Feels like I have back pressure when I let off throttle - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

s13pignose · 04-01-2008, 09:00 PM

My s14 is NA/KA/Auto. I got an intake and 3" exhaust as my only real power adders right now if you will. For some reason ever since I got the car, say if I'm cruising around 20 through a parking lot and let off or coming to a red light on the street and let off, the rpms will drop but seem kinda slow. Also there's this feeling like my exhaust is not flowing enough since I added the intake or something. Kinda hard to explain. I also get like a slight srt-4 pop'n sound like they make on downshift, but ALOT quieter in my case.

I recently changed plugs, wires are good can't be more than a year and a half old. I've cleaned my maf, even again today since I added the intake like 3weeks ago, and I cleaned the inside of the TB best I could with out taking it off. Also pcv valve can't be more than a year old, and fuel filter is like a couple months old. Last but not least the 02 on the ex manifold is like 4 months old. Also checked the vac lines on the intake to make sure they're still on. NO CEL btw.

Could it be the cat is clogged...if so I guess I could wait and see what happens when I ever get hold of a decent header, since I plain to get a high flow cat as well.

Also on a side note anyone with a aem or injen felt like they lost some bottom end for top end with the short ram design, and no cold air extension...cus I did. I reset the ecu b4 the qst drive with it on, and I felt the gains, but now it seems like something is taken away from them...I don't get it

cronux · 04-01-2008, 09:59 PM

so what exactly are you asking? you lost low end in exchange for high end? thats fairly common...

LongGrain · 04-01-2008, 10:02 PM

i had a 3" N1 with 3" test pipe on my NA KA. it would shoot flames on decel, lots of burbling and popping. not to mention the earth shattering roar at WOT. thats what happens with a huge exhaust on a big NA dinosaur motor.

s13pignose · 04-01-2008, 11:24 PM

I dunno it feels like there is some restriction somewhere, it's kinda hard to explain. It almost feels like I can still hear the intake after I let of the gas or something lol. I wish I had a better way of explaining it.

Alright well maybe you guys can answer this question...and maybe it's just automatics that do this. But how come I can floor the car sometimes, and it responds right away and pulls hard, and then sometimes when I go WOT it just gradually pulls. Would that be tps related?

OfourTHREEfive · 04-01-2008, 11:32 PM

Take that 3" off, you need some back pressure for all motor.

s13pignose · 04-02-2008, 08:33 AM

Huh...they're have been numerous dyno sheets on here showing no loses running 3" on NA. I didn't even feel a lost with mine, even before the intake. The intake is the only thing that I felt changed my low end, but kinda helped top.

So back to the question about tps..thanks for posting though

verge513 · 04-05-2008, 05:53 AM

these import engines pretty much require back pressure...running big open pipes mufflers and headers just make them louder with less hp.. i mean come on i had a 1980 camaro with a 454 big block and it even lost power with a 3 inch.. 2 3/4 was its sweet spot and it was a big block...my 240's factory exshaust is tuned to that engine..only thing i see really making ay diffrence is a 4-2-1 header and maybe a high flow cat i leave the rest alone.. anything else and it just kills mids and lows..unless u like to drive around town at 3500

apex · 04-05-2008, 02:44 PM

Quote:

Originally Posted by verge513

these import engines pretty much require back pressure...running big open pipes mufflers and headers just make them louder with less hp.. i mean come on i had a 1980 camaro with a 454 big block and it even lost power with a 3 inch.. 2 3/4 was its sweet spot and it was a big block...my 240's factory exshaust is tuned to that engine..only thing i see really making ay diffrence is a 4-2-1 header and maybe a high flow cat i leave the rest alone.. anything else and it just kills mids and lows..unless u like to drive around town at 3500

read this
"Cylinder Scavenging, backpressure etc all has to do with manifold/extractor/header design, once your past that point, you want as much flow as possible even in an NA car."
read this
http://www.zilvia.net/f/showthread.p...=back+pressure
and this
"Now to the hard question, why do people believe backpressure is necessary? Because they've read in some magazine that an oversized exhaust can negatively affect performance from a loss of backpressure. Nothing could be more inaccurately stated. As Tchi mentioned, an overly large exhaust affects exhaust velocity. One of the fundamental exhaust concepts is the exhaust is a series of pulses, not a continuous stream. Those pulses can be engineered to control exhaust pressure through the exhaust system, and pressure control for blowdown is critical to performance."
this
http://www.yoursciontc.com/informati...kpressure.html
now this
http://www.mx6.com/forums/general-au...st-theory.html

verge513 · 04-06-2008, 02:08 AM

well with big pipes the air moved though them moves slower..its like blowing thought a big pvc pipe compaired to a straw..when one cylender does its compression stroke and does the exshaust stroke as the exshaust escaped it also pulls more exshaust from the combustion chamber and awell from the other 3 cylenders. keeping everything moving. thus improving lower rpm and also at idle.there is such thing as to small..but the 240sx has the right size .but like many people say its youre car.. talk to a professional engine builder and dynoist.. cant always believe what you read on the internet.. and a scion tc...hardly a car.

