|Bijan|

iTrader: (12)

For some who dont know, I'm employed at a muffler, bending, distributing shop here in GA. We're the only Mandrel bending shop in GA. One of the biggest, and most used distributors nationwide.

There are 2 "shops" - 1 is the bending shop, the other is the install shop. I work in the bending shop.

Yesterday, I was making a 3" test pipe for Jay, then I was about to make myself one. A guy (Coop is his name) came up to me and asked me what the hell I was doing. So I told him I was makin a test pipe. And this is how the conversation went:

Coop: Why the hell are you making a test pipe?

Me: So it will flow out better, no restriction.

Coop: Big mistake, your gas mileage will turn to sh*t, you will not have ANY gain besides if your WOT doin about 110+mph, your ECU will go wack and not know what do, throw all these different codes.....and you're wasting your time.

Me: Well, uh...my muffler isnt enough...

Coop: Yes it is, thats as "straight through" as you need. You're a normal aspirated car. All these pipes for this car are totally useless unless you're boosted.

(I had a 3" Y-pipe in my trunk, I went and got it)
Me: So, you're saying...even if this Y pipe were 2.5" -- It wouldnt give me any gain.
Coop: Unless you see a before and after dyno...and that 1hp difference...you tell me.
Me: Then why do all these companies make cat-back systems and these Y-pipes?
Coop: Cuz people buy them...



Any insight on this? I believe the guy, he's been doing this for 15yrs, and he's not there to bullshat me. We get along good, but if your car is not a V8 american from 1979 or younger...he doesnt like it. Hah

DA-MAX

iTrader: (10)

IMO on an N/A car test pipes are pointless...all they bring is noise and backfiringand flames (at least mine did)...big deal you gain 1 mule power if that. boosted cars need more exit flow because air is exiting faster...cats are restrictive for boost and for some setups that run rich can kill the cat when pure unburnt fuel hits it.

as far as the Y-pipes...he obviously has no clue because they are already dyno proven with their gains....theres no argument there. show him a dyno, you won that one

as for the catback systems on n/a cars...IMO eventhough we all buy them and we all get out custom 2.5" catbacks made...its no secret that you don't gain more than probably 5hp if that, but of course combined with a Y it gives that extra kick over what it would if combined with the stock exhaust.

all in all modding n/a cars is a complete and utter waste(I'm not joking either)

DAVE Sz

iTrader: (2)

I think he was saying that the 3" y wouldn't do much on an n/a car over let's say a 2.5".

rik3k

Anything more than a 2.5" on a NA Maxima is useless. You need a little backpressure anyways (or so I've been told). It's like people putting 3" pipes on their riced up NA Civics. Useless.

Kevlo911

iTrader: (36)

I am pretty sure he is right about hte test pipe on a N/A car.

Stephen Max

iTrader: (59)

That's how I understood it, and I think he's right. Boosted is another matter, though. Iansw gained all through the rpm range going to a 3" system.

Quicksilver

iTrader: (1)

You've been told wrong. There is no such thing as backpressure in any N/A 4 stroke motor. You can have back pressure in certain parts of a turbocharged vehicle's exhaust, but not throughout the entire system.

Remember...backpressure is a MYTH in our cars (especially in N/A modes)

Jeff92se

iTrader: (82)

Not to mention if you get caught taking off the cat or even making a test pipe that eliminates the cat, it's a HUGE fine.

Dude. You don't know about Y pipes??

Jay93SE

"You've been told wrong. There is no such thing as backpressure in any N/A 4 stroke motor. You can have back pressure in certain parts of a turbocharged vehicle's exhaust, but not throughout the entire system.

Remember...backpressure is a MYTH in our cars (especially in N/A modes)"

Then how do you explain the gains from a y-pipe? Any gain you get results from the elimanation of restrictions in the exhaust. Restrictions reduce flow and power.
Therefore: Reduced flow due to restriction= backpressure.

