Dart Short-Block Motor - Power in a Package
We Combine Dart's New 400ci Short-Block With One Of Their Top End Induction Kits For 523 Hp The Easy Way.
From the January, 2010 issue of Popular Hot Rodding
By Steve Dulcich
Photography by Johnny Hunkins
Dart's SHP short-block allows...
Dart's SHP short-block allows the builder to fill out the details with parts to meet his exact needs. We built this one with a Dart top end, Holley carb, and a hydraulic roller COMP Thumpr cam to make 523 reliable horsepower.
There are many paths to putting together a true high-performance street engine, but when starting from scratch, you'd better be ready to set aside plenty of time to go along with the money. As seasoned car guys, we've been down that road plenty of times, from rummaging the boneyards and swap meets looking for a suitable core, to tearing down the greasy mess and trucking it down to the machine shop. Still, it's a roll of the dice until an inspection gives the basic core components the nod, then it's a matter of gathering parts, machining, and waiting. Many guys will look to sidestep the entire process, instead looking toward the convenience of crate engines, however, the Dart SHP (Street High Performance) short-block offers yet another approach that is definitely worth a look.
While a crate is certainly the most time effective way to a high-performance engine, this approach typically comes with some compromises. Often, the engine combinations are "one size fits all," or based upon a limited number of combination choices. With the SHP short-block, what you get is a brand-new Dart SHP iron block filled with a brand-new balanced rotating assembly. When you really look at a short-block assembly as an alternative to a full build or a crate, there are some real advantages. With the bottom end built, the most time-consuming aspect of the engine project is already done and ready to go, but the options on heads, cam, valvetrain, and other go-fast ingredients are still wide open to build the combination any way you want. Really, the combinations are virtually limitless, resulting in an engine equipped with the components that are right on target for your combination.
We combined Dart's SHP short-block...
We combined Dart's SHP short-block and their top end kit to find the fast track to street performance. PHR's Tech Editor Liz Miles unloaded the goods at Outlaw Racing Engines where Andy Mitchell bolted it all together.
Whatever engine package you might conjure up will only be as good as the foundation it is built on. If that foundation is a stock 40-year-old block, it's already compromised. For serious power and long-term reliability, there isn't any production core that is a match for today's breed of aftermarket blocks. Dart is a leader in this field, with a long history of manufacturing Chevy blocks in a wide range of configurations. The SHP block is targeted right at the high-performance street enthusiast, taking some of the best features of Dart's race castings and offering them in an affordable block built to take the punishment.
These Siamese-bore castings have the extra meat where it counts, with thick cylinder wall spec'd out at a minimum of .230 inch at a bore size of 4.165 inches, heavier main webs and deck, and all poured from a superior grade of iron alloy. All of the boltholes are blind, eliminating the need to slop sealer on the cylinder head fasteners, and the bottom end is secured with splayed-cap four-bolt mains. Add in the priority-main oiling system, revised cooling jackets, and you've got a foundation that puts the engine in another league compared to a junkyard block.
We covered the fine points...
We covered the fine points of the Dart short-block assembly in the December 2009 issue of PHR. Here, Mitchell does a quick "nut-and-bolt" check of the main and rod caps to double-check Dart's work. Everything was in fine order.
The SHP short-block program takes the SHP block to the next level, providing a fully machined assembly precision-fitted with the basics of a solid performance engine. The short-blocks are available in 4.125x3.48-inch, or 4.125x3.75-inch bore/stroke specifications, producing 372 or 400 cubic inches. The base package here includes a cast-steel crank, I-beam rods, and hypereutectic pistons priced at $2,975 for the assembled 372-cube package. From there, options include forged pistons, a 4340 forged crank, and H-beam rods, allowing the short-block to be tailored to your specific needs.
