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"I have read the addition of an H or Y pipe can increase performance by reducing back pressure. As the exhaust still has to go through the mufflers how can those pipes reduce back pressure"?


In a "true dual" exhaust system the exhaust gasses of both banks are separated from each other. An H or Y pipe allows exhaust gas pulses from one bank to sneak in between the pulses from the other bank. Kinda like cutting in line at Disneyland. The other advantage of an H or Y pipe is they make the exhaust noise a bit more mellow.

In a true dual system the exhaust pulses of both banks are uneven because of the uneven firing order. On the left bank it's #1 then two 90 degree pauses then #3 then another 90 degree pause then #5 and #7 then another 90 degree pause. So an H or Y pipe allows the gasses of the left bank to sneak in between the pauses of the right bank and vice-versa; making the sound more even while reducing the back pressure. Without an H or Y pipe the back pressure from each bank pulsates but with an H or Y pipe the back pressure in each bank becomes steady.

After I fabricated then installed a 2-1/2" H pipe on my 71's 454" I noticed a definite difference in the sound as it wasn't quite as harsh as it had been. Any performance gain would only be realized under full power and high rpm. And of the two styles the Y pipe would be better because the exhaust doesn't have to turn two 90 degree angles.
 

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