Exhaust Modifications and Horsepower
Although a new GT owner, my associates and I have done a substantial amount of Corvette work in the exhaust area. Rule #1 for the serious upgrader is to always do a dyno baseline on your stock car when contemplating exhaust changes. Rule #2 is an experienced tuner who has installed a number of a given exhaust modification, and experimented with tuning parameter changes across the realistic operational ranges for the octane rating of the gasoline being used, can generally provide a pretty good idea of the realistic %-tage HP increase from a given baseline. Rule #3 is you generally always need a re-tune when replacing cats (or adding headers with cat replacement) ,or you can face the prospect of horsepower reductions typically due to possible lean operating conditions from more efficient flow/scavenging (the factory generally calibrates for a richer A/F mixture to avoid any chance of preignition/detonation with the stock operating calibrations). Rule #5 is there is generally some HP left to gain from the factory tune to custom tune regardless of any cat-back replacement (ie., a retune with no changes to the stock exhaust). In general, we have seen minimal HP increases with the replacement of the stock system back of the converters, except mufflers/resonators which choke down the diameter of the exhaust pipe when entering the muffler or resonator. Loudness and resonation are typical results of unresearched cat-back replacements. Some catback pipes, like Accufab for the GT, use a shared cross chamber to reduce resonance. I'm not familiar with the IDA, yet. I'm guessing the stock GT, with a good tune from an experienced dyno-based tuner, could deliver additional horsepower in the 20-35 HP range. Cat backs would yield a deeper louder sound,and maybe some resonance (which has been reported elsewhere on this forum), but minimal horsepower increase (although some would be present). Given the cost of catback systems, an $2000 investment in a vented variable pulley supercharger kit is a far more effective HP producer...albeit a little more complicated install. A full header system without cats and using either the stock or performance FRP muffler would also yield a HP increase in the 10%+ range (with a big increase in loudness, perhaps objectionable to most). Certainly, with the GT, the stock system weighs more than catback replacement systems (muffler about 43-45 lbs. versus, for example, an Accufab at around 15 lbs with hardware; stock exhaust manifolds and cats weigh considerably more than FRP long tube headers). In this game, its always been an issue of substance versus image with a lot of products. It ends up being a personal choice, but the choice should be made based upon fact not hype. Forum members starting with baselines and using subsequent dyno measurements of product add-ons and retunes should post results to the forum, so we all can get a more consistent range of outcome estimates for any exhaust changes.
