You guys crack me up!! Add water jackets to the aluminum blocks that top fuelers have and you have one bad ass block! Could you imagine a forged aluminum Gen 3 block? I'll still take aluminum over cast any day! You know the ole' saying about opinions right?

I like aluminum and will stick with it but here's a good cut and paste to help out the debate.

Cast Iron or Aluminum?
While the truck versions of the Gen III, which were phased into the truck/SUV lineup beginning in 1999, are made of cast iron for severe-duty applications, they have the same set of features as found on LS1 aluminum blocks. The SSR and some of the midsize SUVs are now built with aluminum-block Gen III engines, but the fullsize trucks and SUVs still have iron blocks. The iron version of the Gen III block is approximately 88 pounds heavier than the svelte, 116-pound aluminum block. The iron block is a not-too-shabby 204 pounds, so you’re probably wondering why anyone would run an iron block if the aluminum block is so lightweight.

Well, if you are making more than 700 hp (900-plus is routine today), the aluminum block starts to become a liability to long-term health. Simply put, the aluminum block was not designed to support the power the rest of the engine has shown itself capable of producing. The iron block, though, has been known to withstand well over 1,400 hp with a few additions/modifications, and it will handle a little over 1,000 hp in factory trim.

While these big dyno numbers might seem ludicrous, they’re not. The rotating/reciprocating assembly, cylinder heads, valvetrain, oiling system, ignition, electronic management and other base systems on the Gen III V-8 are the best pushrod design and execution ever. Period. They allow massive amounts of power to be generated in very civil packages, which is why block selection is critical to ending up with an engine that makes ridiculous power with a minimum of heartache.



The engine block’s casting number on the rear of the driver-side cylinder bank of every Gen III/IV V-8 block will tell you what you’re looking at. Refer to the accompanying chart for block casting numbers and service part numbers.So what does this mean to the average enthusiast? Well, if you’re modifying a ’98 Camaro with a set of CNC heads, a cam, and some headers for a 520hp package, your stock aluminum block should work just fine. But if you’re bolting a big centrifugal supercharger on your C5 Vette with visions of a streetable 700 hp, you should make some modifications to the block. In fact, if you want factory durability, you should be considering taking the 88-pound hit to swap in the iron block.

But that’s getting ahead of many choices that are on the table. While the overall horsepower you intend to make with your Gen III V-8 is key, issues like the engine’s purpose (drag, circle track, or road racing; offroad or street usage), required durability, cost, and allowable weight need to be addressed.

Anger Time
If you are building an all-out drag racing engine, the length of time the engine will be used in anger will be much, much less than a circle track or road racing engine or even a hot street engine. This means the necessary foundational stability of the block and the amount of heat the engine will need to shed during its usage is very different, as is how much wear and tear the engine will need to withstand between servicing. In general, the stock LS1 block is not used in high-powered drag racing applications, not because of the block’s strength, but because the bores cannot be machined out much beyond their stock 99mm sizes.

There are inserts available for a block that has had a lot of material removed from the cylinders to achieve a larger bore. They seem to work in short-cycle, drag racing applications, but for more street-type use, it’s probably a better idea to order a 4.125-inch bore C5-R GM Racing Gen III aluminum block or wait a few months for the new LS7 block (derived from the C5-R block). These blocks come with siamesed bores and bore liners pressed into the block, unlike production aluminum blocks that have cast-in-place liners.



The gerotor oil pump for the Gen III and IV V-8s slides over the snout of the crank and bolts to the front of the block. The driver side of the pump casting has a passage to feed pressurized oil through a galley in the block that runs to the back. Before getting to the back of the block, the oil travels down through the oil pan rail to the oil filter, then back up through the rear block cover to the lifter galleys and main bearing galley. From there, the oil follows the standard small-block Chevy split route of through the pushrods to the top of the engine and down to the cam, main, and rod bearings.Something else to take into account is the fact that aluminum blocks require greater clearances than the cast-iron blocks because aluminum grows approximately twice as much as iron. This means the major clearances (crankshaft main and rod, camshaft, piston to bore, and so on) need to be almost double what they would be in an iron block. We’re only talking thousandths of an inch (0.001) here, but it affects oil consumption, the required startup procedures (to bring everything up to temperature), and component life. The last issue is especially critical with solid-lifter cams. With the advances of hydraulic-roller lifters, the use of solid lifters is not as common. But in some full-on racing engines that use these blocks, the valve lash will start out very tight with the engine cold. Then, as the deck height grows from the engine coming up to temperature, it will be at its proper valve lash. Any hard use before the engine is up to temperature could lead to broken valvetrain parts.

Application Suggestions
In general, normally aspirated street engines below 650 hp can be supported by mid-’99-and-later stock LS1 aluminum blocks with just oil pressure/volume increases and basic blueprinting (honing cylinder bores with a deck plate, ensuring the bottom end is machined correctly, and so on). Notice that the ’97 through early ’99 blocks are excluded. If you’re going to start adding power, these blocks become a liability. They aren’t as strong as later blocks, and the oiling passage at the back of the block is restrictive to high-horsepower applications.

Centrifugally supercharged street engines of 700 to 1,200 hp should consider the cast-iron 6.0L truck block, with 800-plus horsepower engines using dowel-pinned steel main caps in iron blocks. In fact, above 650 hp, steel main caps are good insurance, but the stock powdered metal caps will work fine below this hp level on everything but nitrous’d engines. Also, on supercharged, turbocharged, or nitrous engines above 900 hp, a main-cap girdle should be used to stabilize the caps. Head studs of 11 mm or greater should be installed above 700 hp for maximum clamping force and used with performance multilayer-steel head gaskets. Boosted and dry-sump-oiling-system engines should use front and rear cover crank seals designed to handle the positive and negative crankcase pressure found in these applications.

Ultra-high-rpm applications should seriously consider using the ’01-and-later LS6 block and possibly dry-sump oiling. The LS6 block is suggested because the bay-to-bay breathing is much improved over previous Gen III blocks. The production wet-sump oiling system is good, but a dry sump is better because it can be designed to scavenge from the separate bays and eliminate this issue completely. This is one detriment of a deep-skirted block that GM has addressed with the new Z06 Corvette LS7 engine—it is lubed by a dry-sump oiling system from the factory.

Overall, the LS1 is superior to any small-block that has come before it. GM set out to build on the incredible legacy of this engine family, and the block design shows the company’s commitment to that goal. Without a doubt, this engine architecture will become the benchmark performance engine for years to come, and now you know the details behind its foundation. Enjoy the power. HRM