A Big-Power 358 Small-Block Dirt Late Model Spec Engine Build

Over the past several years the popularity of dirt track racing has increased significantly. Nationally, dirt track racing is more popular than NASCAR these days. Dirt track racing is where it all began and maybe spectators are focusing more attention to grassroots style venues and wholesome entertainment for their family. 

There are several different classes in dirt oval racing, ranging from an entry-level Hobby Stock/Pure Stock class to a pro-series Super Late Model platform. For this engine build, we’re going to focus on the Late Model class because they get a lot of attention at the local tracks. Late Models are fast, but racing sanctions tend to be more regional rather than national, with a few exceptions. 

Our primary goal was to build a max effort dirt Late Model engine on a modest budget to be reliable and consistent within the limitations of the rules. This is no easy task but can be done if you choose the right parts and components. (In order to dyno test the new 358ci dirt Late Model engine, we had to mount the dry-sump pump to the front of the engine using adaptors and pulleys).

Keeping On A Budget

If you read the rules for your local dirt Late Model racing sanction, you will likely find many different engine combinations are allowed. The reason for these different engine combinations is to try to even the playing field and still maintain a “sportsman” budget.

With the use of crate engines, the cost of dirt Late Model racing can be affordable and fun. Over the past several years, crate engines have made their way into dirt Late Model classes and have proven to be reliable. But, there are still dirt Late Model racers who want more power without going into the ranks of a Super Late Model budget.

Take, for example, one local racing association known as the Blue Ridge Outlaw Dirt Late Model Series. In the rule book, there are four different engine combinations allowed. Each engine option has its power potential, so more power will incur penalties such as additional weight, or the mandatory use of harder tires. This is done to keep the competition even and not let money take control to overrun the budget-themed program. Racers often choose the spec-engine package because the engine combination works with the car’s current total weight with the driver.    

The Dart Iron Eagle 215 cylinder heads are offered with a shallow 49cc combustion chamber. Notice the edge of the intake valves sticks out right above the deck of the cylinder head. We felt it would be a great choice in achieving our desired compression ratio since the rules state that you have to use a flat-top piston. There is no rule on valve size and pocket porting is permitted, the heads were assembled with 2.08-inch intake and 1.60-inch exhaust valves.

Nationally, there are two basic Late Model spec engine combinations to choose from. These combinations are differentiated by which cylinder head you choose. The first option is to build the engine with Brodix spec-aluminum cylinder heads. A lot of racing classes use the Brodix spec cylinder heads because they are cost-effective and durable for racing applications. They are outfitted with 2.08-inch intake and 1.60-inch exhaust valves and feature a 64cc combustion chamber.

The second option is the use of Dart 215 Iron Eagle cylinder heads. These cylinder heads are made of cast-iron. They are quite heavy, outfitted with 2.05-inch intake valves and 1.60-inch exhaust valves. Deciding which cylinder head combination to choose is the primary concern. 

We know what you are thinking, “use the aluminum cylinder heads,” right? While these heads may weigh a lot less than the iron heads, they are not the best choice for our application. The rules state the maximum allowable cubic inches is 365, and you must use a flat top piston. To build power you have to make compression.

Even though the Iron Eagle cylinder heads are heavy, they are offered with a 49cc or 64cc combustion chamber size. More compression equals more power. If the rules allowed the use of a dome piston, then the aluminum would be a better choice. But, there is power to be made with a cast-iron cylinder head and we will dive more into the physics of that in the future.

The use of an aftermarket block is necessary, not only for the strength and rigidity but also for the use of priority main oiling. Dart released a new small-block Chevrolet (SHP PRO) that is 25 pounds lighter and has a big-block Chevrolet cam tunnel with .904-inch lifter bores which will offer some stability in the valve train for the high-rpm engine build.

Big Power In A Small-Block

Our goal was to build this spec engine using the Dart Iron Eagle 215 cylinder heads and try to make as much power as we could within our cubic-inch limits. The rules do not give any specifics on how you can obtain the maximum cubic-inch. In other words, they do not list a specific bore-and-stroke combination for the 365 cubic-inch rule. A big bore with a small stroke would be the ideal choice such as a bore of 4.165-inches and a stroke of 3.25-inches.

Even though bigger-bore engines do have their advantages, we were still trying to keep a budget. The reason I say this is because of parts availability. With the shallow combustion chambers of the cylinder heads, the valve reliefs become a problem, especially when using a small piston-ring package.

The largest bore size piston offered from most manufacturers using this cylinder head is 4.155-inches. If you want a larger bore size, you have to get a custom set of pistons which means more money. You could choose to use the 4.155-inch bore piston but you’d need to get a crankshaft with a stroke of 3.35-inches. While they are also available, these crankshafts are double the price.

The crankshaft and connecting rods were purchased from Liberty Engine Parts. The 4340 forged crankshaft is a 3.50-inch stroke ultra-light design weighing in at 46 pounds and the connecting rods were a forged H-beam design 5.700-inches long.

