2.5" or 3"

[Kreator]

If i go with 2.5" mandrel bent piping, would it create too much backpressure or not? I'm planning to go with 10psi on t3/t4 with wastegate dumped directly out. I know a number of people who run 2.5" on their turbo 240s, but some people say it's gonna be really bad for the turbo.... advice?

[DrDubbleB]

3" is definitely better for turbo applications. It's not so much for the health of the turbo though, as the wastegate should get rid of any excess flow of air. It does however greatly improve throttle response to have a 3" for the application you are looking for.

I don't know if there are any negative effects except for performance loss (on the 2.5"), there are none that I can think of, but I'm no expert on the matter.

[SR20Fastback]

DrDubble got it right, you will be happy with 3 inch. Although on NA applications you will experience lots of low end torque gone, it will go great with a turbo &nbsp;<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>

[Kreator]

There is one thing that really upsets me about the 3" though. even with the turbo, i'd be doing daily driving (e.g. not using turbo at all) 85% of the time. That means that i'd be running an N/A car with 3" exhaust. Now the torque is gone, and i really got annoyed from my friend's 3" dunk (when i test drove his car) after like 5 minutes of driving it.

What i thought though, if the wastegate woud gradually open up as i'm hitting boost, the total area of flow for the exhaust would gradually increase. So at full boost i'll still have something like 3"+ of total area flow... now if someone with a good knowledge of wastegates could tell me if the wastegates are actually pushing enough for me to benefit from the direct dome (flow area wise).

[adey]

Someone correct me if I'm wrong.

Actually Kreator, you shouldn't be losing ANY low end torque on a turbocharged car w/ 3" exhaust. In fact, the exhaust size (after [meaning bigger] maybe 2.5") doesn't even matter for low end torque.
Turbocharged cars can run a 5" exhaust (exaggeration here) and not lose any low end torque because there is still "backpressure."
'How can you have any backpressure on a 5" exhaust!?' you ask me with a bewildered look... it's quite simple actually.
You're forgetting there's a TURBO with TURBINES directly in the path of the exhaust gasses.
What I'm shooting at is that the turbo itself provides all the "backpressure" in the world to retain low end torque; any lack of torque you feel (or perceive to feel) is solely due to the fact that when you're off boost, you're basically running on a 2.0 (or 2.4) litre naturally aspirated engine... one with plenty of backpressure.

<img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/thumbs-up.gif" border="0" valign="absmiddle" alt=':thumbsup:'>

[240S15DETE6K]

Adey is right.

[240 2NR]

Except you're not looking for backpressure. &nbsp;ALL cars do better with less backpressure. &nbsp;What you do want is a good exhaust flow velocity, which actually decreases with a very large exhaust at low engines when NA (think of a pipe, if too big, water will slowly trickle through until enough water is forced through so it's sized properly, but that same pipe at 1/2 the size will be flowing like crazy, go too small and it starts backing up). &nbsp;Without this exhaust velocity, the scavenging benefits at low engine speeds is lost which in turn reduces engine torque for the gains at mid and high rpm. &nbsp;

The reason backpressure is confused is that a system with high backpressure can still provide flow characteristics that promote high exhaust flow velocities at low engine speeds despite choking the engine up high.

Most people I know with large exhausts on turbo cars have few complaints. &nbsp;It's not like a 2.5" straight through won't work (still worlds better than stock), but everyone seems to agree that a 3" will work better (plus there's a lot more options avaialble for the 240 in that size).

[Krunko]

I have 3" on my NA s14....i don't think its too much of a loss...I didn't really notice one...but I drive an auto so that might be why.

[DrDubbleB]

Sometimes your mind tricks itself also and thinks that there is low end loss when there really isn't. &nbsp;I think someone mentioned that, but it's a very valid point. &nbsp;

Either one will work, but under boost the 3" will give you loads better throttle response, and much better flow, and that's the point I made in my first post, and why I recommend the 3".

[boosteds14]

i had a 3" on my ka-t and i hated it so much i had to get rid of it. i bought the 2.5" and noticed that there is no spiking of the boost and it is alot,,alot,,alot quieter. which i like.
i also noticed on the highway that going 70mph coasting and wanting to get to like 80mph, i do not have to step into it as much or actually come close to using boost. yet i did notice a loss in the top end but nothing extreme. like a loss of 1 pound of boost

now with the 3" yes i did notice a difference. it was loud and i had to step into it more(opposite of what i explained above).
i also did notice a gain in power top end like as if i ran another pound of boost.

[DrDubbleB]

Do you have a dyno chart comparisson of the two?

[boosteds14]

</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote </td></tr><tr><td id="QUOTE">Do you have a dyno chart comparisson of the two?</td></tr></table><span id='postcolor'>

No sorry, i only had the 3" for like 2weeks
i have a dyno chart for the 2.5" but i dont have a scanner

i bet i lost a max of 20hp. which is nothing really at high boost levels.

[DrDubbleB]

Yeah it just woulda been pretty cool to see the power curves of the two different sizes on the same exact car. &nbsp;Ahh wells, it's all good, don't feel bad, I don't have a scanner either! <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt=';)'>

[tzonni]

since we are on this topic already, isnt the A'Pexi N1 exhaust for the 240sx a 3inch system?? SCC said they got a 15.9 HP increase back in the day when they had their project 240sx (97 SE) they had it late 98-mid 99 issues, i havent seen it after that again, i have the 3 issues of it upstairs, i just dont feel like getting them. they said when apexi made the exhaust they just took the same design of the silvia so it &nbsp;is overkill for the US application. now, i want to know if the APexi N1 exhaust sold here IS the exhaust for the silvia, AND if it is, is the s13 N1 ALso from the 180sx??