2000Accord5sp

If my downpipe won't fit unless we use 2.5" pipe until after the oilpan, would it be pointless to widen it out to 3" after that? The oil pan is too close to the front cross member and a 3" 90 degree bend just won't fit.

Kack

no i did the same, cept mine flared out halfway down the engine block not below the pan. Gave me 50 horses compared to 2.25 in piping with stock cat.

BoOsTiN Dc2

im planning on doing the same thing and i want to know what people think also

FOrSfEd

2.5-3" gives you really good spool times and good power. 3-3 gives a lil slower spool times but better power.

BROOD

2.5" to 3" transistion downpipes work really well. They merge into a full 3" exhaust easily, clear areas near the engine that a 3" has difficulty doing, makes bolt clearange on 4bolt turbine housings easier than 3" DP's, etc.
I would opt for the full 3" DP only if you were concerned with extracting max power from your set up.

IntegracinGSR

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by FOrSfEd &raquo;</TD></TR><TR><TD CLASS="quote">2.5-3" gives you really good spool times and good power. 3-3 gives a lil slower spool times but better power.</TD></TR></TABLE>

With a 3-3 I thought that it gives you really week bottom end until it starts spooling but then would spool up faster once it gets going.. and on a 2.5-3 the bottom end would be better than a 3-3, but the 2-3 would spend more time spoolin up to full boost.. enlighten me..

951

2.5 - 3 is better, your downpipe(if sized properly) should be a little smaller than subsequent sections of the exhaust. the gas getting pushed thru the downpipe has a TON of velocity, and moves alot quicker than the gas after it has passed thru the cat, muffler etc. to relate, if you've got traffic moving at 125, they're going to need less lanes than traffic traveling 45, because they're moving faster. conversly, when your DP is too big, the gas expands and cools a tiny bit quicker, causing it to lose its velocity, and create more restriction in the exhaust as a whole. if your DP is too small, at high RPM's(like frosted said) its going to restrict because theres just tooo much gas trying to stuff itself through the DP.


Modified by 951 at 5:16 PM 2/2/2004

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by FOrSfEd &raquo;</TD></TR><TR><TD CLASS="quote">2.5-3" gives you really good spool times and good power. 3-3 gives a lil slower spool times but better power.</TD></TR></TABLE>

Wrong.

The larger the exhaust, the faster the turbo will spool, regardless.

However, a larger exhaust *might* mean less torque out of boost because of lesser scavenging. I doubt that would be the case though, with the turbo restricting the exhaust.

Bigger means faster spool and more power, always.

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 951 &raquo;</TD></TR><TR><TD CLASS="quote">2.5 - 3 is worlds better, your downpipe(if sized properly) should be a little smaller than subsequent sections of the exhaust. the gas getting pushed thru the downpipe has a TON of velocity, and moves alot quicker than the gas after it has passed thru the cat, muffler etc.</TD></TR></TABLE>

WTF?

You're saying that you want a smaller downpipe is better because you want a higher velocity? Why not use a 1" downpipe then, you can break the sound barrier yO.


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">to relate, if you've got traffic moving at 125, they're going to need less lanes than traffic traveling 45, because they're moving faster.</TD></TR></TABLE>

WTF? (I'm beginning to see a pattern here).

Why the hell does traffic moving faster need less lanes? That makes no sense at all. None. No matter how fast cars are going, they are always the same width...


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">conversly, when your DP is too big, the gas expands and cools a tiny bit quicker, causing it to lose its velocity, and create more restriction in the exhaust as a whole.</TD></TR></TABLE>

If it expands, it will lose velocity? Wrong. If it expands, the volume will be larger but the mass flow rate will be the same, which requires a HIGHER velocity.

I also would like to know how the rate at which the gasses cool (dT/dt) is related to the diameter of the downpipe?


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">if your DP is too small, at high RPM's(like frosted said) its going to restrict because theres just tooo much gas trying to stuff itself through the DP.</TD></TR></TABLE>

Big = good. Done.

IntegracinGSR

Thanks for clearing that up KPT.. i also coulda swore that the larger piping would give faster spool times along with more power.. but less power when out of boost..

Bailhatch

TurboXS makes an exhaust-cat-DP combo for WRXs. It starts as a 4" DP then a 3" cat and mid pipe then a 2.5" muffler rear section. What is the theory behind that? it sounds nice and adds a ton on WHP on a stock car.

