Exhaust update. I now have dual 2.5" exhaust, very similar to my custom dual 2.25" exhaust. I have logged a handful of pulls and went through my logs to find a few with comparable ambient conditions. I don't have good data below 4000rpm to see what the low end loss might be, but it looks like some decent gains are had up top. From 4000-6000rpm the average gain is ~2%, followed by 3%+. If we assume I make ~300hp, this is a potential gain of 10ish HP, not too bad. I have even included an average out of a couple dozen logs and the gain is clear. I have logged higher values but I am sure under ideal conditions I will see better numbers with the 2.5" exhaust.


I am not sure how much longer I have to wait until my headers arrive, or how much longer after that I will wait until installing them. Until then I will add a handful of logs with the new exhaust to build a better "before" comparison for the headers. I really hope that I see a gain through the mid range without a loss up top. I have spent a fair bit of money on a gamble that some oversimplified math suggests my car can make just slightly more power.

 

Well I received my second set of headers a couple days ago. The overall quality and finish of these is much better than my first but I am still going to clean them up before installing due to what looks like inadequate back purging. It isn't a big deal and isn't likely detrimental to performance, I just have the free time and ability to clean it up. These headers are not coated, I am not sure if I feel like spending the money this time. I have had time to get a small handful of logs and so far the results all fit very closely to the above graph from 4000 - 7500rpm. I am going to have to go out in snow mode to get some low RPM logs to get a good feel for what the low end gains might be with the smaller headers. Due to a recent shoulder surgery, I am not sure how soon ill be able to get the new headers in.

 

Whoever owns PPE Headers for IS350, can you PM me. I want to confirm something before purchasing

 

You can post your question here and we'll try to answer it for you. We have quite a few folks with PPE headers on their IS350.

 

Let me know what you need to know!

 

Not too bad? I'd say this is excellent, thanks! A 10hp gain when the optimal pipe size was only a bit bigger is pretty significant. Btw, I haven't seen torque numbers published for the headers. From the graphs they appear to be around the 12-15ft-lb range?

I had a V6 3.8l Mustang on which I changed its exhaust from a 2.0/2.25 Y-pipe to OEM GT 2.25/2.5 duals and it had noticeable gains. Considering our 3.5 is close in displacement I was pondering whether to go 2.25 or 2.5 piping, but the decision was made for me when the Tsudo system became available for cheap, and it has 2.5 throughout. The stock system appears to be 2" at best, so 2.25 piping alone should give a decent increase, not to mention the removal of the secondary cats. 2.5" sounds optimal now, and pretty much necessary for anyone thinking about supercharging.

Sound is more important than performance for me, so I'm considering keeping the stock resonators but may go with the Vibrant Ultra Quiets if they'll fit. Heard one guy's with that setup and it was far FAR quieter and more refined than any full cat-back I've heard; it was basically like an axle-back. The UQs not only reduce sound level but basically eliminate rasp completely. In fact, it's almost getting me to consider headers(!). I see that RR recommends putting on high-flow cats after the collectors to reduce volume, and Vibrant recommends resonators to be as near to under the passenger seats as possible so will have to see if bottle ones will fit there. Would be nice to hear if anyone has done something similar, as any IS I've heard with headers is far louder than I'd want to go.