# Back pressure? How does it feel?



## Slug (Apr 16, 2003)

Hey Im still pretty new to all this and I was wondering how you could tell if you have too much back pressure?

Can you just "feel" it or is there some way to measure this (cheaply)?

the reason i ask is because i just installed HS header along wiht CAI and a new cat to my existing Greddy exhaust and I feel no increase - just sound which is actually too loud for me.

I have heard talk of back pressure that maybe i dont have enough, or that i have too much. Could it be the cat? the resonator? 

Thanks to any who can help!!


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## fastpakr (Sep 18, 2002)

You _NEVER_ ever want backpressure. That being said, pipe that is too large will _create_ backpressure because velocity drops at low RPM, allowing the exhaust to cool and slow down in the pipe. You need to figure out what RPM you're tuning the exhaust to be most efficent for and then do the calculations for the right pipe size.


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## Myetball (Dec 15, 2002)

Check out this post .


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## Guest (May 21, 2003)

Please site your source for this statement...



fastpakr said:


> *That being said, pipe that is too large will create backpressure because velocity drops at low RPM, allowing the exhaust to cool and slow down in the pipe.*


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## fastpakr (Sep 18, 2002)

What is your argument with it? It's a simple principle that as volume increases, pressure, velocity, and temperature decrease. When that happens, backpressure is created. As long as there is enough volume being exhausted from the engine to limit temperature drop adequately, pipe size is fine. What is it you disagree with?


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## chimmike (Oct 17, 2002)

fastpakr said:


> *What is your argument with it? It's a simple principle that as volume increases, pressure, velocity, and temperature decrease. When that happens, backpressure is created. As long as there is enough volume being exhausted from the engine to limit temperature drop adequately, pipe size is fine. What is it you disagree with? *



hhaha so you're trying to tell me wiht a larger pipe you get MORE backpressure???? HAHAHAHHA OK buddy. Try and tell that to me with a straight face. Or any turbo guy, or any N/A V8 guy. lol


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## fastpakr (Sep 18, 2002)

Yes, that is EXACTLY what I am telling you. IF the pipe is too large for the motor, it will create backpressure. There is a proper tube diameter for any motor, and going too far on _either_ side of that will create backpressure. A turbo motor is a little bit different because most backpressure is created by the turbocharger itself, going large post turbo isn't that critical.


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## Guest (May 21, 2003)

This is what I disagree with... state your source for this please.



fastpakr said:


> *Yes, that is EXACTLY what I am telling you. IF the pipe is too large for the motor, it will create backpressure.*


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## dschrier (May 6, 2002)

Well I would agree with the fact that increasing the diameter will decrease the temp and velocity, but I'm not sure about the backpressure part, but I do not know the true answer myself so I won't dispute the post. From what I've always been told about how the exhaust works and the right size etc is to look at it like a straw. Too big of a straw and you won't be able to blow, or suck the exhaust out unless you have more strength, which a 4 cylinder car doesn't have much of for over a 2" exhaust, but the turbo generates more power so you can go 2.5" or more. Too small of an exhaust and you can't get the exhaust out quick enough. My analogie could be completely wrong as well, that's just how I look at it though.


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## fastpakr (Sep 18, 2002)

I will try and dig up some articles on it - I don't have any links offhand. What is the question though?
> pipe volume = < exhaust velocity
< exhaust velocity = more temperature drop in pipe
more temperature drop = denser air, which the next cylinder's exhaust slams into. at that point, backpressure is generated because the hot exhaust from the next cylinder is being restricted by the overly cooled exhaust from the previous.

If the volume exhausted is sufficient to keep it from cooling excessively in the pipe (because velocity remains high), then the pipe size isn't too big. For this reason, an engine turning high RPM can use a significantly bigger pipe than one built for towing, even though they're the same or similar displacement (Ford 4.9I6 vs. a 5.0HO for example).


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## zeno (Sep 17, 2002)

Here we go on another physics debate    

The point is, on a NA motor:
-too small of a pipe (in this case <2") BAD
-too large of a pipe (>2.5"), BAD
-between 2"-2.5", we have a winner!

Turbo: need 3" or bigger. Bigger is better (not how you use it...).

