Unfortunately, all this put us in a crunch to completely disassemble the engine to clean it (and another set of cam bearings), fix any other damage, and reassemble it so we could test again all within a rapidly closing deadline. There was no joy in Mudville.
Drilling for Steam
All 400 small-block Chevy engines use siamesed cylinder walls, creating pockets in the cooling jackets that require venting. These holes allow steam and air bubbles to vent vertically out of the block and into the heads. Unfortunately, neither the stock Vortec nor the Bow heads come with these steam holes. That means you need to perform this step. We used a stock 400 head gasket to mark the position of the holes, but note that holes closest to the intake side must be drilled at a 45-degree angle to contact the water jacket. The exhaust side holes can be drilled straight in. It's best to do this using a drill press, but since we didn't have one, we managed with a handheld drill motor.
We masked off the chambers to prevent metal from contaminating our new heads and started w
When we began this project, it appeared that the static compression would be ideal at around 9.5:1 with the Vortec 64cc heads and a 22cc dish in the piston. This was assuming a roughly 0.020-inch deck height. So imagine our shock when we checked the deck height after reassembling the 400 to find the best cylinder was 0.062 inch down in the hole and No. Eight was buried a stupefying 0.074 inch below the deck. This created literally an 8.5:1 ratio in the "good" cylinder and a measly 8.39:1 in the worst. We also discovered that no one makes a thin steel shim head gasket for a 400, so we couldn't use a thin head gasket. We considered running a steel-shim 350 head gasket, but the gasket would protrude into the bore and cause serious detonation problems.
We did some further calcs, and with a 0.030-inch overbore, a zero deck height, and a smaller, 15cc dish custom piston and 0.040-inch gasket, we'd have a much stouter 10.46:1 compression, which is a full two points better than we have now. In all our years of assembling small-block Chevys, we've never seen a small-block with such miserable deck heights.
We were very careful with the installation of the new, nitrided Comp Thumpr cam, using the
After much chasing around to hot-tank the block again and install new cam bearings, we carefully reassembled the 400, this time with the new nitrided Comp Thumpr cam (actually, the Mutha' Thumpr) and the Comp break-in oil. We also reinstalled the lighter, 80-pound seat pressure Vortec springs, and the break-in, thankfully, went according to plan by using Comp Cams' new oil and break-in additive. This allowed us to get back to the original effort, which was to see how much power this beast could make. The power was not as impressive as it could have been because of the poor deck height and therefore the low compression, but the small Vortec heads still produced some decent torque. The power curve numbers reveal that this initial effort produced 448 lb-ft of torque at 3,800 rpm with a marginal 376 peak horsepower at 5,000 rpm. All this required 40 degrees of initial timing to achieve. While disappointed, we swapped directly to the Bow Tie Vortec iron heads to see how those larger, 225cc-intake-port heads would fare on our 400ci engine.
We also ensured the cam's survival by using Comp's new 10W-30 break-in engine oil designed
The results were less than spectacular, mainly because of our dramatic compression handicap. The bigger heads demanded even more timing, with 44 degrees to achieve best power. With timing and jetting maximized, the 400 managed to produce 458 lb-ft of torque at a still low 4,000 rpm, while kicking up the peak horsepower to just 401 at 5,400 rpm. As is typically the case with larger-port heads, we traded some low- and mid-rpm torque in exchange for much more horsepower farther up the rpm chart. Comparing power numbers, you can see that at 3,600 rpm is where the big-port heads began making more torque over the smaller Vortecs, carrying that power all the way up through 5,400 rpm at peak horsepower.