Ive heard different sides of preplacing stock exhaust. No back pressure is good and back pressure is also good. The thing that gets me is that some say that the turbo needs back pressure and no back pressure will damage it. Im not really sure the whole turbo thing is new to me. I have a cat back system and just recently purchased a new down pipe to remove the cat. Someone please fill me in!

Ok, here is goes. The reason that not enough backpressure is bad, I believe, is because the turbo cant spool. If it is just all free flowing, the turbo cant spin, and everything slows down. To much backpressure, and the turbo spins, but the exhaust piles up, and the turbo has to spin against too much pressure, and it creates friction, which raises exhaust gas temps. An aftermarket downpipe is fine, but a 5" system all the way through is too much. Actually, in the GT40R turbo kit, the downpipe is stock sized, but less restrictive, so it gives it a little back pressure, just enough to spool the turbo. Now I am pretty sure that is right, not completely sure, and hopefully someone can correct me if I am wrong.

The turbo itself provides all the back pressure you need. Back pressure beyond the turbo would just cause it to spool up more slowly.

OK... Im still not convinced... I agree... 5" is too much...

On my stock LBZ, Im thinking of pulling the muffler off from the band clamp and re-shaping the intermediate pipe between the muffler and the cat. I ALSO thought of removing the cat... BUT then I have to deal with the fingerstick and blocker... I dont want codes... obviously!

Will I have enough back pressure if I get rid of the cat and muffler on a stock 3.5" system? OR will I need to replace the stock muffler with an aftermarket!


ALSO... ANYONE know who can bend 3.5" or 4" pipe in the NE Indiana area! I will travel if need be!

I don't know of any kind of engine that needs back presure. You need to build velocity. If we could all run just a 4" stack out of the hood that would be great. When we have to run 17' of tubing to rid of the exhaust the last thing you ever want is back presure. What happens when you go too big too quick you loose all of you velocity. The exhaust will slow down and cool off. The exhaust will have to be pushed down the pipe by the next pulse or the turbo. It's the same affect as being too small and restrictive. The trick is finding the right diameter of tubing and length to acomplish this. Unfortunatly the GM's are limited as to what you can do right off the turbo. If you can build a velocity greater than what the turbo produces, you'll see much quicker spooling. Thanks, Jerod Jardine

hey Jerod, do you guys make the Jardine motorcycle exhaust as well?

the only engines that i can think of that need back pressure are two-stroke engines. look at any expansion chamber on a racing dirt bike. the pipe is large and the outlet is a small diameter pipe that they call a stinger. I makes use of pulses and back pressure.

An expansion chamber on a two stroke actually turns the exhaust around and re-burns it. The pipes differant diamaters are tuned to acomplish this. They also use sound pulses. freed929, The Jardine you're talking about is our old company. We sold that off in the mid eighties. It's owned by Sumit industries. Not the catalog Summit. They own several other companys in the automotive and motorcycle industy. Thanks, Jerod

The stinger acts as a pressure bleed, allowing pressure to escape from the pipe. Back pressure in the pipe, caused by a smaller-diameter or longer stinger section, helps the wave action of the pipe, and can increase the engine's performance.

http://www.motorcycle.com/mo/mcnuts/em-pipes.html

I agree with Cheyenne19 you dont need any back pressure post turbo

Like Jardine said, if we all could just run a short exhaust pipe straight off the turbo out the hood like on tractor pull trucks, we would be better off, but we can't. Any normal aftermarket exhaust bolts on to the down pipe (which is just bigger than 3") so you are getting all the velocity you would need. Then, it expands into 4" front pipe for clearance, and after the crossmember, it goes to 5".
So basically, that small downpipe that comes on all our trucks blows your "not enough backpressure" theory out of the water. In reality, it would be better if that downpipe was bigger.

When I had my 4" turbo back on, 1 thing i noticed is that i lost lowend torque. High end was ok! Put my stock back on, and WOW! What a difference it made to the power in low/high, Say what you want,but these trucks runs better w/ little back pressure. also pulls more boost. W/my boost vavle w/4'' I could pull 28 psi w/stock exhaust 32psi

When I had my 4" turbo back on, 1 thing i noticed is that i lost lowend torque. High end was ok! Put my stock back on, and WOW! What a difference it made to the power in low/high, Say what you want,but these trucks runs better w/ little back pressure. also pulls more boost. W/my boost vavle w/4'' I could pull 28 psi w/stock exhaust 32psi

What kind of dyno was that on?

What kind of dyno was that on?


I didnot dyno it???? I could tell by the way it would not get up,and go,vs the stock when it would run off the line like a raped ape!

