Engine Durability Secrets

Tricks to help you engine last an entire season.

Horsepower wins races, but so does durability.  If an engine can’t hold together, you’re not going to consistently see the finish line. And you’re probably going to break your bank account as well as your motor. Building an engine that will last an entire season of drag racing begins with the block. The block is the foundation upon which everything else is based. If the block isn’t rock solid and absolutely bulletproof, metal can shift and something will eventually break.

If you’re building a stock block with two-bolt main caps, 550 hp is about the limit for this type of main cap configuration. Factory production blocks with four-bolt main caps can reliably handle up to 750 to 800 hp, in most cases. So too can a two-bolt main cap block if the main caps are converted to splayed four-bolt aftermarket caps (which are stronger than both two-bolt and four-bolt factory main caps). For higher outputs, a siamesed bore Chevrolet Bow-Tie block or Ford SVO block is recommended to pro-vide added rigidity. 

METAL IN MOTION

A block seems like a pretty rigid chunk of metal, but most are not. Today’s thin wall castings actually flex quite a bit and not just when the engine is under load. Just torquing down the head bolts can distort the upper area of most cylinders a couple of thousandths or more. Tightening the main bolt caps, bolting on the bell housing or even attaching the motor mounts can also produce unwanted bore distortions.   Don’t believe it? Put a bore gauge in a middle cylinder, then push in on the outside of the block. Doing so will usually produce enough distortion in the cylinder too not only register on the gauge, but to cause the gauge to loosen and drop down the bore.  Most drag racers prefer “seasoned” blocks rather than [new] virgin castings because seasoned blocks have settled after repeated thermal cycling and are more dimensionally stable. 

Once you’ve picked a block, the first thing you want to do with it is have it sonic checked. The sound waves generated by an ultrasonic tester will reveal the thickness of the cylinder walls. Thickness should be measured at the top, middle and bottom of each cylinder on all four sides, because cores can shift during the casting process, resulting in less than perfect blocks. Some of the cylinders may be slightly off-center and dangerously thin on one side. Thin metal is weak metal, and if the block is bored or honed to oversize you may end up with areas that are paper-thin and could blow out under the rigors of drag racing. So if a sonic check reveals uneven cylinder walls or thin spots, don’t waste your money trying to build the block. Find another. Next the block needs to be thoroughly inspected for cracks or other flaws.  The cylinders, deck surfaces and web areas around the main crankshaft bore and caps all need to be inspected for hairline cracks.  Magnetic crack detection equipment works well on cast iron, but you’ll need penetrating dye if the block is aluminum.  Pressure testing the water jackets in the lock is also recommended to reveal “hidden” cracks as well as porosity leaks that may not show up by other test methods.

If the block passes inspection, the next step is to completely debur everything. Burrs, nicks and similar surface imperfections can become stress risers that may later turn into cracks. A couple of hours invested here in deburring can significantly reduce the risk of cracks later on.  Oil drainback holes should also be chamfered or blended to promote good oil return, and stock pressed-in oil plugs should be replaced with threaded plugs.

STRESS RELIEF

One way to improve the stability and durability of any block, as well as the pistons, rods and crankshaft is to put them through “cryogenic” stress relief. Freezing can improve the dimensional stability, rigidity and durability of most parts. The process uses liquid nitrogen refrigeration to slowly cool parts down to about 300 degrees below zero. Once the temperature has stabilized, the parts are then allowed to slowly warm back up to room temperature. The process can take 24 to 36 hours and, in some cases, may be followed by additional stress relieving treatments. Those who use this trick swear it makes parts almost unbreakable.

Another process that can accomplish similar results is “vibratory” stress relief. In this process, the parts to be treated are placed on a computer-controlled vibrating table and oscillated at a frequency that produces the right harmonics.  The process takes only about half an hour, but also helps shake residual stresses out of castings and forgings. Shot peening is another process that has long been used to improve durability. By blasting parts with steel shot at high velocity, the surface of the metal is compressed slightly. This increase the tensile strength of the metal underneath and makes the part much more resistant to fatigue failure. Shot peening can quadruple the cycle life of many parts, particularly connecting rods and valve springs - which is why most high performance rods and valve springs are shot peened when they’re manufactured. But if you’re running stock rods or valve springs, chances are they’ve not been peened.  Gears also respond well to shot peening, including not only camshaft and crankshaft timing gears, but also transmission and differential gears. Shot peening will make the gears more resistant to breakage and able to handle higher loads.

