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Duramax Iron Heads Upgrade

Inside PPE’s New Cast-Iron Duramax Heads

Despite the fact that the Duramax’s OEM cast-aluminum cylinder heads hold up well in many applications, at the high-end of the performance spectrum several problems can surface. For starters, the different expansion rates between a cast-iron block and aluminum cylinder heads can lead to uneven clamp loads—which eventually means blown head gaskets. Then, in extreme heat environments the factory heads are prone to cracking. Over the years several companies have released cast-iron replacement cylinder heads for the iconic 6.6L GM V-8, but Pacific Performance Engineering’s latest product may just be the homerun everyone has been waiting for. The company has designed and manufactured high-quality cast-iron heads that should be on the market by the time you’re reading this.

For a bit of comprehensive, pre-market validating, PPE shipped a set of heads to LinCo Diesel Performance—a Troy, Missouri based company that’s not only a PPE dealer but that boasts a machine shop that rebuilds Duramax engines on a regular basis. After examining PPE’s new cast-iron heads in every way it knows how, LDP shared its findings with us at Diesel World. Guide clearance and valve recession specs on the heads were found to be ultra-consistent, and their profilometer-verified surface finish was found to be just about as close to perfect as it gets. In addition, they feature de-burred ports and smooth radius’s on the valves and valve pockets, upgraded and fully nitrided intake and exhaust valves, and come with 125-lb valve springs. If you’re searching for a one-and-done type of head package (heads that support horsepower and reliability), PPE’s cast-iron heads look to be solid performers right out of the box.

Duramax Iron Cylinder Head Upgrade
It’s clear that PPE designed its cast-iron heads with both maximum durability and performance-potential in mind, but the fact that they can be bolted onto a block right out of the box with no additional machining is perhaps their biggest high mark. Just throw them on and go. PPE’s heads arrived at LinCo Diesel Performance fully dressed, like this, and then LDP began disassembling them, carefully inspecting each piece to find out just how good they are.
GM Duramax Cylinder Heads
Cast in the USA from a top-quality, alloyed grey-iron casting and then machined in the states as well, PPE developed its cast-iron replacement heads to stop scrubbing issues, which are caused by the different expansion rates of steel and aluminum. These differing expansion rates often lead to uneven clamp load—and ultimately, head gasket failure. On top of that the high-quality casting is said to experience vastly reduced distortion, be less prone to cracking due to extreme heat, and be more resistant to lifting.
Structural integrity is another area that exceeds the factory aluminum heads. PPE claims that its heads will hold up much better to higher head stud torque, especially when Custom Age 625+ or 14mm diameter studs are employed. The casting material plays a huge role in these heads’ superior strength.
Conveniently, PPE’s heads ship with the rocker shaft and rocker box dowels already installed, so there is no need to transfer them over from your old ones. For anyone who’s been around Duramax heads, this is a big deal, as it’s almost impossible to swap the rocker shaft and rocker box dowels over without damaging them.
PPE’s cast-iron heads also come with all freeze plugs installed, and this includes the freeze plugs in the ends. They’re also stamped right and left, which makes installation all but fool-proof. Externally, and as you can see here, the heads wear a coat of semi-gloss black paint.
6.6L Duramax Performance Valve Springs
Tearing the PPE heads down, effectively reverse-engineering them, LDP got started by inspecting the valvetrain. Going over them with a fine-toothed comb, the clearances were checked on every valve and valve guide. Plus, with a truck pulling customer interested in trying a set, LDP had a vested interest in finding out if PPE’s new heads were the real deal or not. What they found pleasantly surprised them.
The valve springs PPE uses in its cast-iron heads are a definite upgrade over the factory units. Conical style, progressive frequency springs, they provide a natural damping effect without producing extra friction (which leads to heat) and suffering added wear. These valve springs also weigh 11 grams less (apiece) than the factory valve springs.
Using its Longacre digital coil and valve spring tester, LDP checked the closed seat pressure of each valve. Each one checked in at 125 lbs (give or take 1-lb). By comparison, factory Duramax valve springs have a 75-lb closed seat pressure. Making use of stiffer springs will be ideal in applications that see higher boost, drive pressure, and engine speed as they will help rule out valve creep (caused by high boost), reversion (caused by elevated drive), and valve float (caused by excessive rpm).
Performance PPE Power Vs. Factory 6.6L Duramax Valve Springs
In this photo you can see the PPE valve spring and retainer (top) side by side with a factory spring and retainer (bottom). Notice how much smaller the PPE unit is? This smaller, stiffer spring will react quicker, force the valves to seat faster, and will also help with harmonics. For added insurance, upgraded (stronger) pushrods are recommended with these valve springs.
High-strength, chromoly retainers and spring locators—which are CNC-machined, not cast—are also part of the PPE heads’ valvetrain. But not only are they stronger than OEM equipment, they are considerably lighter at just 12 grams apiece. That’s roughly half the weight of a factory retainer in an LB7 (24 grams) and a newer Duramax (21 grams).
PPE Power Performance Valve Springs
Here you can see one of PPE’s exhaust valves. Each one is made from a premium 21-4N super alloy, which contains nearly twice the nickel content of 21-2N and effectively gives them a higher temperature rating than standard Inconel. The exhaust valves are also stellite-reinforced to withstand excessive exhaust gas temps. All exhaust valves are fully nitrided, too, not just the tips.
If you sensed a common theme of lightening up the valvetrain here, you’d be correct. Lighter (and stronger) parts will ensure the valves close faster, and their reduced mass will translate into an ability to support more rpm. Whether the end-user plans to run PPE’s heads as drop-in replacements for stock heads or add more fuel, boost, drive, and rpm, its cast-iron heads are designed to handle either job.
Putting a profilometer to use, LDP checked the surface finish of the heads (you may have also seen this instrument used during honing). Both heads ranged from 8-26 Ra, with the maximum allowed by Cometic being 50 Ra. Once again, PPE’s heads proved winners right out of the box. The folks at LDP, a company with its own machine shop and steady supply of Duramax customers, were impressed with the high-quality surface finish.
In the middle of checking the heads’ surface finish, the guys at LDP reminded us that the 26 Ra number collected (which again is still only half of Cometic’s requirement on surface finish) was only discovered in one spot. Long story short, if they ship like this anyone can slap a set on just about anything and run them with no additional machining required.
Powdered metal valve seats provide superior wear and heat resistance, and PPE utilizes them in its cast-iron heads. On top of their proven durability, powdered metal seats typically have a smoother surface and require less machining once installed.
Very smooth radius’s are present in all of the valve seats, with no sharp, jagged edges disturbing or restricting airflow. In addition, LDP found valve recession to be not only well within allowable specs, but consistent across the board.
Notice that the valve guides are recessed and well out of the way in the exhaust valve bores. Valve guide clearance was well within spec and—like valve recession—extremely consistent. And not only was the valve guide clearance better than satisfactory, but it didn’t vary much from end to end.
To test valve-to-seat seal, a vacuum test was performed on all valve seats, and every one of them held 17 inHg of vacuum. According to LDP, anything over 13 inHg is considered good on valve jobs. They elaborated by remarking that these particular heads were on the high end of what they usually see.
Overall, PPE’s cast-iron heads have nice, smooth intake ports that appear to have good flow. Additionally, the valve pockets all have smooth transitions, no sharp angles or restrictions, which promotes optimal flow. It should also be noted that plenty of room exists for porting…
It’s the same story on the exhaust side. The exhaust ports are all de-burred and smooth—much more so than what you see on most off-the-shelf heads. Notice that the valve guides are visible in this picture.