I think I'll go back to college and major in physics just so I can resolve this stuff.


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## Slug (Apr 16, 2003)

Im glad there is a great debate going on here! Now I dont know half as much as any of you on this matter, so can someone tell me exactly HOW you can tell/feel this backpressure? or do you just assume it is going on when your car doesnt go as fast as you thought it would...


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## Guest (May 21, 2003)

You can roll your eyes all you want but that solves nothing. You need to prove your point stated below or just admit that you are just "jawboning" us...



zeno said:


> *   Here we go on another physics debate
> 
> The point is, on a NA motor:
> -too large of a pipe (>2.5"), BAD
> *


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## fastpakr (Sep 18, 2002)

BGriffey, explain your disagreement. I have posted a very basic physics explanation of it and you haven't made any attempt to explain why you disagree. At this point, you're accomplishing nothing more than being a troll unless you have a reason for disagreeing.


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## zeno (Sep 17, 2002)

> You can roll your eyes all you want but that solves nothing. You need to prove your point stated below or just admit that you are just "jawboning" us...


What do you mean prove my point? I don't have to, thousands of people have proven this point by simply trial and error. This is standard info. on exhaust recommendations for the sr20de engine recommended by NPM, se-r.net, Mike Kojimo, and thousands of members on Nissan Forums and sr20de forums.

This is silly, I'm not saying you guys don't have great info. but the guy wants to know about why he doesn't feel a difference w/ his header and exhaust not a physics lesson.


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## Guest (May 21, 2003)

I claim that larger is better. I see no scientific reason that there would be a "sweet spot" in pipe diameter for optimum performance. After all, why would race cars run open headers at the drag strip if they could get better performance with a full exhaust system of a certain diameter? Doesn't make any sense to me... or to the drag racers!


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## fastpakr (Sep 18, 2002)

There is also virtually zero length to the exhaust systems on all out drag cars - cooling within the system is a non-issue.


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## zeno (Sep 17, 2002)

Ok, go throw a 4" pipe on your car and tell me what happens.


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## fastpakr (Sep 18, 2002)

BGriffey, here's a good one for you. http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

Some more (haven't read through all of these)
http://www.teammatrix.com.my/CarMods/exhaust_1.html
http://www.shotimes.com/SHO4exhaustjohnh.html

Again, outside of the first one I can't guarantee how well written / useful they are.


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## Guest (May 21, 2003)

I did read the link you posted... some of it was interesting but the gentleman who wrote it was definately not an exhaust expert... by his own admission. I believe the point about the cooling of the exhaust... I just don't know what the magnitude of the effect is. I suspect this effect is "mouse milk" (insignificant)...



fastpakr said:


> *There is also virtually zero length to the exhaust systems on all out drag cars - cooling within the system is a non-issue. *


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## zeno (Sep 17, 2002)

Taken from se-r.net and NPM
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"I agree that backpressure isn't relevant; the less the better. The hitch is that pipe sizing is very important to the flow of the system. The bigger the pipe is the less pressure is there, however if the pipe is too big for the amount of air an engine can move, you'll lose power, even though there isn't any backpressure. If you run a 3 inch system on the car, the velocity of the exhaust gases is going too slow down, because the gases have to expand to fill the pipe. And slowing down the exhaust obviously isn't what you want."

"For you do-it-your-selfers here are some general guidelines. If your engine is stock or if you are limiting yourself to bolt-ons, limit your pipe diameter to no more than 2" or you will lose some bottom end with no applicable gain on top. If you are to the point where you are getting into camshafts and headwork, 2.25"-2.5" from the cat back piping is appropriate. For turbo applications, 3" is the way to go."

"You need to have the lowest backpressure possible to produce the maximum power by keeping pumping losses low. Too big of an exhaust pipe causes power loss, especially low-end torque. This is because a big pipe has less exhaust stream velocity than a smaller pipe. Velocity is essential to get the best scavenging (or sucking) effect from tuned headers, which we will discuss in more detail later in a future installment. To simplify things, if the exhaust gas flow is kept high with good velocity, a vacuum can develop behind the closed exhaust valve allowing even better scavenging when the exhaust valve opens on the next exhaust cycle. Good scavenging is even more critical on valve overlap, the part of the 4-stroke cycle where both the intake and exhaust valves are open, especially with longer duration performance camshafts."

