Hot rodders played with blocks as kids, and still play with them as adults. This vast level of experience notwithstanding, enthusiasts often insist on monkeying around with production blocks. They might be cheap, but after you've paid a painful $1,000 to $2,000 machine shop bill, it's still a production block that was never designed to handle the rigors of a street/strip or race application. While a stock block might work fine for a mild rebuild, with the power potential bequeathed by modern cylinder head and valvetrain technology, not many people are content with mild rebuilds these days. A cool 600 hp is becoming the new benchmark for small-block performance, and unless that Rat motor build hits 800 hp, a big-inch Mouse could very well put the hurt on you at the track. For serious stroker engine combos, the option of investing in a high-quality aftermarket block makes too much sense to ignore. Not only are they far stronger than the typical stock casting, they offer thicker cylinder walls and taller deck heights for increased displacement potential, improved oiling and cooling systems, and costly machining operations have already been performed prior to shipping. To get a handle on what to expect when stepping up to an aftermarket block, and how to choose the right options for your application, we consulted with Jack McInnis of Dart, Jason Neugent of Brodix, and Rocko Parker and Jeff Kettman from GM Performance Parts. Here's what we learned.
Aftermarket Block Basics
Jack McInnis: "Many people use production blocks to build performance engines, but aftermarket blocks offer several advantages in terms of durability and value. Aftermarket blocks are designed and manufactured for a completely different purpose than production blocks. Stockers were made with cheap production cost as top priority and were never intended to hold the kind of power we're making today. When the production small-block Chevy block was first designed, no one could have dreamed of the power potential today's cylinder heads and valvetrain components would provide. Aftermarket blocks benefit from decades of technology, both in racing and manufacturing, that has transpired since the original designs were made. Metallurgy, machining techniques, and overall design features are all vastly improved. The castings are designed with thicker cylinder walls which allow larger bore sizes and superior ring seal. Dart blocks also have thicker decks, and various deck heights are available to allow more flexibility in designing engine combinations. Likewise, the main caps are much stronger, and the water jackets are enlarged to enhance cooling. Another consideration is that decent used cores are getting harder to find, and by the time you have one cleaned, machined, and checked, you've spent very close to what a new aftermarket block would have cost. With an aftermarket block, you're putting your money towards hardware instead of labor."
Jason Neugent: "An aftermarket block is usually made at a slower pace and is a product of greater attention to detail than a production unit. Unlike a production block, strength and accuracy are the primary goals. Block tolerances must be tight, and stability in the cylinder sleeves and mains is critical as well. All these issues are addressed with an aftermarket casting. Additionally, aftermarket blocks have several options available, such as a variety of bore sizes, cam heights, deck heights, and lifter bore diameters. That just gives an engine builder much more flexibility in terms of displacement options, bore and stroke configurations, and valvetrain selection. Our blocks also feature reinforced lifter valleys, splayed billet mains, increased pan rail clearance, and revised oil passages. By far, the most compelling reasons to go with an aftermarket block instead of an OEM unit are because of their much tighter tolerances and stronger casting design."
Rocko Parker: "Production GM LS-series engines have an incredible track record for allowing customers to enjoy high performance as well as high efficiency from their production vehicle. Our engineering team took a look at the LS7 block specifically and adopted many of those features into the LSX Bowtie block, which is designed specifically for high-horsepower street and racing applications. The LSX block has several features that our customers demand for in their project car builds. It has six bolts per cylinder to assist in sealing the heads to the block for boosted applications, as well as priority oiling to the main bearings to assist in lubrication for high-rpm applications. It has additional bay-to-bay breathing, similar to the LS7, that production iron blocks do not require. Additionally, we have added two drilled and tapped holes in the front of the block for use with a front engine plate, and the LSX block has extra bulkhead material to endure greater crankshaft loads. The LSX block also features siamesed cylinder bores that allow for a larger piston than a production aluminum block. The LS7 aluminum blocks are also siamesed, but it has iron bore liners that limit the bore size to 4.125 inches."
