Some things in life work out exactly as planned. Our Mean Little Rat has turned out to be just one of those examples. In the February issue we showed you how to build a 500hp "small" big-block using nothing but off-the-shelf parts for around $5K. This month we decided to turn up the wick and got this little beast to grunt out 540hp with a slightly different, albeit not much more, expensive combination.
The key to the newfound power was a simple lesson in good old-fashioned hot-rodding know-how. We swapped the intake and cam, added a trick new carb spacer, and then tuned on it until it gave all it could give. It was that simple. Follow along as we outline the details of our mini-Rat, and what we learned from our changes and testing.
First, a brief history of the engine to get you reacquainted. The basic short-block consists of a 31-year-old, two-bolt main, 396-cid big-block, which we had bored 0.060-inch to yield a final displacement of 408 cid. We paid $75 for a used GM forged crank from The Balance Shop in Reseda, California, and had them balance the entire rotating assembly. Then we hung full-floating 10.25:1 compression Speed-Pro forged pistons on a set of polished and shot-peened factory rods and sealed them in the bores with Total Seal gapless moly rings.
On top of the short-block went a pair of Brodix -1 OEFI oval-port aluminum heads that came bowl-blended and intake port matched from the factory. A new Comp Cams Xtreme Energy Street roller cam was slipped in and Comp's 1.7:1 aluminum roller rockers were lashed to open the 2.25/1.88-inch Brodix valves. An Edelbrock Performer RPM Air gap intake manifold was bolted on to distribute the fuel after it was mixed with the correct amount of air by the 750-cfm Speed Demon carb. MSD ignition fired the Autolite No. 53 plugs and we came away with 500hp at 6,250 rpm and 483 ft-lbs of torque at 4,750. Not too bad for a baby Rat that tips the scales only 40 pounds heavier than an iron-headed small-block would, and does it all on pump gas with off-the-shelf parts.
Like most enthusiasts, we wanted to make more power this time and in pursuit of that goal we swapped in Comp's next size larger XR280R Xtreme Energy roller cam and replaced the dual-plane Performer RPM Air-gap intake with Edelbrock's 454-O Victor Jr. single-plane. Then we slipped Wilson's trick new four-into-one tapered carb spacer under the same Speed Demon 750 carb that we used in the last series of tests and with just a little tuning on Westech's Superflow dyno we found more than 540hp and 500 ft-lbs of torque! If you do the math you'll see that this 510-pound, pump-gas Rat made a sturdy 1.3 horsepower-per-cubic inch with out-of-the-box parts. And it didn't require a second mortgage to build either. Where else can you find a bargain like this?
The Dyno Dance Pt. II
The 408-cid big-block huffed and puffed its way into the horsepower hall of fame this time and we didn't have to consult with a loan officer to get it there. Power was up across the board after we swapped in a bigger cam and bolted on a single-plane intake. If in fact, the Rat was able to crank out more bottom-end torque than it ever had before. Dyno readings began at 3,200 rpm where the fat-block punched out a gut wrenching 433 ft-lbs of torque. And this wasn't just a spike on the horizon either, because the engine carried more than 430 ft-lbs of torque from 3,500 to 6,500 rpm which is where horsepower finally peaked. Even after power began to drop off at the top end the engine still made 417 ft-lbs at 6,750 rpm. Here is a sampling of one of the best power pulls with 36 degrees total ignition timing, and a 1.50-inch tall, four-into-one tapered Wilson spacer.
Peak HP = 542 @ 6,500 rpmPeak TQ = 501 @ 5,000 rpm
Average HP = from 3,500 to 6,500 rpm = 450
Average TQ = from 3,500 to 6,500 rpm = 473