BLOCK WORK

Regardless of how much horsepower you’re building, always chase and clean all threaded holes in the block for the head bolts and main caps. Clean, straight threads will help ensure accurate torque loads when the heads and main caps are installed.  Boring and honing should always be done with a torque plate and head gasket attached to the deck surface.  Many engine builders also recommend installing the core plugs and main caps, torquing the main caps to specs, and bolting on the front cover, bell housing and any other external pieces that may affect block distortion.

The idea here is to duplicate the stresses that distort the block when it is installed in the vehicle so the cylinders can be cut and honed to as round a condition as possible. Some engine builders even circulate hot water through the block while it is being honed to simulate the thermal stresses the engine will experience when it is running. Once the cylinders have been finished, they need to be cleaned - not just wiped out, but scrubbed vigorously with a stiff bristle brush and hot soapy water to get out all the honing residue. Petroleum-based solvents won’t remove the kind of residue that sticks to the surface and can damage the rings and bearings if it remains in the block. The best way to see if the bores are really clean is to wipe them with a clean, white cloth when you think you’re done. If you see any dark smudges on the cloth, the bores are not clean and need to be scrubbed again. Main bores and cam bores in the block also need to be checked for straightness, and should not vary more than .002-inch overall or more than .001-inch between adjacent bores.

CRANKOLOGY

A strong crankshaft is also a must for drag racing, especially if you’re using nitrous oxide. The more power the motor makes, the more stress there is on the crank, especially critical in stroker motors because of the longer throws on the crankshaft. Increasing stroke increases engine displacement, power and torque, but trying to squeeze every last cubic inch out of a given block and crank combo may be asking for trouble. If you’re serious about building a motor that will last the season, don’t push the stroke to the limit. Cast iron cranks are suitable for daily drivers and engines up to about 500 hp, but they’re not the best choice for a drag motor. Factory-forged steel cranks are usually reliable up to about 700hp.  Beyond that, you’ll need an aftermarket steel crank (forged or billet).  For small blocks with strokes of less then 3.750 inches and big blocks with strokes less than 4.500 inches, a forged steel crank made of 4 130 alloy should be able to reliably handle 600 to 650 hp. For longer strokes and/or higher outputs, you should probably consider a crank made of 4340/3000 alloy. If you’re running a stock crank, make sure it is crack-free and has less than .001-inch of runout. 

Some performance engine builders reduce the diameter of the crank journals to reduce friction and gain a few extra horsepower. But smaller journals are weaker journals, and require tighter bearing clearances and thinner oil. If durability is your goal, stick with a standard journal diameter of no more than .010-inch undersize. When the crank is machined, careful attention must be paid to the journal fillets. The proper radius is needed to maintain strength in this critical area. If the radius is too small, it can create a weak point that may eventually lead to crank failure. And if the radius is too large, it can interfere with the bearings. The journals should also be micropolished to provide the smoothest surface possible, and the oil holes should be chamfered to maximize oil flow to the bearings. After the machine work on the crank is done, all the oil holes need to be thoroughly rinsed and cleaned with a bristle brush to make sure no metal chips are left inside. Just blowing out the holes with com-pressed air isn’t good enough. Next, the crankshaft, rods and piston assemblies (pistons, rings and wrist pins) and flywheel need to be balanced. Balancing won’t add any horsepower, but it does reduce vibration and stress that can shorten the life of any engine, especially at higher rpm.

The force created by any imbalance grows exponentially with increased speed, so minimizing imbalance greatly reduces the stresses the engine must handle at higher speeds.  Another modification that will improve durability is to replace the stock harmonic balancer with an aftermarket balancer to minimize vibrations.  The fluid-filled variety provides good dampening across a broad rpm range.

