Jretal

I was browsing the Magnaflow page trying to figure out what I'm gonna be doing with these headers of mine.

During my search, I noticed magnaflow sells prefab Y pipes that go from 2.5" dual to a 2.5" or 3" single outlet (p/n 10768 and 10778 respectively). I was considering using this instead of having a custom Y pipe fabbed up since it would be a ton easier, and probably much less expensive. That way I will just put the balance pipe between the flex joints and the prefabbed Y pipe. Now, since the headers have 2.5" outlets, I am going to run a complete 2.5" system to stay consistent. With this in mind, would it make sense for me to be putting in a 3" cat with the dual 2.5" Y to single 3" then a reducer at the end of the cat to 2.5", or use the 2.5" dual to 2.5" single with a 2.5" cat, keeping all the tubing consistent through the entire system? Would the 3" flow better? Just trying to figure out where your thinking was on this matter.

My take on it is that the combining air would flow better into a 3" area and THEN getting reduced to 2.5" after the cat, but you tell me what you think. You got more experience than I do messing with this stuff I've already made my mistakes with my exhaust, so I want to over-research the hell out of this one.

I would then reuse the remaining items in my exhaust system but have them piped with 2.5" mandrel bent business. This would also involve having the exhaust shop open up my resonator to accept the 2.5" tubing, as well as my existing muffler. Having them open that up is a feasible thing right? I watched the guy who put my old resonator back in open it up to fit the 2.25" in it, so I figured another ¼" isn't difficult.

Thanks for the help!

odelay12v

i am going to adjust mine soon before the track date and right now they are as they came out of the box i was thinking 3/4 turn but i really have no clue how that would affect it wether it would be enough or not.. they are alittle bouncy at this time.. i also have the H&R sport springs Thanks

Jretal

that's what the guys at RPI told me they set them to when I got them. I didn't even think about adjusting them before I put them on the car because that was before I started really getting into all this stuff. I'd say make the backs a lil bit stiffer than the fronts... say 75%/50% or 100%/75% (Rear/Front). That will help ya control some of the understeer the car is subject to.

VAP

a shop can do a far better job. My system had 3"-to-2.5" reducing cone welded to the 3" cat oulet.

Jretal

for me to get a 2.5" in/out cat or a 3" in/out cat? There a reason you picked one over the other?

Since I'll be running 2.5" down pipes instead of 2" downpipes that the OEM car runs, what would be more ideal?

Thanks,

odelay12v

tonight since i found out my wife will be gone at a meeting until 845.. gets me time to take it apart then when she gets home i cant stop at that point..

VAP

at high RPMs without dramatic back-pressure unless you went with a Random Technologies or similar truely hi-flow cat and that will likely give you fits with yout post cat sensors/OBDII system.

And you have to remember that the Magnaflow line (AutoSound) cats are not truly "high-flow" to begin with. They have approx the same cell-count and flow-thru opening cross section as a gazillion other (including stock) cats that are out there. There "may" be some additional benefit to their case designs tapered inlets/outlets but other than that there's nothing really "free-flow" about them. Maybe a teensie bit better than stock but I'm talking single digit flow percentage increases over stock at best. And maybe not even that.

Jretal

would help reduce the negative effects that a larger piped system would have on low end torque? In essense restore some of the backpressure lost due to the big piping.

I want the car to function with all its proper safety features, emmission sensors, etc without having to 'fool' it into thinking everything is ok. So I guess what I'm saying is that I want something to perform and function as well as or better than stock... and if the race cats won't allow me to do that, I'll pass. The performance difference between the 2 is probably so little on our cars I won't notice it.

But that was your reasoning behind the 3"... not enough flow through the 2.5". Thanks for the info. I'll stick with the 3" and just get the reducers from burnsstainless.com.

That's funny though cuz I always thought magnaflow cats were 'high flow cats' I guess maybe compared to other OEM cats they are?

Thanks for the info Mance!

Jretal

I am not motivated enough to dismantle my suspension to adjust those things. I was under the impression that I'd be able to adjust them on the car... I was not very happy when I found out I had to pull them off to adjust them... but whatcha gonna do right?

Have fun

VAP

its all a compromise. You cant have back-pressure down low that disappears without a trace up high. Build it to breathe and let the low-end chips fall where they may.

My reasoning behind a single 3" over dual 2.5" cats was closely related to the above thinking, ie; dual 2.5" is too much coupled with twice as many parts to fail. My new motor will also be a single 3" Random Technology cat in conjunction with the equal length headers and stock resonator with Remus muffler. But all my exhaust tubing will stay at 2.5" because any larger would cost me precious "velocity" which would hurt be all the way upstream to the compression chambers. I think for a closer-to-stock system that 2.25" is totally adequate but a little more expensive (comparatively) and harder to find than half-inch increments due to customer demand.

Nothing at all wrong with 2.25" at anything less than 250HP long as everything down-stream is free-breathing.