I disagree, when i cut out my cat and muffler it was a difference of night and day in spool up, power and egts. Yeah i lost a couple of psi in boost but thats to be expected, boost is a measure of restriction. Take away the restriction loose boost. The bottom end is where i noticed the extra power the most the power came on much much faster!

I disagree, when i cut out my cat and muffler it was a difference of night and day in spool up, power and egts. Yeah i lost a couple of psi in boost but thats to be expected, boost is a measure of restriction. Take away the restriction loose boost. The bottom end is where i noticed the extra power the most the power came on much much faster!
Hmmm, not really following what you are saying here. By restriction, you mean that you are cramming more air into a fixed volume cylinder? What does a little more free flowing exhaust have to do with this? I am just trying to understand what you are saying.

A slight amount of back pressure keeps more heat in the turbo and lights it slightly sooner....

Dan

What do you mean by lights it?

I didnot dyno it???? I could tell by the way it would not get up,and go,vs the stock when it would run off the line like a raped ape!I tried a full 4" exhaust on my 03, you don't need a dyno to feel the loss in torque. My 1/4 mile trap speed droped almost 3 mph. I put my stock front pipe back on and left the 4" muffler and tailpipe in place and it picked back up. Lots of people will argue that this cannot be true, I got flamed really bad for saying this before, but I know for a fact the 03's don't like a 4" front pipe mine was sick with it on. When I had my exhaust issue I did'nt have any other mods. Things may have been different with the bighead, intake and 6-gun.

Lights= spools up

read here: http://www.dieselplace.com/forum/showthread.php?t=135669

Yeah I read all that before. Just trying to understand how some backpressure would allow the turbo to spool more quickly. How does keeping a little heat in the turbo do shorten spool time?

As far as I can tell---back pressure caused by down-pipe or front-pipe restriction simply cannot help anything.

I have yet to read any sensible explanation or actual test measurements to prove this wrong. Exhaust velocity is not affected positively by down stream restriction so how exactly can it help to spool the turbo.

I can see where some restriction entering the turbo may increase velocity if it acts like a nozzle in a rocket engine. That would have to do with the opening where gasses enter the turbine to drive the fan. This would be unaffected by any modification to the system exiting the turbo assembly.

Slight back pressure with quicken spool time, but will hamper high flow power.

You still have not given an explanation as to why slight backpressure will quicken spool time. I understand the high flow power bit, but not the first part.

I wish Diesel Power Magazine would read this thread and do an article testing everything to do with back pressure, exhaust size, etc...

I wish Diesel Power Magazine would read this thread and do an article testing everything to do with back pressure, exhaust size, etc...

Diesel Power is just one big ad, they usually don't do things like that.

I find it interesting how NO vendor wants to make ANY bolt on 3.5" muffler delete pipes or cat pipes... They all just wanna push these 4" and 5" kits. I have said it for some time... Some of these exhausts are too open...
5" is over-kill... You need to have some turbulence in your system... ALOT different than the earlier motors without all the electrical ####. The only thing I dont like on the stock exhaust is the restrictive bends in the back pipe (by the support).

You still have not given an explanation as to why slight backpressure will quicken spool time. I understand the high flow power bit, but not the first part.

I am with you.

All I see is declaratives in any of these "backpressure is good" threads. No explanations and no tests. I remain unconvinced that any backpressure or restriction in the exhaust system from the turbo back (downpipe, front pipe/cat, exhaust pipe, or muffler) can have any positive effect on spooling, boost, torque, power or anything except possibly (muffler and cat) to make the truck less noisy.

I wouldnt believe it either but on my LBZ it seemed to have lost some outa the hole when I went from stock to 5". I couldnt believe it. My "test" was after I did the exhaust it couldnt light up my rear tires like I could before just nailing it from an idle. I'm normaly not into SOTP testing but it did seem less peppy down low. Once you got moving you couldnt tell. Dont ask me to explain why as I cant :think:

I don't doubt what people are saying, I would just like an explanation. The reason this happens is not intuitive to me, and I would like to know why.

Its very simple, I don't know why you guys doubt that a little back pressure is ok. I rode in a truck with a 4" exhaust, this one, and then the another truck, with pretty much the same mods, with a 5" exhaust, and this one seemed faster ou tof the hole.

... I don't know why you guys doubt that a little back pressure is ok. I rode in a truck with a 4" exhaust, this one, and then the another truck, with pretty much the same mods, with a 5" exhaust, and this one seemed faster ou tof the hole.


How about because SOTP is proved to be horribly inaccurate and that there are no rational explanations offered for physically how the backpressure would be a performance booster?

Sorry I still call bovine excrement on this whole idea until some evidence or rational explanation is offered.

