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I recently stated that my 500hp 468 has 2.25" exhaust piping. Duh, I measured the section from the muffler back. Anyway, I got several Emails from guys telling me it should be at least 2.5". Well, rolling around under it tonight playing with trans speedo gears, I noticed that from the header collector to the muffs, it is 2.5". I have two sizes of pipe in the system. Hmmmm...did someone actually know a trick?

It's been 15 years since thermodynamics classes in college, but I remember a formula for gases which goes PV=nRT. Looking at this, we see that volume is proportional to temperature. Comparing the cross sectional areas of the two pipe sizes, it sure looks like the exhaust gases would just have to cool 23% as they travel from the header collector to the output of the muff in order to allow maintain the volumetric flow and velocity (and hence, back pressure) using the smaller tail pipes.

Sooooo...is this correct, am I ok with the smaller tail pipes? Or is there one of those real world parameters which makes the my arguement silly and false?
 

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The exhaust does indeed cool as it travels back. The biggest cool down occurs in the mufflers. That is why you typically see mufflers with smaller outlets. This helps keep the flow smooth as it exits otherwise turbulence occurs. As for pipe size, a good rule of thumb is 2.5" pipe will handle up to 450HP. Higher HP requires 3".
 

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This is exactly the explaination I got from a muffler man and it makes sense. Further, there are claims that the staggered size may actually give more torque to a specific engine. e.g., Torquetech found that the 3" headpipe w/ 2.5" tailpipe was actually better in some high torque applications ( specifically on a 450 hp 455 and a configuration that sounds particularly well suited to your application).

you should know they also told me that 500 hp was the pivot point between needing 2.5" or 3" pipe. Something you may want to consider the next time the pipes need replacing.



[This message has been edited by Gene Chas (edited 07-25-99).]
 

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Gene was right about the size of your pipes. It was nice to see the combined gas law quoted here in chevelle tech, lots of my Physical Chemistry classes apply to our cars.

I will ditto that your pipes are more suited for a healthy 350 instead of a big block. But if you don't race the extra torque may be more your taste but your back pressure at peak HP RPM is on the high side with that healthy 468

Frank

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