BEARING DOWN

Bearing selection and installation is another area in which attention to detail can make a big difference in how well your engine holds up under punishment.  A typical “trimetal” engine bearing has a three-layer construction. The steel backing plate is covered with a layer of copper/lead overlaid with a thin (.0005- to .0008-inch) coating of babbitt to provide a good combination of strength, surface action and embed ability.

Copper/lead can carry 12,000 pounds per square inch vs. about 7,000 to 8,000 psi for an aluminum bearing, and can better resist wiping and scoring under hard acceleration, according to those who manufacturer this type of bearing.  That’s why trimetal bearings are used in almost all racing applications.  Some performance trimetal bearings are not overplated with tin. While a thin flash plating of tin gives a bearing a nice bright appearance, and is used on many standard replacement bearings, under the extreme conditions of racing environment, tin can migrate and cause problems. Most aftermarket trimetal bearings work fine up to about 500 hp, but for higher outputs you’ll probably need a “high-performance” bearing. Such bearings typically have more eccentricity to handle rod distortion that occurs at higher rpm. One brand of performance bearings also uses a thinner babbit overlay (only .0005-inch) to reduce fretting and fatigue under high loads.

When the bearings are installed, the recommended clearance for drag racing is about .0005-inch more than the standard .00075- to .00l-inch per inch of crank journal diameter (.0025-inch total for a crankshaft with 2-inch diameter journals). This is somewhat looser than a typical stock motor, but allows more oil flow to the bearings. Bearings should be select fit to get the most accurate clearances (which means buying several sets of bearings and trying different combinations until you get the exact fit). To help the bearings live, you’ll also need to install a high-volume oil pump. A high-volume pump is necessary for two reasons. One is to compensate for the slightly looser bearing clearances in a drag motor, and the other is to provide sufficient oil flow at high engine rpm - especially if you’re running a low-viscosity oil. To prevent oil starvation during a hard launch, the oil pan must be baffled and deep enough to keep the oil pump pickup submerged. If you have the bucks for a dry sump oil system, oil starvation shouldn’t be a problem.

ROD MODS

Stock connecting rods can typically handle up to about 500 hp as long as rpm remains within the stock redline (5,500 to 6,000 rpm). Above 600 hp and/or 6,000 rpm, stock rods are living on borrowed time. Stock rods tend to pull apart if the redline is increased significantly because they lack the tensile strength to withstand the stretching forces that come with higher rpm.  If you’re running stock rods, check them carefully for any bend or twist, if you find any bend or twist, they should be replaced. Rods can be straightened, but tend to return to the same shape as before because of residual stresses in the metal.

There are a wide variety of aftermarket performance rods from which to choose. Steel rods are less expensive than aluminum rods and typically hold up much better over the long run. Aluminum rods are lighter and hence a little quicker, but typically need to be replaced after 150 or so runs because of stretch. Steel rods, on the other hand, can usually go up to 750 or more runs in a typical bracket car. So if you’re making a lot of runs and don’t want to pull the oil pan until the end of the season, steel rods would be your best choice. Regardless of what type of rods you choose, new rod bolts and nuts are a must. Some engine builders will even replace the bolts and nuts after checking bearing clearances with the rod caps installed just to make sure the fasteners haven’t stretched.

PISTON POWER

Piston selection will depend on the compression ratio you’re running, bore size and rod length. Lighter is usually better, but don’t use ultra light pistons in a nitrous motor because pin beam strength is critical in this type of application. Ultralight pistons are designed to survive in high rpm, naturally aspirated motors.

Don’t get carried away with increasing the compression ratio. More compression equals more power, but when you get above 13.5 to 1, the dangers of detonation begin to outweigh the gain in horsepower. If you want your motor to last, keep the compression under 13.5 to 1. And if you’re driving on the street, keep it under 11.5 to 1. Hypereutectic alloy pistons are a good upgrade from stock cast pistons and offer improved strength and durability, plus a little less weight. But for serious racing, forged pistons will be the most durable.