Yeah I read all that before. Just trying to understand how some backpressure would allow the turbo to spool more quickly. How does keeping a little heat in the turbo do shorten spool time?

Heat=Energy, more energy=more velocity, more velocity=more PSI (in very simple terms).

If the exhaust loses too much heat, the exhaust has less energy to spool the turbo. It also slows down the gas exiting the pipe. It sounds weird, but even though we're talking about back pressure behind the turbo, it will cool the gas enough to lessen the velocity through the turbo.

I have a banks intake, 4" MBRP cool duals without the catalytic converter. I have 285 tires and have never been able to spin the tires without first getting boost, the truck was completely stock when i bought it. this past weekend i installed the muffler delete pipe. just from removing the muffler if i punch it i start to feel and hear slippage at the tires all the way through 4th gear. my fuel mileage is also now the highest that i have ever seen it on my truck.

Heat=Energy, more energy=more velocity, more velocity=more PSI (in very simple terms).

If the exhaust loses too much heat, the exhaust has less energy to spool the turbo. It also slows down the gas exiting the pipe. It sounds weird, but even though we're talking about back pressure behind the turbo, it will cool the gas enough to lessen the velocity through the turbo.Also where the front pipe necks up from 3.5" to 4" there is a disruption in air flow which really screws up velocity.

Heat=Energy, more energy=more velocity, more velocity=more PSI (in very simple terms).

If the exhaust loses too much heat, the exhaust has less energy to spool the turbo. It also slows down the gas exiting the pipe. It sounds weird, but even though we're talking about back pressure behind the turbo, it will cool the gas enough to lessen the velocity through the turbo.
OK, I think that I understand now. The cooler exhaust flow has a lower velocity thereby increasing the spool time slightly. I was thinking strictly in terms of exhaust velocity being the same in both cases. Since a cooler gas occupies less volume than one that is hotter, the velocity of the cooler gas will be lower, thereby allowing for a longer spool time. Hope this is a correct interpretation of what you are saying.

How about because SOTP is proved to be horribly inaccurate and that there are no rational explanations offered for physically how the backpressure would be a performance booster?

Sorry I still call bovine excrement on this whole idea until some evidence or rational explanation is offered.

How can you say thats "Inaccurate" I could tell as soon as i put my turbo back 4" on. It was sluggish, as before w/the stock it would sit you back in the seat out of the hole,

Heat=Energy, more energy=more velocity, more velocity=more PSI (in very simple terms).

If the exhaust loses too much heat, the exhaust has less energy to spool the turbo. It also slows down the gas exiting the pipe. It sounds weird, but even though we're talking about back pressure behind the turbo, it will cool the gas enough to lessen the velocity through the turbo.

Bingo! we have a winner. You typed almost word for word what i was going to type last night..... LOL

OK, I think that I understand now. The cooler exhaust flow has a lower velocity thereby increasing the spool time slightly. I was thinking strictly in terms of exhaust velocity being the same in both cases. Since a cooler gas occupies less volume than one that is hotter, the velocity of the cooler gas will be lower, thereby allowing for a longer spool time. Hope this is a correct interpretation of what you are saying.

You guys are "on it".


By the way we offer 3.5" cat delete pipes (for off road use of course):D

Heat does not translate to higher physical energy (velocity). Heat is heat and turbos work off of physical energy - pressure. The heat differential from engine gas to exhaust gas would not cause lower velocity.

A pressure differential pre-to post turbo is what will spool the turbo faster. The higher the incoming pressure and the freer the outgoing pressure the faster that turbo will spool. Therefore, any post turbo back pressure is going to work against spool up. The laws of fluid dynamics dictate this to be the case and common sense says the faster gas can get out of the turbo the more gas can come into the turbo and the faster that fan will spool and turn.


LWatson - the neck up from 3.5 to 4 at the front pipe won't cause more back pressure than having 3.5" all the way back. Stepping pipe diameter up relieves back pressure - it never increases it. Also, if it did increase it why wouldn't that be a good thing? ;)

You guys can go on believing this stuff about needing back pressure but so far I have heard not one explanation that jives with the laws of thermodynamics, mechanics, or fluid dynamics.

Think of the engine as a stadium full of people and the people are the exhaust gas molecules. At the exits there is a long hallway 10 feet wide. At the end of the hall there is a turnstile. The hallway is the pre-turbo exhaust route and the turnstile is the turbo. After the turnstile you can put either a 10 foot-wide hallway or you can out a 20 foot wide hallway. If there is a fire inside the stadium and everyone wants out - which system will produce the faster turnstile spool up and speeds?

In what way would putting an 8 foot hall way after the turnstile help get people out faster and turn the turnstile quicker?

This is sort of what I was thinking, which is why I was looking for an explanation.

