Print

Email

More Power From Your Mopar

By Dave Emanuel

Chrysler Corp. hasn't built a rear wheel drive, V8-powered car (with the exception of the Viper) since the turn of the decade. And the cars that harbored V8s beneath their hoods during the late 1980s were about as appealing to performance enthusiasts as the front-wheel driven vehicles that replaced them.

In spite of that fact, there's a surprisingly strong demand for high performance Chrysler small block engines. Certainly, much of that demand is fueled by truck owners as the 318- and 360 cid variants of Chrysler's "LA" small block have been offered in both Ram and Dakota models for several years.

The "LA" family of small blocks emerged in 1964 when a 273 cid model was introduced. In 1967, Chrysler stepped up the small block's performance potential with a 318 cid model. And this particular engine over the years has generated much confusion.

Chrysler's original 318 engine, which was produced between 1957 and 1966, bore the "A" series family designation and was a larger displacement version of the 277 and 301 engines introduced in 1956. Very few parts are interchangeable between the old "A" engines and the newer "LA" powerplants. Few performance parts are available for the old "A" series engines, primarily because nobody cares.

Like its ancestor, the "LA" engine didn't have much of a performance orientation until 1968 when displacement reached 340 cubic inches. In 1971, a 360 cid version was introduced, but initially it was simply a torque generator for Chrysler's larger vehicles.

Blocks

Chrysler "LA" blocks are easy to distinguish from one another because displacement is clearly noted on most castings. Although all blocks are virtually identical in physical appearance, 340 and 360 cid versions accept a different left-side motor mount than their 273 and 318 cid counterparts. All production "LA" blocks have two-bolt main bearing caps, and although kits are available to convert these blocks to a four-bolt main configuration, they should be avoided. The production blocks don't really have enough material in the web areas to comfortably handle the added bolts.

If four-bolt mains are required, so is additional block strength. The only way to get both is with a Mopar Performance "X" race block (p/n P4120637). It's also possible to add four-bolt mains to an old Trans/Am block or a p/n P3870873 race block.

All 273-360 production blocks were cast using a thinwall process and, as such, a .030" overbore should be considered a maximum. Some older blocks (cast before 1976) may tolerate up to a .060" overbore, however, whenever you plan to machine the cylinders of an "LA" engine more than .030" over size, it's advisable to sonic check the cylinders to ensure that core shift isn't excessive. You may find, especially with later blocks, that .020" is the maximum safe over bore.

With a small block Chrysler, standard high performance block preparation is no different than with any other engine. Typically, machining includes removal of casting flash from the lifter valley area, enlarging of the oil return holes in the valley, cleaning all critical threaded holes with a tap, machining of the deck surfaces to assure they're square and flat, and chamfering the head bolt holes after decking the block.

As a general rule, high performance small blocks should be machined to yield a zero deck clearance between the piston top (the flat portion, not the dome) and the block when the former is at TDC. A "zero deck" improves power output and also makes the engine less likely to detonate. However, many "LA" cylinder heads have completely round combustion chambers, so the only way to achieve a "zero deck" is to have the pistons actually stick out of the block .018" to .020" at TDC.

Regardless of the desired deck clearance, take the time required to assure proper machining of the deck surfaces. Small block Chrysler engines have only two rows of head bolts so the block and head mating surfaces must be absolutely flat if the head gasket is to enjoy a long and successful life. Suggested torque for head bolts on high performance Mopar small blocks is 100 lbs./ft., so thread integrity and head bolt quality are critical.

Without question, honing the cylinders with torque plates in position is an essential part of machining any high performance engine. Garry Grimes of Grimes Automotive Machine, Alpharetta, GA, notes that cylinder walls can move as much as .004" when the head bolts are tightened so use of torque plates during honing is absolutely essential if optimal ring seal is to be achieved. Grimes typically hones at least .004" to .005" after boring to assure optimum cylinder wall finish. Align honing is another operation that's standard procedure when machining a block for high performance usage.

