Not necessarily, but the pipes off the manifold flanges don't need to be very big or you'll mess up the back pressure and possibly draw cold air back to the exhaust valves as the pressure pulses cycle. A split manifold only has half the pulse frequency in each pipe as a single exhaust, and the pulses occur every 240 degrees of crankshaft rotation. That's what gives a split manifold six the characteristic "BRAAAAAPPPPP" sound, but it can also warp valves with cold air if the pipes are too big.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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