well and have the added benefit of chemically intercooling the air charge. The low-pressure zone created by the venturi, along with the latent heat of vaporization induced as gasoline is atomized at the top of the intake plenum draws significant heat out of the intake air charge. The downside is that carbs are dumb. They often don’t start well when the engine is cold and they aren’t particularly forgiving of altitude and ambient temperature changes. Comparably, fuel injection is smart, it can adapt to changing conditions and alter fuel delivery accordingly. In the big picture, especially when boost is in the mix, it is the better option and yields superior driveability compared to even the best blow-through carbs, though it does often come with a bit of a price premium. CHP
Photos by Evan Perkins and courtesy of the manufacturers