exhaust theory queestion

[darklynoon]

I'm about to swap my exhaust, today actually, on a 93 S13, DOHC. Got a high flow cat on it, but after that the stock pipes run to a stock muffler. I don't have emissions requirements though, so I was planning on just running straight pipes to a monza glasspack tip(for looks mainly). I was worried about back pressure though. Its a n/a engine, and even thought its running msd and all that garbage, the exhaust parts involved are all still stock, except for the new cat. Do anyone have any thoughts on the back pressure issue? I have been told that too little back pressure is really bad for low end torque, but i was also thinkin that the stock pipes and the tip would create enough back pressure on their own, even without a muffler, to compensate for the difference. Even though I know it's gonna be LOUD, the goal is not the sound, but performance.

[navysnail]

the backpressure idea works in theory i guess, but look at some dyno charts, many times larger piping will net more gain. as far as an exhaust system, get somthing that is mandrel bent all the way back. brm makes nice things, spl has a nice one too, although i cant remember the name of it.

[namwons_240]

Yes youll lose some low end torque but gain more high hp the lost torque and thats what u wanted right, performance.

Less back pressure will net u a tad less gas milelage too i think.

[le_ryan]

i got a 91 240sx dohc, no cat, just a single glass pack in the middle, and straight pipe the rest of the way... i did notice a drop in low end rpm, but, it revs faster, and 2nd and 3rd pull is so much stronger now. you can hear my glass pack in my signature.. oh yeah, and i upgraded to 2 1/2 inch pipeing and 3inch tip, you can see the set up in my sig. too

[InsanityInc]

rglarhagahragahgrhaghgrahgarhaghraghrgahgbr/1`qi48y1`~~!!!!!!!!!!!!!!!!!!!1

Backpressure is never good. Ever ever ever ever EVER.

Backpressure is pressure that resists exhaust scavenging, and scavenging is ALWAYS wanted, because otherwise you get spent exhaust in your cylinder, which is crap for power. The reason why smaller diameter pipes tend to improve low end power somewhat more is due to the bernoulli effect which states that as the VELOCITY of the gas increases, the PRESSURE across it decreases. Meaning that if you can get your exhaust gases moving quickly (and keep them moving quickly) you can actually form a vacuum (read: NEGATIVE pressure. Less than 0 backpressure) in the exhaust pipe which aids in scavenging even more.

However, the backpressure of a stock system is very high compared to a non restricted aftermarket one, meaning regardless of the piping size you will see gains EVERYWHERE over stock because even with improved velocity of the smaller pipe the restrictions in the stock system will stop the pressure from ever dipping below even the non-velocity-modified backpressure of the exhaust in the larger pipe.

STOPBUYINGTHISMYTH.

[s13sr20chris]

insanity is correct(mostly )

[darklynoon]

well, i went ahead and did it. low end power actually increased tremendously, removing the clutch fan helped too, im sure ( it still has the second electric fan which cools it just fine) heres the problem though, around 4,000 rpm....nothing. the engine sounds like its trying, but even on a slight uphill, the car wont rev higher then 4 grand. standing still i can red line it though. now im running stock piping still, except for the high flow cat and the monza glass pack resonator tip. I do have an extra one of those 3A adjustable free flow mufflers (pep boys garbage), but I got it since it actually had the adjustable backpressure/silencer disc on it. is it possible that the resonator tip is screwing things up and i should really be using a free flow muffler? I was under the impression that free flow muffler and no muffler were pretty much equivalent. could this be my problem? also, should i leave my AIV valve hooked up, since its main use is to inject air into the exhaust to even out positive and negative pressure, or should i plug that sucker?

[s13sr20chris]

i hate aiv. i have seen perfectly good maf meters busted by bad aiv. is it a soft 4000rpm or is it sudden? does it repeatedly whack the 4k limit or just kind of hang there?

[darklynoon]

it just hung there. I actually just removed the whole unit. I went searching for another thread and found out how. i need to check my throttle valve stopper screm and TPS sensor i think as well, cuz it seems to surge when i lift my foot just slightly off the pedal. Takin the AIV valve out did wonders. Its still not right, based on my 89 sohc that i used to have. It was an automatic and it would've ran circles around my newer one, it seems like it has a LOT more low end torque, but the top end is gone on it. The monza tip is the small single tip, probly a 2 in diameter opening, a foot or so before it, there is a "bubble" in the pipe where i had welded on a 2 1/2 '' high flow muffler, cut it off, then put the other pipe on. so its going from 1 7/8 to a 2.5 section about 6 inches long, then back down to 2 in for the rest of the way. i didnt think that that would effect it tho, maybe i was wrong.....

[darklynoon]

heres a pic of my ghetto exhaust

[s13sr20chris]

that exhaust setup is not quite optimal but its not the end of the world either.

i would start checking ign timing and valve timing. i am leaning towards the latter.

[darklynoon]

Just thought I'd clarify...It will go higher then 4K, i just have to be going downhill, or on a flat, and its hurtin by that point. Streets i could hit 90 on, i can only hit 60 mph now. you can feel it want to go, but it feels like its towin a boat behind it. Does anyone know what the proper setting is for the main butterfly valve? i heard it should close so that the pin hole is just barely exposed, but no further. This, of course, was my starting point for the IAC and the idle speed, the TPS, the timing, and so on and so forth. everyone i talked to says it feels like a bad air/fuel, but im just about outa ideas on what could be causing it.....and yes i have a new O2 sensor, yes i checked for intake/exhaust leaks. im about to go ohm-meter all my pieces, but wouldn't a faulty MAF show a check engine light?

[darklynoon]

valve timing or valve clearance?

[s13sr20chris]

a jumped t-chain will cause lack of power at certain sections of hte powerband. it depends on how it jumps.

try monitoring maf voltage when she runs out of steam.

[darklynoon]

a jumped timing chain would seem kinda unlikely since i had the tensioners replaced at a nissan dealership in 2002, and the car has sat mostly since then. I should say tried to have it replaced, since it was still under some no name used car place warranty that nissan was forced to honor. Anyway, they refused to replace the chain, or even the tensioners since they said it was a "wear-outable" item. They claimed that they took off the timing chain cover and one of my tensioners was all beat, so they took it off, claiming it was...get this...EXTRA!!. Anyway, they charged me 3-400 dollars to do it, or pretend to do it anyway, and it runs the exact same with the exact same sounds as before. Some background on this car might help too...I bought it in Miami from a not so reputable dealer, not realizing that the red marker scribble on the intake manifold, was actually junkyard inventory scribbling. so I have no idea what is in this engine, but it has run wrong since the day i bought it, but not wrong enough to make me wanna sell it, just wrong enuff to make me pull my hair out in frustration. Its the second 240 I have owned, so I know what should and shouldnt be, something in that damn thing just shouldnt be......