"If the exhaust pipe is too large, the flow will be sluggish with low velocity and the scavenging will not be good. Remember that a good exhaust has low backpressure and high velocity. The only possible exceptions to this rule are for turbocharged or nitrous motors. It is almost impossible to put too big of an exhaust past a turbocharger as a turbo depends a lot on the pressure differential across its turbine to get power recovery efficiency. A turbo engine can have an exhaust gas volume about 1.5-2 times more than an equivalent displacement naturally aspirated motor. NOS motors also have a pretty high exhaust volume and require a bigger exhaust if they are to be optimized for NOS operation."


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## Slug (Apr 16, 2003)

You guys all know a thousand times more than I do about the whole exhaust system so can someone please explain to me about "feeling" this backpressure? In particular, CAN you feel it or is it something that has to be measured by some expensive equipment?


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## dschrier (May 6, 2002)

You could probably measure the velocity of the exhaust out of the tail pipe with some meter, but other than that just gotta see how your car feels to you, and of course to actually see if you gained or lost power run it on a dyno.


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## Slug (Apr 16, 2003)

THANK YOU dschrier. That is exactly what i wanted to know. 

To everyone else, by the time youre all done with the subject of exhuast and pipes, etc, I feel like I will be an expert!! Keep it up.

Thanks to all


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## Guest (May 21, 2003)

Everyone seems to be hung-up on this velocity thing. Velocity doesn't mean crap... volume does. You want to get as much volume of exhaust out of the engine as fast as possible. Who cares about what velocity the exhaust is flowing at! We are concerned about getting the largest volume out! There is a HUGE difference in velocity and volume. In a larger pipe... the speed may be rather slow... but the volume is more.


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## fastpakr (Sep 18, 2002)

No, you're missing the point. If the volume increases enough to slow velocity, it causes resistance on subsequent exhaust pulses. If temperature were not an issue then a larger pipe size probably wouldn't be a problem. If streetable power weren't an issue, you would pick exhaust that has peak efficiency at the hp peak (as you will find on anything used solely for performance purposes). However, temperature IS an issue and we are discussing exhaust systems for use on street vehicles.


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## Guest (May 21, 2003)

Slug said:


> *You guys all know a thousand times more than I do about the whole exhaust system so can someone please explain to me about "feeling" this backpressure? In particular, CAN you feel it or is it something that has to be measured by some expensive equipment? *


You can put a fitting on the exhaust right after the header and measure the pressure with a momometer (sp?). But that is not going to tell you much since a gasoline engine has such a broad RPM range, you can have a lot of pressure at 5000 RPM's, and non at 6000. You have to decide on where you want your performance. Do you want lower end, or upper end? All out peformace means RPM's. So you are going to need to rap it up to get there. Of course, you may or may not feel a difference. I never trust the ass dyno.



I won't neccesarily agree that you get back-pressure for too large of a pipe, but it will have similar effects. If you can not move the exhaust out, then it doesn't matter if it is from too small of a pipe, or not enough velocity.


You can not compare a race car to a street car. Race cars have very narrow power bands. A top fuel car is desgned to run peak RPM's (somewhere around 15,000 RPM's if I remember right). There concern there is reversion. Those pipes are the proper length to take advantage of the reversion created for the additional natural supercharging, above what the blower adds to it.



Even a turbo can suffer from too large of a pipe, on either side. The effects are smaller, but still there. Bigger is NOT better.


Tuning is a thing that is rarely taken seriously on a street car. People don't understand it, and don't want to take the time to learn it. The turbo system that I am still planning will have an individual runner system coming out of a plenum (with velocity stacks) where the water to air intercooler is located. The runners will be of proper length to take advantage of reversion (probably for third phase, not much, but better then nothing). The cam will be matched with the intake, turbo and exhaust (turbo will be on a merged header collector) to for absolute peak power. Yeah, it will take a lot of time and calculations, but when done, it will be fun to drive. Definitely peaky, but that is what I am going for on this car.

By the time I am done with that engine, I plan to pull 400 HP out of a 4 cylinder, air-cooled VW engine. But that is still a ways down the road...


