Chevrolet 406 Big Block - The Impersonator

Big-Block Torque In A Small-Block Package

Scott Crouse Jun 1, 2003 0 Comment(s)
148_0306_406_lead_1_z 1/13

Big-cubic-inch small-blocks are now all the rage for Chevy street-driven heros. With an excellent power-to-weight ratio, parts availability, and pricing, it just makes sense to go big. Whether you want a lot of torque to move your non-overdriven, 3.08-geared, heavy Chevy around, or you just want to do award-winning burnouts, more inches are the way to go.

Horsepower is a multiplier of torque, but it's usable torque and horsepower that matters. Street-driven vehicles rely on torque to make them fun to drive, and we decided to build a practical street small-block with lots of usable power. While the big boys play around with 434ci, 440ci and even 454ci small-blocks, we decided to stick with the classic 406. But instead of fixating on horsepower, we wanted to emphasize torque. To this end, we matched up a long stroke with the iron Vortec head to see just how much torque we could make with an eye toward decent horsepower.

Our quest pointed us directly at the classic 400ci small-block. With a 4.125-inch bore and 3.75-inch stroke we had all the basic ingredients. The original plan was to make a few phone calls, find a standard-bore 400 block, pick it up at the wrecking yard, and do a budget rebuild. While this sounded good, it wasn't reality. After several trips to different wrecking yards we realized that finding and rebuilding a 400ci short-block isn't easy. By the time we found a block, paid for the machine work, and spent hours building it, we would be better off with a Coast High Performance 406ci short-block. What's especially cool about this short-block is that it can be ordered with 9.5:1-compression dished pistons for the Vortec cylinder head's small 64cc combustion chambers. This particular Coast short-block features a Scat 9000 series crankshaft, Scat 5.7-inch long I-beam connecting rods, and a set of Probe-designed forged dished pistons. The greatest thing about this motor is that Coast High Performance offers the assembled short-block for $2,358 with a limited warranty.

As soon as we heard about this, we brought one home and buttoned it up with a complete Milodon oiling system to combine reliability with power. We then added a TD Performance two-piece timing cover, Lunati flat-tappet hydraulic camshaft (see sidebar for specs), and a timing set, sealing it all up with Fel-Pro gaskets and ARP bolts. When building a 406ci small-block, the long 3.75-inch throw of the crankshaft's rod journals can cause the connecting rods to come in contact with the camshaft's lobes. To avoid this problem, we made sure to order a small-base-circle camshaft. Without the small base circle, the rods can be clearanced, but for an extra $20 added to the camshaft price we felt it would be best to use a small-base-circle camshaft. By now our engine was starting to look pretty awesome with its cool forged pistons, freshly painted block, and deep-sump oil pan.

With only the induction left to assemble, we added a pair of stock GM Performance Parts Vortec iron cylinder heads, an aluminum Edelbrock RPM Performer Vortec Air Gap intake manifold, and a 750-cfm Holley mechanical-secondary carburetor. While we went to the effort to machine our heads to accept the 0.480-inch-lift hydraulic flat-tappet camshaft, you can also order the Vortec heads pre-machined with stronger valvesprings from Scoggin-Dickey Performance Center. Regardless of which heads you choose, they will have to be drilled with steam holes in order to >> match up with the cylinder block's steam holes (see "The 400 Drill," Feb. '03). This also requires special 400ci Fel-Pro head gaskets with a 4.200-inch bore and the appropriate steam holes between the cylinders. Once the heads are on the engine, the intake manifold requires special Vortec bolts and gaskets to hold it in place. By this time we prelubed the engine to make sure it had oil pressure and called dyno man Ed Taylor to schedule some brake time on the Duttweiler dyno.

