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What changes running with and without Cat??


Patrick Garvan
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I am looking at getting a new exhaust system going up to a larger size. I have been running a full 3" from turbo all the way back without a Cat, but I have decided to get a removable high flow Cat that I can use on the street, but change out for a straight pipe for the track. What I'm wondering is how much difference can I expect in a/f's and Egt's etc? I realize that this is way to open a question but at a guess is this something I would need to dyno test and make a different map for, or could I just play with the a/f's and watch the egt's? To explain a little further. I currently am running a MAF with chips and piggyback. I can also use an SMT6 to tune the a/f's and timing. Once I get my new motor later in the year I will be using the G3+. Any advice is welcome.

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The changes required will depend on how much of a restriction the cat really is.  Maybe you should measure the exhaust back pressure at full power before and after installing it.  On a turbo engine the back pressure seen by the engine is that of the exhaust manifold anyways.  In any case you should probably get a tuner to check your tune after adding the cat anyways.

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  • 3 weeks later...

Ashley, I am now rather hurriedly going to be installing a G3+ w knockblock system. The exhaust may not be ready in time for the first tunes. Also I am waiting on a custom header set AND change of turbo. My current turbo is reasonably large, but I'm switching to a GT35r. Is it pointless to do some tuning with the older exhaust and turbo or will it allow us to set up some good base maps, and then do some tweaking when the other goods are installed? I am working to a deadline so I may have to do some tuning next week or so before the new mods arrive. thx.

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Things like off power tuning, idle control and all that sort of stuff will not change much with your mods.  Full power tune will require a bit of tweaking after your mods.  Depends on if you want to pay to have it tuned twice or just wait until its all good.  At least it would allow you to see what gains your extra mods make...

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