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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”

 

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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.

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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Lucky enough for me there is no need for cats where I am @Thatblueis !!!!

I am looking to go to stainless headers, Cat delete, Resonator delete, with a Borla or similar muffler.

The only sticking point is the exhaust diameter - I don't care about the noise (and I'm in my 40's for those that feel age and sound crossover at somepoint)

Yeah my original plan was turbo which is why I got the 3". One thing to note is because the 3" supports so much flow there is no backpressure which puts all the strain on your cat. This has caused my cat to start to blow to bits slowly so be aware this will destroy your cats.

 

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I typically will fab my own exhaust from scratch - its really easy to add a resonator if the sound/drone is terrible.

I have lots of 3" mandrel and 4" mandrel bends and a bunch of straight too - nothing I have worked on for many years had so little power that it needed 2.5" - Unfortunately I don't have any 2.5" lying around.

🤣😂🤣😂

So having essentially a straight pipe in terms of flow definitely hurt my low end torque a bit. Don't get rid of your resonator, your resonator is what helps reduce rasp and drone. Your muffler will dictate how your exhaust sounds and how loud it is. Keeping your cat will help keep some back pressure the engine likes.

 

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Damn @01 SolarYellow I missed the IS300 heyday apparently, should have bought one of these when they were newer 🤣

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.