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I've got the resonated midpipe and muffler delete installed on my Fiat 124, and I am having some buyers remorse. The exhaust looks and sounds great, is very high quality, but I have had a noticeable loss of performance below 3000 rpm. The car has gone from fun to dud in the low end. I do also have the 2.25" crosspipe (I have an auto), but I haven't installed it, yet. I'm hesitant to install the crosspipe now because I'm concerned that I'll lose even more bottom end.
I'm very surprised by this result. People have raved about this exhaust, and I'm not aware of anyone reporting losing performance with it. I'm thinking 2.5" may be too large of an exhaust for this car. I'm looking at other exhausts now. Remus is 60 mm, rather than 2.5".
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Like the website says, the Crosspipe is the KEY to most early turbo spool.
Putting that aside for the moment, if there is some performance loss below 3000 it is not because you have let the turbo breath easier. Think or read about how turbos work, and why reduction of backpressure after the turbo allows the spool to be faster. There is also a ton of data out there for this turbo 1.4 motor and pipe size choices in the prior Fiat 500 application, and based on that data we decided 3 inch was overkill and 2.5 is just right (though as you note Crosspipe just 2.25 for automatic).
If your Fiat is falling on its face below 3000, and it did not before, it is far more likely that it is going rich or lean and you are feeling that flat spot where fueling is either trying to catch up with the flow of the faster spooling turbo....or is overshooting and gone too rich. These cars are run by half a dozen sensors and there is a lot of variation in the sensors sample to sample. Got a piggyback on this car? Have you done anything to check the AFR? Stock ECU should adjust to flow assuming all your sensors are working well, but some cars are adjusting better than others. One customer reported his car didn't tune right until he disconnected the battery for a few minutes and then the car 'learned' the new setup in a few cycles (this should result in the car starting a bit rich and tuning down to the new setup using feedback from the first O2 sensor and fuel trims). We have seen other examples where the customer had to change the first O2 sensor to another of the same factory part number before the car would do what it should do and adjust the fuel trims...but it might also be that they damaged the original O2 during install.
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