LTChip I did'nt say necking down from 3.5 to 4" would increase back preasure, what I ment was the sudden change in diamiter caused a disruption in the flow and had ill effects on velocity. I see you have an LB7 and a 4" exhaust, I'm sure you did'nt throw your stock front pipe away so why don't you do a little test. Go to your local drag strip run your truck with the full 4" system, install the stock 3.5" front pipe and run it again. I did'nt do my test on a track I wish I did, I had a friend clock me with a radar. I know this does'nt add up, so just put your calculator asside and do some real world testing. You seem very knowlegable about turbo diesels I'm not trying to insult your intelligence, this does'nt seem logical to me either, but my 03's performance suffered with a 4" front pipe.

Heat=Energy, more energy=more velocity, more velocity=more PSI (in very simple terms).
.

That is one of the most innaccurate things I have ever read, to put it nicely. Heat IS energy, but it is a byproduct of a reaction, therefore it is an inefficient energy. Everything else doesn't apply after that.

Dosnt going up to 4" at the flange cool the exhaust as it expands there? :D Dont think its adding turbulence but its velocity will slow every time its opened up. It will also speed up any time its necked down ;)

Like I said I cant explain it and I hate to use SOTP.....SOTP it felt like it was a bit weaker from a dead idel nailing it. But my "proof" or "testing" was on my LBZ bone stock I could get scratch from a no boost nail it and go launch, i put the exhaust on it and I couldnt get it or it would happen a bit latter than it did stock. Like I said....dont ask me to explain. Some have reported the oposite. I dont know....all I know is what happened on my truck. That "test was also with just exhaust and no blocker plate or finger stick but I didnt noticce anything one way or the other realy with that stuff installed.

If I had to bet i would think alot of it may be programing related with the VVT? I dont remember my LB7 acting that way.....

Dosnt going up to 4" at the flange cool the exhaust as it expands there? :D Dont think its adding turbulence but its velocity will slow every time its opened up. It will also speed up any time its necked down ;)

Like I said I cant explain it and I hate to use SOTP.....SOTP it felt like it was a bit weaker from a dead idel nailing it. But my "proof" or "testing" was on my LBZ bone stock I could get scratch from a no boost nail it and go launch, i put the exhaust on it and I couldnt get it or it would happen a bit latter than it did stock. Like I said....dont ask me to explain. Some have reported the oposite. I dont know....all I know is what happened on my truck. That "test was also with just exhaust and no blocker plate or finger stick but I didnt noticce anything one way or the other realy with that stuff installed.

If I had to bet i would think alot of it may be programing related with the VVT? I dont remember my LB7 acting that way.....My LB7 acted exactly like your LBZ, my LB7 was also bone stock when I installed my exhaust.

As far as I recall, my LB7 got quicker and louder when I went from a stock system to my 4". Trouble is that I added the Edge the same day so I don't have any recollection of any power loss.

I can think of a way you might explain the reported but not measured low-end power loss in a rational and consistent way.....

How about the turbo comes on too fast? Yup - exhaust is freer...turbo spools up too much for the fuel early in the power curve and eats engine power rather than adds to it. This is why you can easily tune for this thus making a 4" nothing but positive so long as you are not running stock tuning. BTW - I am not convinced that this is the case but at least my explanation of your reported experiences fits with the physical realities of the system in question and is not based on some voodoo magic where back pressure causes improved turbo performance.

As far as I recall, my LB7 got quicker and louder when I went from a stock system to my 4". Trouble is that I added the Edge the same day so I don't have any recollection of any power loss.

I can think of a way you might explain the reported but not measured low-end power loss in a rational and consistent way.....

How about the turbo comes on too fast? Yup - exhaust is freer...turbo spools up too much for the fuel early in the power curve and eats engine power rather than adds to it. This is why you can easily tune for this thus making a 4" nothing but positive so long as you are not running stock tuning. BTW - I am not convinced that this is the case but at least my explanation of your reported experiences fits with the physical realities of the system in question and is not based on some voodoo magic where back pressure causes improved turbo performance.Your explination is as logical as any I've heard. You mention reported but not measured low-end loss. When I installed my system it felt sluggish, I installed it at a friends shop who also owned an x patrol car that still had the radar in it. The 1/4 mile is marked off on the road in front of his house, he clocked me running 78 mph with the full system. I put my stock front pipe back on and left the 4" muffler and tail pipe in place. One hour, mabe a little less he clocked me at 81 mph. I know this is'nt a very accurate way to test but at the time I was looking for anything just to make sure I was'nt going crazy. By the way putting the exhaust issue asside I still think I'm going crazy:eek:

You are right - your clocked runs are a type of measurement that do tend to show some mal-effects of going 4" but it definitely wasn't a controlled study. Were you doing boosted launches or did you start from idle?

