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MerCruiser



                     GM V8 BIG BLOCK EXHAUST SYSTEM EVALUATION



                               PORT SIDE                              STARBOARD SIDE
                         Firing Order Cross Section                Firing Order Cross Section









                     1         3         5        7              2        4         6        8

              Separating Exhaust Gas Pulses
                 All V8 engines have a firing order that fires two cylinders within 90 degrees of each other
              on the same bank of cylinders.  With a firing order of 1-8-4-3-6-5-7-2 we have #8 and #4
              firing on the starboard side of the engine within 90 degrees of crankshaft rotation of each
              other.  Of course, this also means that the exhaust gases for these cylinders immediately
              discharges into the exhaust manifold one after the other.  It is at this point, when the two
              cylinders fill the manifold in immediate succession, that the exhaust manifold becomes
              overwhelmed with exhaust gases, creating excessive back pressure.  In fact, these two
              cylinders on each bank firing in succession, not only create excessive back pressure in the
              manifold, but all the way down the exhaust pipe.  Excessive back pressure from an over-
              whelmed exhaust system will not only keep spent gases from leaving the cylinder head
              exhaust port, but in certain ports, exhaust gases from adjacent cylinders may find their way back in.

                 The solution is to isolate each exhaust gas pulse as much as possible for as long as
              possible in the exhaust manifold.  There are aftermarket manifold manufacturers that go to
              great lengths to isolate each exhaust pulse the full length of the manifold and there are
              some that barely address the problem at all.
              Divided Exhaust Manifolds
                 Because the starboard side of the engine fires cylinders 4 & 8 in succession, there is a
              cylinder (#6) separating the two.  Dividing this manifold into two different sections by group-
              ing cylinder #2 and #4 in the front half of the manifold, and #6 and #8 into the back half of
              the manifold with a divider, works relatively well in isolating cylinders #4 and #8.  On the port
              side of the engine, we have a problem.  The cylinders that fire in succession, 5&7, are next
              to each other.  Dividing the port manifold  into a front half and back half groups cylinder 1&3
              together (which is okay) but also groups cylinder 5&7 together (definitely not okay).  This is
              exactly what we are trying to prevent.  The correct way to build the port side manifold is to
              isolate the exhaust gases from cylinders 5&7 all the way to the manifold exit.  This can not
              be done with a simple divider in the middle of the manifold.
              Lowering Back Pressure
                 Lowering back pressure in the exhaust manifold is a combination of
              isolating the exhaust pulses as discussed in the previous paragraph
              and simply making the exhaust system larger.  Large passages
              in the exhaust manifolds, a large exit hole out of the manifold, a
              large exhaust pipe and large radius curves in the pipe wherever
              the exhaust gases have to change direction are the secrets to
              lowering back pressure.”




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