At 2.190 diameter, the stock...
At 2.190 diameter, the stock intake valve is too small even for a stock-displacement 454, but the room to put in bigger valves is limited. After installing a valve as large as possible, the only solution is to develop the port's high-lift flow to the greatest extent possible, and even then, employ a high-lift valvetrain.
3. What are some cost-effective alternates to porting stock heads?
The answer to this question is easy, and there are three choices I've had hands-on experience with that I feel confident in recommending. [Editor's note: These are based on the author's own dyno data, and do not take into account all available possibilities.] Dart's Iron Eagle, World Products' Merlin, and RHS's iron offerings represent cost-effective, entry-level, high-performance heads for a budget-constrained big-block engine builder. But there is a point here that needs to be made really clear. The term "entry level" refers only to the price, because as far as performance is concerned, these heads, when used in the right combination, will produce nothing less than pro results. As far as "used in the right combination" goes, we will cover the principle issues as we move along.
By purchasing heads as a complete package, we get not only a set of new, high-flow castings, but also new hardware. Aftermarket head manufacturers are well aware that without springs with a high natural frequency, the valvetrain will ultimately let down the head's ability to deliver optimal results. Purchasing a package means you get the benefit of both their knowledge in this department, and their bulk buying power.
4. If I replace stock heads with aftermarket iron heads, what's the payback?
Most of my recent experience with aftermarket iron heads has been with Dart's Iron Eagle, however, all three (Dart, World Products and RHS) can currently be had at street prices of a little over $1,500, ready to install. My test data on the Dart heads is well worth studying. First, let's look at the flow figures comparing a factory "049" casting versus Dart's Iron Eagle 308cc rectangular port (the smaller of their two offerings).
The Dart Iron Eagles are a...
The Dart Iron Eagles are a cost-effective way to get a new set of heads equipped with quality hardware. As-cast or ported, they can be part of a big horsepower combination.
From the graph, you can see what a $1,500 investment buys in terms of airflow. Though instrumental toward the production of more power, flow is far from the only factor. At the time I did the following dyno test, Dart had just introduced their wet-flow technology to the Iron Eagle casting. Good swirl, a port area well suited for a street engine between 454 and 530ci, and good wet-flow characteristics deliver some good dyno results. The following dyno test shows what you can expect from a set of these heads should you replace a well-prepped set of 049 castings.
Although these are some impressive gains, they may not have been as good if some aspects of the flow characteristics had not been taken into account. Note from the flow curves that most of the Iron Eagle's extra flow happens above about 0.450-inch lift level. If the valvetrain I used did not generate sufficient lift to access that flow, a sizable proportion of the extra power would've been absent. In this instance, the cam used was a single-pattern Comp Xtreme street roller that delivered some 0.638 inches of lift after lash had been figured in.
If the budget runs to an aluminum head, then all the power advantages of a high flowing set of iron heads can be had with a weight reduction of some 80 pounds. Some recent dyno tests with as-cast AFR 325cc port aluminum heads showed that 675 streetable horsepower is practical from a 10.5:1-compression 505 buildup. As gratifying as these numbers are, it needs to be made clear that they only come about by virtue of an understanding of what it takes to put together a successful combination.
To get results from a set...
To get results from a set of ported factory heads requires a lot of time, and a flow bench to verify the results. Even then, you still only have 30-year-old heads.
5. Can the current generation of aftermarket big-block heads be ported cost effectively?
Yes they can, in fact, they are very novice friendly, and can be given an effective basic porting job without the aid of a flow bench by anyone with average dexterity. The payback for even a basic porting job is good because no matter what we do, any big-block over about 427ci is short of flow. Personal experience with Dart's iron and aluminum heads, and AFR and RHS's aluminum heads, show that thinning up the guide bosses, then blending out any irregularities, nets as much as 30 cfm of extra air at 0.700-inch lift on both the intake and exhaust. On a nominally 650-horse 10:1-compression 500 incher, this translates into an additional 25 or 30 hp.
6.I just bought heads that flow much more air. What parts will make the best combination?
Because a big-block is always starved for air, it's far more sensitive to certain aspects of the build. Appreciating these acute areas makes for a significantly greater output without spending a dime more. The first rule is that if you have high-flow heads, don't starve the heads of air by using a carb that is too small.
Ninety percent of big-blocks are 454 inches, and up. They are not 350 inches, and they do not like small-block-sized carbs! If a 350 warrants a 750-cfm carb, its 500-inch big-block equivalent will need something on the order of 1,100 cfm just to stay on par. That's Dominator territory. In practice, we find that big-blocks really respond to Dominator-sized carbs on high-flowing intake manifolds. If you must use a two-plane intake for hood clearance, Edelbrock's RPM Performer Air Gap is a sound choice, but because it is designed to take a 4150 carb, it needs to be used with a really high flowing example of such. I would recommend a 900 cfm minimum here. As for single-plane intakes, Dart, AFR, and Weiand each have manifolds that work well with either a 4150 or 4500 series carbs. [Editor's note: Once again, we'll remind readers and manufacturers that these recommendations are based on the author's independent experience.]
Cylinder Flow Dart Iron Eagle...
Cylinder Flow Dart Iron Eagle Versus Stock "049"
Here's the intake flow of an "049" casting's good port (black curve) versus that of a 308cc Iron Eagle (blue curve). The "049" casting has had the benefit of a good valve job, allowing it to stay close to the Iron Eagle's flow at lower lift, but when the valve clears 0.400 inch, the Iron Eagle just runs away from the stocker. On the exhaust side, the seat/throat design of the port benefits from Dart's Pro Stock experience. The result is higher-than-average flow right off the seat, with a substantial increase over stock at high lift.
7. Other than induction flow, what else do I need to take care of in the cylinder heads?
When choosing your cylinder heads, there are two principle factors you should consider. The first is the intake port size, and the second is the compression ratio. First, the intake port size. Do not fall into the trap of thinking that bigger must be better. To get the best combination of torque and horsepower, select the port volume required using the accompanying chart. Don't overestimate the power your engine is likely to make. Doing so will mean the port selection will be too big, and as a result the output will be less. It's best to underestimate by a small margin.
Another important consideration is the compression ratio. Big-block Chevys like all the compression you can give them. For the street, it is good for mileage and output, especially torque. For a race motor, it's a key factor for winning races. With the stock chamber at typically 119cc, your first move is either to mill the head down so the chambers are as small as possible, or use plenty of inches under the heads, or both. Do not go overboard on piston dome, as excess here can cut combustion efficiency. Most heads will cut to about 102-105 cc; go that route first, then choose a piston crown that delivers the required compression ratio.