Jack McInnis: "Every company takes its own approach to the amount of prep work their blocks receive before shipping, but Dart blocks are fully machined and only require a finish hone to suit your pistons and ring package. The only reason we don't finish the cylinders and lifter bores is because every piston and lifter manufacturer has their own tolerances, so there isn't a universal specification to finish to. Even so, we do offer custom machine work for specific purposes per customer request. A light deburring is a good safety precaution, and it is always important to thoroughly clean and inspect any block before assembling an engine. We also provide an option for completely finished and washed blocks based on customer supplied specifications. These are bagged, sealed, ready to assemble, and save you from having to drag your block to a machine shop and back. Some aftermarket blocks will require fairly extensive machine work before use, so that's an important factor to consider when comparing different blocks."
Rocko Parker: "The LSX block comes fully machined except in a couple areas, where engine builders perform their own prep work to suit their specific engine combo. The head deck surface is left at 0.020-inch taller than a production block, which allows the engine builder to fit the piston-to-deck surface to his liking. The crank main bores are also left with an additional 0.002-inch of material so that the engine builder can fit main bearing crush and clearance to his preferred values. All other machining is complete, and this adds a tremendous amount of value for consumers. When you start doing the math of a custom Gen III/IV engine build, our LSX Bowtie is a real value because of the extra machining we provide."
Jason Neugent: "The materials and alloys used during the casting process plays a big role in block strength, but blocks must also be reinforced in high-stress areas to optimize strength. This is another huge perk of an aftermarket block, as stock castings seldom have sufficient ribbing. Brodix blocks have strengthening ribs in the lifter valley area of our blocks, which substantially reduces deflection. We also offer half-way filled water jackets to increase the lower end strength of a block. Ultimately, block integrity all goes back to using good materials, quality foundry work, and proper heat-treating. If any of these elements are absent, the casting quality will be compromised. To ensure quality and strength, Brodix continuously dissects its castings to properly place the correct amount of material in the essential places."
Jack McInnis: "The alloys and metallurgy used in blocks is a complex subject which has many variables. All of our raw materials come from U.S. suppliers, and the quality of what goes into an alloy dramatically affects strength. Just like in a crankshaft, small amounts of different elements are added to the primary metal to alter its characteristics. Material selection is very important. For example, by adding just 0.02 percent copper to our aluminum alloy blocks, their strength at elevated temperatures is greatly improved. We do not use any remelted scrap in our aluminum because it tends to introduce contaminants. Another drawback of remelted metal is that it tends to have areas which are harder or softer in the finished piece. Variances in the casting process and post-process can also have dramatic effects on strength. Basically, OE blocks use alloys which favor reduced cost while providing adequate strength for their intended purpose, and aftermarket blocks utilize alloys which give elevated performance and strength levels. There are also some aftermarket blocks appearing on the market from offshore sources which are made from alloys based more on OE specs, but are very inconsistent in their strength and hardness properties.
"In our aluminum blocks, we use C355-T6 aluminum over the more common A356-T6, which is the strongest material available for manufacturing blocks and yields a highly consistent casting. A C355-T6 alloy remains very strong until 300 degrees, whereas 356-type alloys begin to lose their strength at 200 degrees. That 100 degree difference is very important in a high-performance engine. Plus, C355-T6 is extremely crack resistant, easy to repair, and has excellent tensile elongation properties."
Jason Neugent: "Brodix offers a block for small-block Chevys with a 4.500-inch bore spacing, and a block for big-blocks with a 5.000-inch bore spacing. This rewrites the book as far as displacement potential. These two blocks include the same design features and strength that Brodix blocks are known for. The difference is that their larger bore spacing allows for bigger bores and greater displacement figures. The 4.500-inch bore space blocks include features such as a 0.391 raised cam, deck heights from 9.000 to 9.625 inches, and lifter bores up to 0.937-inch. This block also includes reinforced lifter valleys, billet main caps with splayed bolt patterns, and sleeves with thick registers. The 4.500-inch bore space block will allow bores up to 4.250 inches, and gives you the potential to build a 500-plus-cubic-inch small-block. Likewise, the 5.000-inch bore space Rat motor block allows bore diameters up to 4.750 inches, which when combined with its taller deck height offers the potential of 800-plus cubic inches. This block features a 1-inch raised cam, billet main caps with a splayed bolt pattern, sleeves with thick registers, and a reinforced lifter valley. Deck heights from 11.200 to 11.625 inches are offered, and lifter bores up to 1.062 inches are available options. The larger bore spacing of these blocks is achieved by spreading out the center mains, so they require a custom crank, heads, and intake manifold. We already have heads for our 5.000-inch bore space blocks, along with adapters that allow running a standard BBC intake. These parts are also under development for a 4.500-inch bore space Mouse motor block."
