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N/A 2.5 is ideal. If you are going turbo then 3.. If you have cats. It will be fairly quiet. No cats gets noisy. Specially on 3 inch. No mater the muffler.I have a couple of cars with Greddy SP systems in non turbo for now and no cats. They are fairly noisy idle and specially at high rpm. Did not see power changes from 2.5 to 3 in N/A.
 

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I got a 2.5 inch exhaust right now Meagan racing catback. you can definitely hearing it coming but once the car warms up its not as loud as I would like it. If I just have the radio on I can't really hear it at all. Would like it to be louder but mostly on the deep side. Would switching out the midpipe make any difference. And also when you do turbo it what should be added for as less rasp and as deep as possible?
If you are still running cats. That is why it is not loud. If you go to a non catted y pipe. Yes it will get louder. The Megan y pipe is only 2.25 diameter. Shade12Fab on Facebook makes a 2.5 that is nice.
 

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So your resonator controls the pitch ad drone and a little bit of noise and your muffler reduces and sets the loudness of your exhaust. The metal you use can also play a factor. Deleting your cat in the mid pipe will add rasp and drone. To really get that loud sound at wot and quiet cruising you'll have to change out your muffler and resonator.
That member is running a Megan Racing cat-back.
 

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Sorry to be bringing this up from the dead…………

2.5” single exhaust supports +/- 230hp, if stock the IS300 2JZ puts out 218hp, then the 3” should be the go to as a base exhaust. No?

A “good” cold air intake, with headers (no cats, even better!) and a 3” exhaust with a high flow straight through muffler (like Borla?) should be able to more than exceed 230 +/- hp. Why would anyone want to bottleneck the output by going 2.5”
NA you lose some bottom end torque going with 3 inch with no cats and header. No significant power gain in hp. Proven already. Other than that louder noise levels. Most go with 3 inch as a temp till they get a turbo. So NA only 2.5 would be the best over all.
 

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Hey @01 SolarYellow - are there any dyno charts on this to show the lack of delta between 2.5" and 3" on a N/A car with supporting mods? I would be really quite curious to see it, as I come from a different world of larger displacement vehicles where they are probably less efficient.
Way back in the days of the forum. I do not know if they are still posted from old members that are not active anymore. A lot of that stuff was not hosted by the site. So if they deleted it. It would not show anymore.
 
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