Several days later we arrived at the dyno with a set of 15/8-inch Hedman headers, plug wires, an HEI distributor from Pertronix, and a 750-cfm HP Holley carburetor. With everything in place, we fired the 406ci and broke in the Lunati hydraulic flat-tappet camshaft and lifters. After the 20-minute break-in period, we entered the dyno cell, checked for leaks, burnt wires, or anything out of the ordinary, and set the total timing to 35 degrees before top dead center. Since we would be running this engine on the street, we decided to start the dyno pull at 2,300 rpm and watch the engine work. Taylor let the dyno handle fly and recorded information from 2,300 rpm to 5,500 rpm where the engine showed off with 525 lb-ft of peak torque at 3,500 and 428 peak horsepower at 5,000 rpm. Impressive numbers like these made our dyno day go by pretty fast. The small-block beast delivered an incredible power curve that looked more like what a fat Rat would produce. At 2,300 rpm, the engine was already making 471 lb-ft of torque. Can you say burnout? We also tried several tricks including more timing, changing carburetors, and adding a 1-inch spacer, but none of the changes improved upon the initial numbers. The Coast High Performance 406ci put up its best power numbers running 91-octane, 35 degrees of total timing, 15/8-inch headers, and a 750-cfm HP Holley carburetor.

On the drive home from the dyno, we couldn't help but think about how awesome this motor turned out to be. It made 525 lb-ft of torque and 428 hp all under 5,500 rpm. The fact that we were able to do it for under $6,000 was even better. Had we scrimped and used a few less-expensive parts and been willing to sacrifice a little horsepower and torque, this same motor could be reproduced for under $4,500 (for details, go to www.chevyhiperformance.com). No matter how you look at it, big-inch small-blocks offer outstanding power-to-weight characteristics and it's hard to beat the Coast High Performance price. We dubbed this engine The Impersonator because it thinks it's a big-block, but at a small-block price.

9

Before any short-block is assembled, it's crucial to check every clearance. All the main journals and rod journals should check between 0.0022 and 0.0028 inch of clearance.

Make sure to install a pipe plug under the No. 5 main cap (arrow) closest to the oil filter or the engine will not direct its oil through the filter. It's also important to use Fel-Pro's oversized rear-main seal when the 400 block is align-bored.

Before you install the cylinder heads, make sure to plug only the outer portion of the top driver-side oil hole on the fire deck. Do not drive a plug deep into this hole or all of the engine's upper-end oiling will be blocked.

With a 3.75-inch-stroke Scat 9000 crankshaft and 5.7-inch I-beam rotating assembly bolted in place, we finished our Coast High Performance 406ci off with a complete 7-quart Milodon oiling system and a TD Performance two-piece timing cover.

A 406ci Coast High Performance short-block can be yours for $2,358. All you have to do is add an oiling system, heads, induction, and ignition parts and you will be doing burnouts in no time.

When installing a timing chain, make sure the camshaft gear markings properly line up. This is how your timing gear alignment should look when the camshaft is installed straight up. The innermost mark on the timing gear shows you which keyway to choose while the uppper timing mark shows you at what angle to place the camshaft.

When building a 400ci small-block, you can either use a standard or a small-base-circle camshaft. Installed correctly, the small-base-circle camshaft will clear without grinding the rods. If you insist on using a standard-base-circle camshaft, the rods will have to be ground before the engine is balanced.

Production 400ci GM blocks feature two holes between adjacent cylinders to vent steam between the large 4.125-inch bores. When building a street-driven 400, it's essential to drill matching holes (arrows) in your new heads. The hole closest to the intake side must be drilled at a 45-degree angle.

In order to bolt our Vortec induction system onto the engine, we had to use special intake bolts and gaskets. We also finished the job off with an ARP accessory-bolt kit and some new GMPP valve covers.

On Dutwelier's dyno, we bolted on a set of Hedman 15/8-inch headers, Pertronix wires, an HEI distributor, and a 750-cfm Holley HP carburetor to make a monstrous 525 lb-ft of torque.

With 9.5:1 compression, we ran our 406ci engine on 91-octane fuel with 35 degrees of total timing and never heard a single knock.

We ran our engine with a set of aluminum 1.5:1 self-guided Proform roller rockers in search of max power, but if you want to save a few dollars, you can choose from an assortment of less expensive self-guided stamped rockers.

COMMENTS

TO TOP