Stephen Max

iTrader: (59)

I thought he meant the NEED for backpressure is a myth. Surely he didn't mean there is no backpressure in an exhaust.

Jeff92se

iTrader: (82)

Actually there is no such thing as "backpressure". Only exhaust gas velocity.

Stephen Max

iTrader: (59)

I don't know where you guys came up with this idea, but backpressure surely does exist and is a measurable quantity.

Dave B

1) The VQ does have backpressure just like any other gasoline combustion engine.

2) Back pressure is never a good thing.

3) The goal to developing the best exhaust is to keep the exhaust velocity up and keep the exhaust gases hot. A good exhaust doesn't necessarily mean the largest piping you can stuff under the car. Too large of piping causing the exhaust gases to become turbulent and cool which can cause poor low to midrange performance. Turbo cars generally do quite well with overly large exhausts because the turbine housing acts as a good exhaust velocity creator and once under boost the turbo can take advantage of the large piping.

4) A y-pipe does work on the VQ and 2.5" is the maximum you want to go for an NA application.

5) Mandrel bent pipes are overrated when you're talking about NA motors that don't move a lot of air (ie street driven VQs). I've watched firsthand the dyno difference between a WSP non-mandrel y-pipe vs a WSP mandrel y-pipe. Same car, same dyno, same conditions.....1-2hp at peak and throughout the powerband.

6) Test pipes are worthless on todays NA cars. First reason being catalytic converters aren't restrictive. You're talking 1-2hp at best. Second reason being most late model cars use an O2 sensor that sniffs the exiting gases. If the sensor gets incorrect readings, the ECU may dial back engine performance in an effort to reduce emissions because it believes the cat is failing. Turbo typically respond favorably to a removed cat because of the amount of exhaust gas the turbo can move.



Dave

Jeff92se

iTrader: (82)

Some explain "backpressure" then. I don't see anything that exerts a reverse pressure inside the exhaust. Maybe exhaust restriction maybe. Is that the same? I don't see it that way.

Ask youself "why is backpressure good for a na engine?"

Stephen Max

iTrader: (59)

Okay, I thought it might be a semantics issue.

Backpressure, as commonly used, is the resistance to flow that exists in a pipe with a fluid being forced through it. It is due to nonconservative losses in a piping system and is due to friction of the fluid against the pipe walls, frictional losses within the fluid, changes in fluid momentum (such as when flow velocity or flow direction suddenly increases or decreases) and probably some other stuff I'm not thinking of right now. A perfect pipe with zero losses will have no static pressure differential between the fluid in the pipe and the outside atmosphere. In real world exhaust systems, frictional losses result in 3-5 psi of pressure in the exhaust system (more or less) above ambient. This pressure differential resists flow out of the cylinder heads and is what we mean by backpressure. Backpressure is never good.

Jeff92se

iTrader: (82)

Can you agree that backpressure is never good but exhaust gas velocities are? If you can keep the exahust velocities up throughout the rpm range w/o restriciting flow, you can have in theory any large pipe you want.

DAVE Sz

iTrader: (2)

I wouldn't agree completely as ZERO backpressure is not a good thing at all. You need SOME backpressure, not too much and not too little.

Stephen Max

iTrader: (59)

Absolutely.

Exhaust velocity helps evacuate the cylinder during the exhaust stroke, due to the momentum of the air column in the exhaust pipe creating a certain amount of suction at the exhaust valve. At low-mid engine speeds the effect of the exhaust in aiding cylinder evacuation can be very pronounced.

Furthermore, the less backpressure you have the more suction there is to help draw out the exhaust.

But when exhaust pipe diameters are sized so large that backpressure all but disappears, the exhaust velocity (and therefore momentum) also drops, resulting in an unintended adverse affect on the exhaust's ability to aid in cylinder evacuation.

So the need is an exhaust system that maintains exhaust velocity, epecially at low-mid engine speed, while minimizing backpressure.