verge513 · 04-06-2008, 02:24 AM

The simple truth is that SOME backpressure is essential to proper engine functions. Go stick some stub pipes off your exhaust ports and tell me how well the engine runs. You probably won't even be able to start it.

Each pulse of exhaust helps pull the pulse from the next cylinder along with it. The reason larger exhausts are bad is because they decrease exhaust gas velocity. This means the engine must work harder to expel waste gases which is why you lose low end torque.

On the other hand you gain top end power as at high RPM the exhaust gases are travelling fast enough to make the larger diameter pipe efficient. Essentially changing exhaust diameter shifts the power curve of the engine to different RPM ranges. You open the exhaust, you get more of your power at higher RPM, but you lose some in the low RPM.

The best exhaust is not the shortest or largest diameter, the best exhaust is the exhaust that maximizes exhaust gas velocity at the RPM levels you need. IE: drag racers would want a large exhaust as they're at WOT all the time. Autoxers would want a smaller exhaust as low end power is essential to get out of a tight corner. Road racers need a compromise as they need top end power as well as low end power.

Finally, the blanket statement "backpressure is BAD" is completely wrong. Backpressure is necessary to proper engine operation, removing all backpressure would cause your engine to run like crud, or not at all.
and this came from youre examples of higher backpressure is bad...atleast read youre examples all the way though...

apex · 04-06-2008, 02:35 AM

to: op sorry for the thread jack im out

to tell u the truth i really dont care bout that backpressure just posted that to bust ur balls
u said "running big open pipes mufflers and headers just make them louder with less hp"
this is not true if u look around on here u can find dyno sheets that prove on n/a ka's no loss in
hp just sum in tq. with 3" thats what im talkin bout is wrong

verge513 · 04-06-2008, 11:43 AM

ok sorry if i made you mad. i just know theres diffrent benifits with 3in and factory..i like the way nissan design there cars and i feel everything they put into there cars are there for a reason..

murda-c · 04-06-2008, 11:56 AM

The exhaust sizing of the KA is there for three reasons.

1.emissions compliance-slower exhaust flow means more effective catalytic converters.
2.sound - smaller piping is much quieter
3.price - smaller pipes are cheapers and easier to bend.

Backpressure is bad. Scavenging is good.

murda-c · 04-06-2008, 11:58 AM

to the op:

my car does the same shit, it spits and pops and burbles when i coast in gear, and it seems like it loses engine braking when i get down below about 2k rpm on decel, but i think that's just because of the ecu adding fuel to the mixture again. Because it cuts off the fuel flow on decel until you apply throttle or the rpms drop below a certain point.

***S14DB*** · 04-06-2008, 12:27 PM

KA needs back pressure is bullshit. All cars need flow bigger the pipe, more flow. It's a 2.4L not a 1.5L, needs a bigger pipe then a honda.

Back to the OP:

Stock ECU has a Decel Fuel "map". It dumps some fuel on decelation then cuts. This is why it makes a back fire sound. SRT's have this programed in for sound effects. KA does this not to stall out.

The lag issue could be many things. I would lean more to the Auto more so the TCU not keeping up with the improvements.

mifesto · 04-06-2008, 11:00 PM

wish you could borrow a maf from a buddy, maybe the intake you put in was over-oiled and fouled your maf. happens on evos after people cake the cone filter w/ the k&n oil....

really hard to say since you havent given us much info. did you install the exhaust after you bought the car or it came with it?

