i have read a lot of threads where somebody has posted pics of their downpipes they made, most have been bigger than the outlet of the turbine. there is always the flamers that say why make it bigger than the outlet? i see all these topmounts that use a 4in dp but they are always ran under the oil pan, if you need that big of a dp is the car really streetable, or is it a track car? if it is a track car why not just use a side exit(less backpressure) and if the bigger the better theory is true has anyone dynoed whithout a downpipe at all? it wouldnt be hard at all just use good ventalation and make a simple open top box to shroud flames.

 

downpipe makes power.. Dont want that heat in the bay.

some tracks require you to have exhaust or mufflers. Along with class rules for events.

 

ok the class rules makes sense but has anyone actually dynoed somethig with no downpipe to prove that bigger is better, because if it made no difference it could save some money in a build to go with a downpipe that is the same size as your tubo outlet right?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nateg &raquo;</TD></TR><TR><TD CLASS="quote">ok the class rules makes sense but has anyone actually dynoed somethig with no downpipe to prove that bigger is better, because if it made no difference it could save some money in a build to go with a downpipe that is the same size as your tubo outlet right?</TD></TR></TABLE>

larger exhaust increases turbo spool. and in high hp cases reduces back pressure freeing up horsepower. there have been plenty of threads with people comparing 3-4 inch exhaust setups that show dramatic improvements with the larger setup. the main reason people dont go bigger is space. and the main reason there is an exhaust at all is because who the hell wants the exhaust to exit under the hood?!! and most street classes require a full exhaust for drag racing. not to mention the under hood temps would be insane

this question is just kind of stupid and should be in the forced induction forum anyway. just common sense if you ask most people

 

from my understanding, in turbo applications your dp size is one of the important factors in limiting the potential of the turbo. your exhaust can only flow as quickly as the turbo can expel it and if there is any kind of obstruction/restriction in this path it will only hinder the turbo from reaching its full effeciency. so in short, the theory of bigger is better is the one to follow.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by weiRtech &raquo;</TD></TR><TR><TD CLASS="quote">from my understanding, in turbo applications your dp size is one of the important factors in limiting the potential of the turbo. your exhaust can only flow as quickly as the turbo can expel it and if there is any kind of obstruction/restriction in this path it will only hinder the turbo from reaching its full effeciency. so in short, the theory of bigger is better is the one to follow.</TD></TR></TABLE>

a bigger downpipe also will not get near as hot as a smaller one b/cthe same amount of exhaust gas has much more room to expand/transmit heat. at the same time, gives the exhaust much less resistance and allows it to flow much more freely, so your turbo can spool up faster and less resistnace=more power.

on my new setup i'll be running a 4''dp v band'd to a 4-3'' reducer to a 3'' exhaust, it'll sound mean on the street but when it's time to get serious, i can disconnect the exhuast quickly

 

The problem with that temp idea - the turbo puts out the same amount of thermal energy at 400hp with a 2" exhaust or 400hp with a 4" exhaust. You just have more surface to distribute the heat.

The main thing to know is that on turbo exhausts: the manifold sees anywhere from 2-5x (or more) pressure than the intake is seeing; any increase in post-turbo back-pressure results in a greater amount of energy being put into the turbine wheel. In other words, instead of using energy to push gas, its used to spin the turbine. That leads to quicker spool/less lag. Less backpressure overall will allow for more hp at upper-rpm's.

 

No one is debating that less backpressure is a good thing. His question is are you benefiting from having a dp that's way bigger than the turbine outlet. It's like putting 3" charge piping on a 2" throttle body and 2" intercooler inlet/outlets or putting a 5" pipe on a 3" compressor inlet, is that extra inch or two really doing anything for you? Everyone cries about "smooth transistions," well how smooth is a 2.5" turbine outlet dumping straight into a 4" downpipe? Sounds like a lot of turbulence to me. And don't say it's so big it's like dumping into open air, because it's not. It still has to flow through the pipe. Smooth flow at the turbine exit matters too. Transitions need to be smooth whether the air is coming in or going out, most people overlook the "going out" part.

Personally, I agree with the big dp idea, but I can see where the OP is coming from and it's definately a valid point.

 

Where's the eyeroll smilie?

