Full-Bore Manifold


A sure cure for a hidden Duramax exhaust choke point; BD Diesel performance manifold restores flow balance and lowers EGTs


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 efficiency.

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 owner’s 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.  

“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 those EGT restrictions can be obvious to the eye if looking at the manifold at night because the two driver’s-side exhaust ports before the restriction turn yellow  while the rear tports show no signs of such heat build-up.

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 and is even drilled and tapped to accept the most popular EGT probes. It retails for about $300. 

Driver’s-side Duramax 6.6L exhaust manifold (bottom) flows 20-percent less than the passenger’s side. BD Big Bore (top) restores full flow and lowers EGTs.

We want the Duramax in our 2008 GMC Sierra 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, AL, 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.

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 dyne tests on the Duramax under load is about a 200-degree drop in 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.

BD Full Bore exhaust manifolds come pre-drilled and tapped to accept standard thermo couple probes.

There’s another benefit: Roth says the thermo couple 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.  

“For guys installing chips or gauges they don’t have to drill and tap the stock one, worry about the metal debris chipping the turbine wheel once the engine starts back up. So the labor they save on not having to drill and tap can be applied to the installation of the manifold.”

BD’s Big Bore exhaust manifold upgrade might be one to consider on your fleet’s Duramaxs 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. – Pro