You definitely have me thinking about this and I am not sure I buy my own explanation because I am not sure how it would be robbing power exactly. I am also still not convinced it is robbing power.

I don't do a lot of LLY tuning but I do know that bringing the turbo on too early can cause poor mileage so there has to be a negative effect of too much air (actually it would be too much turbo drag early in the power curve). With LB7s you cannot tune the turbo directly (fixed vanes) so you tune boost basically by tuning how much fuel, timing etc. there is (and therefore how much exhaust pressure there is).

If this effect is real and my theory is correct, then it would be the case that some power is robbed by too much turbo drag or something like that.

I won't be satisfied until I figure this out.

Need to think about tuning an LLY some more. The answer might be there.

SOOOO, if what you guys are saying a non programed LBZ performs better with a stock exhaust. But a LBZ with a prgrammer will perform better with a 4" downpipe back because the programmer allows enough fuel to compensate for the loss of backpressure and therefor the motor produces more raw power without the turbo and then you can't tell you lost anything in the lower rpm's. Then when the turbo spools up in the higher rpms you really feel the power. Thus keeping the egt's lower and allowing more right pedal for an extended time than a programmer with stock exhaust. Does this sound even remotely plausable? This is why I love this forum and read it with the intensity worthy of a divorce from my wife!

An exhaust will help a "tuned" truck more than a stock truck. I also belive the stock truck will see an overall improvement from an exhaust. To see the full potential of your tune you should run a 4" exhaust, to see the full potential of your 4" exhaust you should tune your truck. The whole back pres. deal has been hashed over quite a bit and I think years ago it's goten confused with velocity. Thanks, Jerod

Some things to think about:

In a fixed volume, cold air weighs more than warm air. Warm air weighs more than hot air.

Your tailpipe never really gets "hot" compared to the downpipe. The air cools rapidly as it travels down the exhaust pipe. The more surface air of the pipe, the quicker it cools.

It takes more energy to move something heavy than something that is light. Cold air is HARD to move, hot air is easy.

Turbo race cars run very, very short exhausts for a reason, and you will see them wrap the feeder pipes to keep them from bleeding off heat. Heat is your friend when it comes to getting gasses out of an engine.

"Backpressure" is never good in a turbo engine. What some are calling backpressure is simply keeping the gases hot so they move easier.

We can make stupid power on a 4" exhaust. But we can make even more with a short 4". "Casper" doesn't lack torque, yet has very little backpressure.

[quote=McRat;1690558;]Some things to think about:

In a fixed volume, cold air weighs more than warm air. Warm air weighs more than hot air.

It takes more energy to move something heavy than something that is light. Cold air is HARD to move, hot air is easy.

Some interesting points, however, one has to be careful how this is interpreted. It is true that cold air is more dense, but the actual volume of air (or in this case exhaust gases expelled by the engine) is going to be roughly the same whether you have a 4" pipe or a 31/2" pipe. Since the starting volume is the same, when you compare a 4" to a 31/2" pipe, the mass of exhaust gas exiting is the same. To satisfy the gas laws, the velocity of the gas in the 4" pipe is lower, and the temp also drops as the gas slows down, however, the mass (or weight) remains the same. This is where I disagree with the second bold statement above. I do agree with the explanation about the lower backpressure allows the turbo to spool more quickly, thus increasing the drag produced without the fuel to compensate. This makes the most sense to me at this point. Just my thoughts at this point.

[quote=LTChip;1689677;]You are right - your clocked runs are a type of measurement that do tend to show some mal-effects of going 4" but it definitely wasn't a controlled study. Were you doing boosted launches or did you start from idle?

When I clocked my runs it was from a dead standstill from idle. When I added my exhaust I had put about 10,000 miles on my truck with no mods. I instantly could tell without a doubt my truck was running bad, it did'nt feel a little slower it felt allot slower.

I will go to track and tell you what mine did as soon as weather is good.
I went from stock with out muffler to 5 inch all the way with out cat and muffler. Sounds wicked, lots of turbo sound, turns heads like crazy, have to watch for law. It went 14.17 at 97.8 before at 58 degrees and about 50-60 % hum. If it still runs that or better at a warmer temp. well out goes all the guesses.Watch for results in my sig. Zank

I wouldn't worry about it. Backpressure with a standard exhaust does different things than it does with a turbo. It doesn't much matter anyway because that pipe from the turbo down is a squashed restrictive-looking thing and changing it would be a whole lot of trouble.

