We tend to think of Dart as a cylinder head manufacturer, but the truth is they also make some outstanding intake manifolds. With four years of engine building experience, Dusty had found this fact out for himself. Drawing on this experience, he selected a Dart intake manifold (PN 4112400). This intake was to be used with a Dominator-style carb, so it has to have plenty of flow for the job. So often, big-blocks end up being undercarbureted. For a big-block targeting over 600 hp, it almost inevitably runs best on a Dominator-size carb. Just looking at airflow proportionally to a small-block, we can see that if a relatively stout 355 needed 800 cfm, a 500-inch big-block would, on a pro rata basis, need 1,125 cfm.
The carb Dusty chose was a relatively new AED Dominator. AED has made some great strides with these carbs in driveability and low-speed output. Just in case you think 1,200-plus cfm is too much carb for a 492ci street big-block, try this for size. The last engine Dusty built was a 355-inch small-block. On this he tested AED's claim that this carb allows much more flow for a strong top end without necessarily compromising the low end. The carb ran flawlessly on a 355 incher. It was only down about 5 lb-ft over a purpose-built 900-cfm 4150 at 2,000 rpm, but was up over 20 hp at peak, and about 30 up at 8,000 rpm.
After mocking up the heads on the block, the Dart intake was first port-matched to the heads. Only a minor amount needed to be removed to achieve this goal. The intake runners were dressed out with 60- and 80-grit emery rolls to remove casting flash. The last operation on the runners was a coarse bead blast. Last item on the agenda was to modify a Moroso 1-inch four-hole spacer. This is worth about 5-7 hp from about 5,000 on up, with no measurable downside at lower rpm.
Flow tests of AFR's 325cc big-block head are among the best of a series of heads tested. G
Cam And Valvetrain
If the valves are opened and closed at the wrong time, no matter how good the heads, intake, and carb are, the motor will not produce anything like its full potential. The factors affecting what the engine needs in the way of valve events are the cubic inches, valve sizes, compression ratio, rocker ratio, rod length, and a lot more. All too often, cams are chosen on the basis of prior luck. In my shop, we use a Cam Master computer program that, give or take a degree or two, gives the opening and closing points needed for a specific peak power rpm. It also delivers the lift figures for maximum output, and identifies the LCA within one degree. We were looking for peak power at 6,000 rpm. The Cam Master program predicted that the engine needed a cam of 283 and 282 duration (intake and exhaust) at the effective lash point and on a 107.9-degree LCA at 4 degrees advance.
With this information, a COMP Cams single-pattern 280 XRS street roller, ground on a 108 LCA 4 degrees advanced, was ordered. This street roller profile gives about 285 degrees duration at the lash point. A street roller profile was chosen because it would be reliable over a very long haul. As for the rest of the valvetrain, lifters were COMP's Endurex items, which actuated the rockers through 3/8-inch Magnum pushrods. The pushrods were length-selected to optimize valve lift and the sweep geometry across the top of the valve stem. As for rockers, it should be said that COMP's aluminum rockers are more than up to the job. Nevertheless, we had a set of their Pro Magnum stainless rockers that we've been wanting to use, so that is what we used here. For sheer good looks, the valvetrain was topped off with a set of Moroso cast aluminum valve covers with a black crinkle enamel finish.
For spark, a Performance Distributors HEI was selected, and although it may seem out of step with a big-block build, it was selected with fuel economy in mind. We ordered our HEI from Performance Distributors with a vacuum advance. This little upgrade is often shunned by hot rodders, because it's seen as an unnecessary addition to the distributor. The fact of the matter is that at cruise, when the engine is pulling vacuum, the timing needs to be as much as 40-45 degrees advanced. If the vehicle is only turning 2,000 rpm at 70 mph, there will only be about 20-25 degrees advance if only static and mechanical are used. The difference between what timing the engine wants at cruise, and what it's given if no vacuum exists, costs between 2-4 mpg! A vacuum advance HEI is the fix.
This particular KB piston is a relatively new part number in KB's line of inexpensive forg
In addition to a vacuum advance, the HEI was also ordered with a clear plastic cap so we could easily see that the rotor aligned OK with the pickup points in the cap. This is an easy way to see what they do during the build. The clear one can always be swapped out for a regular one right after alignment has been verified.
As mentioned earlier, carburetion is by a high-flowing AED Dominator-style carb. With its radically revised circuitry and calibration, this has shown to be versatile enough to produce outstanding results on engines as small as 355 inches, when appropriately calibrated. For a big-block, low-speed output with a Dominator-style carb is rarely in question. The engine is usually big enough to need all the carburetion it can get from a carb of this size. With the carb installed and a few other little details taken care of, our 492 was ready for the dyno.
As is so often the case, our dyno testing was done at T&L Engines, in Stanfield, North Carolina. We used this opportunity to do a little impromptu testing of intakes and carbs, but time was limited, so any real fine-tuning was out of the question, except for the final spec we intended to use. First, the porting on the Dart intake looked to be worth about 10 hp, with gains starting at about 5,000 rpm. Also, the use of a two-plane intake and a high-flowing 4150-series carb are worth considering. This engine made low-speed torque like a steam loco, even with the single-plane intake and the big Dominator carb. So much so that, even when put on full load, it blew through the dyno's absorber to a lowest "hold-down" point of 4,500 rpm. If the two-plane made any more torque below that, we were not in a position to measure it. In the normal course of events, this combo probably did make more low end, but any extra torque could prove to be more of an embarrassment in all but the heaviest of vehicles. Above 4,500, the two-plane/4150 combo started to lag behind the single-plane/Dominator combo. By the time 6,000 rpm came around, it was down by about 50 hp. All this pointed to one thing: the spec we originally conceived was on the money. The conclusion is that the AFR heads work just fine, thank you, but any successful engine is a combination of the right parts, so take note of the cam, intake, compression, and pistons that were used with these heads. If you are in the market for a set of highly functional and cost-effective big-block heads, the suggested retail price for these AFRs is $2,485 a pair ready-to-install, but we've seen street prices below this. The tale of the tape showed the Dominator-topped, single-plane combo feeding the AFR pieces to the tune of 632 lb-ft at 4,600 rpm, and 656 hp at 6,100 rpm.
At the end of the day, we learned three important things about stroker big-block Chevys. First, a Dominator-style carb on top of a single-plane intake is preferable over a 4150 on a dual-plane, unless there are class rules or physical constraints, such as hood clearance. Secondly, AFR's box-stock 325cc big-block head is a relative bargain when compared to many of its contemporaries in the same price range. And finally, you can get an exact replica of this engine in crate form (less headers and water pump) from T&L for about $8,145-which, in big-block parlance, is an absolute bargain.
Here is the ported and port-matched Dart Dominator-flanged intake for our 492. Note the in
Here's a move we picked up from the guys at T&L. Take a four-hole Dominator spacer and mod
Our 280 street roller cam was timed in to four degrees advance. In this instance, that pro
For roller lifters, we used COMP's solid Endurex design. In any sane valvetrain, we have y
Because of the expansion of the aluminum heads, the rockers need to be cold lashed at abou
Most budget builds like ours would use the aluminum COMP Cams rockers (not shown). On this
Our distributor was a custom unit with the appropriate advance curve already built-in from
A custom Dominator from AED handled the fuel delivery and metering chores. This was one of
Our project motor was used to run a number of tests before finally buttoning it up with th
Tests with a 4150-series carb on a two-plane showed just what we have been saying for a lo
An 850-rpm idle, torque from about 1,000 rpm, and a midrange output that pulled right thro
This was not an expensive engine, as big-blocks go. If you want one like this, T&L will bu