The perfect exhaust is one that maintains high exhaust velocity (equal to maximum piston speed) with zero backpressure.

Jeff92se

iTrader: (82)

You don't understand exhaust theory. If you had absolute 0 "backpressure" as you call it but were still able to maintain exhaust gas velocity and flow, there would be no need for "backpressure" whatever that term means

Jay93SE

Yes. Check PM.

Edit: This was meant to end up a few posts above!

Stephen Max

iTrader: (59)

I think what he means is that real world exhaust systems are a compromise, and you can't have adequate exhaust velocity at low rpm without suffering a little backpressure at high rpm. That doesn't make backpressure good, just something we have to put up with in order to have an exhaust that works well for a broad range of rpm.

Jeff92se

iTrader: (82)

I still think "backpressure" is an incorrect term. Because the loss of torque down low from running too large of pipe is not from the lack of "backpressure" it's from the reduction of exhaust gas velocity (as stated from yourself very well above)

Stephen Max

iTrader: (59)

Most people mean it as the pressure in the exhaust pipe above ambient that resists cylinder evacuation. At low-mid rpm it is going to be negligible, and exhaust velocity is what is important, like you say. Back pressure becomes a power robbing factor at high rpm and especially with boosted cars having exhaust systems sized for NA operation.

Bags

iTrader: (3)

This is what is, and has been, missing from this forum.

Thanks for bringing it back

DerekJ212

iTrader: (1)

Dont all the gains from the ypipe only come from the removal of the precats and nothing else?

Derek

Quicksilver

iTrader: (1)

Backpressure is not the correct term...not by a long shot. And no, there can not be "backpressure" in a N/A 4 stroke fuel injected motor (I left out the fuel injected part in my above post, and for that, I apologize). What you're referring to is gas exit velocity, or exhaust energy. It's true that opening up and exhaust system too much in any N/A 4 stroke can cause a reduction power, just the same as too narrow a system will cause an inability to flow properly...but the exhaust gasses do not flow backwards through the tailpipe and back into the combustion chamber. This would cause a severe spike in exhaust manifold and cylinder temperature, possibly leading to cylinder destruction through pre-detonation when the next "batch" of fuel/air is delivered. Also, remember that the engine in our cars is nothing more than an air pump. It sucks air in one side and blows it out the other (OK, that's over simplified, but you catch my drift). And air is a fluid, similar to water in nature (flow wise).

Say you have a 1" pipe, and you try and cram 1000 gallons per second thru that pipe. Do you think that pipe can actully flow 1000 G/sec ? No way in hell. Now say that pipe is 1 mile wide. You can easily flow 1000 G/sec thru that, but it's not going to flow very fast (nor draw as much water into the pipe at the source). The water just meanders into and out of the mile wide pipe with no energy. It won't scavange the "headwaters" very well. Now say you have that pipe sized perfectly for maximum flow and volume of water. Not only will you get good water speed (energy) through the pipe, but you'll also get a good amount pressure as well. This properly sized pipe will be more energetic vs. the mile wide pipe or the 1" wide pipe. This properly sized pipe will scavange MUCH better as well.

"It all started back in the day when the government started regulating vehicle emissions. The first catalytic converters were incredibly restrictive, and most cars were still runing carbs.

The cats were so restrictive that at low engine rpm, air would sometimes pass through the carb twice! Carbs were then re-jetted to compensate for this - to keep the air from getting a double-dose of fuel.

When you take one of these cars and remove the restrictive cat/exhaust to replace it with a free-flowing system, the stock carb will not work right anymore - you will not be getting enough fuel at lower rpms and will get a loss in torque. The solution - change the carb!

Modern catalytic converters are free-flowing compared to early pellet-filled versions. Modern fuel injection systems are not designed to compensate for super restrictive catlysts. Modern fuel injected cars do not need backpressure.

The truth - exhaust gas velocity

Here's the real deal about choosing an exhaust that is too big. I'm not a physicist, my understanding of fluid dynamics is very limited.... but here's my understanding.