Crankshafts

All "LA" engines produced prior to 1973 were equipped with forged steel crankshafts. Since 273, 318 and 340 small blocks all have the same stroke and main journal diameter, crankshafts are interchangeable between these engines. Production 360 engines, however, were never blessed with a forged crank and since they have 2.81" diameter journals, rather than the 2.50" journals found in smaller displacement "LA" blocks, retrofitting isn't an option.

When interchanging crankshafts, keep in mind that the balance factor will vary considerably depending on the connecting rods and pistons used in the original configuration. The addition or removal of an excessive amount of material may be required when using a crank in an engine with a bore size different from the one to which the crank was originally mated. Therefore, the most efficient approach is to use a crankshaft that was originally installed in an engine of the same displacement as the one being rebuilt.

When preparing a crankshaft for use in a high performance engine, it's best to "buy a little insurance" by taking the extra time required to radius the oil holes, polish the journals and add a generous fillet radius where the journals meet the flank. Obviously, it will be necessary to chamfer the bearings so they're compatible with the new radii.

The crankshaft numbers of interest are: 2843868, 1968-'72, 340; 2843883, 318 truck; and 3751162, 340 cast iron. Although a forged crankshaft is obviously stronger than one manufactured of cast iron, crankshaft strength is rarely a problem in a high performance street engine. However, when selecting components, be aware that cast iron 340 crankshafts are externally balanced, while "LA" forged cranks are internally balanced.

Consequently, the vibration damper and flywheel or torque converter designed for a cast crank must not be installed on a forged crank, and vice versa. The vibration dampers installed on 360 crankshafts are unique and are not suitable for other assemblies.

Another caveat regarding "LA" vibration dampers is that 1972 and later dampers (p/n 3614369) have a symmetrical pulley bolt pattern while the 1971 and earlier dampers (p/n 2951673) have a non-symmetrical pattern. The 340 cast crank requires Chrysler damper p/n 3751169; 360 engines require either a 3512972 or 3751808 damper. Another option is to install a viscous vibration damper. Fluidampr p/n 71420 fits all internally balanced 318, 340 and 360 engines.

Bearings

Don't scrimp on either the rod or main bearings. Install top quality parts. Main bearings with a grooved upper half and full width lower half are recommended by most high performance Mopar specialists. When trial fitting the upper halves, make sure that the oil hole in the bearing is aligned with and is as large as the oil supply hole in the bearing saddle.

If it's necessary to drill a larger hole in the bearing, be sure to deburr it. Bearing clearances are typically .002" to .0025"; tighter clearances - .0015" to .0018" are certainly permissible, however, when the accelerator pedal is beneath the foot of an uneducated driver, tight clearances can prove detrimental to the life of the bearings; engine oil must be thoroughly warmed (above 200° F) before escapades are made into the land of full-throttle operation.

Oiling system

In years past, high pressure and high volume oil pumps have been standard issue for high performance engines. But metallurgy and lubricating technology have come a long way, eliminating the need for extremely wide bearing clearances. Tighter clearances eliminate the need to flood an engine with oil to compensate for internal leakage.

If a small block Chrysler is assembled properly, and all recommended clearances maintained, a standard oil pump will provide more than adequate pressure and volume. In fact, high volume pumps can be a liability if crankcase capacity isn't adequate because they can literally suck the oil pan dry - resulting in oil starvation - while filling the upper part of the engine with oil.

The old guideline of 10 psi for every 1,000 rpm is entirely applicable to Chrysler small blocks. Chrysler's oil pump blueprinting specifications call for clearance between the rotor and oil pump body to be less than .014" and tip clearance between the inner and outer rotors of less than .010". A clearance of .004" is specified between the pump face and rotors. Standard pumps properly prepared should provide the required amount of pressure in engines that won't operate above 6,000 rpm for extended periods.