Forgings are ductile and can often survive a valve hit without shattering like a cast piston. Forged pistons are also more conductive and less likely to cause detonation in a high-compression engine. If you don’t think stronger pistons are a must, consider this: In a stock 350 Chevy V8 with a compression ratio of 8 to 1, the combustion pressures generated at wide-open throttle typically peak out at around 700 psi with a total force of about 8,800 pounds pushing down on the top of the piston. By comparison, a high-performance 350 engine with a compression ratio of 12 to 1 can generate upward of 1,200 psi with a total downward force of 15,000 pounds on each piston (nearly twice that of the stock engine).  To minimize the risk of scuffing, use pistons with a graphite/moly coating on the skirts. The coating not only prevents metal-to-metal contact, but also allows much tighter piston-to-bore clearances to reduce blow by.

Ring selection is a matter of preference. Chrome- or molyfaced will both work, but moly is the more durable material of the two. Ring sets with a ductile iron or steel top ring will be more resistant to breakage than those made of cast iron. Follow the ring manufacturer’s recommendations for end-gap clearances.

COMBUSTION SEAL

To keep the combustion chambers sealed, you’ll probably need some type of performance head gaskets.  Performance gaskets are made of premium materials to meet the tougher demands placed on them in the racing environment.  Graphite is one such material that is now used in many such gaskets because it is a natural lubricant and can take the heat. But there are also improved non-asbestos materials that offer superior torque retention and strength for performance engines. One difference you’ll find in the design of some performance gaskets is the use of a steel or copper wire fire ring inside a stainless steel combustion seal.

The ring concentrates loading in the critical area around the cylinder and can triple the sealing strength. Such gaskets can withstand combustion pressures of up to 2,000 psi!  The use of this type of gasket may also eliminate the need for special machining that’s required to O-ring a racing engine. Steel wire rings work best with cast iron heads while copper is better for aluminum because it is softer and reduces the likelihood of brinelling (indenting) the heads. If you’re building a street engine, make sure the gasket is not a “race-only” design. Some race-only head gaskets do not have steam holes that are necessary for street operation. Other gaskets may have relocated or enlarged coolant passage holes to increase cooling when aftermarket aluminum heads are installed.

FINAL ASSEMBLY

When you assemble the engine, measure, remeasure and recheck everything one last time to make sure all clearances are correct. New fasteners are recommended for all critical applications (connecting rods, main caps and head bolts). Follow the recommended tightening procedure from the bolt manufacturer because they vary. Some aftermarket performance fasteners are torqued with the threads lubricated rather than dry. Adding a little oil or other lubricant to the threads will decrease friction and significantly increase the load so make sure you follow the directions and recommended tightening procedure. Use an accurate torque wrench and angle indicator. When the engine is installed in the vehicle, use a front-mounted motor support plate rather than stock side-mount motor mounts. This will reduce block distortion and do a better job of controlling motor torque.

Solid mounts are great for the strip, but are not recommended for the street. Before the engine is first started, pressurize the oil system with a pre-lubricator. A dry start can be very damaging to the bearings and camshaft.  As soon as the engine is first started, it should be run at 2,000 rpm for up to 30 minutes to assure proper break-in of the cam.  Valve train, timing and carburetion adjustments can then be made to fine-tune engine performance. Most performance engine builders prefer to do all of this on a dyno in the shop because a dyno gives them much more control over variables and allows them to achieve optimum results in the least amount of time.

BEETWEEN RUNS

Many drag racers will check valve clearances between every run, and depending on the class (and budget), some will drop the pan and inspect all the bearings. The valve springs and retainers probably deserve the most attention because valve springs can lose tension if the engine is over-rewed and aluminum retainers can pull through at high rpm. If a spring is weak or a retainer is riding too high, they should be replaced immediately. Circulating coolant through the block with an auxiliary electric pump after a hard run can also help lessen the thermal shock on the block and other parts. Just don’t pump cold water into a hot block unless you like to hear the sound of cracking metal. As for oil and filter changes, it depends on how much power you’re running, the type of fuel and whether the engine is blown. The greater the blow by, the greater the rate of oil contamination.

Today’s NHRA

Winter 2001

By Larry Carley, Contributing Editor