Jack McInnis: "The perception that aluminum blocks lose power through decreased ring seal, as compared to iron blocks, is largely based on early aluminum blocks, which lacked the integrity we can achieve today. Aluminum blocks might have sacrificed power many years ago, but that's not the case anymore. A properly engineered modern aluminum block will make the same power as an iron block. We were recently involved in a direct comparison test with a big-block dragster engine making in excess of 1,100 hp. The engine was assembled and dyno tested with an iron Dart block, then disassembled and rebuilt using an aluminum Dart block. At the end of the day, the power figures were nominally identical for the iron and aluminum blocks."
Jason Neugent: "In the early days of manufacturing aftermarket aluminum blocks, the material that was used did indeed move around a lot under heavy abuse. This caused the cylinder sleeves to move, sink, or go out of round. The consequence was that this deflection would cause water leaks and poor ring seal, which are prime ingredients for poor reliability and power loss. Today we have solved these problems with the use of our virgin A-356 aluminum recipe, tighter tolerances, and more rugged sleeve materials. In independent testing, Brodix has witnessed our own aluminum blocks seal as well as a quality cast iron block."
Jack McInnis: "Core shift can be a major issue on some production blocks, as it results in inconsistent cylinder wall thickness from one block to another. Dart addresses the core shift issue in a couple of significant ways. We design and machine our own tooling for greater control of the mold-making process. Most importantly, we qualify and machine every block to maximize the cylinder wall thickness in the finished piece. Dart's CNC machining centers are designed to hold extremely tight tolerances over long production runs. Likewise, our fixturing process cuts down on the number of setups required and reduces stack-up tolerances. We also inspect our cutting tools at specific intervals to ensure that surface finishes are optimal in the machined parts, and continually check the finished blocks for quality control. In comparison, an OE block would be located for machining based on an external surface to speed production, which makes cylinder wall thickness very inconsistent. Dart blocks are actually located to maximize cylinder wall consistency during machining. Our block castings are made with much more attention to detail than the mass produced OE blocks, and this hands-on approach keeps core shift to a minimum."
Jason Neugent: "At Brodix, we have the advantage of having our own in-house foundry. As a result, our core shop is right next door, making it easier to monitor the entire casting process. With this arrangement, making changes to the tooling or machining is much easier and more efficient. Consequently, our aftermarket blocks are made with much tighter tolerances and much better core prints. Our system for coring blocks is very modern and watched over daily. Unlike the OEMs, we're not making thousands of blocks per week, so this lower production volume allows us to maintain better quality control. If there is a problem, we can catch it early on and scrap the problem blocks. On the other hand, if you're paying an outside foundry and don't catch a problem until several hundred blocks have been cast, you might be more inclined to try to fix the blocks and put them into circulation."
Rocko Parker: "Core shift is unacceptable in a performance engine build, so the new GMPP LSX block has five additional core plugs. This allows for the sand casting cores to make contact between the outside of the block and the cylinder liners. In total there are six supports inside and outside of the block when the iron is poured into the sand mold at the foundry. These additional efforts help keep the core from shifting."
Jack McInnis: "Stock blocks are often plagued by poor oil routing. In a typical OE design, the oil feed is routed through the cam bearings first, then splits off to the mains and valvetrain. This can lead to inadequate lubrication to the critical main bearing area, especially at high rpm. The stock systems also have quite a bit of restriction due to the way they are machined. High-volume oil pumps are commonly used to try and compensate for these restrictive oil passages. We relocate the main oil gallery alongside the cam tunnel so that the oil can be fed directly to the main bearings first. We also eliminate a lot of the restrictions so that oil flow is improved enough to where a high-volume oil pump is not required. Dart race blocks are machined to simplify installation of dry-sump oiling systems as well."