If oil pressure isn't adequate, a Mopar Performance high pressure oil relief spring (p/n P3690944) can be substituted. Additional volume can be obtained by installing a long rotor kit (p/n P3690944).

If at all possible, install a windage tray. Chrysler offers p/n 2531945 for 273, 318 and 340 engines and p/n 3577794 for 360 engines. In both cases, a hardware installation kit (p/n P3690939) is required.

Keep in mind that oil pan selection for a Chrysler small block engine is dependent upon the chassis into which the engine will be installed. Vans require a front sump pan, passenger cars call for the sump to be in the middle, and trucks require a pan with a rear sump.

Connecting rods

Chrysler small block connecting rods are essentially all the same. And, Mopar connecting rods do have a good bit of integrity. All are forged, but the 340 rods (p/n 2899495) are bigger and stronger than their 273/318 counterparts (p/n 2406785). Both part numbers have full-floating wrist pins. The 360 four-barrel rod (p/n 3751015) is also a heavy duty forging, but has a pressed pin. All small block rods have a 6.123" center-to-center length, but 1974 and later 318 and 360 rods are of the pressed pin design.

Pistons

This is one area in which attention to detail is critical. The compression distance of replacement small block Chrysler pistons can vary from 1.740" to 1.840". That being the case, some pistons will yield a deck clearance of .082" while others will stick out .018" above the block's deck surface. Be sure that the pistons selected are compatible with the cylinder heads and will provide the desired amount of deck clearance.

Also note that 4.040" is the standard bore diameter for a 340 engine while a 360 has bores that measure 4.000". Consequently, if a 360 is bored .040" oversize, 340 pistons would fit the bores, but the compression height would be wrong because a 340 has a 3.31" stroke, a 360's measures 3.58".

To avoid problems, verify piston compression distance as early in the building process as possible. It will also be necessary to determine combustion chamber volume so pistons can be selected to provide the desired compression ratio. Most performance pistons are of the flat top persuasion, with either two valve reliefs, a trough, or a dish.

With the large combustion chambers found in many late model heads, compression ratio will be well below 9.0:1. Depending on camshaft duration, most street performance engines should have a compression ratio between 9.0:1 and 9.5:1. Pistons should be fit according to the manufacturer's recommendations. Most forged pistons require .004" to .005" piston-to-cylinder wall clearances, although some "street type" varieties are designed for .0015" to .002" clearance.

Piston rings

Standard fare is a Speed-Pro® plasma moly top ring, cast iron second, and low tension oil ring. As far as cylinder wall finish, simply follow the recommendation of the ring manufacturer. The love affair many race engine builders had with exotic stones, mirror finishes and other "tricks" evaporated years ago.

Ring width selection is determined by the pistons and a 5/64", 5/64", 3/16" combination is generally preferred for street engines; 1/16", 1/16", 3/16" is commonly used in oval track, road race and bracket-style drag race engines. As usual, there's a trade-off. Wider rings offer better durability, but they're heavier, so they don't seal as well at high rpms (above 6500).

High performance engines tend to build more heat than their grunt-and-groan counterparts, so ring end gap is more critical. Common practice is to use "file-to-fit" rings (.005" larger in diameter than the bore) so that end gap can be precisely set. Minimum end gap dimensions are .016" for top rings and .012" for second rings, but end gap is typically much wider when hypereutectic pistons are used.

The hypereutectic material doesn't transfer heat through the piston as rapidly as a standard cast or forged piston. The advantage is that heat trapped above the piston can be converted to power; heat transferred through the piston is lost. The additional heat retained in the ring area requires end gaps of .022" to .026" for the top ring. This information can be found in the piston installation instructions that rebuilders/technicians often toss in the trash without reading.

Be sure to use a proper ring expander during installation and don't wind the rings onto the pistons. For best ring seal, make sure that vertical clearance between each ring and its groove is tight. Ring grooves for 5/64" rings should measure .0785" while grooves for 1/16" rings should be .0635" tall. In both instances, tolerances are +.0005", -.0000".