04-01-2008, 09:00 PM	#1
s13pignose Zilvia FREAK! Join Date: Dec 2005 Location: ?? Posts: 1,324 Trader Rating: (2) Feedback Score: 2 reviews	Feels like I have back pressure when I let off throttle My s14 is NA/KA/Auto. I got an intake and 3" exhaust as my only real power adders right now if you will. For some reason ever since I got the car, say if I'm cruising around 20 through a parking lot and let off or coming to a red light on the street and let off, the rpms will drop but seem kinda slow. Also there's this feeling like my exhaust is not flowing enough since I added the intake or something. Kinda hard to explain. I also get like a slight srt-4 pop'n sound like they make on downshift, but ALOT quieter in my case. I recently changed plugs, wires are good can't be more than a year and a half old. I've cleaned my maf, even again today since I added the intake like 3weeks ago, and I cleaned the inside of the TB best I could with out taking it off. Also pcv valve can't be more than a year old, and fuel filter is like a couple months old. Last but not least the 02 on the ex manifold is like 4 months old. Also checked the vac lines on the intake to make sure they're still on. NO CEL btw. Could it be the cat is clogged...if so I guess I could wait and see what happens when I ever get hold of a decent header, since I plain to get a high flow cat as well. Also on a side note anyone with a aem or injen felt like they lost some bottom end for top end with the short ram design, and no cold air extension...cus I did. I reset the ecu b4 the qst drive with it on, and I felt the gains, but now it seems like something is taken away from them...I don't get it

04-06-2008, 02:35 AM	#11
apex Zilvia FREAK! Join Date: Aug 2004 Location: palm springs Posts: 1,569 Trader Rating: (44) Feedback Score: 44 reviews	to: op sorry for the thread jack im out to tell u the truth i really dont care bout that backpressure just posted that to bust ur balls u said "running big open pipes mufflers and headers just make them louder with less hp" this is not true if u look around on here u can find dyno sheets that prove on n/a ka's no loss in hp just sum in tq. with 3" thats what im talkin bout is wrong __________________ Red Rocket

04-06-2008, 11:56 AM	#13
murda-c Post Whore! Join Date: Jan 2007 Location: Montgomery County, MD Age: 36 Posts: 3,197 Trader Rating: (1) Feedback Score: 1 reviews	The exhaust sizing of the KA is there for three reasons. 1.emissions compliance-slower exhaust flow means more effective catalytic converters. 2.sound - smaller piping is much quieter 3.price - smaller pipes are cheapers and easier to bend. Backpressure is bad. Scavenging is good. __________________

04-06-2008, 11:58 AM	#14
murda-c Post Whore! Join Date: Jan 2007 Location: Montgomery County, MD Age: 36 Posts: 3,197 Trader Rating: (1) Feedback Score: 1 reviews	to the op: my car does the same shit, it spits and pops and burbles when i coast in gear, and it seems like it loses engine braking when i get down below about 2k rpm on decel, but i think that's just because of the ecu adding fuel to the mixture again. Because it cuts off the fuel flow on decel until you apply throttle or the rpms drop below a certain point. __________________

04-06-2008, 12:27 PM	#15
*S14DB* AFC #1 Join Date: Oct 2003 Location: 昨晩あなたのお母さんの家 Posts: 20,181 Trader Rating: (3) Feedback Score: 3 reviews	KA needs back pressure is bullshit. All cars need flow bigger the pipe, more flow. It's a 2.4L not a 1.5L, needs a bigger pipe then a honda. Back to the OP: Stock ECU has a Decel Fuel "map". It dumps some fuel on decelation then cuts. This is why it makes a back fire sound. SRT's have this programed in for sound effects. KA does this not to stall out. The lag issue could be many things. I would lean more to the Auto more so the TCU not keeping up with the improvements. __________________ Comments should be taken as Opinions not as Statements of Fact

04-01-2008, 09:59 PM	#2
cronux Zilvia Junkie Join Date: Oct 2006 Location: wI Posts: 300 Trader Rating: (0) Feedback Score: 0 reviews	so what exactly are you asking? you lost low end in exchange for high end? thats fairly common...

04-01-2008, 10:02 PM	#3
LongGrain Post Whore! Join Date: Sep 2006 Location: Midwest Posts: 2,741 Trader Rating: (5) Feedback Score: 5 reviews	i had a 3" N1 with 3" test pipe on my NA KA. it would shoot flames on decel, lots of burbling and popping. not to mention the earth shattering roar at WOT. thats what happens with a huge exhaust on a big NA dinosaur motor.

04-01-2008, 11:24 PM	#4
s13pignose Zilvia FREAK! Join Date: Dec 2005 Location: ?? Posts: 1,324 Trader Rating: (2) Feedback Score: 2 reviews	I dunno it feels like there is some restriction somewhere, it's kinda hard to explain. It almost feels like I can still hear the intake after I let of the gas or something lol. I wish I had a better way of explaining it. Alright well maybe you guys can answer this question...and maybe it's just automatics that do this. But how come I can floor the car sometimes, and it responds right away and pulls hard, and then sometimes when I go WOT it just gradually pulls. Would that be tps related?