Playing With Blocks

Our best option was to choose an engine combination using a 4.030-inch bore and a stroke of 3.50-inches. These parts are readily available and the cost for forged pieces is very reasonable. This bore and stroke combination may sacrifice some horsepower, but picks up a little increase in torque. The additional torque would help out because most of the tracks range from 1/4 to 3/8 of a mile. 

Dart blocks introduced the SHP “PRO” series small-block Chevrolet which works great for our combination. The “PRO” block is lightened, weighing in at around 175 pounds and is offered with a big block Chevrolet cam tunnel and .904-inch diameter lifter bores. This allows us to get some weight off of the front of the car and also the use of bigger cam and lifters helps stabilize the valvetrain. Dart offers this block with two different size bores of 4.125-inches or 4.030-inches.

The camshaft, lifters, and valve springs were purchased from COMP Cams. The camshaft is a special solid-roller grind with a 4/7 cylinder swap having 258- and 266-degrees duration @.050-inch with .705- and .712-inch valve lift. The larger diameter lifters are part number 96904B6.

Our next quest was to choose our connecting rod length. Connecting rod length is a touchy subject because every engine builder has their theories about this subject. The rod ratio should always be considered when building an engine, but this will always depend on the application.

For our application, we choose to build the engine with 5.700-inch length rods. Most engine builders will use 6.00” rods. I would have to agree because I have used this length many times in a lot of engines. The reason for the 5.700-inch length was to minimize some of the piston rock in the cylinder bores. When you move the wrist pin higher in the piston, you lose some of the piston skirt.

Skirting The Issue

A little more piston skirt might be beneficial for a dirt engine. Unless there is a mechanical failure, most Late Model dirt engines are refreshed once a year. A dirt engine lives a very crude life. It doesn’t matter how good of an air filter you run, or how many times you change it, there will be dirt ingested into the engine. When tearing a dirt engine down you will always find dirt in the venturi of the carburetor, intake plenum, and intake runners of the cylinder heads.

Mahle‘s flat-top pistons are made for the shallow combustion chambers of the Dart cylinder heads. These pistons are an off-the-shelf item.

As for the upper components of the engine, the rules are pretty lenient. You have to run a cast-aluminum, production-style intake with a 4150 carburetor. The intake manifold can be ported and there is no rule on the CFM size of the carburetor. We choose a Holley intake (P/N 300-110), in which we did some minor port and polishing, and a Holley carburetor (P/N 80496). We took the carburetor to Maloney Competition in Martinsville, Virginia, and had modifications done to the baseplate, venturi, and boosters.

Wet Or Dry Sump?

Rules also allow for the use of a wet-sump or dry-sump oiling system. Even though wet-sump oiling is cost-effective, it is hard to control the oil and windage inside the engine of a dirt car. Think about it this way, the car goes into the turn and you have eight quarts of oil in the pan. It doesn’t matter what oil pan you use, the oil in the pan is going to be sitting at roughly 45-degrees when the car is in the apex of the corner.

Because the engine block’s lifter-valley design features windows between the lifter bores, we pulled the three scavenge stages from the oil pan. We also used Cometic .040-inch thick 4.060-inch bore head gaskets.

The crankshaft is churning the oil like butter which is slowing the engine speed down. Not only windage but think about the air getting whipped into the oil. It’s like a super-high-speed blender whipping it into a froth which is horrible for the bearings. Air ingress into the oiling system is only asking for trouble. Also, the vacuum being pulled on the crankcase by the scavenge stages helps relieve crankcase pressure which promotes better piston-ring seal. Dry-sump oiling systems help alleviate these problems associated with wet-sump oiling.

We were able to find a used Barnes 4-stage dry-sump oil pump which we sent to Barnes to rebuild and make sure it was ok. Because the lifter valley of the block has windows which allow the oil to go to the pan, we only need to pull our scavenge stages from the oil pan. The major factor of dry-sump oiling systems is usually the expense which is not really considered in sportsman racing classes. But, for our new engine build, we wanted to get the best insurance possible. If you factor in the pump, lines, tank, filter, and cooler, the cost will be around $5,000.   

The 358 dirt spec engine was built for David Minton’s 3D Motorsports car. David Minton was the points champion last year and competes regularly in the Blue Ridge Outlaw Late Model Class. (Photo courtesy Brian Metski)

The dry-sump oil pump is mounted to the transmission bellhousing. It is driven from a belt that runs from a pulley mounted to the flywheel. This location of the dry-sump system is purposely done by the manufacturer to keep the weight of the dry-sump components in the middle of the chassis.

For dyno purposes, we were able to find a front engine mount from Jones Racing Products so we could attach our pump to the front of the engine and drive the belt from the front of the crankshaft. During our dyno testing, the Late Model engine produced 649.5 horsepower @ 6800 rpm and 510 lb-ft of torque @ 5200 rpm using Driven Racing brand break-in oil and VP 110 racing fuel – and we did it on a sportsman budget.      

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