Soooo, i was wondering. If I have a 3 inch down pipe, does it really matter what size the rest of my exhaust is as long as it 3 inch also. A 5 inch exhaust wont make a difference if my choke point is 3 inch. The reason I ask is that I am making two trucks out of one. The truck thst gonna be driven has a rag-tag 4 inch straight pipe exhaust but the donor truck has a new 3 inch exhaust with muffler. I just have the down pipe and a edge programmer that I leave on the tow setting. Any thoughts?

 

You can run any size exhaust you want

 

Personally, I'd step it up to 4" after the down pipe. 3" may have a slight restriction, and will be louder then 4".

 

I would go from the 3" to a 4". The 4" will open things up offering a bit less resistance.

 

Resistance is addative. That is the resistance from each component (or section) is added together to give total resistance. The resistance of each section is proportional to it's length and (disregarding boundry conditions, etc.) inversely proportional to the square of the diameter. so... if the resistance of the 3" is 1/9 units (.111) then the resistance of the 4" is 1/16 units (.0625).

example: if the three inch down-pipe is 3' long and the rest is 9' long,
A 3" exhaust would have .111 X 12' which = 1.332 units of resistance.
A 4" exhaust w/3" d.p. would have (.111 X 3') + (.0625 X 9') = .8955 units.
(Very roughly speaking)

 

Ummmm. I thought this wasn't rocket science but maybe I was wrong. Lol. Makes sense, just hate to ditch a good exhaust but also don't want to go backwards in performance.

 

spray paint a strip down the pipe. start and run the truck. where ever the paint stops burning of you can go as big as you want.

 

I was told that the turbo (on a diesel) provides enough back pressure and therefore you can immediately go as big as you want.

 

You were told correct.

The turbo is all the back pressure you need. Anything beyond that is a restriction.

However, nothing larger then 4" will show any improvements in performance... The 5" systems you see out there are for looks and sound.