DaX

iTrader: (1)

Like Pontiac says, wider is better.

IntegracinGSR

That piping is still large piping.. aside from it seemingly being capped off at the end with a 2.5" exhaust.. however it will still flow well.. the only reason i can see for doing this is so that you still get fairly good bottom end.. along with spooling benefits of larger piping of the DP and Cat..

951

no, i said you want a properly sized downpipe, ill get to what properly sized means in a second.

okay, ill clear this up a bit. if you have x amount of cars on two seperate highways, a 2-lane highway(speed limit = a), and, a 4-lane highway(speed limit = b), where a is always greater than b. traffic on the 4-lane highway will be able to move smoothly, even at low velocity, where as the traffic on the 2-lane highway will still move adequately, because the velocity is greater. but, as the speed limit decreases, it will become less and less effective. so im basically saying that the 2-lane road is the downpipe, and the 4-lane road is the muffler/rear section of the exhaust. not trying to patronize you, i just wanted to make it a little clearer, my previous post did suck.

keep in mind traffic does not behave like exhaust. also, i dont know what size turbine/housing you have. im guessing its nothing insane(200-225whp? fair?), which is still a good amount of power. from a theoretical standpoint, you would be fine with a 2.0" downpipe, as a guy named Bernoulli pointed out in the 1700's, the volumetric flow over that distance compared to the increased volume of the 2.5" pipe would be inconsequential. the reality of that might be disputed by someone testing on a dyno, although i am guessing that it would be a pretty small amount of difference either way.

[rant - you've been warned ]

the issues with the exhaust are velocity and pressure

1. gas velocity relates to volume flow per second per length of tubing. when the gas cools and loses energy as it goes towards the back of the tubing, it slows down. this reduces the volume flow per second past any given check point in the tubing at the rear. in order to keep volume flow the same at a lower velocity, you have to increase the diameter of the tubing.

this is similar to the traffic lanes and # of cars equation. if you have a two lane highway moving at a mph, you have a certain number of cars passing under a bridge per second. as the cars slow down, to say b mph, you have to double the number of lanes to 4 in order to keep the flow the same.

same thing with the exhaust. it's moving out of the turbo at super-high velocity and a certain x-volume per second flows past the tubing at the front. by the time it reaches the back, it has slowed down quite a bit, thus you need to open up the tubing or else the slow column near the end of the tailpipe will impeded the stuff coming from the downpipe.

anyone familiar with road-racing will see this as being analogous to the 'yo-yo' effect as the front of the pack slows down for a corner, yet the tail-end is still moving at 35-40mph!

another example would be the burning nightclub. what happens when a crowd rushes towards the exit down the hallway? yet the people further out are taking their sweet time because they don't realize that the club is on fire?

2. pressure is also related to velocity. if you tap into the sides of a straight-pipe exhaust (2.5" or 3", etc.) and measure the pressure, you'll find that at the downpipe, you'll have the lowest pressure and at the tail-end you'll have the highest pressure (pressure is inversely proportional to velocity). this high-pressure at the tail-end impedes the flow of the whole system because it's the rate-limiting step. in order to reduce the pressure at the end of the tailpipe then, you want to increase the diameter of the tubing to reduce the back-pressure.


EDIT: not done yet... notice no "[/rant]

951

keep in mind that this is based on a porsche 951. but, its all relative.

"how do you find the optimal size exahaust for optimal performance?"


on a Turbo, there are actually two exhaust systems to consider. the first is the section between the exhaust-valves and the turbo. one of the keys here is keeping exhaust velocity high to drive the turbo. thus the heat-shielding around the headers and crossover pipe. however, the exhaust drops 300-degrees on its way over to the turbo(on a 951, sorry no experience /w boosted honda's). so, adding a ceramic coating will help keep the heat in the pipes and away from the engine-bay.(a little tangent there).

one other factor in this section of the exhaust is the size of the turbine on the turbo. the smaller #6 used on non-S turbos are fine up to 15psi of boost. but if you try to use more than that, the rotational-force required out of the turbo causes excessive back pressure in the exhaust headers. this causes exhaust pressure to actually fight the incoming intake air under boost. this can be especially bad if you're using aftermarket cams with lots of overlap.