We were looking to put together a nice 400-cube small-block Chevy, to build a hot-street pump-gas combination with a minimum of 500 hp and reliability that we could count on for the long haul. After exploring the options, the Dart SHP short just made sense as the starting point. Considering the scarcity of production big-bore 400 blocks, and weighing that against the improvements with the Dart unit, it really didn't take much to tip the scales to favor the SHP block as the basis of the build. Ultimately, we went all the way with the assembled short, considering it to present a good value for machining and assembly given the quality components used. Working the numbers, we would have a hard time equaling the short-block assembly for the money, not even accounting for the additional time required when starting from a bare block. To make the bottom end as stout as possible, we ticked all of the option boxes when making the order, upgrading to the forged crank and pistons with H-beam rods (bringing the short-block total to $4,184.66). And while we didn't see these upgrades as an absolute necessity for the power level achieved in this story, we do have our eyes set on upgrading to a more potent top end and valvetrain at a later date.
Naturally, the camshaft selection...
Naturally, the camshaft selection will have everything to do with the power production and personality of the completed engine. We went with a COMP Big Mutha Thumpr hydraulic roller, because the extended duration and tight lobe centers make power and hit with the sound of plenty of attitude. We went with the largest of the three Thumpr grinds-just as we did with our Howitzer 496 big-block. These things work!
With the short-block taken care of, we could focus our attention on the power parts to complete our combination. Since dependable street use was our target for this story, the camshaft would be a hydraulic roller. The roller cam would eliminate the need for a tedious cam break-in procedure, and provide more lift than a conventional flat-tappet cam. We weren't shy when it came to our cam choice, going with a healthy COMP Cams Big Mutha Thumpr stick, part number 08-602-8. Spec'd out at 299/319 gross duration (243/257 at .050), with .533-/.519-inch lift on a 107-degree lobe separation angle, the tight separation and extended exhaust duration would definitely have some bark. The Dart block is already configured to accept the full OEM hydraulic roller system, but instead we opted to step up to COMP's new short-travel hydraulic lifers, which are available as retrofit-style lifters only. [More on these extraordinary pieces to come in a future issue.-ed] The limited-travel lifters allow the cam to act more like a solid at high rpm, reducing the likelihood of lifter instability problems when turning tight.
As with the camshaft, by starting with the short-block, that left the cylinder head options wide open. Here, we looked no further than the Dart catalog, going with the Dart Pro-1 200cc aluminum heads with 64cc chambers. These as-cast Pro-1 heads offer a good balance of cost, flow, and runner size, putting them right on target for our displacement and rpm range. To accommodate the heads, the springs were upgraded to COMP number 930-16 pieces (a 1.550-inch diameter dual-spring assembly). The large-diameter springs can create clearance issues with some rockers, but we found they worked fine with a set of COMP Ultra Gold aluminum rockers in a 1.5:1 ratio. To provide the link between the rockers and the lifters, we went with a set of Manley hardened 5/16-inch pushrods in a length of 7.250 inches.
A COMP No. 7136 billet timing...
A COMP No. 7136 billet timing set was dialed in to an installed centerline of 102 degrees. Mitchell aligned the upper and lower gears to perfection by machining the GM thrust plate.
To round-out our power-parts combination, we still had the induction system to consider. Here, we weighed the potential benefit of a single-plane manifold for top end power against the stronger overall torque curve of a dual-plane. Within the realm of street rpm of under 6,500 rpm, a modern dual-plane will almost always show more average power over the engine's operating range. To keep the major castings all in the same family, we went with a Dart dual-plane, an intake manifold that is regarded by many engine builders as one of the best of this configuration. (Note that you'll need to chase down the proper pipe plug fittings for the manifold and cylinder heads ahead of time.) To top the manifold, we had a couple of Holley carburetors to try, a brand-new 750-cfm Ultra Double Pumper (anodized blue to match our Moroso ignition gear), and one of our old favorites, an 850 Street HP.