Going to the 4" cat back made almost no difference in loudness. It did add a little power.....not much, just enough to where I could definitely feel it. Later adding a 4" cat delete front pipe gave about the same amount of power increase, but the turbo definitely spooled quicker. That front pipe is wrapped with exhaust header tape.....this was done to keep heat away from the transmission hydraulic lines.

The exhaust was done long before installing the Edge box.

Part of this "loss of power" could be because of the hp and tq curve being skewed upward. Either way there is no evidence that backpressure is needed.

The only explanation of why there may be some loss in low end power that makes some sense is that if you are running a factory truck with an exhaust upgrade is that the turbo will spool up faster and the engine doesn't get enough fuel to match the additional air being pushed into the engine. When I put my 5" on, I noticed an increase in low end, and a much faster spool up. I was running a programmer though. There is NO possible way that having a smaller exhaust would cause the turbo to spool up faster.

The other point of the physics of hot air vs warm air weighing less, and therefore making it easier to move...do you honestly think that the slight difference (very slight in the sceme of things) in air temperature is going to make any difference in HP or torque? My 6" exhaust tip still gets hot after driving my truck. Too hot too touch in fact. So I doubt that if it were a 3 1/2", it would be much hotter.
The reason race cars wrap their exhaust is to keep the extreme radiant heat from catching stuff on fire! The headers on race cars get to 1500 degrees and they run underneith the drivers feet. If they didn't wrap them to minimize the amount of heat they give off, it would cause major problems. And they usually only wrap front engine cars. They don't wrap exhausts on INDY or F1 cars because they aren't close to the driver.

People just need to think about how things work a little more. If you put your finger over the end of a hose, the speed of the water coming out is much higher than if not, but you aren't allowing as much water out as if you didn't have your finger there. So just think about if their was a turbine in that hose. It would spin at a much higher rate if you didn't have your finger at the end of the hose because you aren't restricting the flow of water behind your finger. It is simple.

I just did the downpipe on mine. I think it made a noticable difference In a positive manner. I now have a 4 inch straight through exhaust. Its not too loud and I like it. I cant explain how it works. I do know my egts are lower. I thought that was a good thing.
Jon

if we install a bigger down pipe and lower exhaust temps
wouldnt that now need more pressure to push it out?

so now we raise the hp on the trucks and increase the pressure now we are back to square one only with higher horse, does this sound right?

LTchip, you mean we have less differential pressure with the bigger down pipe correct?

People just need to think about how things work a little more. If you put your finger over the end of a hose, the speed of the water coming out is much higher than if not, but you aren't allowing as much water out as if you didn't have your finger there. So just think about if their was a turbine in that hose. It would spin at a much higher rate if you didn't have your finger at the end of the hose because you aren't restricting the flow of water behind your finger. It is simple.
I've used that exact same analogy before.
And no, it won't take more pressure to move more volume through a big pipe. It takes less. That's what this whole discussion is about.

I've used that exact same analogy before.
And no, it won't take more pressure to move more volume through a big pipe. It takes less. That's what this whole discussion is about.


ohhhh sorry, I believe in let er breathe but we have all read in this post that hotter temps flow easier now you cool the flow what is this doing?

i dont know and i am asking what people think about it,maybee im just overthinking it :D

Cooling the air down certainly doesn't have much effect on a 3' long race exhaust, but on a 15' long truck exhaust? Dunno, but nobody has shown a HP increase by running a 5" pipe back to the bumper or duals, so there must be some reason for it. First time I saw someone wrap headers was on the feeder pipe of a turbo bike. He said it was worth .2 in the 1/4mi on his 8 second bike.

ohhhh sorry,:D
I'm sorry, after reading my post it sounded short. By moving the same volume through a bigger pipe velocity will slow as will backpressure. It might move faster if hotter, but I think it would be very small. Sorry for sounding short[-][/-]

if we install a bigger down pipe and lower exhaust temps
wouldnt that now need more pressure to push it out?

so now we raise the hp on the trucks and increase the pressure now we are back to square one only with higher horse, does this sound right?

LTchip, you mean we have less differential pressure with the bigger down pipe correct?


I mean you have higher differential pressure -and better turbo performance with a bigger post turbo exhaust system.

From the engine to the turbo-fan I think there is a certain amount of pressure tuning that can be done by fooling with the diameter, routing, and shape of the orifices (think rocket engine nozzle - or even hose nozzle). That preturbo gas velocity is important - it must have the right pressure to optimize the turbo spinning. That means the right volume and velocity of gas. With a fixed fin turbo the physical energy of the gas (pressure/velocity/volume) is the only factor. With a VVT you can tune the turbo to the appropriate gas pressure.

But after the energy (pressure) is expended on getting the turbo spinning - the gas just has to get out of the way so bigger/freer exhaust (downpipe, frontpipe, exhaust pipe, muffler etc.) is better.