"so should the 2.5" downpipe from the turbo be used in this scenario?"

not necessarily, you really want to match the downpipe's diameter to the turbine's outlet. . most of the Garrett turbos are using the #8 hotside and even the larger turbines only have a 2.6" outlet diameter. dumping these straight out into a 2.5 or a 3" (because "bigger is better") downpipe may not be the best match. the sudden loss in pressure and velocity at this early point will only get worse as you travel through the exhaust. remember that at the downpipe, velocity is king. you only want larger diameter later on in the exhaust flow when the gases have cooled and slowed down increasing pressure.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 2000Accord5sp &raquo;</TD></TR><TR><TD CLASS="quote">If my downpipe won't fit unless we use 2.5" pipe until after the oilpan, would it be pointless to widen it out to 3" after that? The oil pan is too close to the front cross member and a 3" 90 degree bend just won't fit.</TD></TR></TABLE>

in a word, yes. the exhaust column after the downpipe is still very hot with high-velocity and low-pressure. there's really no need for larger diameter than 2.5" in the middle cat-bypass section (even in 300rwhp+ applications). more volume here would just slow the exhaust down and eventually cause more backpressure as this extra volume will move slower and eventually cool off sooner.

not to mention that any abrupt changes in diameter when the gases are at high-velocity would be very detrimental to the overall flow. any distruption in the air-flow early on in the pipe will only be compounded because it will only worsen as you travel down the pipe.


remember that the struggle with designing an exhaust is to juggle velocity with pressure in entire system. changing one will inversely affect the other and you want to achieve a balance in the middle for each section of the pipe in a particular configuration.

so you can't say things in black & white clear-cut terms such as "a 3.0" downpipe is always better than a 2.5" one". because it's not, it really depends upon the turbo you're using and the types of flow you have in the car. the only thing i can probably say for sure, really, is that you probably would never need a 4" cat-bypass section. perhaps maybe if you're running a 700whp+ car...


[/rant]

CLIFFS: velocity stacking = bad. bigger is better = lie.



Modified by 951 at 5:39 PM 2/2/2004

951

how do those feet taste?

2000Accord5sp

well basically I was just wondering if 2.5 flared to 3" would make a difference (advantage more like). But from what you're saying, you seem to indicate the opposite, that it can make the flow worse.

just for an FYI, the exhaust guy told me that the amount he flared is 5% above what the maximum angle of flaring people recommend. Being, if he flared it too much, that would cause turbulence because of flaring out to 3" too quick. So he IS aware of that pitfall but supposedly isn't widening it out to much in too short of a distance for this to become a problem.

However, I just tried to think of this in a real life easy to use example. I tried to think of blowing in a straw from mc donals or something, compared to cutting it in half and putting a huge end on it. Either way, to me, it would seem like the amount of air flow would be the same because of the initial width of the straw. *shrug* but what do I know.

951

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 2000Accord5sp &raquo;</TD></TR><TR><TD CLASS="quote">well basically I was just wondering if 2.5 flared to 3" would make a difference (advantage more like). But from what you're saying, you seem to indicate the opposite, that it can make the flow worse.

just for an FYI, the exhaust guy told me that the amount he flared is 5% above what the maximum angle of flaring people recommend. Being, if he flared it too much, that would cause turbulence because of flaring out to 3" too quick. So he IS aware of that pitfall but supposedly isn't widening it out to much in too short of a distance for this to become a problem.

However, I just tried to think of this in a real life easy to use example. I tried to think of blowing in a straw from mc donals or something, compared to cutting it in half and putting a huge end on it. Either way, to me, it would seem like the amount of air flow would be the same because of the initial width of the straw. *shrug* but what do I know. </TD></TR></TABLE>

alot of the "bigger is better" garbage is under the assumption that the ideal gas law is "ideal" which it is not. it assumes that particles have zero mass and zero volume. this causes alot of discrepancy between, "ideal" conditions, and real world tests. predictions based on this law are soooooo rediculously wrong at high temperature that its not even worth considering.

about turbulance, you may run into problems if you're using mismatched flanges, or other forms of abrupt transition.

the "perfect exhaust" would be a tapered cone, sized corresponding to turbine outlet/power ouput, and no, its not going to make a whole lot of difference either way(tiny tiny tiny 1/4th mile differences).