With the major components of the build selected, the parts were taken to Andy Mitchell's Outlaw Racing Engines for final assembly. Having a complete short-block as a starting point, the assembly process was completed very quickly, with no undue surprises. Mitchell degreed the cam in to an installed centerline of 102 degrees, using a COMP timing set, and locked it down with an OEM GM thrust plate. A COMP two-piece aluminum timing cover (to aid future cam swaps) was use to seal the front of the engine, with a TCI Rattler damper hung on the nose of the crank. The heads went on with a set of ARP bolts, giving a compression ratio calculated at 10.66 by Mitchell based on 64cc combustion chambers, .039-inch thick Fel-Pro gaskets, and .028-inch piston deck. Providing for the lubrication, Mitchell used all Moroso components, including an Eliminator seven-quart pan and standard-volume pump. Turning back upstairs, a Moroso DuraFire HEI distributor and Blue Max wires put the final touches to our completed engine.
On The Dyno
Anytime a new combination is put together it comes down to the power numbers to gauge the success of the effort. We have to admit that this engine assembly all came together with out-of-the-box parts, with none of the custom "massaging" that some of our engines get. Really, to all of us involved, it just seemed too easy this way, and it made us wonder if it could really "turn a number." This apprehension was at its peak the day the engine was bolted to the Westech engine dyno. Editor Johnny Hunkins was responsible for most of the parts selection, and it did look good on paper. With the cubes, heads, cam, and compression, the ingredients were there for power. We were looking for an honest 500 hp on pump gas, and I reassured Hunkins that "anything we see over 500 hp is pure gravy."
Westech's Steve Brule, our dyno operator, had first-hand experience with these Dart SHP engine combinations in the past, having personally put together and tested a 372-cube version. As he put it, "These Dart short-blocks are nice. It's the highest-quality crate short-block I've ever seen." Needless to say, Brule confidently predicted that the engine would make our target with ease.
To seal off the front assembly,...
To seal off the front assembly, a COMP billet two-piece timing cover provides the advantage of not having to disturb the oil pan to access the cam. To ensure accurate ignition timing settings, once the TCI Rattler damper was installed, Mitchell confirmed the TDC setting, and locked down the timing pointer.
With the timing set conservatively at the damper, the Dart SHP fired instantly, and with a few preliminary checks of the timing and fuel, the engine was allowed to seat the rings on the SuperFlow dyno's automated break-in cycle. Next, it was what we had been waiting for: power testing. If the engine build seemed a little too straightforward, the same can be said of our dyno test session. Right out of the box, the dyno readout showed our first carburetor, the Holley 850 Street HP, was very close on jetting, and the power curve was picture perfect. We had an almost ideal dyno graph, all the way from the bottom to the top, with no flat spots, dips, or instability from 3,500 to 6,600 rpm. Better still, we were well past our 500 hp target. With some minor adjustments of the jetting and timing, the printout showed 523 lb-ft of torque at 4,400 rpm, and 523 horsepower at 5,700 through 6,200 rpm. The power curve was fat, with almost 500 lb-ft right at the bottom of the test at 3,500 rpm, and a power peak that just hung on at the top.
We still had the 750 Ultra Double Pumper to test, a new lightweight aluminum-bodied carb from Holley that features an electronic choke for street driving. The 400-cube Chevrolet certainly did seem to like our big 850-cfm baseline carb, and we had to expect a power penalty going with the smaller 750. As we went into the pulls, the mixture numbers were virtually dead-on, and the 750 Ultra's power was within a kitty cat's hair of the bigger Holley. Even with a 750-cfm carb, the combination delivered.
The oil system components...
The oil system components included a standard-volume pump, the pump drive, and stud were all Moroso items, as was the baffled seven-quart Eliminator oil pan. In keeping with the easy bolt-it-together theme, we knew that by staying with Moroso components throughout would promise us no surprises.
While there are still plenty of applications where a fully custom-built engine may be the right choice, we found the Dart short-block to be right on target for our objectives here. What we had was an all-new big-bore, big-inch small-block with power to spare, and components inside we can believe in. With the SHP short-block, we certainly shortcutted the build time and trouble, but the results showed we didn't get shortchanged on power. That's what we call a win-win situation.