Some news! Husam and I have been chatting, and I moved to an ebay exhaust, and turned up the boost. I turned it up the boost from 8 to 10 psi, and lets just say I did a pull, and was pleasantly surprised! I was soooo surprised that that I had thought it was a lot more than two. Then I saw my datalog and it showed another 2 psi!

Currently I'm at 12 psi, on my street tune, and it feels way stronger, but my injector duty cycle is at 80%, so I'm scrambling to finalize my water meth system.

Anybody able to chime in on this ^^^^^

Only 2psi, but opening the exhaust allowed for a lot more overall flow. You would have made a good bit more power without even turning up the boost! :)

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The difference is the 2.5" will spool faster and torque will come on strong due to the back pressure in the 2.5" exhaust. With the 3" you will see higher whp numbers and your tuner should be able to control the torque better which allows for the increase in top end. Spool not really a factor unless you are using a large turbo from what I can tell on our cars.

I disagree with this. There is no benefit to downsizing the exhaust for scavenging on a turbo motor, and it will only harm spool speed. You want to maximize the pressure differential across the the turbo to speed up the spool.. back pressure behind the turbo will only reduce spool, flow, and require more boost to make power.

For the 2.5" vs 3" exhaust question, its really just a question of diminishing returns.... It doesn't appear to me (and I haven't really looked at it with much scrutiny so I could certainly be wrong) that the 3" is going to do a whole lot more for you in a modest 500whp type of build.

I don't disagree with what you are saying as I think we are saying the same thing but you provided the long and more detailed answer. Ultimately the 3" will out perform the 2.5" due to scavenging and the reasons you pointed out, however I have not seen much difference on power, spool or any other performance points on a non-built motor. The 2.5" will produce good numbers up until about 635whp or so before the tq becomes to much and as you stated it will be a question of diminishing returns on a 2.5" if you want big power. As far my comment on spooling faster or more aggressively, that is due to back pressure if I am mistaken then I have been edumicated!:p

6 psi isn't enough to gain much power even after a tune. I had the same thing with my first Z until i put it at 9 psi but even then, 9 psi isn't THAT much in the grand scheme of things. As for the exhaust, your stock nismo exhaust is sitting relatively close to 2.5" so if you dont use the catalytic converters and put on some muffler deletes, i feel you would render about the same results as getting a full exhaust.

With that being said, i would recommend 3" of pleasure to your fun meter. You can hear the turbo's (especially on cold start), sounds better and you will gain a little more from it v.s. a 2.5" exhaust but do not expect anything drastic.

The Nismo dual exhaust (piping) will outflow a single 3" exhaust by about 15% due to greater cross sectional area. A single 3" exhaust will support 600whp.

However, it is not just the pipe diameter that you guys need to consider. The rear muffle is more than likely baffled a fair bit in order to generate a certain tone, and flow which was purposefully engineered by Nissan

There is mention of scavenging, which does not apply in this case at all. Scavenging refers to one exhaust pulse pulling exhaust gasses from the next combustions cycle chamber. This can only be achieved with the use of OEM, or aftermarket equal length headers.

With turbo applications it is a very simple rule, as the principal of operations is primarily based on pressure differentials. The turbo will spool faster if the pressure differential between inlet and outlet is greater, hence the benefit of a larger downpipe. The pre turbine pressure (depending on the kit/turbo/turbine) can easily be double the boost pressure. So if you are at 12 psi boost, your pre turbine exhaust pressure could be 25psi. If you exhaust (post turbine) builds up a resistance of 5 psi, that is a loss of efficiency.

So with a turbo system, bigger exhaust is ALWAYS better.

Hey all, just wanted to update,I haven't been ablet o make it to the dyno yet. I've gotten most of my parts for the methanol, just need time to get it all together. My plans are to get it on the dyno and to leave it as is, and see what my pre-meth numbers are. Hoping to get on the rollers this weekend.