ok this is probably the nastiest D series motor in existance, its from one of our moderators over at turbod16.com.

We were having a breif discussion over there and i want to get to the bottom of this.

That is a 4" DP on there. But as you can see, the pipe starts as a 3" and quickly expands to a 4". What is the point of this if the source is only 3 inches. How will a 4 inch dp reduce any backpressure if the source is only 3".

I see a lot of downpipes that do this. I just want to know why.

 

Um.... show me a Honda turbo that has a 4" turbine outlet

If there was one I am more than sure the pipe would be a straight 4"

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 96 GSR-T &raquo;</TD></TR><TR><TD CLASS="quote">Um.... show me a Honda turbo that has a 4" turbine outlet

If there was one I am more than sure the pipe would be a straight 4" </TD></TR></TABLE>

I can't, and one probably doesnt exist that a honda will support, whats your point?

Maybe i should rephrase for simple folk, all im asking is, "why have a 4" DP when your source isn't that big."

 

Because the Honda gods say it helps.....

why run a 4" Inlet on a turbo when the actual part where the compressor blades are at are only around 3"????

 

because you will still have technically less back pressure, its the same principle if you have a 2.5" outlet on the turbo and run a 3" downpipe. As the exhaust leaves the turbine the gas is still expanding and the larger the downpipe the more room and less pressure for that gas...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Soccerking3000 &raquo;</TD></TR><TR><TD CLASS="quote">because you will still have technically less back pressure, its the same principle if you have a 2.5" outlet on the turbo and run a 3" downpipe. As the exhaust leaves the turbine the gas is still expanding and the larger the downpipe the more room and less pressure for that gas...</TD></TR></TABLE>

Didnt feel like getting that technical

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Soccerking3000 &raquo;</TD></TR><TR><TD CLASS="quote">because you will still have technically less back pressure, its the same principle if you have a 2.5" outlet on the turbo and run a 3" downpipe. As the exhaust leaves the turbine the gas is still expanding and the larger the downpipe the more room and less pressure for that gas...</TD></TR></TABLE>

please dont comment on my artistic skills, i spent maybe 10 seconds on it

http://www.turbod16.com/album_page.php?pic_id=3348
but your saying if you filled bottle 1 and 2 with the same amount of pressurized air, #2 would come out faster?

 

I guess you can look at it that way but......

I believe the variables are different.

1.) The gas is heated and is rapidly cooling as it exits the tube
2.) Its constantly coming out at a different pressure

I could be wrong..... but it must make a difference because alot of cars run it that way, and its standard on a Full-Race Stg 4 kit.

This question would be a great one to ask http://www.evans-tuning.com in the forums section, he will break it down to you properly

 

gt42r has 4 inch outlet

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 96 GSR-T &raquo;</TD></TR><TR><TD CLASS="quote">

I believe the variables are different.

1.) The gas is heated and is rapidly cooling as it exits the tube</TD></TR></TABLE>

I'm aware of that, and as it cools it becomes more dense, which means it could use an even smaller DP.


<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 96 GSR-T &raquo;</TD></TR><TR><TD CLASS="quote">This question would be a great one to ask http://www.evans-tuning.com in the forums section, he will break it down to you properly</TD></TR></TABLE> I'm a member there, honda-tech usually answers the questions for me faster then that forum.

 

a 3 inch downpipe with a 2.5 inch exhaust makes huge power gains over a straight 2.5 all the way back exhaust. i'd imagine its the same principal.

and it looks cooler

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Turbo-charged &raquo;</TD></TR><TR><TD CLASS="quote">a 3 inch downpipe with a 2.5 inch exhaust makes huge power gains over a straight 2.5 all the way back exhaust.</TD></TR></TABLE>

WHY!! I've been around turbos for a long time, I have done a lot of hydraulics work in the military. I am also going to school to be an enginneer. I am very familiar with the properties of air flow and fluid flow. I want to know why. Everybody says this, but I really don't see why a DP should be any bigger then the turbine wheel itself.

 

no see the gas is compressed in the turbine housing and when it exits it might be cooling but it is still expanding as it leave if you have a smaller pipe it cant expand and you still have that compression in the gas while if you have a larger pipe it is allowed to expand lowering pressure.

 

I was under the impression that its b/c it allows the gas escape faster. With the gas having more room to expand it will also increase in speed and cause the least amount of resistance. Back pressure or any kind of resistance against the turbine is a bad thing. So going from 3" to 4" would assist with that.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Soccerking3000 &raquo;</TD></TR><TR><TD CLASS="quote">no see the gas is compressed in the turbine housing and when it exits it might be cooling but it is still expanding as it leave if you have a smaller pipe it cant expand and you still have that compression in the gas while if you have a larger pipe it is allowed to expand lowering pressure.</TD></TR></TABLE>

no trying to belittle you, but i think thats wrong, i just can't explain why. But it is wrong. Well i can explain why, but I would take a long time and with many pictures.

