Page 13 - GMC Vintage RVing - Winter 2018

Technical Corner MAINTENANCE, PARTS, TIPS, INNOVATIONS AND MOREHeadersDick Paterson | 1976 Eleganza II | 705-325-4554 | Springfield Ignition | springfield-ignition.comOnce OEM manifolds are cracked, etched, warped, etc. replacements are getting harder to find. One option is to replace the manifolds with a set of headers. Some folks just put them on, harking back to their teens and when they were the hot choice for performance.There are always questions when considering them for the GMC, the following are from my observations – your thoughts may differ.Q: Are they nosier?A: Yes. The younger the ears, the louderthey sound.Q: Do they raise the under the hood temperature?A: Yes. As they have more surface area thanthe manifold. Most evident at idle and low chugging speeds. When on the road there is sufficient airflow through the front to get rid of excess heat. At shutdown the header cools faster than the manifold since it is thinner steel vs. the heavy thicker cast iron manifold.Q: Will they improve the performance?A: Yes, but it is hard to feel a 12,500 lb. lump.Q: Will they improve my mileage?A: Perhaps slightly, since they impact volumetric efficiency (VE) and thus torque. Torque is what moves vehicle under load. Torque is influenced by the breathing of the motor.In other words, its ability to inhale and exhale air. Freer breathing either in or out will increase torque.Header Versus the OEM ManifoldWe can examine the function of the header versus the OEM manifold as it relates to the exhaling side of the motor and its resulting impact on the inhaling side of the motor.The tri-Y design of the headers offered for the GMC are the best design for lower rpm performances improvement.Most motors operate most efficiently around their peak torque RPM. The reason is that peak torque is found at best VE or in simple words at the best cylinder fill of the air/filter mixture. A stock V8 motor of the 70’s era will run about 70-75 percent volumetric efficiency. Asthmatic by today’s engineering technology.The peak torque for the GMC is in the area of the mid-2000 GMC where cruising withminimum throttle (highest vacuum) will give the best MPG.The camshafts design back then was short on duration and long on valve overlap. Overlap is when the intake valve begins to open and the exhaust valve was not yet completely closed. The theory and physics are that the exiting exhaust flow will pull a slight yank on the incoming air fuel mixture helping it to “get started” on filling the cylinder (helping the cylinder fill will raise the VE).Physics has the intake flow parked on the back side of the valve giving it an initial minor tug helps the flow begin. (Too much overlap and the exhaust “pressure” will push back against the incoming flow.)As the piston comes up on the exhaust stroke,it creates some pressure within the cylinder, then as the valve opens it blows down out through the exhaust port and into the exhaust manifold. Any reduction of pressure in the exhaust loop will lower the residual pressure remaining in the cylinder when the intake valve begins to open. Purging this residual exhaust from the cylinder allows more room for the incoming charge.If you visualize the volume in each cylinder as a 50 cubic inch “slug” of air (about the size ofa round salt box) you can follow its path as it gets pushed (squeezed) out through the exhaust valve into the line and begins its long journey towards a GMC back bumper.The major work done by the exhaust blow down capitalizes on heat retention and its resulting velocity of the gas flow weakening as it gets further from the port.Outside the exhaust valve, 14.7 PSA atmospheric pressure waits to offer some resistance to the “slugs” flow. The exhaust port and beyond havea direct influence on the efficiency of the flowin terms of volume and velocity, which in turn affect “pumping losses” that rob gross power.The OEM manifold is a log style that requires the exiting exhaust flow to make an immediate 90 degree turn before continuing its course. This results in back pressure and slightly adds to pumping loss.Considering the common chamber in the log manifold, there is no internal baffling to direct the flow once expelled. The center four cylinders can initiate some exhaust flow ineither direction. These center cylinders (2 & 4 – 3 & 5) exit through a common single opening. Their consecutive firing order adds to the initial confusion going on within the log. Visualize the “slug” mimicking a squirrel crossing the road.In the case of the OEM manifold for the 403, the passenger’s side manifold has a pinch (say “major” restriction) in it to accommodate the oil filter. AAARRGGG.... What a relief the header offers there.Headers by their design offer a straight pipe extension with identical cross section area of the port to each individual port. Flow is less restricted with no immediate 90 degree turn. This results in freer flow for the “slug” as it heads by itself down to merge in the collector. (The major benefit from the tri-Y layout.)The resulting pull goes all the way back up to the exhaust valve producing a stronger yank on the incoming mixture flow helping the VE without altering the exhaust CAM timing. This will benefit a stock motor but a rebuilt motor witha different torque CAM will benefit even more from the increased flow offered by the header.Exit at the center four cylinders is now improved by the “individual” pipes flow characteristics in contrast to the common port convergence.During a motor rebuild, building out the center divider will improve the function of the header even more.Port miss-matching of headers to motors 455 and 403 has been resolved now for several years and as produced for either motor will have the correct openings to match the cylinder head exhaust ports.External coatings will add to longevity. External and internal coatings add to some degree heat retention/flow velocity within the header. Not that you will notice.The installation of headers does require good gaskets and the re-tightening of mounting bolts after some drive cycles.In ConclusionExamine the shape of exhaust manifolds on current production vehicles, almost header-like and striving to meet the mandated mpg regulations. The OEMs don’t swallow the added costs of dual exhaust systems without the same ensuing benefit. It is all about the “breathing”.WINTER 2018 GMC MOTORHOMES INTERNATIONAL 13