No professional engine builders here. Just a guy in a greasy sweatshirt torquing head bolts on a Sunday night at 11 p.m.
'Smith is from the Michael Jackson school of latex gloves.
'Face it: Bigger is better.
Since it's a jungle out there, where the meek get eaten for lunch, if you wanna be Tarzan, you'd better swing a bigger club. The days of a 396 or even a 454 as king of the jungle are long gone. When we stumbled upon this used 496ci Rat motor for a righteous price, we snapped it up thinking we could build an affordable big-inch Rat. The problem comes from using the terms "affordable" and "Rat motor" in the same sentence. Nevertheless, that's the challenge we decided to take on. We think Tarzan would approve.
We knew that bitchin' cylinder heads, a good cam, and a healthy carb and intake would be where we would have to spend our money, and that's exactly what we did. There are dozens (if not hundreds) of ways to build a strong, normally aspirated big-block Chevy, so this opened up a ton of options. And since we made over 700 hp on the first try, this is not the last time you will see this motor.
After JGM had cut the stock Speed-Pro hypereutectic pistons, we double-checked our valve-to-piston clearance to ensure the valves would not crash the piston party at high rpm. Unfortunately, this also reduced the static compression ratio down to a realistic 9.9:1 We installed the pistons with a 4.310-inch Powerhouse tapered ring compressor.
The Beginning
It was a typical Sunday at the swap meet. There, among the cheap rags and rusty Buick valve covers, was a lump of black-painted iron. It didn't look like much, really. Taped to its side was a hand-scrawled sign that proclaimed this particular round mound of poundage as a 496ci Rat motor (4.310 bore, 4.250 stroke). The owner couldn't remember who made the crank ("Scat . . . maybe," he said), and he wasn't sure if it was even a four-bolt main block. Despite his vague description, he looked honest. Even its history was dubious: "When we pulled it, it wasn't running right." But the price was right at $800, so we took a chance and loaded the lump in the back of our GMC pickup.
A quick autopsy revealed the deck was anything but square, but at least the cam wasn't flat. We found a 1-inch length of broken pushrod lodged in the oil-pump pickup, and most of the bearings were hideously scored. It looked like our Rat had been assembled on the floor of a rodeo ring with all that bulls%*# and dirt in the motor.
We took our orphan Rat to the wizard machinists at Jim Grubbs Motorsports, where we discovered the four-bolt main block needed deck work and an align-hone to correct the oversized main-bearing-housing issues. JGM also resized the stock connecting rods that had previously been fitted with ARP rod bolts. At least the 0.060-over Speed-Pro hypereutectic pistons were in good shape. But we also discovered the valve pockets would have to be machined to clear the 2.250-inch valves used in the AFR heads we planned to run. A good portion of our budget was invested in JGM's quality machine work, but that also meant we now had control over all the clearances in the short-block, creating a solid foundation from which to build some power.
This is how our Rat ended up on our shop floor. The short-block consists of a cast 4.250-inch crank, stock rods, and 0.060-over Speed-Pro hypereutectic pistons with 51/464-inch rings.
Games Pushrods Play
The engine assembly went fairly routinely until it came time to set up the valvetrain. First, we had to adapt a 0.125-inch-thick aluminum plate to the inside of our stock timing-chain cover to get the proper 0.005-0.008 inch of camshaft endplay using a Comp Cams cam button. Then we ran into a snag with the pushrods.
The stock-length pushrods we thought would work fell far short, and we were puzzling until we did what some car crafters don't do-we read the directions. That's when we discovered these AFR heads come with 0.250-inch-longer intake valves and 0.100-inch-longer exhaust valves. This dictated custom-length pushrods to make the rocker-arm geometry work with the longer valves.
We used a pair of Comp Cams adjustable pushrods to determine the proper lengths. We started by adding 0.250 inch to the stock length of the intake pushrod and 0.100 inch to the length of the exhaust pushrod. Remember that Rat motors use different lengths for the intake and exhaust pushrods.