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## GA16DESentraXE (Dec 17, 2002)

I don't feel like typing, I've done my share of it when it comes to bitch slapping, er, i mean educating misinformed people about exhaust piping... So I'll just say

[jaded bastard] bgriffey, you're wrong. deal with it. stop being an ass, open your mind, and learn something. [/jaded bastard]


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## Guest (May 27, 2003)

Well, as long as you're getting nasty about it... you educate someone... Surely you jest. This is your first post on this thread, what is your point? Don't just start your namecalling imtimidation crap.


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## chimmike (Oct 17, 2002)

bgriffey said:


> *Everyone seems to be hung-up on this velocity thing. Velocity doesn't mean crap... volume does. You want to get as much volume of exhaust out of the engine as fast as possible. Who cares about what velocity the exhaust is flowing at! We are concerned about getting the largest volume out! There is a HUGE difference in velocity and volume. In a larger pipe... the speed may be rather slow... but the volume is more. *



bigger exhaust doesn't mean the engine is going to put out more volume...................if you have a bigger exhaust it's going to take MORE of the gases to fill it up to create the velocity to force it out..................this drains power. 

Think of turbo. the more velocity the better, so the better it can flow out of the turbo. slower velocity with an exhaust that is too big takes more time to fill the pipes, uses more gases, and creates backup at the turbo.


seriously man. I've tested this on my car when I was n/a. 2.25" piping on my car lost power as opposed to 2".


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## Pretty White (May 6, 2002)

zeno said:


> *   Here we go on another physics debate
> 
> The point is, on a NA motor:
> -too small of a pipe (in this case <2") BAD
> ...


Yet the Greddy exhaust for a b13 is 2"???


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## zeno (Sep 17, 2002)

Sorry....to clarify, the info. I gave was for a b14 sr20de engine.


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## Lurker Above (Apr 21, 2003)

a smoker could do this experiment pretty easily  using the same effort to exhale, change the size of the opening at the lips. if you crack pursed lips, exhaust velocity is low, you feel back pressure, and at the end of your breath you're left with a mouthful of smoke. open your mouth wide and repeat, and again you're left with a mouthful of smoke despite the greater diameter. open your lips to usual puffing size (slightly pursed, no more than a fingerwidth) and you get good velocity and scavenging with no lingering smoke in the mouth. same concept?

don't take up smoking though...nasty habit...


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## Guest (Jun 2, 2003)

Lurker Above said:


> *don't take up smoking though...nasty habit... *












Would it tell you anything if I were to say that I got the muffler welded on my Bronco at the same place that Mr. Bubb Rubb got his whistle tip?  (no, seriously, I did).


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## Lurker Above (Apr 21, 2003)

sorry, i wandered off and got lost in the forums...

heh...does your bronco go 'whooo whooo'?  i looked up the kron news movieclip...man, that reminds me of my old r30 turbo back on okinawa! wasn't quite that piercing, but the turbo was jacked so it made a 'hooo hooo' sound (and it ain't just for decorashun!)


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## Guest (Jun 9, 2003)

Lurker Above said:


> *sorry, i wandered off and got lost in the forums...
> 
> heh...does your bronco go 'whooo whooo'?  i looked up the kron news movieclip...man, that reminds me of my old r30 turbo back on okinawa! wasn't quite that piercing, but the turbo was jacked so it made a 'hooo hooo' sound (and it ain't just for decorashun!)  *


Mine id more of an "I think I can, I think I can..." with that big mean nasty 0-60 in 16 (something like that) 2.9...


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## C-Kwik (May 1, 2002)

I think there may be confusion from a technical article that described backpressure being caused by low-velocity turbulence in an exhaust system. It may have even been SCC magazine that stated this. It's been at least a few years since I read this so don't quote me on it. 

