Y-pipe cat-back, good or bad idea?

[Talesin]

I'm about to get a cat-back fabricated, and I figured that since my 1G GSX is already dropped by an inch and a half, it might be a good idea to split a 3" into a pair of 2.5" or 2.25" pipes, either on the same side, or on either side of the gas tank (assuming clearance will work on the left side).

I understand that it will flow less effectively than a single 3", but how much worse would it flow? Would that ground clearance override the possible damage a single pipe would suffer? (Stock exhaust will occasionally scrape on some rain guttered intersections in the area)

Or would it be a better idea to simply get the rear axle traversing section crushed into an oval, maintaining as much of the same volume capacity as possible while allowing it to be tucked up further under the vehicle?

 

[95blackGsTurbo]

I would stick with a single catback. Maybe just go with 2.5" all the way back. My car is lowered about 1 inch now and I have a custom made 3" mandrel bent exhaust and it doesn't rub. Just make sure that you tuck it as close the the rearend and lower control arm as possible. Mine sometime's bumps the rearend but I have a strip of dynomat on it to kill the sound. Make sure when putting the exhaust on to have the back tire's off of the ground so you con compensate for full suspension travel as well. That way your lower control arm doesn't come down and pinch the piping. You SHOULD be fine with 3" if you want it depending on the roads where you live but if you are worried just stick with a 2.5" catback then maybe a 3" downpipe or something.

 

[Talesin]

Yeah, I'm eventually going to be headed for 400+, so I might as well buy it once, and do it right. Why I'm trying to get input on the different options.

 

[DSMJim]

To figure out the rough flow of a pipe compare them with their sized squared. The number represented doesn't mean anything but as a comparison number to calculate differences between sizes.

A 3" pipe would have a relative flow of 9 (3x3 or 3 squared)
2.5" pipe would flow 6.25 or 20% less then 3"roughly.
2.25" pipe would flow 5.0625 35% less than 3"

With a number of 5.0625 it's almost less than half of the 9 for a 3" pipe meaning you would basically need two 2.25" pipes to flow the same amount as a 3" pipe.

Just as an interesting tid bit of info it would take four 1" pipes to flow as much as one 2" pipe.

 

[Talesin]

Wouldn't you need to figure out the inner diameter cross section? Those calcs would be for a square pipe, rather than pi(r)^2... so a 3" would have a 'surface' of 7.065. A 2.25" pipe would be 3.974.

According to that, even if I did branch a 3" into a pair of 2.25" pipes, I'd still have more overall volume. I'm just wondering if the Y will cause too much turbulence to make it worth my while.

 

[xveganxcowboyx]

I think you're going to run into issues fitting it under there without lots of bends, which of course are restrictions. I also think that trying to run smaller pipe in non-conventional locations is likely going to cause you more clearance problems than a single 3 inch with stock routing. Of course I've never tried though, so i could be wrong, but I looked into it for another car of mine and after 1/2 hour on a lift it began to look worthless.

Remember, you have limited space, because you want to stay pretty clear of gas tank, brake lines, etc....