New Guy Here...both to the forum and Japanese cars, but I've been involved with cars for about 55 years now.

My question is about a 2012 G37 Coupe I bought yesterday which is my first Ricer. The previous owner is a mechanic at the dealership I bought the car from, and he had installed a full I.S.R. Performance Exhaust system all the way from engine to exhaust tips (and of course no cats) I'm just curious about how much H.P. gain that would give, the engine is originally a 320 H.P. V6 and has only 35,000 miles on it. I doubt anyone will call the car fast, but it did impress both in handling and acceleration.

I tried to include a short video of as much of the system as I could film but I guess I can't upload MP4...

 

Welcome to the club! Yes these cars make +/- 320HP at the crank. The cats being the most restrictive part of the exhaust, it should feel more responsive with them out of the way. From what I've read, there are claims you could get 15-20hp gains from cat back exhaust, however, I don't think those numbers are very realistic unless you're tuned after the exhaust is installed.

 

For what it's worth, most reports of wheel horsepower on a stock car are ~287 (about a 13% friction/drivetrain loss). That was for a new engine though.

With a long-tube CAI and high flow cats, and FI catback exhaust, I made 299 untuned. 315 tuned. Using that 13%, that's 340ish to the flywheel. YMMV based on auto v manual, etc.

Edited to add: I've got a G35 intake mani which lowers peak HP (but gives more torque around 5K) so that will throw those numbers off. Sorry, apples to oranges.