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LinCo Diesel Performance owner, Jeff McCord, was very upfront about his team’s findings in PPE’s new Duramax heads. McCord summed things up by calling them the highest quality replacement cylinder heads he’s seen yet. But on top of that, McCord added that with very minimal time involved in blending them, there is zero doubt that they would outflow a good set of stock heads. Again, and as you’ve already seen, there is plenty of room for porting. McCord concedes that at 85 pounds apiece (fully assembled) the cast-iron heads are considerably heavier than stock (just 35 pounds), but he also knows that that won’t be a big deal to a street truck owner or especially a truck puller.
This specific set of PPE’s cast-iron heads will be reassembled, fastened to a Duramax, and dyno-tested for some out-of-the-box results vs. remanufactured or stock aluminum heads. However, a second set of heads has already had its water jackets filled with concrete, been O-ringed, CNC ported, and will soon be hitting the dyno to help a truck pulling customer squeeze as much horsepower as possible out of his turbo/air-limited application.
6.6L Duramax Diesel Heads
As we went to press, MSRP on PPE’s cast-iron heads was forecasted to be $3,399 (there is no core charge). These heads, which fit LLY to LMM engines (’04.5-’10) are set to be available by early summer, and may have even been released by the time you’re reading this. At a later date, PPE also plans to bring cast-iron LB7 heads to market. The LB7 heads will come complete with cast-in (and machined) injector seats, which means no more injector cup issues.
Whether you’re after a bolt-on performance option or a direct replacement set of cylinder heads, PPE’s cast-iron units are as practical as they are beneficial. If you have a cracked or damaged head, they’re a no-brainer—especially when you factor in the upgraded valvetrain, with roughly $600 worth of valve springs and retainers alone.
While the drag racing crowd might be slower to embrace cast-iron cylinder heads due to the added weight (although they have been tried), the pulling world should have no problem with them. If PPE’s units can resist cracking and hold up to the extensive heat a pulling application produces, we think they will be a hit. The fit and finish, race-ready valvetrain, and solid machining on these heads not only screams quality, but it also makes them worth the money.

SOURCES

LinCo Diesel Performance

636.528.1577

lincodieselperformance.com

Pacific Performance Engineering (PPE)

725.238.2002

ppediesel.com

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