Camshaft

Every gearhead loves the sound of an engine that harbors a long duration camshaft. Unfortunately, the sound of performance doesn't guarantee that an engine will produce the desired amount of power. Consequently, the safest bet for an engine rebuilder is to be conservative with camshaft duration, making sure it's compatible with static compression ratio.

High compression and short cam duration lead to detonation. Low compression and long cam duration zaps cylinder pressure, which in turn zaps power output. The idea is to select complimentary components that result in an engine that pops the gauge to 165-175 psi in a cranking compression test - that's the target to assure compatibility with pump gas. (Competition engines that are fueled by high octane racing gas should register about 220 psi).

Compared to a stock camshaft, a high performance grind typically increases horsepower by holding the valves open longer and raising them higher. There's no mystery there, but the latest computer-aided designs offer significant power increases without having to rely on excessively long duration which results in a rough idle.

Cam lobes designed within the past few years open and close valves at higher velocities so the valves reach full lift sooner, stay there longer and close quicker. That translates into increased air flow potential which leads to more power, all other things being equal. However, current camshaft designs deliver this increased power while still maintaining strong low speed torque, good drivability and acceptable idle quality.

What about mechanical lifters? Unless you're building a race engine - avoid them. They require extra maintenance and also increase maximum potential engine speed. Hydraulic lifters are hard pressed to deliver much more than 6500 rpm and that safety valve will serve you well - especially with customers having more money than sense.

Cylinder heads

Selecting cylinder heads for a high performance Chrysler small block is relatively easy. The best choice for 340 and 360 high performance engines is a large port casting with 2.02" intake and 1.60" exhaust valves. Heads fitting this description were originally installed on 1968-'71 340 engines (c/n 2531894).

In 1972, intake valve diameter was decreased to 1.88", but port size remained virtually unchanged. Large port heads with the smaller intake valve (c/n 3671587) are suitable for 318 engines, while 273 engines are well served with small port castings having 1.75" intake and 1.50" exhaust valves (c/n 2465315, 2536178, 2658920 or 2843675).

For serious street performance and racing applications, the W2 head (p/n P3870812 for the standard W2, p/n P4120043 for the "econo" version) is the obvious choice. These heads were never installed on a production engine, so the chance of finding a pair through normal rebuilder channels is pretty slim. However, they are still available from Mopar Performance and can frequently be found at Chrysler-oriented swap meets.

Regardless of the castings used, preparation should include a multi-angle valve job (45°, 60° and 70° cuts below the valve and a 30° top cut) which is also part of the program. Under no circumstances should you sink the valves. If the seats are worn excessively, install new ones (which is often necessary with older castings not originally designed for use with unleaded fuels) or oversized valves, which improve performance in two ways.

A larger diameter not only increases air flow potential, it also moves the valves higher in the chamber, thereby unshrouding them. As a general rule, best performance is achieved when the distance from the valve seat to the short turn radius is at least .100". Installation of oversized valves makes it easier to achieve this dimension.

To make the most of each port's flow potential, clean up the valve bowl area. The goal here is to smooth and blend the areas where the machined and as-cast surfaces meet, and minimize any obvious flow restrictions. Extensive porting and polishing are not required for street performance engines, though most customers will expect the port openings to be matched to the gasket size. Except in extreme cases, the effect on performance is marginal. However, the effect on a buyer's perception of power potential is significant.

Installation of valve springs is part of any cylinder head reconditioning, but spring usage is dictated by the camshaft. Most of the milder performance cams require nothing more than a decent quality stock diameter valve spring - provided that spring offers a minimum of 100 lbs. of seat pressure at an installed height of 1.700". More radical camshafts typically require stiffer dual valve springs which require spring seat machining because of their larger OD (1.430" versus 1.250"). The heads should also be fitted with premium quality valves.

---