Professional Answer to Exhaust Size Question 2.5" vs 3"
Exhaust Pipe Diameter is comparable to Soda Straws.
You ever notice how when you go to Taco Villa....you get a Monster 44oz cup with a 6" wide lid, and a tiny .25" diameter 12" long straw?
You go over to McDonalds, you get a 32oz cup, a 4" lid and a .375" (3/8") 10" straw.
Which one can you empty the cup FASTER with?
Exactly. The .375" High Flow McDonalds Straw.
Interior Volume of a given size of pipe HAS a Maximum Flow for a given pressure/vacuum.
(CID X RPM) / 3456 = CFM
So for a 360 to breathe IN and Exhale OUT 7000 rpm worth of air....the exhaust system would need to be able to flow 730cfm WITHOUT RESTRICTION....if you choose to small of pipe diameter, the engine will see flow restriction and there will be BACK PRESSURE which will hinder high rpm FLOW.
Choose too big and......the engine may be a little soggy on the bottom end.
By messing around with turbochargers for the last couple of years, I have learned how the diameter and interior VOLUME of a stick of pipe can translate to FLOW.
The chart posted is pretty accurate....if you read the fine print....it said CRANK HP recommendation.
My general rule of thumb is this:
Single Exhaust:
320hp WILL FLOW THROUGH 2.5" pipe
550hp WILL FLOW THROUGH 3"
700hp WILL FLOW THROUGH 3.5"
Dual Exhaust:
450hp WILL FLOW THROUGH 2.25"
600hp WILL FLOW THROUGH 2.5"
750+ WILL FLOW THROUGH 3"
Each one of those listed HP's is the limit for the size of pipe.
You could always put a Pressure Gauge in the rear of your exhaust system.
When the (PSI) pressure of the exhaust system reaches 1.5 - 2 psi the pipe has reached its flow limit and is starting to cause excessive BackPressure.
IMO....you want less than .5 psi of pressure at Max RPM....equalling ALMOST ZERO BACK PRESSURE.
MY Naturally Aspirated 360 will have a 2.5" Dual System, and when I Turbocharge it later on down the road...It will have a single 3.5" system.
The same interior volume/flow application is relevant to Pre-Carb, or Throttlebody Intake Piping as well.
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