Modified by 951 at 5:44 PM 2/2/2004


Modified by 951 at 5:50 PM 2/2/2004

Kataku2K3

With our higher boost motors (say ~24lbs.+) we've done the same thing... We flare our 3in downpipes to 4in just as soon as the pipe gets 90º off the turbine housing and it has made a difference (makes it a little easier with the F-R manifolds as you have some room between the block and the turbo)... We've made more power in doing this but then again 3in was probably a bit too restrictive to begin with... I'm not in anyway saying bigger is always going to be better, it just helped in our situations... Just my $0.02

shermanyang

iTrader: (1)

i know that people think bigger is better, but the damn opening on the exhaust side of my turbo is only about 2.25". so IMO a 2.5" flared to 3" dp is enough for me since it makes more sense to me to create a smoother path for the gas to flow rather than creating more turbulance at the start of the dp. that's just my .02

951

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Kataku2K3 &raquo;</TD></TR><TR><TD CLASS="quote">With our higher boost motors (say ~24lbs.+) we've done the same thing... We flare our 3in downpipes to 4in just as soon as the pipe gets 90º off the turbine housing and it has made a difference (makes it a little easier with the F-R manifolds as you have some room between the block and the turbo)... We've made more power in doing this but then again 3in was probably a bit too restrictive to begin with... I'm not in anyway saying bigger is always going to be better, it just helped in our situations... Just my $0.02</TD></TR></TABLE>

i agree, in your situation(high boost) a 3" DP would be needed. high power situations will amplify the effects i had mentioned earlier.


Modified by 951 at 8:17 PM 2/2/2004

The Internet Tough Guy

i swiped this info but it isn't copyrighted so

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on http://www.impreza.net regarding exhaust design and exhaust theory:

“Howdy,

This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems.

N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.

For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.

As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”

"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.

A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.

If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.

Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.

Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.

Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.

Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.”

"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.

There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.

As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”

"Here's a worked example (simplified) of how larger exhausts help turbo cars:

Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:

(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure

So here, the turbine contributed 19.6 psig of backpressure to the total.

Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).

So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.

This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.

As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?

The advantage to the bellmouth setup from the wg's perspective is that it allows a less torturous path for the bypassed gases to escape. This makes it more effective in bypassing gases for a given pressure differential and wg valve position. Think of it as improving the VE of the wastegate. If you have a very compromised wg discharge routing, under some conditions the wg may not be able bypass enough flow to control boost, even when wide open. So the gases go through the turbine instead of the wg, and boost creeps up.

The downside to a bellmouth is that the wg flow still dumps right into the turbine discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you go too big on the bellmouth and the turbine discharge flow sees a rapid area change (regardless of whether the wg flow is being introduced there or not), you will incur a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust."</TD></TR></TABLE>

kpt4321

If you want to discuss traffic dynamics, then how about you start a thread in the "******* stupid meaningless ****" thread.

Second of all, what the **** does that even mean? Traffic on the 4-lane highway will move smoothly, and traffic on the 2-lane highway will move adequately? The road is the downpipe?

How about you go to amazon.com and buy the book "Making Sense for Dummies."

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">keep in mind traffic does not behave like exhaust.</TD></TR></TABLE>

Right. Then why are you comparing them?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">also, i dont know what size turbine/housing you have. im guessing its nothing insane(200-225whp? fair?), which is still a good amount of power. from a theoretical standpoint, you would be fine with a 2.0" downpipe</TD></TR></TABLE>

It doesn't matter what power I have, we're not talking about my car. We're talking about theory.

I don't try to be "fine," I want to do what is best.


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">as a guy named Bernoulli pointed out in the 1700's, the volumetric flow over that distance compared to the increased volume of the 2.5" pipe would be inconsequential.</TD></TR></TABLE>

What? The volumetric flow compared to the volume? Just FYI, volume-flow and volume are two different units, you can't compare them. Can you say this again, but right this time?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">1. gas velocity relates to volume flow per second per length of tubing.</TD></TR></TABLE>

The length of the tubing does not change the velocity.


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">when the gas cools and loses energy as it goes towards the back of the tubing, it slows down. this reduces the volume flow per second past any given check point in the tubing at the rear. in order to keep volume flow the same at a lower velocity, you have to increase the diameter of the tubing.</TD></TR></TABLE>

That is correct. To keep volume flow up as speed decreases, you need to increase diameter. How this is relevant to the question at hand, I do not know.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">this is similar to the traffic lanes and # of cars equation. if you have a two lane highway moving at a mph, you have a certain number of cars passing under a bridge per second. as the cars slow down, to say b mph, you have to double the number of lanes to 4 in order to keep the flow the same.</TD></TR></TABLE>

Right, because as the cars travel down the highway, they start to cool, and then they slow down.... oh wait, no they don't.

