The no-holds-barred Dirt Late Model engine is one of the most viciously powerful in all of stock car racing. With 800-plus HP, it boasts nearly as much peak power as a NASCAR Nextel Cup powerplant, but the DLM engine works over a much wider RPM range and, thanks to an aluminum block, also weighs significantly less.
The DLM engine easily produces the most power and torque of our three engine builds. This is due to having the most cubic inches, a healthy 14.5:1 compression ratio, and a host of custom-made speed parts. The drawback is that this motor is also by far the most expensive, averaging between $30,000 and $40,000, depending on the builder and parts used. It also takes the most time to build, partly because it requires much custom fabrication.
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To be frank, a full-bore DLM engine is probably outside the range of most non-professional engine builders. This is not because of the skills that must be mastered, but because of the many different pieces of specialized equipment required to complete the job. Still, it is possible to do the assembly yourself if you can farm out some of the machine work. And even if you never build your own DLM engine, there are a lot of tips here that you can take advantage of in any engine you build.
Clements Racing Engines in Spartanburg, South Carolina, is the builder of our full-bore DLM mill. Clements specializes in dirt racing, especially the Late Models, and even has its own CNC machining centers in-house, which it uses to turn raw cylinder head castings into pieces of automotive jewelry. Clements has worked with many of the top DLM racers in the country and produces approximately 250 engines a year.
The engine for this build is a Chevrolet SB2. Originally developed in 1996 for NASCAR Nextel Cup (then called Winston Cup) competition, the SB2 has since been adopted by other forms of racing because it is a more efficient design than the classic small block. The SB2 (which means Small Block Second Generation) has actually been updated since its inception and is now officially called the SB2.2, but most racers still refer to it simply as the SB2.
The SB2 (both versions) is based on the original small block, but it has a few critical differences. First, the valve pattern is different. Instead of a valve pattern with the exhausts on the outside edges (E, I, I, E, E, I, I, E) like a standard Chevy small-block, the SB2 moves the intake valves to the outside (I, E, I, E, E, I, E, I). This is called a “Mirror Pattern” and the intake ports are angled toward the center of the engine to provide a straighter path for the air/fuel charge from the carburetor to the combustion chambers.
The valve angles have also decreased significantly from our 23- degree Street Stock. In this form, the intake valves are at 11 degrees with a 4-degree sideways cant to reduce valve shrouding. The exhaust valves, meanwhile, are angled at eight degrees from vertical.
In addition, the lifter bores are offset for the intake versus the exhaust lifters. This change in angles cleans up the rocker arm geometry and eliminates the need for offset rockers, which are expensive and can cause wear problems. Coolant flow throughout the motor, especially to the cylinder heads, is also improved. Finally, the SB2 is a dry-sump-only engine since there is no oil filter boss on the block casting.
The biggest drawback to the SB2 engine for racing is the cost. The engine is well supported by the aftermarket, but since it is expressly designed for ultra-high performance racing, there is virtually nothing on the economical end of the spectrum for it. Glenn Clements says that it costs Clements Racing Engines approximately $6,000 more to build an SB2 than it does a similarly sized Chevrolet small-block. But a wellbuilt SB2 will always be capable of producing more power than a firstgeneration small-block, so for most teams the extra cost simply comes down to a matter of whether it can be worked into the budget.
Another unique factor of this build is that this is the first time we are using a block not produced by General Motors. The DLM rulebook is surprisingly open, so Clements sources an aluminum SB2 block by Dart. The Dart block arrives with several features that can make an engine builder’s life a little easier. For example, the freeze plug holes arrive with threads cut to accept screw-in freeze plugs. These plugs use O-rings to virtually eliminate leaks. The cam bore is also raised to allow for stroker applications, which Clements regularly takes advantage of.
This particular engine will be given a 4.00-inch stroke, and along with a final bore diameter of 4.145 inches will have a displacement of 431 cubic inches. Even though the block is designed to accommodate extra crankshaft throw, that kind of long stroke still requires a little extra work cutting clearance for the counterweights.
An aluminum engine block requires cast iron cylinder sleeves no matter if you are racing an SB2 or a standard small-block. Glenn Clements and engine builder Chuck Pridgeon say they have noticed that sometimes the sleeves won’t be fully seated from the foundry, and this is true for any brand of aluminum engine block. When this is the case, the sleeve will eventually settle, which can lead to cylinder sealing problems. To counteract this potential problem, Clements says you should find some way to ensure that every sleeve is completely seated before decking the block.
Clements Automotive uses a homemade tool that seats against the main bearing housing bore at one end and the corresponding cylinder sleeve, on the deck, on the other. The two sides are connected by a large piece of threaded rod, which can be used to compress the tool and ensure the sleeve is completely seated. After all the sleeves have been treated to this process, the cylinders are honed before the block is decked.