Jason Neugent: "Production blocks were designed based on the oiling demands of engines producing far less power than a typical performance engine. In many instances, the oiling system is simply antiquated, and routes oil to the cam bearings first instead of the mains. On the other hand, Brodix blocks have priority main oiling systems. This means that the oil will travel from the oil pump to the mains, then to the camshaft, and then finally to the lifter bores. This will ensure proper oiling for the mains, not rob the top end of oil, and still produce very good oil pressure. We have also enlarged some of the oil journals in our blocks from 1/4 to 5/16-inch for improved flow."
Rocko Parker: "The oiling system on the LSX block is different than GM's production LS-based engines. It has an additional oil feed drilled directly to the crank main bearing. This allows engine builders to reduce the feed to the lifters and use more of the oil to lubricate the crank and rods. This is something that we do not do with a production LS block because they use hydraulic lifters. The typical racer that buys an LSX block will probably opt for mechanical lifters. Likewise, the oil feed hole from the oil pump to the oil filter has been moved outboard from the centerline of the block. This allows for a larger crank throw by machining rod clearance into the block without contacting the feed supply to the filter."
LSX Block Design
Rocko Parker: "The LSX block design does not include any additional reinforcement ribs for structural support compared to a production block. The reason for this is that the production LS design underwent extensive analysis and testing to develop its strength and rigidity. The production LS7 has many improvements from past engines such as a deep-skirt block with six-bolt mains, and 10 additional head-to-block fasteners to stiffen the top of the block. These improvements have been maintained within the LSX block, which makes it a more solid combination to prior GM small-block designs. Furthermore, the LSX block design uses the architecture of the LS7 block, so the majority of the weight difference is attributable to its heavy-duty iron construction. The LSX block weighs 125 pounds more than the production LS7 block, and we have removed items like the lugs that the production machining process uses to transfer the block from station to station. Also, most of the gusset ribs have been removed, as they are not required due to the additional strength of cast iron. We assume all of our customers want eight cylinders firing all the time, so the bosses for the cylinder deactivation manifold have been removed as well."
Jeff Kettman: "In addition to the LSX block, GMPP offers a wide variety of Mouse and Rat motor blocks originally developed for racing. All the various Bowtie, Race, and Sportsman small-blocks that are offered by GM Racing were originally developed, tested, validated, and used in GM racing programs. For example, all the Bowtie blocks (1248xxxx part #) are based off the same casting that was used for our NASCAR engines when teams ran 18-degree cylinder heads. Then we moved on to the SB2.2 engines, and those blocks are now available as well. The ROx block was actually the first generation of the RO7 block that is now used in NASCAR. Of course, all the small-blocks ended up in use in many other forms of racing such as in NHRA, IMSA, Trans-Am, and SCORE.
"The Bowtie big-blocks were developed for Pro Stock drag racing and we now also offer DRCE blocks, which are a very specialized big-block used in today's Pro Stock cars. The ZL1 block is a different story. Most everybody is familiar with its history from the '60s and early '70s. What happened was that some GM race engineers were reviewing the list of tooling when Winter's foundry-who cast the original ZL1 block-was shutting down. Being gearheads, they recognized that some of the part numbers were for the ZL1 block tooling. Since the ZL1 was a quasi race engine to begin with, and it was produced in low volumes, it made sense for GM to reacquire the tooling. We refurbished it, brought it back to the market, and you can still buy an aluminum block 427 big-block."
Full Race Blocks
Jeff Kettman: "Two very interesting small-block Chevy blocks that GMPP offers are an 8.325-inch short-deck unit as well as a block with 283 mains. In an era when hot rodders want the tallest deck and max displacement possible, this might seem odd, but keep in mind that they were developed for very specific racing applications. The short-deck blocks were originally developed for Trans-Am racing where the displacement limit was 310 ci. In order to keep weight to a minimum and optimize the bore/stroke ratio, the GM Racing engineers came up with the short-deck engine. We also offer an 8.700-inch medium-deck blocks, which were designed for the NHRA Pro Stock Truck program. The 283-sized mains on these blocks came about as race engine builders looked for every advantage possible to make more horsepower. They determined that the smaller mains had less friction and therefore made more power. With today's engineering tools like FEA, they were able to see that the smaller main crankshafts could handle the loads with the smaller mains, and they could also get the added benefit of a lighter crankshaft."