I think basically, my theory is,

turbo with 2" turbine wheel

a 2" exhaust with 0 bottle necks and perfectly smooth will have the same amount of resistance on the turbine as open DP

 

using the same 2.5 inch downpipe ive seen 75whp gain going from 2.5 inch to 3 inch exhaust. much quicker spool time, more power, more torque, down low, and up high. a bigger exhaust WILL make more horsepower

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 95GSRTT &raquo;</TD></TR><TR><TD CLASS="quote">I was under the impression that its b/c it allows the gas escape faster. With the gas having more room to expand it will also increase in speed and cause the least amount of resistance. Back pressure or any kind of resistance against the turbine is a bad thing. So going from 3" to 4" would assist with that.</TD></TR></TABLE>

a bigger DP will slow the gas speed.

this is exactly what happens when u stick ur thumb in a garden hose. The smalller opening forces the water too move much faster.

go turn on ur kitchen sink. As the water falls faster as it comes close to the sink. The stream will become thinner.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Turbo-charged &raquo;</TD></TR><TR><TD CLASS="quote">using the same 2.5 inch downpipe ive seen 75whp gain going from 2.5 inch to 3 inch exhaust. much quicker spool time, more power, more torque, down low, and up high. a bigger exhaust WILL make more horsepower</TD></TR></TABLE>

please, this thread is for people who think about why stuff works.

it appears to me you just know what works, but not why.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DaZman69 &raquo;</TD></TR><TR><TD CLASS="quote">no trying to belittle you, but i think thats wrong, i just can't explain why. But it is wrong. Well i can explain why, but I would take a long time and with many pictures.

I think basically, my theory is,

turbo with 2" turbine wheel

a 2" exhaust with 0 bottle necks and perfectly smooth will have the same amount of resistance on the turbine as open DP</TD></TR></TABLE>

well it isnt wrong lol, Think of it this way you have 4 seperate exhaust charges entering the turbine housing and are compressed against the backs of the turbine blades, now if you kept the piping size the same as the blades that same compression would continue as it entered the exhaust piping and as it flowed. Now if the piping flares this is pretty much the opposite of the turbine intake and will relieve the pressure built up in the system.

Edit: and in this case a larger downpipe will not effect gas velocity because the slower velocity is offset by the expanding gas essentially cancelling each other out

edit #2: BTW good discussion

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Soccerking3000 &raquo;</TD></TR><TR><TD CLASS="quote">

well it isnt wrong lol, Think of it this way you have 4 seperate exhaust charges entering the turbine housing and are compressed against the backs of the turbine blades, now if you kept the piping size the same as the blades that same compression would continue as it entered the exhaust piping and as it flowed. </TD></TR></TABLE>

ok now where getting some where
now heres a little picture I made
http://www.turbod16.com/album_page.php?pic_id=3349
you see the two pipes, the first one is 1" and is long and will have the same pressure the whole time, the second one is 1" and then expands to 5". During its time in the 5" part, i completely agree the pressure will be less.

But according to what your saying. The one with the 5" outlet should expell the air faster. But that is not true. both hoses will expell exactly the same amount of air at the same rate.

 

but that doesnt take into consideration expanding rates of the gas leaving, if the pressure was constant entering the pipes that would be true

 

http://www.turbod16.com/album_page.php?pic_id=3350
there this one makes it a little easier. The gas will expand as soon as it hits the bottom opening.

Which is the same thing as having a 4"DP on a 3" turbine

Both exits in that picture will expell at the same rate

 

will you still have pulses, since the turbo is basically chopping the hell out of the air.....

I don't know about the 2 in exhaust having less resistance then an open downpipe. I don't know, I need to think about that one some more i guess

 

We use larger tubing to gain volume carrying capacity by lowering air pressure. Remember air is compressible, if we can make the air less dense, than it becomes lighter. The lighter it becomes the faster it moves. Although, In lower WHP setups(we'll say 400whp and less on on true 1.8's) I don't see anything over 3" gaining any power. I would suspect it would have to do with the volumetric efficiency of the engine, displacement and power output but, I'm not an engineer yet so, I'm not 100% on this theory.

Phil

 

http://www.turbod16.com/album_page.php?pic_id=3350

did u look at this yet?

Is this not the same princible?

Picture the top tube as a 2" DP on a 2" turbine wheel

and the bottom a 4" DP on a 2" turbine wheel

THESE TWO OPENINGS WILL EXPELL AIR AT THE EXACT SAME RATE