A buddy of ours has a '68 Valiant that he takes out to track days at the local road course whenever possible. His Valiant is fairly quick with a cast iron 360, but the new Z06 Corvettes were beating him on the straights, so he's ready for more power. The Valiant is a four-speed car with a 2.94 rear end ratio, so the new engine needs to make loads of torque. After thinking it over, we decided the best way to make a small-block act like a Viper engine was to increase the displacement. In our search for displacement, we uncovered some tips and tricks as well as stubbed our toes a few times. The purpose of this article is to show everyone what we've learned on this engine project so far, to help you guys when you decide to go big with a small-block.
Given our power goals, we knew that we needed to build a short-block with at least 420 cubic inches. Going that big with displacement requires either a much larger bore size than stock, or a much longer stroke. Most production blocks are limited in possible bore size to just a 0.030-inch overbore, but they will physically accept a lot of stroke. However, the production blocks were not really designed to handle the stress put out by a really long-stroke engine, so we decided it would be safer to start this project off with an aftermarket block. Fortunately for us, Mopar Performance has a selection of race blocks available for the small-block engine builder. There are a lot of options available including different deck heights, different lifter angles, dry sump or wet sump, and siamesed or non-siamesed cylinders. We reviewed the various block choices and eventually decided to purchase a 340 restoration block from Mancini Racing.
Our 340 restoration block...
Our 340 restoration block weighed 211 pounds as delivered from Mancini Racing. That makes the Mopar block about 50 pounds heavier than an original 340 block.
The 340 resto block that we ordered was a non-siamesed, wet sump block with a 9.60-inch deck height and 59-degree lifter angle. Non-siamesed cylinders means that the cylinders have a water jacket all the way around them, just like a production block. These full water jackets provide better cooling, but they also limit the maximum size of the cylinder bores to 4.080 inches. Had we selected a siamesed block, we could have used a 4.125 bore size and increased the displacement to 440 cubic inches, but we didn't think we had enough cylinder head to handle that many inches.
The 340 resto block we ordered from Mancini was part number P5007552AB. It was delivered to us finished honed at 4.040-inch bore size, and came complete with cam bearings, ARP main bolts, and freeze plugs. In theory, these 340 resto blocks are ready for assembly right out of the box, but we knew right away that our first stop would be the machine shop, not the assembly line.
Block Machine Work
Fixtures cut from heavy gauge...
Fixtures cut from heavy gauge sheetmetal were used to hold the block in a Bridgeport mill for machine work. Here an end mill is plunge cutting a clearance relief for an external oil line.
The MP race blocks are designed with four extra head-bolt lugs on each side of the valley. Even though those extra bolts aren't required on the 340 resto block, Mopar does not machine them away. These extra lugs stick out between the lifter bores and consequently prevent the use of tie-bar-equipped roller lifters. Because we wanted to run a roller cam, we knew that the block needed some serious machine work before we could start the assembly process.
Shady Dell Speed Shop in Shady Dell, Pennsylvania, offers a super nice CNC package for these race blocks that removes the extra head bolt lug, as well as other excess weight from the block. In hindsight, we should have had our new block drop-shipped to Shady Dell for their CNC package. but being inexperienced at this, we just assumed that we could do the machine work ourselves. We knew it wouldn't be too difficult to machine away the extra material in the valley once we had the block in the machining center, but we underestimated how complicated it was to fixture the block. Engine blocks are heavy and awkward to hold on to, and our local shop wasn't fully equipped to handle this type of work. To make things even more complicated, our machine work needed to be performed at the same 59-degree angle as the lifters. We eventually figured out how to build a set of custom fixtures, to hold the block in a machining center at the correct angle for the necessary machine work. Our homemade fixtures worked perfectly once we had them constructed, but it did cost us a few weeks of time to design the fixtures and to get them fabricated-lesson learned, just call Shady Dell.
The Mopar block comes with...
The Mopar block comes with massive four bolt main caps on the center three locations. The block came with high quality ARP main bolts which we ditched in favor of ARP studs.
The oil passage that usually...
The oil passage that usually feeds the oil filter was tapped for 3/8 pipe thread for a fitting that will feed oil to the external filter and cooler.
The 340 resto block is cast...
The 340 resto block is cast with provisions for additional head bolts as well as 48-degree lifters. The additional material adds weight and caused plenty of grief for us by getting in the way of our roller lifters.