i found this at http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html
its a reallyyyyyyyyyyyyyyyyyyyyyyyyyyy long write up longer than the last one i posted so im goin to brake it down simpler

Secondaries -- Diameters and Lengths (if u already have headers will be useful)

collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily.

As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. For a motor peaking at around 6,000-8,500 rpm, a collector length of 10-20 inches is effective.

Getting secondary lengths nearer optimal can be worth a sizable amount of extra power. If you want to bump up torque at the point a stock converter starts to hook up the engine, you may want a secondary as long as 50 inches but something between about 10 and 24 is more normal. The shorter of these two lengths would be appropriate for an engine peaking at about 8,500 rpm whereas the longer length would be best for an engine that peaked at about 4,800-5,000 rpm.



Muffler Flow -- How Much is Needed?

The engine cares little what size the muffler pipe diameters are but it certainly does care what the muffler flows and muffler flow is largely dictated by the design of the innards. What this means is that the informed tuner/engine builder should select mufflers based on flow, not pipe size.


Knowing that 2.2 cfm per open-pipe hp means zero loss from backpressure allows us to determine how much muffler flow your engine needs. Just make a reasonable estimate of its open exhaust power potential and multiply by 2.2. For instance, a V-8 making 500 horsepower on open exhaust will require 500 x 2.2 = 1100 cfm. Two 550-cfm mufflers will get the job done and contain the backpressure-induced power loss to 5 horsepower or less. With mufflers rated in cfm, see how easy making an appropriate choice gets?

If u have a 2.5-inch muffler that flows 400 cfm, the engine reacts to this just the same as it would a piece of straight pipe flowing 400 cfm.
At 115 cfm per square inch, that's the equivalent to a pipe only 2.1 inches in diameter.

this is why cheap ass exhaust systems are bad for your car much less the reason why im goin down to a 2.5" full system rather than my 3"

This is an important concept to appreciate. Why? Because so many racers worry about having a large-diameter pipe going to the muffler. This concern is totally misplaced, as in almost all but a few cases, the muffler is the point of restriction, not the pipe. The fact that muffler core flow is normally lower than the connecting pipe can be off set by installing something with higher flow, such as a 4-inch muffler into an otherwise 2.75-inch syste

so i dont wanna sound like an ass to all the guys out there running 2.25" exhaust but if u have headers and there of 1.5 primaries u shuold be running a 2.5" exhaust system to achieve full potential of your engine and correct rated mufflers on your car other wise ur $1000 is a waste



if it start the next day it can be driven harder

Read the same, posted the like in own words months ago. Nice to see someone else keen on this stuff.


Go beyond the "bolt-on".