IMO the idea behind using a bigger exhaust pipe isn't to gain power as in a gasoline engine, but to provide the ability to produce more power. a smaller exhaust will work up to the point where it canot allow the exhaust gasses to escape rapidly enough. this is the point where power production is haulted due to rising EGT. with a larger pipe diameter, more exhaust gasses can escape more quickly, thereby lowering EGT, and alowing more power to be produced without damage to moving parts. the larger exhaust, for the most part, does not produce more power. it simply allows more power to be produced by other modifications.

Just my thoughts,
Mike

I've used that exact same analogy before.
And no, it won't take more pressure to move more volume through a big pipe. It takes less. That's what this whole discussion is about.


That analogy is somewhat incorrect according to Bernoulli's Principle.The conservation of mass states that mass can neither be created or destroyed - so what goes in one end of the tube must come out the other end of the tube. The mass does not change as it moves through a restriction unlees the input mass is changed As a result, the velocity must increase to get that mass of air or liquid through. This is also known as the Venturi Effect.

That analogy is somewhat incorrect according to Bernoulli's Principle.The conservation of mass states that mass can neither be created or destroyed - so what goes in one end of the tube must come out the other end of the tube. The mass does not change as it moves through a restriction unlees the input mass is changed As a result, the velocity must increase to get that mass of air or liquid through. This is also known as the Venturi Effect.
To correct my statement, I meant to say the "same volume through a bigger pipe"
And I was in no way reffering to the venturi effect. I didn't mention mass disappearing. I just stated a bigger pipe could flow more at a slower speed.

We've done allot of dyno work with diferant diameters and lenghts on down pipes. I think it would really surprise allot of you. Their is quite a bit of tourq and horsepower to be made with diferant diam. BEHIND the turbo. On the VVT turbo if it jumps to four inch right away the turbo has nothing to work against. If it's too small it takes the vanes longer to change their geometry and start to build boost. Their's allot of therory out there, but the dyno and 50 yrs experiance throw allot of it right out the window. Thanks, Jerod.

here is a spread sheet i built to determine the difference in gas flow
and tempatures

the numbers are just ball park as i have no way to determine actual Diff pressure....yet or orfice size

this is not to be taken litterly its just showing how temp affects gas flow:D




yes, i constantly over think things

Why are we discussing high flow/High hp??? We are only discussing spool time vs Back pressure. We all know back pressure will hurt high performance setups.

To correct my statement, I meant to say the "same volume through a bigger pipe"
And I was in no way reffering to the venturi effect. I didn't mention mass disappearing. I just stated a bigger pipe could flow more at a slower speed.


A thousand apologies. I could only go by what was stated in your thread about when a restriction (finger) is placed over a water hose then there is lesss water coming out. I was just making sure that it was understood that there is the same amount of water coming out.

A thousand apologies. I could only go by what was stated in your thread about when a restriction (finger) is placed over a water hose then there is lesss water coming out. I was just making sure that it was understood that there is the same amount of water coming out.
No worries, I need to explain myself better. You're right though, same amount just higher velocity.

what goes in one end of the tube must come out the other end of the tube. The mass does not change as it moves through a restriction unlees the input mass is changed As a result, the velocity must increase to get that mass of air or liquid through. This is also known as the Venturi Effect.

That is only true if the force behind the restriction remains constant, but that does not happen in real world applications. Look at a hydrolic dam. If they open up one gate, a little bit of water comes out. But if they open up 5 gates, more water comes out. That is back pressure. The same goes for the hose analogy. If you are only letting the water out a pin sized hole, it will not allow the same amount of water to get out as if you had no restriction. The water would simply slow down behind the blockage, and pressure (back pressure, what we have been talking about) builds up. This happens because of surface tension in water, and to a lesser degree in gasses, friction. Take away friction (which makes the Venturi Effect correct in this case), and my explanation goes out the window.
Another example is with a syringe. Cut the tip off a syringe to make the hole bigger, and it takes less effort to push down the plunger than if you tried to push the same amount of liquid (or air) out of the original small hole. Once again, friction.

Nay hit the nail on the head. In theory volume in equals volume out, but that is in a frictionless environment.

Put 5 inch on and I can feel it lugging when it goes into 6th gear. MPG has dropped at a steady speed lets say 60 mph. Was 22-24, now 18-20.

A week ago with kitty and no muffler with 3.5 inch exh. I ran 3 runs av.
14.24 and 97.78 mph temp 58 deg and 50 hum. Short times 2.0's

To nite 5 inch, temp 72 and hum 75 it ran one run at 14.43 and 95.4 mph. Had the best short time ever 1.92 Now my running gas eng. the different in temp and hum. would be .l-.2 slower.

