i know its suggested to run a 2.25 exhaust with the super six conversion, but anyone running a 2.5?

yes and when I install my dultra duel duels i will be two 2 1/2 stacks...yes I know I will lose some torque

Do you mean 2 1/2" off the stock manifold? It's possible, but on a lower HP motor I don't think you'll see any benefit to power or economy over 2 1/4". The muffler is more important than the larger pipe.

I made a chart showing the cross sectional area of various pipe sizes to make it easier to compare flow capacity. 2 1/2" is 25% greater flow capacity than 2 1/4". But if stock is 1 7/8" then 2 1/4" is 48% more flow capacity. 2 1/4" single will support 185hp with no loss assuming the muffler flows enough. Good stock motors are only about 100hp or so.

Tube OD wall thickness ID Area

1.375 0.065 1.245 1.22
1.5 0.065 1.37 1.47
1.625 0.065 1.495 1.75
1.75 0.065 1.62 2.06
1.875 0.065 1.745 2.39
2 0.065 1.87 2.75
2.25 0.065 2.12 3.53
2.5 0.065 2.37 4.41
3 0.065 2.87 6.47
3.5 0.065 3.37 8.92
4 0.065 3.87 11.76
4.5 0.065 4.37 14.99


I like that it's easy to see than 3 1/2" single is actually slightly higher flow capacity than 2 1/2" duals, but here the muffler(s) becomes critical because of space constraints on most vehicles. Be aware that 2 1/2" duals or 3 1/2" single will support 466hp with no power loss assuming adequate muffler(s).

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Joshua

I just spent an hour on the phone with Mike J last night. We were chatting about my duals. After describing what my engine was doing, he agreed with me on my conclusion.

When we built the head pipes for this engine, we made them from 2-1/4" mandrell bent tubing. I run Dual Dutra Duals; both new castings.
Power was terrible. After wrapping the pipes, power improved markedly. This indicates that the charge was slowing down too much in the large pipe, and scavenging was poor. The engine was having to pump the spent exhaust gasses out of the engine.
By wrapping the pipes, heat was kept high longer, keeping the gas volume high, and therefore velocities in the exhaust stystem high. This improved scavenging, and thereby improved cylinder fill, and reduced pumping losses.

I will be making a new head pipe arrangement in 2" for the Dual Dutra Duals, and wrapping them. This should contribute to much better power across the engine's range.

Going bigger is not necessarily the answer.

CJ

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Part of Tyrde-Browne Racing