Big horsepower, high reliability, and stock-like driveability--in order, those three parameters define the ultimate daily driven street engine. Each are, of course, highly desirable in a street car, and each is fairly easy to accomplish individually. So how is it that three simple characteristics can become so complicated when they're combined?
Even in this day of digitized keystroke tuning, only a select few wicked-fast street GMs possess these three attributes. It doesn't matter if it's big inches, big boost, or a nitrous motor--chances are that there's a spiky power curve here and an all-or-nothing rpm range there. And sometimes, there are parts everywhere.
Contrary to popular belief, the six-bolt-main-equipped LS1 and LS6 is prone to the same shortfalls that lesser GM mills suffer from--and in certain areas, more. Sure, it was both strong and powerful when it was nestled into the F- and Y-body platforms. But the high-tech sleeved cylinder case, a wonder when it is kept near-stock, has its own unique set of problems when the dyno line heads north. And for pure power output, even the mighty Gen. III is subject to the "no replacement for displacement" rule. 346 naturally aspirated inches get issues when 450 emissions-legal horses hit the pavement. Heading down the supernaturally aspirated road means much higher power outputs, as well as considering a forged crank and pistons set in place of the nodular iron and hypereutectic stockers. And therein lies the rub: Once that stock-displacement head/cam LS1 is all out of steam, making serious power live on the street gets to be a serious challenge.
Houston, Texas-based Motorsport Technologies is up for that challenge, which is very good news for you. Proprietor Jayson Cohen and his staff have spent the last 10 years producing some of the fastest Camaros, Firebirds, and Corvettes in the country, and Jayson soon noticed that the stock-cube LS1s could only breathe so hard. Extensive research and development time was spent to bore and stroke the LS1/LS6 cylinder case to 427 cubic inches--providing 50-plus more horsepower than the wildest heads/cam Gen. III, along with stock-like manners and longevity. The idea was to create a quick-revving, big-inch motor that would make 500-plus ponies, with enough low-end left over to get you around town.
"Depending on the customer's cam and exhaust system selections, 525 horsepower and around 500 ft.-lbs. of torque at the tires is within easy reach of this motor," Cohen explains. "Some of our customers who don't like to show their hand about how much is under the hood simply go with a quieter muffler and a smaller cam, for around 480 horses at the wheels."
MTI converts regular LS1s into 427-inch strokers at the rate of two a week. Around 50 percent of these mills are sent out mail-order; In addition to all of the happy Americans, customers as far away as Kuwait, Australia, and Norway have reaped the benefits of big-inch Gen. III power. GMHTP was interested in what it took to transform a 346-inch motor into a big-inch 427, and Pecos Loughlin, MTI's head machinist, gave us the guided tour. If you're ready to go big, follow along.
The factory GM sleeve, identified by its ribbed construction, is on the left. MTI uses a centrifugally cast iron sleeve, since cast iron is the best material for ring seal properties. | 
A side-by-side comparison of the stock 3.622-inch LS1 crank (top) and a 4-inch Lunati unit (bottom). MTI will use the cast stock crank for naturally aspirated 387-cube motors that can push upwards of 460 rear-wheel horsepower. Beyond that and for juiced or force-inducted motors, a forged crank becomes necessary. This Lunati crank for the 427 build is an affordable foundation for the LS1s and LS6s where going to a billet piece isn't crucial, and MTI hasn't had a failure yet. | 
This shot is of the crank's fillets. The stock unit's rolled fillet journal (background) is recessed and prone to cracking, whereas the Lunati crank's fillet (foreground) is a more traditional rounded design for more strength. |
Here is the 4-inch Lunati crank and a set of eight custom-made Wiseco pistons. Lunati 6.125-inch Pro Billet rods will be swinging the Wisecos in this application. MTI's crew doesn't recommend using stock rods in a performance application--they are good rods for factory parts, but Jayson and company has seen the powdered metal units bend and break in nitrous applications. MTI will still build a motor with them at the customer's request, but he or she will be driving away without the benefit of MTI's 2-year/24,000-mile warranty. | 
The crack cap of the stock rods. During the production process this method is cost-effective and easy to do: GM bores out the center to the correct outer diameter, then a groove is machined at the center of the two halves to act as a stress riser. That riser makes the rod break perfectly in half in a later step. This method is good enough for a factory application, but because there is a permanent crack, there is no way to resize the big end of the rod in a performance application. Machining is always needed in a normal small-block GM to verify that they are perfectly round and to get the proper crush on the bearing. | 
The machining process gets going with the block being bored with a Kwik-Way FL boring bar. An interchangeable carbide bit does the cutting. Six passes and 45 minutes are needed to completely bore each cylinder. After the cutting is complete Pecos measures the bore size with a dial bore gauge to verify the press fit--the hole in the block is 2.5 thousandths smaller than the sleeve, the interference fit is what will hold the sleeve in the block. An incorrect size will cause water leakage issues. |
MTI uses a proprietary method to install the new sleeves into the cylinder case. Pecos heats the block in a specially prepared oven to 250 degrees Fahrenheit. The block is removed from the oven and placed on a special fixture in preparation to receive the sleeve. The sleeves are then lowered into a container of liquid nitrogen, thereby reducing their overall size. The sleeves are removed from the nitrogen one by one and are lowered into each cylinder by hand--don't try this at home. With the sleeves in place, Pecos uses the carbide bit in the Kwik-Way FL to cut down the excess sleeve material in each bore until it is flush with the deck. The next step is boring the sleeves from the original size of 4.110 inches to 4.125 in the Sunnen CK-10 boring machine. | 
A block surfacer, in conjunction with a CBN diamond cutter, is used to surface the block. | 
Each cylinder's final hone is done on the Sunnen CK-10 with an aluminum torque plate installed on the block. 70-grit stones are used to get the bore to 3 thousandths smaller than the final bore, and the 280-grit stones gets each bore to the final size. |
The next step is to install the rotating assembly. Federal Mogul mains and Clevite rod bearings are used on all of MTI's engine packages. Here, Pecos has installed the bottom bearings in anticipation of the Lunati crank. | 
He gently places the crank into position... | 
... then prepares to install the cap/bearing combos. |
Loughlin begins the installation of the main caps. The stock bearing cap bolts are tightened from the innermost out. The centers and outers are torque-to-yield spec, while the side bolts are torqued to 18 lb.-ft. MTI looks for 2.5-3 thousandths clearance on the main bearings, 1.8-2 thousandths on the rod bearings. | 
The Lunati rod/Wiseco piston combos are installed next... | 
...with the rod bolts being tightened to 70 lb.-ft. |
The camshaft used in this particular application is MTI's R1 grind: .574/.578 lift and 232o/236o duration at .050 with a 112-degree lobe separation angle. | 
Custom Manley 7.4-inch pushrods, Crane roller lifters, and a Rollmaster double timing chain utilizing a Torrington bearing complement the bumpstick. | 
The two shims are used to space out the oil pump for the wider double chain's clearance. |
Finally, the Rollmaster double roller timing chain is placed and tightened down to 28 lb.-ft. | 
With our bored, sleeved and stroked bottom end in place, we'll leave off for now. Next issue we'll build a set of MTI's hand-ported Stage 3 LS6 heads--how does 330+ cfm with stock manners sound? Next, head dowels are installed to locate the heads, and the crank key is installed. | |