It feels great out the hole, mite be time for lift pump.

What do you all think about wraping the first 6 feet or so with header wrap??

Then I ran bully dog ext and lp the 1/4 mile and it ran 13.78 at 98.52 mph

The trans has yet to limp??? Getting close???

Does no one have actual dyno numbers to prove any of this? I dynoed my truck w/ cold air and stock exhaust and made 285/465. I then put on the 5" exhaust with 4" inter. pipe and saw 295/505. I gained torque at 1800 rpm and hp at 3200 rpm. The 5" didnt hurt me on top or bottom. And people worrying about the lack of temp due to the lack of "restriction", my tailpipe was still at a blistering 400 degrees with my temp gun. I think these motors push our enough gasses that it isnt an issue. But I am also still running a stock downpipe :mad:
just my .02

Does no one have actual dyno numbers to prove any of this? I dynoed my truck w/ cold air and stock exhaust and made 285/465. I then put on the 5" exhaust with 4" inter. pipe and saw 295/505. I gained torque at 1800 rpm and hp at 3200 rpm. The 5" didnt hurt me on top or bottom. And people worrying about the lack of temp due to the lack of "restriction", my tailpipe was still at a blistering 400 degrees with my temp gun. I think these motors push our enough gasses that it isnt an issue. But I am also still running a stock downpipe :mad:
just my .02

THANK YOU!!!! Finally some actual proof to go along with common sense and basic physics. If others did the same thing, they would find the same results! I would hope that all this "air temp", "back pressure", etc... b.s. will stop.

THANK YOU!!!! Finally some actual proof to go along with common sense and basic physics. If others did the same thing, they would find the same results! I would hope that all this "air temp", "back pressure", etc... b.s. will stop.295 hp on a lb7, he's got a tunner or programer. The 4" front pipe killed my power on my lb7 but it was before I installed the 6-gun. I never tried 4" front pipe with my tunner, results would probably been different. Do you think the people who have experienced a loss in power with an aftermarket exhaust just sat down and made up a bunch of b.s. for the heck of it? They did'nt, just because it does'nt sound logical to you does'nt mean theres no truth to their claims. I'll admit I'm completely baffeled, I've got no explanation for my trucks loss in power, but don't you think if someone is going make up a lie it would be about more hp instead of less hp?

295 hp on a lb7, he's got a tunner or programer. The 4" front pipe killed my power on my lb7 but it was before I installed the 6-gun. I never tried 4" front pipe with my tunner, results would probably been different. Do you think the people who have experienced a loss in power with an aftermarket exhaust just sat down and made up a bunch of b.s. for the heck of it? They did'nt, just because it does'nt sound logical to you does'nt mean theres no truth to their claims. I'll admit I'm completely baffeled, I've got no explanation for my trucks loss in power, but don't you think if someone is going make up a lie it would be about more hp instead of less hp?
No I don't think it's made up. I think it's a sotp deal that some people are stating as fact. "Well I know my truck lost power" and things like that. Which basically makes it false, until they can back it up on the lie detector.

You still have not given an explanation as to why slight backpressure will quicken spool time. I understand the high flow power bit, but not the first part.


When i had my turbo back 4" exhaust,I could only pull 28Ibs of boost This was w/my boost increase vavle, w/stock exhaust i can pull 33Ibs.This was w/my bullydog programmer on extreme. I dont understand it myself , but the boost gauge dont lie.

No I don't think it's made up. I think it's a sotp deal that some people are stating as fact. "Well I know my truck lost power" and things like that. Which basically makes it false, until they can back it up on the lie detector.I did'nt go to the dyno or the track, but I ran my truck on a marked 1/4 mile on a back road, lots of people race here it's in the country and no traffic. A friend has an x-patrol car with a radar he clocked me on several runs, my truck ran a consistant 81mph stock. With the 4" Banks exhaust it ran a consistant 79mph. I have no way of knowing what my times were, but the truck felt weaker and ran slower acording to the radar. Like I stated before my truck was completely stock at this point. There is no doubt in my mind a tuned truck will benefit from a 4" exhaust.

THere are dyno numbers, but track numbers are what count. I have heard of an LLY with EFI Live tuning, and they were experimenting with tunes, one dyno'd 445, and on the same day, same dyno, another dyno'd 488 and the 445 tune ran faster at the track.

THere are dyno numbers, but track numbers are what count. I have heard of an LLY with EFI Live tuning, and they were experimenting with tunes, one dyno'd 445, and on the same day, same dyno, another dyno'd 488 and the 445 tune ran faster at the track.
You're right, the track is where it counts. But afterall tuning, is all we are talking about. Using the exhaust to tune with.