Page 55 - Manifold_Catalog_Flipbook

MerCruiser

GM V8 SMALL BLOCK EXHAUST SYSTEM EVALUATION

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

1 3 5 7 2 4 6 8

GLM COMPLETE GLM COMPLETE
EXHAUST EXHAUST
MANIFOLD MANIFOLD
Cross Section

If properly designed, it can achieve this by two ways:
Separating Exhaust Gas Pulses and Lowering Back Pressure.

Separating Exhaust Gas Pulses Continued...
Because the starboard side of the engine fires #4 and #8 in succession, there is a
cylinder (#6) separating the two. Dividing this manifold into two different sections by
grouping #2 and #4 in the front half of the manifold and #6 and #8 into the back half of
the manifold works relatively well in isolating the #4 and #8 cylinders. On the port side of the
engine we have a problem: The #5 and #7 cylinders that fire in succession are next to each
other.

Dividing the port manifold into a front half and a back half groups cylinders #1 and #3
together (which is okay), but groups #5 and #7 together (which is 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 and #7 all the way to the manifold exit.

Lowering Back Pressure
Lowering back pressure in the exhaust manifold is a combination of isolating the
exhaust pulses as discussed in the previous section, and simply making the exhaust
system larger. Some techniques for lowering back pressure include: 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.”

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