Let's start comparing exhaust flow to the rate **** comes out of my ****, since that's about as relevan as this stupid car analogy.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">same thing with the exhaust. it's moving out of the turbo at super-high velocity and a certain x-volume per second flows past the tubing at the front. by the time it reaches the back, it has slowed down quite a bit, thus you need to open up the tubing or else the slow column near the end of the tailpipe will impeded the stuff coming from the downpipe.</TD></TR></TABLE>

I still don't see anything here about why your downpipe needs to be small.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">anyone familiar with road-racing will see this as being analogous to the 'yo-yo' effect as the front of the pack slows down for a corner, yet the tail-end is still moving at 35-40mph!</TD></TR></TABLE>

And anyone familiar with the female menstrual cycle knows that the period flow rate is proportional to the cross-sectional area of the vaginal opening, divided by the.... STOP WITH THE ******* GAY ANALOGIES. We're talking about exhaust flow, not yo-yo's, road racing, or highway traffic.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">another example would be the burning nightclub. what happens when a crowd rushes towards the exit down the hallway? yet the people further out are taking their sweet time because they don't realize that the club is on fire?</TD></TR></TABLE>

You crack me up.

Welcome to http://www.meaningless-comparisons.com


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">EDIT: not done yet... notice no "[/rant]
</TD></TR></TABLE>

Don't worry, I'm not either.

Del_Slowest

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kpt4321 &raquo;</TD></TR><TR><TD CLASS="quote">

You crack me up.

Welcome to http://www.meaningless-comparisons.com
.</TD></TR></TABLE>

your link doesn't work ....hehe

kpt4321

Yeah, a little tangent. Let's talk about the question (post-turbo exhaust sizing).


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">one other factor in this section of the exhaust is the size of the turbine on the turbo. the smaller #6 used on non-S turbos are fine up to 15psi of boost. but if you try to use more than that, the rotational-force required out of the turbo causes excessive back pressure in the exhaust headers. this causes exhaust pressure to actually fight the incoming intake air under boost. this can be especially bad if you're using aftermarket cams with lots of overlap.</TD></TR></TABLE>

Now we're talking about turbine sizing. Sweet. Can you give me some help with choosing injector and writing my timing maps while we're at it?

There was some junk about having the downpipe initially the same size as the turbine outlet, but I'm going to ignore it since it's not as wrong as the rest of this crap.


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">in a word, yes. the exhaust column after the downpipe is still very hot with high-velocity and low-pressure. there's really no need for larger diameter than 2.5" in the middle cat-bypass section (even in 300rwhp+ applications). more volume here would just slow the exhaust down and eventually cause more backpressure as this extra volume will move slower and eventually cool off sooner.</TD></TR></TABLE>

So, you are saying right here that a larger exhaust will cause more backpressure because the flow will be slower, right? Which means that faster flow would create less backpressure? If that's the case, then why don't you recommend a 1" exhaust for maximum velocity? Absurd.

Here's a tip: Flow losses increase as velocity goes up. Larger pipe keeps the velocity down, which will decrease your losses (and subsequently backpressure).

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">not to mention that any abrupt changes in diameter when the gases are at high-velocity would be very detrimental to the overall flow. any distruption in the air-flow early on in the pipe will only be compounded because it will only worsen as you travel down the pipe.</TD></TR></TABLE>

Yeah, so don't make abrupt changes in exhaust size. Duh.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">so you can't say things in black & white clear-cut terms such as "a 3.0" downpipe is always better than a 2.5" one". because it's not, it really depends upon the turbo you're using and the types of flow you have in the car. the only thing i can probably say for sure, really, is that you probably would never need a 4" cat-bypass section. perhaps maybe if you're running a 700whp+ car...


[/rant]

CLIFFS: velocity stacking = bad. bigger is better = lie.</TD></TR></TABLE>

Here is a quote from Corky Bell, the author of the book "Maximum Boost" which is one of the best basic turbo theory books:

"The best exhaust for a turbo car is no exhaust at all."

You want to minimize pressure after the turbine wheel, in order to maximize pressure differential across it. Also, a lower post-turbine pressure means a lower pre-turbine pressure, which increases VE.