Some engines just won’t go away. Back in the May ’11 issue, we brought you the first part of our budget-built 400 small block Chevy. The idea was to bolt a set of iron Vortec heads on a junkyard 400 small block Chevy, add a cam, and enjoy the torque. Right away, we ran into snags. The block was so nasty it demanded help while retaining the stock crank, pistons, and rods. The cam failed with less than a week before our deadline necessitating an unplanned thrash to rebuild the engine a second time because all that dead cam metal wiped out the main bearings. By the time we were finished, we had never worked so hard to produce so little power. The 400 small block Chevy problems were multiple: The cylinders waffled like ice cream cones, preventing anything approaching ring seal, and the compression was a sickly 8.2:1 due to ugly 0.060-inch piston deck heights. In case you missed the story, the best power numbers were gained with a set of GM Performance Parts Vortec heads supplied by Scoggin-Dickey, and even then the 400 small block Chevy managed only 401 hp and 458 lb-ft of torque. We were underwhelmed.
Rather than kick the motor to the curb, we decided to build this engine correctly. The plan was to retain the original test’s cam, Vortec heads, Weiand intake, Holley 750-cfm carburetor, and exhaust headers so we could honestly evaluate the power increase from better cylinder sealing and a bump in compression. The difference was astonishing. You’ve probably already jumped ahead to the dyno test, but if not, the new torque numbers came in at a tire-shredding 501 lb-ft at 4,000 rpm, and horsepower leaped up to 466 at 6,000 rpm. That’s a peak horsepower bump of 65! We did cheat a little with a couple of new parts, but you’ll have to read the whole story to find out what we did.
While we originally planned to reuse the stock crank, Scat’s Tim Lieb suggested adding an important option. All production 400 small-blocks were built as externally balanced engines. The problem with building a performance 400 that might see rpm above 6,000 is the external weight spinning around on the crankshaft snout; it acts like a rock on the end of a string, adding load on the crankshaft that could cause eventual failure of the front main bearing. Lieb suggested an internally balanced crankshaft assembly that would require 6-inch rods and a short- compression-height piston to allow room for larger crankshaft counterweights. This package costs a little more than the standard 5.7-inch rod and cast crank system, but it offers much greater durability. One other possible advantage is the improved rod-length-to-stroke-ratio from the 6-inch connecting rods. Since it’s always a possibility that we will lean on this engine harder at a later date, we opted for the internally balanced rotating assembly and had Scat balance it for us.
The most obvious change to this different rotating assembly is that with a 6-inch rod, the piston compression height becomes very short, which moves the wristpin up into the oil control ring. This requires an oil support ring that is supplied with the pistons. It’s important to note that this support ring comes with a small dimple that should be placed with the raised portion facing down so it will not move relative to the piston.
Once our machine shop, Barrington Engines, ensured that all the cylinders were properly bored, the torque plate was honed, the main bearings were align-honed, and the decks were level, we had a great foundation. By the time Barrington finished squaring this block and making the holes round, the thought occurred to us that we had $800 invested. We looked up the price of a Dart SHP block from Summit. It comes in at $1,499.41 (plus shipping). While this block is more expensive, it’s a new casting with four-bolt mains and 350-size main journals, which means slower bearing speeds over the larger stock 400 size. You will still have to hone this new block, so factor in that cost—it’s worth considering before investing $800 in a 40-year-old block.
We will not spend too much space documenting the assembly other than to mention that we did have to mix in quite a few Federal-Mogul 0.001-inch undersize bearing halves with the Scat rods to get bearing clearances to achieve our ideal 0.0025-inch clearance target. The main bearings were easier because Barrington align-honed housing bores, which established more consistent bore diameters. We also mocked up the engine to measure deck heights and then had Barrington machine the deck surfaces to create a much more consistent deck height.
|Comp Cams Thumpr 287TH7 hydraulic flat-tappet camshaft
||Duration at 0.050
The Scat rotating assembly came with a forged crank and longer counterweights to make it a
A big reason we chose the internally balanced rotating package was to eliminate the eccent
One side benefit of the new pistons is the thinner 1⁄16-inch top and second ring with 3⁄16
These Icon pistons are full floating, which means we had to install the spiral locks. The
Even though we retained the original Comp Thumpr cam from the first test, we treated it ju
Because the longer rods push the wristpin up into the oil-ring land, the Icon pistons come
Don Barrington Jr. showed us a slick way to check crankshaft endplay without having to bol
Once the full floating pins were spiral-locked in place, we carefully installed the Total
Installing the pistons is easy if you have a 4.155-inch tapered ring compressor like this
Here is the forged Scat crank and complete rotating assembly in place in the block. This e
Don’t forget to include the three oil-gallery plugs behind the timing chain. Production en
On the same Westech dyno, we tested the engine with the same Weiand Air Strike dual-plane