The explanation that I understand and that makes the most sense to me is the effect velocity has on scavenging. You have to look at it in terms of velocity and not pressure. Think of the exhaust gases as a column of air moving out the exhaust pipe. This column of air has mass and is moving which means it has momentum. The higher the velocity, the more momentum it has. Higher velocity air will be harder to slow down. As a piston reaches TDC on the exhaust stroke it starts pushing out less air. As the air in the runner slows down, it will be supplying less air to the main exhaust stream. As this happens the main column of air will still have momentum and create a low pressure area behind it. This low-pressure area will find it's way all the way back to the combustion chamber. With some cam overlap, it can even pull some intake air into the combustion chamber, perhaps even as far as the exhaust. The overall effect of this scavenging can vary based on header design, cam profiles and RPM. But without velocity, there will be no momentum to "pull" air out of the combustion chamber so you may lose power due to the loss in scavenging effects. Exhausts that are larger may only see this effect at lower RPM's where exhaust airflow is low therefore, it is seen as a low-end loss. There is a lot more to it than this, such as pulse tuning with header tube lengths play a big factor as well,but this a simplistic view of my understanding of why velocity is important.

As far as turbos, it's more of a pressure differential on each side of the turbine wheel that is important. As an example, if you have exhaust manifold pressure of 10 psi and pressure in the exhaust of 1 psi, you have a 9 psi differential. If you make the exhaust smaller, and increase pressure in the exhaust, say to 5 psi, than in order to achieve the same 9 psi differential, the manifold would have to reach 14 psi. This means the motor is seeing 4 psi more backpressure and has to work harder to push air out. This is why large turbines can be so great and freeing up power. But for a turbo motor, a larger exhaust is typically better. The recommendation I've seen for turbos are run as big an exhaust as you can bear to hear and fit in your car.


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## Lazarus_023 (Jun 3, 2003)

C-Kwik said:


> *I think there may be confusion from a technical article that described backpressure being caused by low-velocity turbulence in an exhaust system. It may have even been SCC magazine that stated this. It's been at least a few years since I read this so don't quote me on it.
> 
> The explanation that I understand and that makes the most sense to me is the effect velocity has on scavenging. You have to look at it in terms of velocity and not pressure. Think of the exhaust gases as a column of air moving out the exhaust pipe. This column of air has mass and is moving which means it has momentum. The higher the velocity, the more momentum it has. Higher velocity air will be harder to slow down. As a piston reaches TDC on the exhaust stroke it starts pushing out less air. As the air in the runner slows down, it will be supplying less air to the main exhaust stream. As this happens the main column of air will still have momentum and create a low pressure area behind it. This low-pressure area will find it's way all the way back to the combustion chamber. With some cam overlap, it can even pull some intake air into the combustion chamber, perhaps even as far as the exhaust. The overall effect of this scavenging can vary based on header design, cam profiles and RPM. But without velocity, there will be no momentum to "pull" air out of the combustion chamber so you may lose power due to the loss in scavenging effects. Exhausts that are larger may only see this effect at lower RPM's where exhaust airflow is low therefore, it is seen as a low-end loss. There is a lot more to it than this, such as pulse tuning with header tube lengths play a big factor as well,but this a simplistic view of my understanding of why velocity is important.
> 
> As far as turbos, it's more of a pressure differential on each side of the turbine wheel that is important. As an example, if you have exhaust manifold pressure of 10 psi and pressure in the exhaust of 1 psi, you have a 9 psi differential. If you make the exhaust smaller, and increase pressure in the exhaust, say to 5 psi, than in order to achieve the same 9 psi differential, the manifold would have to reach 14 psi. This means the motor is seeing 4 psi more backpressure and has to work harder to push air out. This is why large turbines can be so great and freeing up power. But for a turbo motor, a larger exhaust is typically better. The recommendation I've seen for turbos are run as big an exhaust as you can bear to hear and fit in your car. *


HOO-RAY!! we have a winner.

N/A engines:
timing of exhaust scavenging effects using runner lengths/volumes = Helmholz tuning. a personal fave of mine... the rapid change of exhaust gas pressure as the piston nears TDC and the exhaust valve begins to close can produce a low pressure front in the exhaust that can draw a small amount of exhaust out of the cylinder due to the gasses' own inertia. this low pressure front can also be used to draw the exhaust out of other cylinders, if timed correctly.

