Diesel Exhaust Manifold Upgrade
How to maximize Duramax power and improve turbo responsiveness
By Bruce W. Smith
Optimum exhaust flow is critical to maximizing the efficiency of any engine. After all, an engine is just a sophisticated air pump – and the easier it is to move air in and out, the more efficient the pump becomes. 
That’s why any choke points in that flow of air have an adverse effect on engine Âefficiency.
When it comes to diesels we typically think of such choke points as the air intake, the turbo and the exhaust system downstream.
But when it comes to the 6.6L Duramax, there’s a choke point most owners don’t see, yet it has an adverse effect on achieving optimum engine performance: the driver’s-side exhaust manifold.
“The factory manifold is pinched down at its mid-point to allow for steering shaft clearance, although such clearance isn’t actually needed,” says Brian Roth, the founder and president of BD Diesel Performance (dieselperformance.com).
“As a result, the OEM driver’s-side manifold flows 20 percent less than the passenger side, creating an exhaust flow imbalance and elevating EGTs under high-load conditions such as towing,” explains Roth.
Roth says you can see the temperature imbalance difference at night because the front two exhaust ports glow yellow after the engine has been worked while the back two ports remain black (cool). That’s not a good thing.
So the diesel performance specialists at BD came up with the obvious solution: the BD Full Bore manifold.
Made from high-silicon ductile iron, the Full Bore is a direct, bolt-in replacement for the factory manifold. It retails for about $300.
BD Duramax exhaust manifold upgrade is a relatively easy job but it takes a few hours to do.
We want the Duramax in our 2008 GMC ÂSierra 3500, Project Big Red, to be as efficient as practical for any contractor involved in heavy construction.
So we had Truck Supply & Outfitters in Tuscaloosa, Alabama, make the exhaust manifold upgrade while they had that side of the engine exposed to install the BD Positive Air Shutdown kit.
The install is straightforward and takes about 2 1/2 hours.
The most difficult part, other than getting to the manifold, is removing the three bolts at the exhaust flange: They are difficult to reach and even harder to break loose.
But the effort is worth it on several fronts.
Roth says their chassis dyno tests on the Duramax show about a 200-degree drop in loaded operating temps measured at the exhaust flange.
In addition to reducing exhaust manifold heat imbalance/build-up, the turbo response is improved:
“Quicker turbo spool up is the biggest gain I have heard from customers,” says Roth of the Full Bore manifold’s performance value.
“For guys installing EGT gauges they don’t have to drill and tap the stock manifold, either, or worry about the metal debris chipping the turbine wheel once the engine starts back up. Our manifold comes with a threaded port for an exhaust temperature gauge to be installed.”
Speaking of the exhaust temp probe placement, Roth says the thermo Âcouple is often mounted after the turbo, which is the “wrong place to measure exhaust temperatures.”
He recommends it be placed in the driver’s-side manifold, which is why their performance manifold is already drilled-and-tapped for the probe in the correct location.
BD’s Big Bore exhaust manifold upgrade might be one to consider on your fleet’s ÂDuramaxs the next time mechanics are working on the driver’s side of the engines or are in the process of rebuilds.
The new manifold will ensure those GM diesels will be going full-bore when called to do so.
