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Originally Posted by 84LX89GT
Pipe diameter and length for resonance tuning and scavenging is what is important, backpressure slows exhaust from coming out of the head and diluting the air/fuel mixture on the intake cycle. If he went to 2.5" exhaust it would likely kill bottom end completely. The svo exhaust bolts behind the cat and is 2.25" dual, so it has the same effect as a low restriction muffler even with quieter mufflers. You want to keep diameter smaller near the manifold to increase velocity and the effect of the scavenging exhaust pulses. Once he gets a ways past the 105 horsepower range it might be a good idea to increase pipe diameter.
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The 85.5 & 86 SVO had dual 2.5" exhaust past the cat.