Exhaust gases moving out of your exhaust can be used to pull other exhaust gases out of your cylinders. This is why headers are sized and the tubes matched the way they are. This effect is called scavenging. The faster the gases move, the more you benefit from scavenging.

The larger the tubing, the slower the gases move, the less scavenging occurs. (but, more ultimate flow with less restriction is possible)

The smaller the tubing, the faster the flow. (but obviously, the resistance can quickly overcome the benefit of scavenging if you start reducing your tubing size... a drinking straw simply can't flow enough for a motor)

Notice I didn't say anything about backpressure. Backpressure is just a measure of the restriction in the system. It can be caused by bends, narrow tubing, badly flowing mufflers, etc. Backpressure will be present in any system - but the less that is there the better. A large diameter system with a restrictive muffler (and therefore a high backpressure) will not gain any velocity. So now you will have to worst of both worlds - reduced scavenging and a restriction to free flow at higher rpms.

This is the reason that apexi made the N1 dual. The two pipes can support the flow of a larger pipe, but maintain better gas velocity and scavenging.

Some people claim to lose low end power when strapping on a big bore exhaust. This may be true, but it's because of a decrease in the scavenging effect, not because you don;t have enough backpressure."

http://www.uucmotorwerks.com/html_pr...torquemyth.htm

There's some lite reading for you...

Justin

Quicksilver

iTrader: (1)

This myth needs to be disspelled, just like the "wider tires give you more traction" myth, whcih I have elaborated on in the past...

Quicksilver

iTrader: (1)

"There's a science behind this that i only understand the jist of, but basically backpressure is never a good thing. It's the tuned length of each exhaust tube (header primary) and then the eventual diameter of the collecter and exhaust beyond that point.

The ideal length creates the right amount of scavenging that effectively "sucks" the exhaust out, creating power. (that's ridiculously simplified of course). The old hot rod guys used to slap on a new, aluminized full exhaust, and fire up the motor... where the pipes started to turn rainbow colors, they'd cut 'em and that would be the end of the exhaust.

This tuned length idea is what creates the power, not backpressure. The back pressure myth comes from the fact that if you run too big an exhaust, or no exhaust at all, you make less power. That is often misinterpreted as meaning that some back pressure is necessary for power, when really the power loss is from insufficient scavenging.

Ideally, you'd determine the proper "tuned length" (presumably w/ a more modern method than the old hotrodders' ). Beyond that point you'd have no exhaust, or a very large exhaust, since beyond the tuned length the diameter or presence of exhaust is inconsequential, and the most benefit can be seen by getting the hot gasses out of the way as fast as possible.

Regarding the intake gasses sneaking out the exhaust in the absence of backpressure, there's a very good argument for that being mythical, but i can't remember what it is... haha

In any case, whether you call it backpressure or scavenging or whatever, you are better off with some kind of exhaust vs none at all. And minor differences in flow, unless you're REALLY tuning for power, shouldn't keep you awake at nite.

There was a great debate on this on a mustang forum i used to frequent... i wish i could find the link."

Another link on the same subject...

http://www.jeepforum.com/forum/showt...845#post547845

DA-MAX

iTrader: (10)

thats part of the gains, but not all...the 3rd gens have no stock precats, but when a Y-pipe is added it shows the same gains as the 4th gen Y-pipes...so no removing the pre-cats is only PART of the gains, the overall diameter increase and design has a lot to do with it as well

Stephen Max

iTrader: (59)

I agree with everything you have said, including that backpressure is a measure of the restriction in the system.

So I don't understand when you say that there can not be backpressure in a NA 4 stroke fuel injected motor.

Also, backpressure doesn't mean there is flow of exhaust gases back into the cylinder head, it just means there is a pressure above ambient in the exhaust pipe that resists exhaust scavenging to some degree. Maybe that is the crux of the misunderstanding.

