I know that there has been a lot of discussion regarding the merits of a 2.5 vs 3 inch downpipe.
I am, however not planning to mod my car much and would prefer not to have to deal with fitment problems, rattles, hot oil pans, tiebars etc.
My question is, would there be any concerns linking a 2.5 inch downpipe to a 3 inch hi-flo cat and 3 inch catback?
If there is a little loss in high end power, I'm prepared to accept that, but are there other issues that might crop up?

 

Just get the 3" DP.

 

the 2.75 is the best you can get for your car if you have stock turbo

 

2.75 is good if you want to make less power and less torque.

 

Find the Buschur thread on this topic. He's done dyno comparisons of both...
The big restrictions are the turbo housing and catalytic,

 

I am not sure what you mean by that, I have the 3" from BR on a stock turbo?

 

Why would you want to keep the restriction on your motor? the 3" will allow it to spool faster, maybe not a whole lot more power (~5hp) but there will be a big difference in how quickly the thing spools. Second, I had NO fitment issues with the DP from BR (infact that goes for the whole TB), fit great, the only thing is to use the spacers that BR gives you with it. Other than that, I havent had one rattle.

BTW - hot oil pans wouldn't be an issue for you if you aren't going to really mod your evo or do some serious racing.

 

I thought the turbo housing only frees up about 6-7HP.

 

How is 3" exhaust alone gonna make more power and more torque than 2.75" exhaust?

you need 3" intake in order for you to take full advantage of 3" exhaust. And you need an a/f tunning in order for you to take full advantage of the intake.

I'd say just get 3" just in case you're going to do intake mod, then you don't need to mod later on.

 

Less restriction will ALWAYS make more power on a turbo car. You may not FULLY utilize it but it will make more power. There is NO reason power wise to get anything smaller than 3" Most 3" DO have SLIGHT fitment problems (DAMN near rubbing on the oilpan or the tie-bars but its not that bad). If you are honestly that concerned about a tiny rub when going over speedbumps then get a 2.75". But 3" really doesnt have any fitment problems on all the DP's Ive seen or used. Get 3" so you dont sell the 2.75 later Trust me on that one.

 

With the size of the oil cooler the EVO runs, I doubt overly hot oil will be a problem. Not to mention that unless your sitting still, there is air flowing through the engine compartment, so its not like the heat from the DP is sitting there boiling the oil.

 

If that's the case, where can we buy the adhesive heat shield that goes on the oil pan? Just to be safe...

 

even sitting still the air flows through the engine compartment... heat flows outta that big hole in the top... the convection just sucks cooler air in. That being said it's not gonna prevent heat soak in some areas.

Back on-topic I keep hearing that 2.75" will optimize the torque down low and still give a good flow increase. Despite this thought I still bought 3" turbo back from BR.

 

I don't think you guy's have searched....

To keep the exhaust velocity up you need to go from smaller diameter to larger diameter as the exhaust gas cools and expands.

Asuming a 3" cat back, the folowing is true:

The exit hole of the stock o2 housing is 2.5" and the stock cat is 2.5".

If you keep these two items (or add 2.5" hi-flow cat or 2.5"delete), a 2.5" dp. will flow better than a 3" dp.

Keeping the stock o2 and jumping to 3"dp. with a 3" hi-flow or delete will out flow the above set up.

The most power will come from a 3" o2> 3" dp>3" hi-flo/delete.

If this is incorrect please speak up, as this is what I have read from the discussions on this board.

 

Speaking of the tie bars underneath your car (I am assuming those are the 2 foot long or so black bars)...is it necessary to keep them on if your exhaust is too bog and you dont want to put spacers?