Build Turbo Kits

Learn How To Build Turbo Kits

Chapter 9 - Internal Engine Modification - Cranks, Cams, Pistons and Heads for Turbocharging

With a safe tune, stock engines can withstand the punishment of moderate boost and horsepower. Most people however, find that eventually higher boost and horsepower levels coupled with the stock internal pistons, rods and crankshaft will end up breaking parts.

Pistons

Many car manufacturers make their pistons out of cast aluminum (hypereutectic) rather than forged aluminum and cannont withstand the abuse of high boost and horsepower. The pistons are usually the first to go on a boosted engine. This is due to the fact that detonation creates extreme pressure and explosive combustion rather than smooth flame propagation. Common piston failures are broken ring lands, cracks and holes in the crowns.

The cure for broken pistons is a correct tune of course, along with a set of forged pistons. Some people opt to use dished pistons to lower the compression ratio and increase the amount of boost that can be used. Popular piston makers like JE, Manley, Mahle, Diamond and Wiesco make forged flat top and dished pistons. If you are building an engine specically for boost, forged pistons are a must.

Forged turbo pistons installed. Photo courtesy of Nick Nagrodsky aka ddnspider.

See more of Nick’s Trans Am build at http://my.fit.edu/~nnagrods/mp/

Crank and Rods

Quality forged rods are made by Carrillo, Callies, Eagle, Lunati, Oliver, Scat, Katech and Crower among others. Usually, the rods are second to fail after the pistons. A good set of forged connecting rods with high quality bolts such as ARP bolts are a sound investment when building a high horsepower engine, boosted or not.

Most stock crankshafts are very strong and are made out of nodular iron. The last internal engine part to break would most likely be the crankshaft. Forged crankshafts are available from Eagle, Callies, Lunati, Katech and other manufacturers.

Camshafts

Selecting a camshaft for a turbocharged engine can be a harrowing task. Ask ten cam experts to give you advice, and you will more than likely get ten different answers on what duration, lift and lobe separation to use. The camshaft you select can make or break your combination. I will attempt to guide you through the basics of turbocharger cam science. Hopefully you will gain some insight on selecting a cam for your application.

Let’s take a look at some cam terminology and apply it to turbo specific requirements.

Duration

Duration is the amount of time that the camshaft keeps a valve open and is measured in crankshaft degrees. It is applied to both the intake and exhaust lobes of a cam. The longer the duration, the longer a valve stays open. Selecting the correct duration in a turbocharged engine is critical and differs from naturally aspirated engines due to the fact that air is being forced into the engine, rather than being drawn in. If you learn one thing about choosing a turbo cam, let it be this: long duration and overlap are not good characteristics for turbo cams.

Why is overlap bad for a turbocharged engine? The exhaust pressures in a turbocharged engine are generally higher than the boost pressure in the intake. By leaving the exhaust valve open whlie the intake valve starts to open (overlap), exhaust gasses tend to force their way back into the cylinder and out the intake. This is called reversion and it contanimates the intake charge. By delaying the opening of the intake valve until after the piston reaches top dead center, reversion can be eliminated. A camshaft with a reverse split duration pattern will do just that.

Reverse Split Duration Pattern

Turbo cams usually use a reverse split duration pattern. Reverse pattern refers to the intake duration being larger than the exhaust duration. An example of a reverse split camshaft is 236° intake and 230° exhaust duration, denoted by 236/230.

By using a shorter exhaust duration and delaying the opening of the intake valve until just after the piston reaches top dead center, the pressure in the cylinder is at or below boost levels and fresh air can be forced back into the cylinder.

Single Pattern Duration

A camshaft with a single pattern duration will have intake and exhaust durations of the same values. Single pattern cams can be used in some turbocharged applications where back pressure in the exhaust system is at a minimum or near a 1:1 ratio with boost pressure. Engines with high flowing tubular turbo manifolds and larger A/R turbine housings generally have lower exhaust pressures. A single pattern cam such as a 224/224 may work better in this situation.

Lobe Separation

Lobe separation (LSA) is the angle between the centerline of the intake and exhaust lobes measured in camshaft rotation. LSA along with duration and lift determine the amount of overlap that a cam has. A smaller LSA will give more overlap with a given duration. By the same token, increasing duration with a given LSA will also increase overlap. For turbocharger applications, an LSA between 114° and 117° seems to work well.

Lift

Lift is the distance that a valve moves from its seat. Performance camshafts generally have more lift than stock cams. Higher lift will allow more air in, and exhaust out during the intake and exhaust strokes. This holds true for both naturally aspirated and turbocharged vehicles. The problem with higher lift cams is that the duration must increase with lift to keep the angle between the cam and lifter (ramp rate) from becoming too steep. A shorter ramp rate will raise the lifter quicker, giving higher lift with a shorter duration. If the ramp rate is too short, excessive valve train wear will occur.

Aftermarket camshafts for turbo engines are made by Lunati, Competition Cams, Crane, Iskenderien, Edlebrock, Erson and others. You can even tell them what combination your engine has, and they can all grind you a custom cam.

Stock Camshafts

The various stock cams that reside in differing engines usually work great for turbocharged applications. The overlap is at a minimum, idle quality is great and they help make great low-end grunt as well as high end horsepower. Some engine builders suggest keeping the stock cam for mild to medium horsepower turbocharged applications.

Cylinder Heads

Flow is very important for forced induction. Many stock heads will be a limiting factor in your engine’s ability to make a lot of horsepower.

A good set of aftermarket heads, or at least a port and polish job on your current heads will reward you with increased flow and horsepower.

Valve size should also be a consideration. Smaller valves will restrict flow and decrease horsepower. An cylinder head with larger intake and exhaust valves and runners is a good idea if you plan on making serious power. Some heads can be re-worked to allow for larger valves to be installed.

Intake Manifolds

Stock intake manifolds tend to be a bit restrictive. A manifold with larger runners will improve flow and allow more air to enter the cylinders with each intake stroke of the pistons.

Some manifolds respond well to porting, however I have found that the designers of aftermarket intake manifolds have found ways to increase airflow beyond the benefits of porting.