Drjmaxx88

Stupid question coming!!!

So is this an accurate way of describing "Backpressure"?

It could be likened to the air that rises into the water cooler after u open the valve. Without the air coming up you couldn't get the water out.

Does that make any since?

Jeff92se

iTrader: (82)

There is not backpressure. Only maybe a "flow restriction". But there is no pressure that is excerted in a back or reverse direction in the exhaust.

Quicksilver

iTrader: (1)

I'm sorry, but I don't understand what you're getting at. I'm saying there is no backpressure. It does not exist in a N/A 4 stroke fuel injected engine. It does not exist in any self-adjusting fuel meter delivery system in current production.

Those quotes are from some of the several websites I had listed...not my specific definition of exhaust energy. Maybe that's why you're confused with what I stated vs. what I quoted from other technical discussions on other sites. I'm saying that backpressure, which would more appropriately be described as a reversal of the direction of exhaust flow during normal engine operation, does not exist. Backpressure, by this same definition, does exist in some two stroke and some carberator equipped 4 stroke motors with early environmental controls to this very day. Some of these items that use and have backpressure currently are; some current model chainsaws, yard equipment, and some 2 stroke motorcycle engines.

Again I restate that our engines do not have backpressure in a N/A state. Our engines can suffer from restrictions in flow (not backpressure--too small an exhaust, improper header design, exhaust damage) or not enough flow volume (going to an exhaust diameter too large to support proper scavanging). I think the confusion keeps occuring because there really is backpressure, and it is aptly described, but it is being tagged to the wrong symptom in our 4 stroke motors. That is my definition of backpressure...

Quicksilver

iTrader: (1)

That would be the displacement of one fluid by another with different physical properties (my best description)...

SR20DEN

iTrader: (7)

Would everyone be content if I test this the next time I go to the dyno? I will dyno once with my exhaust connected and once with an open collector. Or will you all simply say then that my exuast system was too small to start with?

The irony that I am trying to point out here is that the guy with the "15 years" experience perhaps needs 15 more years of experience but with different vehicles. Just because an open 3" exhaust system might have a detrimental effect on a Honda Insight doesn't mean it will have the same effect on a highly modified 3.5 liter Nissan.

I wonder if some of you really do not understand the concept of backpressure or are you just trying to dispell the so called myths and get everyone else to stop using that term? Perhaps backpressure is a bad term. If that is the case just use the phrase 'exhaust pressure'.
Now I am certainly no expert in this subject and I would love to think that any sort of exhaust pressure or resistance is a bad thing however, if it isn't needed for something then why don't auto manufacturers use larger exhaust pipes and why do all A33 Maximas have a dual stage muffler?

Perhaps an exhaust system is just an inversion of an intake system and follows similar rules for tuning.

Quicksilver

iTrader: (1)

Actually Matt, it is pretty close to an inverted intake system. That's a good way to look at it. And as far as not using the term backpressure, I gave a couple of terms that could be used in it's place to reduce confusion. Exhaust pressure is another good term. They are all correct to an extent...

|Bijan|

iTrader: (12)

You mean, a NICE, intelligent conversation? Or are you flamin my azz?

Jeff92se

iTrader: (82)

Stay outa this, the smart people are talking
haha

You getting this stuff??? It's your thread!

SR20DEN

iTrader: (7)

Stephen Max

iTrader: (59)

Quicksilver,

I think you and I pretty much agree on exhaust system dynamics and the trade-offs involved between flow velocity and exhaust restriction when sizing pipe diameter.

We seem to have different definitions for "backpressure". I have stated my definition, which is consistent with what you are calling flow restriction, and I also stated that backpressure does not by itself result in exhaust gases flowing back into the cylinder (although reverse flow can happen because of a cam with large intake and exhaust overlap, but that is another discussion entirely), which seems to be your point of contention to the term.

I don't know where to go from here, so I'll just say it is nice to have a level-headed discussion without it getting into a flame war.

Peace.