Harey

With the low compression of 8.3 I reckon stay with the 0.82 housing.

Doesnt mean you wouldnt have had good gains with a 3" exhaust instead

250hp at the engine is only roughly 150rwkw. That is what a standard RB25DET makes. A normal setup with a few mods is around 190rwkw so as said by Arthur thats around 300hp at the engine. So going by the quote of 250-300hp you will be right at the end of the range. The article goes into great detail about why having a larger turbo back exhaust can only improve things. So if you are getting a new exhaust made then by the logic of the article you would get a 3" exhaust. You can say you dont care about the article, but then why are you posting in this thread?

Geeze I am glad this article was posted up, seems as though the outcome most people have come to is to buy 2.5" exhausts?????????? You realise it has 3 paragraphs explaining why bigger is better. Only negatives can be sound, so get something thats not loud but still flows well.

So all the effort of posting this article up and not reading it properly yourself?? "At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note." Please show me where it states that having a smaller exhaust after the turbo will help your response and be better at low rpms. The quote above I think is what you are referring to and it simply states that you wont gain much, it does not say you will loose anything.

Yeah I couldnt believe it, how low is that. Prob thought I wouldnt know the difference, but instead I fitted this myself. I am sure it was there beforehand and I am going back to see how I go. Thanks for the info regarding the 5" body metal cat, I just ordered one to replace it. Hopefully, this workshop will pay for it.

Problem is I dont really have proof it was on there just before I took it in. The only other 2 shops I have taken the car to since I fitted the hiflow cat myself are very reputable and have proven themselves reliable to me. So I am pretty certain it was this mechanic but I dont have proof.

Sweet that sounds awesome, just sent him an email!

I have recently been ripped off by a mechanic who switch out my metal hiflow cat for a standard one and then has since left the company GRRRR!!!#$%%$^%$ So I am in the market for a new 100 cell metal cat for my 3" turbo back exhaust. I am looking at the venom range and have been recommended them from some experienced members on this forum. Feel free to recommend another brand. I am looking at either: 3" inlet/outler with 4" body, $179 (no flanges) 3" inlet/outler with 4.5" body, says suitable for 300rwkw+, $210 (no flanges) 3" inlet/outler with 5" body, says suitable for 350rwkw+, $229 (no flanges) I am happy to pay the $ for a 5" body just to ensure minimum restriction but I am worried about space so that I dont hit the floor or create too little ground clearance. They are a round shape. Has anyone had any experience with this? http://www.venomexhaustworks.com.au/products.html

Yes exactly, going bigger then required wont net you any gains but it wont loose anything. These are the 2 paragraphs before your sentence: "Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure. Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range." Not sure how it could be stated any clearer than that?? If you want to discuss this further please create your own thread.

Agree! I dont see the harm in grinding first, seeing if its acceptable noise wise etc. Then if not I purchase another muffler.

Here it is: http://www.teamswift.net/viewtopic.php?f=2&t=7628&view=next

Yeah I realise I wont defy physics and get the full 5psi pressure drop back. I am hoping with a better intercooler the after intercooler shape will be the same and at most be 2.5psi pressure drop in the top end. The history with this intercooler is that I have never been able to run more than 15.5psi above 6,500rpm no matter what I do. That is with 2 different turbos, the other turbo was much larger and was proven to run 20+psi above 6,500rpm on other rb25dets. So I believe this test proves that at the very least that the intercooler is at least quite restrictive at higher airflow levels.

Yes a 5" exhaust would be better. Sure you would not see much of a gain at the power level we are talking about, but you would not see any negatives apart from trying to fit the bastard somewhere! You are getting mixed up with N/A cars like the integras in the link you posted. There is an article on the garrett site confirming this that bigger is better after the turbo.

The factory wastegate runs off a nipple on the hot pipe, well it does on my R34 anyway. The intake pipe has 2 connections, a bov return and a breather hose from the pcv. Not to mention there isnt pressure in the intake pipe, only a possibility of vacuum. If the exhaust is too loud or it is too hard then I will look at a new muffler but I cant see why it cant be done nor am I satisfied with how it is currently.

"if it was a restriction the pressure before the turbo should increase while the pressure after it should decrease reletive to each other" That doesnt make any sense. If what you mean is that you expect the pressure after the intercooler to drop as the airflow increases then yes marginally by not this much. You mention numbers like 7% and 10%. The pressure drop up to 4,000rpm is only 2psi but increases to almost 5psi in the top end. That is a lot more than 7 or 10%. Intercoolers normally have a rating of less than 1psi drop at 15psi etc. Not to mention Trent was the one who was telling me that the shapes should be similar if the intercooler is flowing well. The stock configuration on my R34 GTT definitely gets the pressure signal from the hot pipe (turbo to the intercooler).

The wastegate runs off the hot pipe just like the factory setup. Its a hypergear actuator, not sure what its official rating is.

Nope exhausts after turbos are different. Bigger is better everywhere in the rev range. Better flowing exhaust only has the potential to reduce lag. Whats the point in a 3" exhaust when it reduces to 2.5" in one location, might as well have a 2.5" exhaust.

Exactly why not just put bigger/better mufflers that still flow well. Its not like their cheap!

Just running off the wastegate pressue. The pressure drop includes all intercooler piping. So from a nipple that I installed on the hot pipe from the turbo to the intercooler then the other side is measured by the boost pressure in the manifold. Trent said he wasnt necessarily looking for the pressure drop but more the shape. If both shapes are the same then the cooler is most likely working ok where as if the pressure drops off after the cooler then there is a restriction. And yeah from the dyno sheet above there is a serious restriction!!

Just stuck my hand in there then and its no bigger than 2.5". Measured to ~60mm. My exhaust is based off a 3" exhaust not 3.5". I definitely think its worth it.

Trent at Status did a test for me without the catback and then without the cat. Just without the catback lost between 5-8 rwkw although my cat looks to be restrictive as well. So the cat might be reducing the amount the restrictor is holding power back. We also confirmed that the intercooler is the worst restriction. See the way the boost before the intercooler holds steady but the pressure after the intercooler drops away considerably. I have just purchased a better turnflow intercooler so will fit that very shortly. I would like to sort out both this restrictor and the cat and then go in for a retune and then E85

Yeah you can get your hand in there. Its 5" until the first restriction to 3" and then another restriction to 2.5". Yeah I am happy for it to be louder. I really want the flow of a full 3" exhaust, if its too loud then I will have to get another exhaust or muffler.