NOW, think of it this way: Energy = Mass x Velocity^2
got it? good.

large diameter runner: more mass allowed at a lower velocity. the lower resistance will initially allow more of the exhaust gas volume to flow out of the cylinder. why? less restriction to flow. BUT the pressure front will be damped because of the lower velocity (less energy overall, speed is MUCH more important than mass in an energy equation)... if you have too large of an exhaust runner, the pressure front will have a lowered effect due to the loss of exhaust gas velocity.

small diameter runner: Less mass allowed at a higher velocity. keep in mind that the exhaust gas velocity will only reach the speed of sound. it's not possible to force exhaust to move faster, so if you continue to decrease the diameter of your exhaust pipe, eventually you will restrict flow.

so, what do you want? you want to decrease exhaust pipe diameter enough to take advantage of scavenging effects of the exhaust gas inertia, but not so much as to significantly restrict flow. as far as performance goes, there is no "perfect" solution, you'll have to compromise based on the engine speed you want to tune for.

to put it in simple, practical use? ouch. no such thing. in my experience, it's best to eliminate large restrictions, such as catalytic converters and non-flow-through mufflers to reduce the flow restriction as much as possible, and to err slightly on the larger side (DO NOT TAKE IT TO EXTREMES, HOWEVER... 4" exhaust on a 2.5L is idiotic.) of pipe diameters, and hope that the increased flow will help more that the lost scavenging effect.

Turbo engines:
all about pressure drop over the turbine. on the high pressure side, there may be pulses (which can be limited by header design to provide near constant flow through the turbine at a given engine speed), but they'll be lessened by the inertia of the turbine and the restriction of the turbo nozzle. on the lo pressure side, there'll be very little to no pulse, therefore no scavenging to take advantage of, so for the most part, larger = better... same thing, though 4" pipes are stupid.

NOTE: There is validity to the LARGER IS NOT BETTER side of this argument, and if somone is willing to send me enough chocolate chip cookies or tollhouse bars, i may be willing to come up with some supporting equations.

I would like to also reiterate my support of nissan's design teams and state that, if you're leaving the stock cats, expansion chambers and exhaust manifold in, increasing pipe diameter is gonna do you shit for performance. i'm sure somone at nissan is getting paid a lot more than me to calculate the optimum pipe diameter for our cars....


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## chimmike (Oct 17, 2002)

I agree. If you really want a new muffler, just buy one and don't do a cat back until you do a header as well. 

Sometimes cat-backs on these newer cars LOSE power in certain places where we actually could use it the most!


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## cargeec 99xe (May 10, 2002)

Milliuns and milliuns of mags from import mags to dem V-8 mags have dyno'd same results. from small pipe to bigger pipe = more low end torque. from big pipe to too big pipe = loss of low end torque w/o improvement in mid & high end, except at very top. There is ideal size to maximize torque over entire rpm range of engine. Size of pipe depends on engine displacement and other factors.

A turbo is stuffing more air into chamber, is like bigger cylinder, just like bigger engine, spits out more gas per cycle, so ideal pipe is bigger, which is why V-8's with 2 inch piping is too small and need bigger pipe than puny 1.6l.


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## 200sx98fl (Jan 5, 2004)

*reading all these terms is confusing....*

i have a b14 ga16de i put a 2.25 inch pipe from cat back going into a bomz exhaust...which is 2.5 inlet and 4 inch outlet....also have a tuner to make it 2 inch.....first Q did i go right with a 2.25 and next would it be better to leave the tuner on or take it off....As of right now all i have is a intake....planing on putting headers in at the end of the month....Can anyone answer this question simply? 



98 200sx se


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## Gsolo (Sep 11, 2003)

200sx98fl said:


> i have a b14 ga16de i put a 2.25 inch pipe from cat back going into a bomz exhaust...which is 2.5 inlet and 4 inch outlet....also have a tuner to make it 2 inch.....first Q did i go right with a 2.25 and next would it be better to leave the tuner on or take it off....As of right now all i have is a intake....planing on putting headers in at the end of the month....Can anyone answer this question simply?
> 
> 
> 
> 98 200sx se


yeah, almost to big of pipe for n/a. I run 2.5 on my chevy V8 which is perfect for low end torque, if i wanted more horse i'd do 3 or 3.5 ....remember though that that is pushing a hell of alot more air than yer 1.6 liter. i've found that 2" is perfect for most 4cylinders. 