04-01-2008, 11:32 PM	#5
OfourTHREEfive Zilvia Member Join Date: Dec 2007 Location: Florida Age: 36 Posts: 174 Trader Rating: (1) Feedback Score: 1 reviews	Take that 3" off, you need some back pressure for all motor.

04-02-2008, 08:33 AM	#6
s13pignose Zilvia FREAK! Join Date: Dec 2005 Location: ?? Posts: 1,324 Trader Rating: (2) Feedback Score: 2 reviews	Huh...they're have been numerous dyno sheets on here showing no loses running 3" on NA. I didn't even feel a lost with mine, even before the intake. The intake is the only thing that I felt changed my low end, but kinda helped top. So back to the question about tps..thanks for posting though

04-05-2008, 05:53 AM	#7
verge513 Leaky Injector Join Date: Mar 2008 Location: memphis tn Age: 37 Posts: 54 Trader Rating: (0) Feedback Score: 0 reviews	these import engines pretty much require back pressure...running big open pipes mufflers and headers just make them louder with less hp.. i mean come on i had a 1980 camaro with a 454 big block and it even lost power with a 3 inch.. 2 3/4 was its sweet spot and it was a big block...my 240's factory exshaust is tuned to that engine..only thing i see really making ay diffrence is a 4-2-1 header and maybe a high flow cat i leave the rest alone.. anything else and it just kills mids and lows..unless u like to drive around town at 3500

04-06-2008, 02:08 AM	#9
verge513 Leaky Injector Join Date: Mar 2008 Location: memphis tn Age: 37 Posts: 54 Trader Rating: (0) Feedback Score: 0 reviews	well with big pipes the air moved though them moves slower..its like blowing thought a big pvc pipe compaired to a straw..when one cylender does its compression stroke and does the exshaust stroke as the exshaust escaped it also pulls more exshaust from the combustion chamber and awell from the other 3 cylenders. keeping everything moving. thus improving lower rpm and also at idle.there is such thing as to small..but the 240sx has the right size .but like many people say its youre car.. talk to a professional engine builder and dynoist.. cant always believe what you read on the internet.. and a scion tc...hardly a car.

04-06-2008, 02:24 AM	#10
verge513 Leaky Injector Join Date: Mar 2008 Location: memphis tn Age: 37 Posts: 54 Trader Rating: (0) Feedback Score: 0 reviews	The simple truth is that SOME backpressure is essential to proper engine functions. Go stick some stub pipes off your exhaust ports and tell me how well the engine runs. You probably won't even be able to start it. Each pulse of exhaust helps pull the pulse from the next cylinder along with it. The reason larger exhausts are bad is because they decrease exhaust gas velocity. This means the engine must work harder to expel waste gases which is why you lose low end torque. On the other hand you gain top end power as at high RPM the exhaust gases are travelling fast enough to make the larger diameter pipe efficient. Essentially changing exhaust diameter shifts the power curve of the engine to different RPM ranges. You open the exhaust, you get more of your power at higher RPM, but you lose some in the low RPM. The best exhaust is not the shortest or largest diameter, the best exhaust is the exhaust that maximizes exhaust gas velocity at the RPM levels you need. IE: drag racers would want a large exhaust as they're at WOT all the time. Autoxers would want a smaller exhaust as low end power is essential to get out of a tight corner. Road racers need a compromise as they need top end power as well as low end power. Finally, the blanket statement "backpressure is BAD" is completely wrong. Backpressure is necessary to proper engine operation, removing all backpressure would cause your engine to run like crud, or not at all. and this came from youre examples of higher backpressure is bad...atleast read youre examples all the way though...

04-06-2008, 11:43 AM	#12
verge513 Leaky Injector Join Date: Mar 2008 Location: memphis tn Age: 37 Posts: 54 Trader Rating: (0) Feedback Score: 0 reviews	ok sorry if i made you mad. i just know theres diffrent benifits with 3in and factory..i like the way nissan design there cars and i feel everything they put into there cars are there for a reason..

04-06-2008, 11:00 PM	#16
mifesto Zilvia Member Join Date: Jul 2007 Location: NJ Posts: 209 Trader Rating: (2) Feedback Score: 2 reviews	wish you could borrow a maf from a buddy, maybe the intake you put in was over-oiled and fouled your maf. happens on evos after people cake the cone filter w/ the k&n oil.... really hard to say since you havent given us much info. did you install the exhaust after you bought the car or it came with it?