I needed to call AFR boss Rick Sperling to give him the great results I had just seen with a pair of his company's small-block Chevy heads on a 350. In a way, I was just bragging, because our combination had exceeded his predicted numbers by almost 30 hp. After we were done on that subject, Rick asked if I would like to test the latest version of their street as-cast 325cc runner big-block heads. Sure, but there was a problem. I was on deadline, and time was not on my side. But I had an idea. My co-conspirator on many engine build projects over the last four years, UNC Charlotte engineering student Dusty Kennett, had accrued much experience with pretty racy small-block Chevy and Ford engines. But his big-block experience was minimal, and I had a suspicion he was hot to trot on something bigger than just a regular 350 or 383 small-block. So I asked Dusty, "You wanna build a 500-inch big-block Chevy to test AFR's new heads?" Response: a big smile, and, "Yeah, sure!"
Never one to needlessly reinvent the wheel, Dusty started by calling budget big-block building expert Lloyd McCleary of T& L Engines for some basic advice. Here is what he got: "Big-blocks love compression, but in a 454 or smaller motor, that means a big piston dome, which they don't like so much. To avoid this, stroke it. From here on out, it's a question of good induction, good heads, and the right cam." Yeah, Dusty, no problem. Go for it!
As it happens, Dusty had located a Gen IV block, just in case a build opportunity came his way, so at this point it was a question of dragging it out of storage and checking it out for suitability. It proved to be crack-free, and the sonic tester showed it was OK for our intended build. The next job was to drop it off at T&L for a top-to-bottom machining job. T&L put the block through the system, and when it came back from boring (.040-over), honing, decking, etc., we checked it out with some high-tech equipment, and it was right on the money. The crank we chose was a 4.25-inch stroker from Scat. There were many reasons for going with the Scat brand. First, all of these cranks are ground in their Redondo Beach, California, plant on the same grinders that do their Cup Car, Top Fuel, and Pro-Stock cranks. Secondly, the Scat cranks are cast from an alloy that is about the top of the line in cast steel, and runs some 10,000 psi stronger than many of the cast steel cranks available today. Third, Scat has a $250 balancing service that has much to commend it. You can get an entire rotating assembly from Scat as a balanced kit, or you can have the crank balanced to suit whatever pistons you have. The real beauty of having Scat balance the rotating assembly is that they machine off most of the excess material from the counterweights in a lathe. This means that each counterweight not only gets the right proportion of metal removed to keep the intended mass distribution, but also very little drilling is needed to finish off the balancing. All this, plus a windage radius on the leading face of the counterweights, makes for a good-looking crank that is lighter and stronger than average at a very reasonable cost.
The basis of our 492ci buildup was a Gen V 454 big-block. Here's how the block looked as w
As for rods, it is a good move to go for a longer-than-stock rod, especially if a stroker crank is used. Big-block Chevys already have a rod that is a little on the short side to begin with, so it makes no sense to aggravate the situation by adding stroke and retaining a stock-length rod. We used a set of Scat's 2-ICR6385 rods. These 4340 forged I-beam rods sport a 7/16-inch cap bolt, and though they will go higher for a drag race-only application, are about bulletproof up to 7,500 rpm and 850 hp. With the stroke in our crank, they allow the rod/stroke ratio to be put back to almost stock-1.5 for our stroker versus 1.53 for a stocker.
At this point, we come to the pistons. As it happens, KB had just introduced a new design of inexpensive heavy-duty forged pistons, and we were eager to try a set. In the line-up, they had a spec (PN KB-789) that gave us precisely the 10.5:1 pump-gas ratio we wanted. This piston is machined from a nice-looking and robust forging. The crown itself is reinforced with stiffening ribs that indicate great nitrous capability without much of a weight penalty. Rings are of a conventional 1/16-1/16-3/16-inch pattern, so not much out of the ordinary there, but when all is said and done, this is a proven ring pack that is relatively inexpensive. At face value, the pistons looked good, but at the end of the day, the dyno numbers will be the deciding factor.
The first test would be to see how freely the assembled short-block turned when tested with a torque wrench. Dusty makes a power move that, though time consuming, is worth passing on. He takes a Scotch-Brite pad and gently polishes the rings so they feel smooth. This takes about 60 seconds a ring. As for the bores, the treatment depends on the quality of the bore finish. If done to Cup Car quality, then leave as-is, but for a regular street hone job, make a few passes with the Scotch-Brite in a vertical motion up and down the bores. This takes the micro-sized ragged edges off the top of the hone pattern. This speeds break-in, and causes less wear. It also cuts friction, and that means a little extra power. A post assembly torque turning test showed our 4.25-inch stroker, with street-oriented clearances, turned at a very creditable 28 lb-ft, whereas 33-35 lb-ft is about normal for a typical street build.
This is the latest iteration of Scat's cast steel 4.25-inch stroker crank. A relatively re
Last item on the agenda for the bottom end is the pan. Any time a stroker crank is used, the chance of the crank picking up oil and entraining it is very real. Personal experience has shown that it's easy to lose as much as 15 hp when this happens. The fix is a good pan. We used one of Moroso's basic pans, which did the job nicely.
AFR Cylinder Heads
Because of its large displacement, a big-block Chevy really needs a lot of airflow. But this is far from the only factor that dictates success. To see what we were dealing with, our AFR heads (PN 2101-1) were tested on the bench along with some other brands that we've used to successfully make strong torque and power curves. What we found was that flow was up with the best. Also, swirl was good, and port velocity and velocity gradient were a little better than most. So things looked good. As far as volumes were concerned, the intake ports (good and bad) averaged 319 cc, while the chambers, which are CNC machined, checked out at 119 cc. For our 10.5:1 compression, we planned on milling to 112 cc. But there's more. AFR has established a reputation for using top-quality hardware on their heads, and these big-block heads are no exception. Instead of the more normal 3/8-inch valve stems, the AFR heads sport much lighter 11/32-inch valve stems. Additionally, AFR uses a more costly spring package. Because these springs can be run at a higher stress, they are lighter for a given poundage, and this results in better valve action, especially with a hydraulic roller. The smaller springs also mean a lighter valve retainer, and all this weight reduction adds up to superior valvetrain control, and that's all good for more power. By any standard, these AFR heads are a great deal, and promised to deliver well on the dyno.