The rotating assembly swings on a custom 4.00-inch stroke crank created for Clements Automotive by Crower Cams and Equipment. The forged crank uses pendulum-cut counterweights to help achieve proper balance with minimum overall weight.
Although the typical DLM rulebook might allow ultra-small journal sizes like you will find in a Nextel Cup engine, you might be surprised to see the larger, relatively standard size main and rod journals on this crank. The mains measure 3.500 inches while the rod journals are Chevrolet standard at 2.100 inches. The extra beef is necessary because of both the large quantities of torque this engine puts out and because most DLM teams simply cannot afford to rebuild their engines after every race like a Cup team. This engine, in fact, is expected to run between 1,000 and 1,200 laps before a rebuild is required. Main bearing clearance is maintained between 0.0018 and 0.0021 of an inch.
The rods are 6.00 inches from center to center and produced by Dyer’s Top Rods. A 6-inch rod is about all you can pack into the block with a 9.00-inch deck height, when combined with a 4-inch stroke. Even at that length, the piston pin intrudes into the oil ring land and requires a few extra precautions.
Rod bearing clearance is held between 0.0024 and 0.0026 of an inch, but Pridgeon looks for that clearance in a very specific location. The big end of the rod flexes and deforms slightly into an oval shape every time the crank pulls the rod and piston down into the bore on the induction stroke. Because of this, Pridgeon measures the rod journal housing bore vertically (parallel to the beam of the rod) and 45 degrees to either side. If the vertical clearance is correct, the variance at the 45-degree measurements is allowed to be larger by as much as 0.0005 of an inch, but no smaller then the vertical measurement. For example, if the crank’s rod journal measures exactly 2.100, then the housing bore of the rod with the bearing in place should measure no less than 2.1024. The bore diameter 45 degrees from vertical may be as much as 2.1029 inches, but if it is any smaller than the target size of 2.1024, the rod must be re-honed.
The pistons receive the greatest amount of work. In order to reach the 14.5:1 compression ratio, the pistons have a small dome. They are quite beefy with a full skirt, which Pridgeon says is necessary in order to live in an 800-hp environment for 1,200 laps. The 1.00-inch compression distance is so short that the pin bore intrudes into the oil-ring land, and the oil ring requires its own support rail. Finally, to maximize ring seal in the bores, which is an area of abuse for most dirt motors, each piston has eight gas ports cut into the top ring land. The ports are cut so that half of the 0.060-inch drill bit cuts into the aluminum that forms the top of the ring land while the other half is in the gap of the land. These ports allow combustion pressure to get behind the rings and press them outward, improving ring seal against the cylinder walls. Once all of that is done, every surface of the piston must be massaged with a Scotch- Brite pad to eliminate any sharp edges that may cause detonation.
Clements uses a custom ground cam supplied by Comp Cams with a 55-mm core for ultimate rigidity. The cam spins in the bore on roller bearings for extra protection, and a gear set from Shaver-Wesmar controls the timing. This is necessary because the aluminum block undergoes so much thermal expansion that a traditional timing chain can cause valve control problems. The Shaver-Wesmar gear set maintains good control even with the extreme expansion inherent in the aluminum block. It is also quite durable when set up correctly.
One of the strengths of the SB2.2 engine is the revised valve angles, but this also creates a few unique requirements. For example, the lifter bores are offset, which requires special matching lifter sets. The SB2 lifters are solid rollers supplied by Crower and are oversized to 0.937-inch diameter to allow all the largest rollers possible. The lifters activate the 1.75:1 shaft-mount rockers through 7/16 inch, 0.165-wall pushrods, which are two different lengths because of the lifter-bore offset.
Another unique facet of this motor is its crank-trigger controlled ignition. Clements uses the crank trigger because it is more precise than using the distributor to control spark timing. This is because there are lots of opportunities for tolerances to stack or flex to be introduced between the crankshaft, timing assembly, camshaft, distributor gear, and finally the distributor. These variances are more than enough to significantly affect when the spark ignites the air/fuel mixture.
A crank trigger assembly eliminates nearly all of those mechanical connections. It is connected directly to the snout of the crank and uses a magnetic pickup to signal the ignition exactly when to fire each spark plug. Proof that this system can be beneficial shows up not only on the dyno, but also when setting the engine timing. In many cases you can see that the timing is much more stable when shining a timing light on the advance scale scribed on the side of the balancer.
Although Clements sometimes experiments with different firing orders for their Chevrolet engines, this SB2 utilizes a standard 1, 8, 4, 3, 6, 5, 7, 2 firing order. The spark plugs are gapped to 0.035 of an inch.
Written by Tony Huntimer and Posted with Permission of CarTechBooks