Larger = less backpressure. Or does it? Well what if you don't have any backpressure with a 3" dp? Then how is going to a 4" going to benefit you at all. This is the point that the OP was making and you seem to be missing. Hence why I used the example of a 5" intake on a 3" inlet turbo. Hurray, less pressure drop is good, but does that mean a 7" intake would make even more power than a 5"? No. Why don't we all start running 10" exhausts? Bigger is better, right? You probably make 5hp extra for every inch that you add. *eyeroll* Again, no one is debating a larger dp being better, just throwing out the idea that it might not be nessesary or even better in some cases.

But if you really want to get stupid about bigger = better.

Most reducers are very steep angle, which is NOT a smooth transition. Just because it feels "smooth" when you rub it with your finger doesn't mean the air flows over it smoothly. You're obivously a very smart person, so why don't you tell me how much the exhaust gas slows going through a typical 45* (or &gt 3-4" transition. And seeing as how you have that transition about an inch from the turbine exit, you now have a bunch of stagnated air and extremely turbulent air right at the turbine outlet. I wonder how good that is for flow and turbine efficiency? Care to enlighten me? As much as you'd like to believe it is, a 4" dp is NOT the same as venting the turbine to open air. Yes, it's big, but it's still a pipe the air needs to find it's way through. Less back pressure doesn't always mean bigger pipe, it means getting the air away from the turbine as quickly as possible. And that means a smooth flowing (I.E. shallow angle &lt;15*, if you didn't catch that) transition into whatever size exhaust you're running. Not a huge step up to a 4" exhaust out of some tiny 2-2.5" turbine outlet. If you have a 3"+ turbine outlet, obviously it's a different story.

Think before you just go with the typical, "Hey it's bigger, it must flow more."

 

dont get butthurt because hes right.

also, his question wasnt WHY it was HOW MUCH.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nateg &raquo;</TD></TR><TR><TD CLASS="quote">i have read a lot of threads where somebody has posted pics of their downpipes they made, most have been bigger than the outlet of the turbine. there is always the flamers that say why make it bigger than the outlet? i see all these topmounts that use a 4in dp but they are always ran under the oil pan, if you need that big of a dp is the car really streetable, or is it a track car? if it is a track car why not just use a side exit(less backpressure) and if the bigger the better theory is true has anyone dynoed whithout a downpipe at all? it wouldnt be hard at all just use good ventalation and make a simple open top box to shroud flames. </TD></TR></TABLE>

clearly hes qouting what other people say. HIS ******* question is IS IT ACTUALLY BENEFICIAL. retard.

 

Calmn down, guys.

 

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

Yes, and the ******* answer is YES, BECAUSE THE EXHAUST GASES ARE STILL EXPANDING AFTER THEY EXIT THE TURBINE.
</TD></TR></TABLE>

As if that has anything to do with what was asked. Do please explain what this supposed "expansion" has to do with making a 4" better than a 3" and a 5" better than a 4" and a 6" better than a 5", etc etc. Are you assuming because they're "expanding" then they must need a bigger space to move through? Wrong. Did you read that in Super Street? Sure, they're under pressure while in the turbine. How big is the end of the turbine volute (highest pressure point)? MAYBE 1/4-3/8" diameter on a typical .63 T3. Then you expand that out to 3".... is that not enough expansion for you? Do you honestly believe that even 5% of the people in the Honda community are outflowing a 3" open dumped dp?? Give me a break. Post a before/after dyno chart of a 0-400hp car (that includes 99% of the people on the road today) with a 3" open dp and a 4" open dp and see if you gain anything. I'll put money on no. Prove me wrong.


Modified by AWDstylez at 10:15 AM 2/7/2007

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by George Knighton &raquo;</TD></TR><TR><TD CLASS="quote">Calmn down, guys. </TD></TR></TABLE>

*Calm


A simple answer for everyone is that a larger DP increases the pressure differential across the turbine.

Turbos work off of flow and pressure.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .EnzoSpeed &raquo;</TD></TR><TR><TD CLASS="quote">
Question: "Why is it beneficial to run a downpipe that's larger than the turbine outlet?"

Answer: "Because the exhaust gases are still expanding after they exit the turbo."
THAT'S IT. Stop talking.</TD></TR></TABLE>

The problem with this answer is that most people including yourself do not even understand what this means.