Doing the R&R is easy except for the rear bolts at the exhaust flange, which are hard to reach and remove.
How to install a BD Diesel performance manifold to restore exhaust flow balance and lower EGTs while towing
by Bruce W. Smith/Senior Editor
One section of the OEM manifold on this 2008 Duramax 6.6L is constricted to allow clearance for the steering shaft. So instead of a nice round exhaust passage it creates a choke point that reduces overall exhaust flow by about 20 percent.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 from intake to exhaust have an adverse effect on engine power and fuel economy.
Heavy-duty GM pickup owners may not know it, but the Duramax 6.6L has a hidden choke that 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).
BD Full Bore manifold lets exhaust move unrestricted end-to-end. Such temperature imbalance isn’t a good thing when one is looking for optimizing engine performance.
BD’s solution: the BD Full Bore exhaust manifold. It’s from high-silicon ductile iron and is a bolt-in replacement for the factory manifold. The manifold retails for about $300.
We installed one on a 2008 GMC Sierra 3500 during the installation of a BD Positive Air Shutdown kit for a contractor working in the gas/oil fields.
The upgrade 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.
Not only does the new manifold balance out the OEM temperature imbalance, it improves turbo response and drops loaded EGTs at the exhaust flange by nearly 200 degrees (tested while truck was on a dyno under heavy load.)
“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 chips or 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. We supply a threaded port for an exhaust temperature gauge to be installed in that manifold.”
BD Full Bore manifold (top) is made from high-silicon ductile iron and is a direct bolt-in replacement for the factory manifold.Performance tip: 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 in the correct location.
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.
