If you're looking to squeeze more life out of your small-bore LS, getting your 706 heads ported is honestly one of the best bangs for your buck. These little castings are everywhere—junkyards, Facebook Marketplace, and probably sitting under the hood of that high-mileage Silverado in your driveway right now. For a long time, people just viewed them as "budget" or "throwaway" heads compared to the more famous 243 or 799 castings, but the performance world has finally woken up to the potential hidden inside these things.
The 706 head, typically found on 4.8L and 5.3L LM7 engines, is famous for its small combustion chambers. That's a massive win for guys trying to bump up compression without spending three grand on aftermarket aluminum. But, as with anything factory-made for a truck, the ports are designed for efficiency and low-end grunt rather than screaming top-end power. That's where the porting comes in.
Why Everyone Is Obsessed With 706 Castings
Let's be real for a second: compression is king when you're building a street car. The 706 heads feature a roughly 61cc combustion chamber. When you slap those onto a flat-top piston 5.3 or even a 6.0L, your compression ratio jumps up significantly compared to using a head with a 64cc or 70cc chamber.
The downside? The intake and exhaust runners are a bit restrictive from the factory. They're great for getting a heavy truck moving from a stoplight, but they start to choke out once you get into the higher RPM ranges. By getting your 706 heads ported, you're essentially keeping that high-compression "snap" while opening up the lungs so the engine can actually breathe when you're mashing the pedal. It's the best of both worlds.
The Difference Between CNC and Hand Porting
When you start looking into getting work done, you'll hear a lot of back-and-forth about CNC porting versus hand porting. If you're talking to an old-school guy who's been grinding on heads since the 70s, he'll tell you there's no substitute for a steady hand and a die grinder. And to be fair, a master porter can work wonders.
However, for the average enthusiast, CNC is usually the way to go. Why? Consistency. When you get your 706 heads ported via a CNC machine, every single intake and exhaust port is identical. The machine follows a digitized "master" program that has been flow-tested and perfected on a dyno. You don't have to worry about cylinder number three flowing slightly differently than cylinder number seven.
Hand porting is great for "cleaning up" the casting flashes—those little bumps and ridges left over from the factory mold—but if you want maximum CFM gains, the precision of a CNC program is hard to beat. Plus, most shops that offer CNC porting for 706s will also include a multi-angle valve job, which is where a lot of the actual power is found anyway.
Let's Talk About Valve Sizes
The stock 706 heads come with 1.89-inch intake valves and 1.55-inch exhaust valves. They're tiny. If you're going through the trouble of having your 706 heads ported, it is almost always worth it to upgrade to a larger intake valve.
Most porters will recommend bumping the intake up to a 2.00-inch valve (the same size found in the 243/799 heads). This requires the machine shop to cut the valve seats to fit the larger diameter, but the airflow increase is substantial. You're moving from a straw to a garden hose. Just keep in mind that if you go too big on the valves with a small-bore engine (like a 4.8), you might run into some shrouding issues where the valve is too close to the cylinder wall, but a 2.00-inch valve is generally considered the "sweet spot" for 5.3L builds.
The Elephant in the Room: The Castech Issue
We can't talk about 706 heads without mentioning the Castech cracking problem. It's the one thing that scares people away. Some 706 heads were manufactured by a company called Castech, and a certain percentage of them were prone to developing a hairline crack near the head bolt pillars. This would cause a slow coolant leak into the oil, which is a recipe for a spun bearing and a very bad day.
Before you spend a dime on getting your 706 heads ported, check for the Castech logo. It looks like a little battery symbol or a tiny "notched" square under the valve cover area. If you have those, you need to have them pressure-tested by a professional. If you don't see that logo, or if they pass the pressure test, you're good to go. Most of the ones that were going to crack have already done so by now, but it's always better to be safe than sorry before you invest $800 in machine work.
What Kind of Power Gains Are We Talking About?
This is the question everyone asks. "If I get my 706 heads ported, how much faster is my car going to be?"
The answer, as annoying as it is, is that it depends on the rest of your setup. However, on a typical 5.3L with a decent cam and an LS6 intake, a set of well-ported 706s can easily add 40 to 60 horsepower over the stock configuration.
Because you're keeping the small chambers, you aren't losing any of that low-end torque that makes a street car fun to drive. Often, when people swap to "better" heads with larger chambers, the car feels a bit lazy at low RPMs because the compression dropped. Ported 706s avoid that trap. They feel punchy the second you touch the throttle and just keep pulling until you hit the rev limiter.
Choosing the Right Cam to Match
You shouldn't just pick a random cam out of a catalog if you're running ported heads. You want something that complements the increased airflow. Since your 706 heads ported for high flow will likely have a much better intake-to-exhaust flow ratio, you can get away with a more aggressive cam profile.
Talk to your porter or a cam specialist. Tell them exactly what's been done to the heads—the valve sizes, the port volume, and the flow numbers if you have them. A "Stage 2" or "Stage 3" truck cam usually pairs beautifully with these heads, providing a nice lopey idle without making the truck undrivable in traffic.
Is It Worth the Money?
In a world where aftermarket heads like Trick Flow or AFR cost upwards of $2,500, getting your 706 heads ported usually costs somewhere between $800 and $1,200 depending on the shop and the hardware you choose.
When you look at it from a dollars-per-horsepower perspective, it's one of the smartest moves you can make. You're getting performance that rivals many aftermarket offerings while using a factory casting that fits perfectly and uses all your original accessories.
For the budget-conscious builder who wants a reliable, high-winding LS engine, the 706 is no longer the "junk" head. It's the secret weapon. Just make sure you find a reputable shop that knows these castings inside and out. Don't go for the cheapest guy with a Dremel in his shed; find a place with a proven CNC program and a history of making LS engines scream. Your butt-dyno will thank you the first time you hit the highway on-ramp and realize just how much air those ported heads are moving.
Final Thoughts on the Build
At the end of the day, hot rodding is all about making the most of what you've got. The LS platform is legendary because of its interchangeability and the way it responds to basic airflow improvements. While everyone else is fighting over overpriced 243 castings at the local yard, you can grab a set of 706s for next to nothing, send them off to be worked over, and end up with a setup that's arguably better for a street-driven 5.3.
Just remember: check for cracks, go with the bigger valves if the budget allows, and don't forget a good set of valve springs. Once those 706 heads ported and polished are bolted down, you'll have a high-compression, high-flow monster that proves you don't need a massive budget to make serious power. Now, quit reading about it and go find some castings!