Page 54 - JML_ManifoldCatalog_Flipbook

MerCruiser

GM V8 SMALL BLOCK EXHAUST SYSTEM EVALUATION

OEM PORT SIDE STARBOARD SIDE

Exhaust Gases Exhaust Gases

GOOD

1 3 5* 7* 2 4* 6 8*
“OTHER”
“OTHER” 1-8-4-3-6-5*-7*-2 1-8*-4*-3-6-5-7-2
*Cylinders fire in succession *Cylinders fire in succession

PORT SIDE STARBOARD SIDE
BETTER Exhaust Gases Exhaust Gases

JML

1 3 5* 7* 2 4* 6 8*
JML
1-8-4-3-6-5*-7*-2 1-8*-4*-3-6-5-7-2
BEST! *Cylinders fire in succession *Cylinders fire in succession

A high performance aftermarket exhaust manifold can deliver
a performance increase over a stock OEM exhaust manifold.

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. On the
port side, we have #5 and #7 firing within 90 degrees of crankshaft rotation of each other.
Of course, this also means that the exhaust gases for these cylinders immediately
discharge into the exhaust manifold one after the other.

At this point, when the two cylinders fill the manifold in immediate succession, the
exhaust manifold becomes overwhelmed with exhaust gases, creating excessive back
pressure. In fact, excessive backpressure from an overwhelmed 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 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. Choosing a high performance manifold that isolates each runner is the
key to building horsepower with the exhaust system.

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