And i'm gonna through another mix into this old debate of exhaust....using small pipe for awile then go to bigger pipe. A stepped header is good example of this, small pipe creates velocity for low end torque then goes into big pipe increasing volume capacity which still allows high rpm power. this is why alot of cars i've seen run a bigger pipe after the cat....which yeah isn't good cause the cat alone has already restricted it but you get the idea. And from what i read i dont think anyone brought up this but a bigger pipe will not increase backpressure, but it will impede the velocity. i know it sounds like a contradiction but when thinking about scavenging its clear why to big of pipe loses power. 

and most cars made have the correct pipe size for general power across the rpm range. 

Although i'd still like to get my hands around the neck of the guy at GM in 1970 who designed 2.5" midpipe going to a 1 3/4" tailpipe


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## 200sx98fl (Jan 5, 2004)

Gsolo said:


> yeah, almost to big of pipe for n/a. I run 2.5 on my chevy V8 which is perfect for low end torque, if i wanted more horse i'd do 3 or 3.5 ....remember though that that is pushing a hell of alot more air than yer 1.6 liter. i've found that 2" is perfect for most 4cylinders.
> 
> And i'm gonna through another mix into this old debate of exhaust....using small pipe for awile then go to bigger pipe. A stepped header is good example of this, small pipe creates velocity for low end torque then goes into big pipe increasing volume capacity which still allows high rpm power. this is why alot of cars i've seen run a bigger pipe after the cat....which yeah isn't good cause the cat alone has already restricted it but you get the idea. And from what i read i dont think anyone brought up this but a bigger pipe will not increase backpressure, but it will impede the velocity. i know it sounds like a contradiction but when thinking about scavenging its clear why to big of pipe loses power.
> 
> ...





So would it be better to leave the sliencer on and keep it at 2 inch or let it be the 4 inch outlet????????


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## Gsolo (Sep 11, 2003)

200sx98fl said:


> So would it be better to leave the sliencer on and keep it at 2 inch or let it be the 4 inch outlet????????


if the 4 inch is just for the very end of the system it doesn't really affect performance at all. depends on what you want it to sound like.


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## Standard Games (Dec 7, 2003)

this is a good thread, they should all be so entertaining.


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## 91SERHondaKiller (Aug 28, 2002)

I know this thread is old but I read most of it and had a question about the piping. I have 2.25 press bent piping and since the size decreases in the bends will that mess up the velocity, since it would speed up in the bends and slow down in the straights. I've been told by a few mechanics that instead of paying the money to get 2.25 mandrel bent, just get 2.5 press bent, since the corners will be about 2.25. But wont that cause problems since you want a smooth continous flow thruought the exhaust?


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## Gsolo (Sep 11, 2003)

91SERHondaKiller said:


> I know this thread is old but I read most of it and had a question about the piping. I have 2.25 press bent piping and since the size decreases in the bends will that mess up the velocity, since it would speed up in the bends and slow down in the straights. I've been told by a few mechanics that instead of paying the money to get 2.25 mandrel bent, just get 2.5 press bent, since the corners will be about 2.25. But wont that cause problems since you want a smooth continous flow thruought the exhaust?


I doubt you'd ever feel the difference but if you can afford it go with mandrel


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## broaner22 (Jan 5, 2004)

Most mechanics are wrong. You are totally right. The exhaust energy will decrease as i comes out of the bent section. A basic rule of thumb is 1" for every 100HP. Thats one in of straight, not crumpled piping, as in mandrel. 

I didn't read this thread except for the first page. Backpressure is for old foggies and two strokes. The correct term is exhaust energy. Too large of a pipe will lower the exhaust energy, thus reducing the power of the engine becuase it used more effort to expel the exhaust gasses out of the combustion chamber.


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## Gsolo (Sep 11, 2003)

broaner22 said:


> The correct term is exhaust energy.


never thought of wording it that way. works though and makes more sense


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## chimmike (Oct 17, 2002)

kenbarls said:


> I am totally agree! Nothing to oppose with you.


I am curious as to the reasoning for your reviving a 7 year old dead thread.

Are you preparing to spam?


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