This page contains information about the creation of some custom 4-2-1 headers for the second generation B16A in my Honda Civic SiR. This was brought about after the decision to do a B16A Engine Swap. The stock cast iron manifold looks somewhat restrictive even though it is rumoured to flow quite well. The cast iron exhaust manifold is also very heavy, so just removing it and replacing it with an equivalent header that weighed much less would have some performance benefits.
I looked around for a little bit, and found that most of the aftermarket B16A exhaust manifolds didn’t even have equal length primaries. The last time I checked, having equal length primaries/secondaries was what made a good manifold (assuming a tuned length). Also, any reasonably good looking and well constructed exhaust manifold cost a small fortune and was usually made out of stainless steel (which can be prone to cracking or localised corrosion unless the welding is spot on. So, once again I looked into fabricating my own. I decided on a 4-2-1 configuration because the PR3/PW0 ECU requires two oxygen sensors, which is almost impossible to get working correctly with a 4-1 configuration. A 4-2-1 configuration is also supposed to be better for retaining bottom end torque compared to a 4-1 configuration.
My father knows the owner of a local speed shop, so I sent the B16A VTEC cam timings to Edgell Automotive to obtain the optimum exhaust dimensions for a B16A 4-2-1 exhaust. The measurements were to be tuned to the stock standard peak torque that occurs at 7200rpm. The numbers returned were interesting because they were much larger than I expected. For example, the primaries are nearly 2” and feed into some larger secondaries and then onto a full 2.5” exhaust.
With the optimum exhaust sizing sorted we built the exhaust directly on the new engine to make sure that it would fit nicely. Click on any of the images to get a larger version.
The custom B16A exhaust manifold is made out of mild steel mandrel bends and has been ceramic coated. Below are pictures of the nice new custom B16A 4-2-1 headers after being coated. Note the two bungs for the oxygen sensors are at the end of the primary collectors.
The new headers are much larger than the stock ones and required us to take a pretty good sized notch out of the front cross-member to get the engine back into the engine bay. There is also minimal clearance to the air-conditioning cooling fan (although some might argue that removing the air-conditioning would be an acceptable solution). I actually had to take one of the metal brackets off the back of the fan to stop the custom B16A headers consistently rubbing against it.
If I was going to do the headers again I would look at getting them tuned for a lower RPM. With the existing set-up, both the peak power and torque occurs at red-line as shown in the latest B16A Dyno Run. Lowering the peak torque RPM would mean that I could more effectively use the potential power of the engine. Of course upgrading the head so that it could handle a higher RPM is also an option, but typically that is also an expensive one.
I’d also look at wrapping the headers if I had to put them in again. These new B16A headers give off a lot more heat than the cast iron manifold with heatshield. In fact, they give off so much heat that I installed some Bonnet Vents to help remove the excess heat from the engine bay.