so my trust downpipe came in today. looks great, 3" downpipe, except im surprised to find that it concaves in at the dumppipe to utilize the oem 2.5".
so my question is, how much of a loss will there be, if any, utilizing a 2.5" dumpipe with the 3" setup?
will this resitrct flow over using a 3" dumpipe significantly?
if the flow isnt changing direction or hitting some obstruction at the divergence point, then the flow shouldnt be interrupted cuz it will flow faster to compensate for the decrease in pipe diameter. i dont think 1/2 of an inch will make a huge difference anyway. feel free to correct me if im wrong.
This is a subject of enormous debate. You’ll hear 10 sides of every story. I’m not going to say this is true, then pad what I say with 30 pages of other people’s borrowed information.
But I’ve been told a restriction in the exhaust is better for flow than a straight pipe. Basically due to the restriction, it accelerates exhaust flow past that point.
To me, that makes sense.
I’ve been told a thousand times turbo cars will be faster with the less exhaust they have. My 200 was slower with the downpipe and small straight pipe extension than with a full 2.5" exhaust with highflow cat and unsilenced muffler.
And no, you’re not likely to notice any difference at all.
I have actually been thinking about exhaust flow lately…
I have a full 3" turbo back system and due to the GF finding it loud I have added added a 1 foot 3" straight through vibrant resonator and replaced the stock resonator which cracked with a 14" long 3" straight through magnaflow resonator, she still thinks it is loud… 
After reading some stuff on exhaust flow, they suggest that although big exhaust piping is good for turbo motors (for turbo spool and all the restriction is at the turbo), you do not want to go too big or else the Exhaust Gas Temperatures will decrease as they travel down the pipes which cause the gas to become heavier and harder for the motor to push out.
I do not think it will restrict too much exhaust flow if it concaves to 2.5, it might even be better
now my questions are…
Is a 2.5" ehaust system better or worse then a 3" system on our SR20 motors? which makes more power?
I haven’t been able to really find any info
With small diameter piping, the car feels more repsonsive and torqi in the low-mid range…
Once you open it up to 3 plus inches, u will get less response, and less instant power at 1000-2200 rpms…once ure turbo spools though, the 4000rpm’s plus is far stronger with a 3 inch compared to the stock one that nissan supplies with the s13’s in japan.
Couple of things. There’s a big difference in opinion because there are many ways to look at this. A very basic approach would be to look at it from a flow standpoint, treat the pipe as a control volume and ignore the fact that the gases are being provided by an internal combustion engine, or turbocharger, etc…
There are several variables in a flow problem such as this, massflow, density, velocity and diameter. Massflow is provided by the motor, turbo, whatever, and doesn’t change, so the variables become density, velocity and diameter. Since flow out of the motor, turbo, whatever is pretty slow (I realize it’s NOT slow, but it’s definitely less than mach 0.3) one can assume incompressible flow and generally get away with it. So, density aint changing either, since the air is not being compressed (yes, I realize that air is being compressed, but it’s at another point, and doesn’t relate to this, it’s not being compressed at the downpipe at all) That leaves us velocity and diameter. Diameter goes up, velocity goes down. So now that we have that cleared up, lets look at things like pressure and shit, b/c I know everyone loves that. There’s two pressures to be aware of, static pressure and dynamic pressure. Together they form the total pressure. Static pressure is just what the pressure of the air is… (I know it’s ambiguous as shit, but when you see what dynamic is, it’ll make sense). Now dynamic pressure is the additional pressure that you get from all that fast moving air smacking into you. If you put them together, you get total pressure. Total pressure is constant, because you’re not changing anything at this point, it’s just air flowing through a pipe. Therefore, higher velocity, higher dynamic pressure, LOWER static pressure.
So wouldn’t that mean that you would want highest velocity possible, therefore smallest diameter? So why does the stock exhaust suck? Because of a little phenomenon called choked flow. You can kinda notice this with a straw. Put it in your mouth and blow a little… Now blow more… and more… More air is coming out. But after a certain point, no matter how hard you blow, the air coming out won’t increase. It’ll stay the same. Same with a car. At a certain point the mass flow becomes too large for the diameter of pipe and the velocity can’t go up anymore, so the pressure starts building, and building and your power sucks.
So now you know why certain exhausts are good for certain applications, but not others. When the massflow is high at higher rpms, the big exhaust will let it all flow out and make nice power, where the little exhaust will choke and you can kiss your top end goodbye. However, in the low rpms, when massflow is low, the big exhaust is lowering your exhaust velocity and costing you power.
This part isn’t all that useful:
Now for a bit on efficiency. There’s another thing that makes our lives an annoyance, and that’s called the no-slip condition, and it is due to viscosity. This means that air touching the walls of the pipe gets slowed to a stop, and air right above it gets slowed down as well, by friction between the particles. So you get a velocity distribution inside your pipe, with the highest velocity being in the midsection, and the velocity being zero at the walls. Now if you have a bigger diameter exhaust, there’s a much smaller area being affected by this, therefore less losses. This is the head loss that people talk about. There’s also other loss factors. Like bends for example, and expansions and contractions (like the one in your downpipe), this is why an exhaust will flow much better if it is all the same diameter, rather than expanding and contracting. So you gain a bit from expanding to 3" from 2.5", but you lose a bit from the expansion as well.
And now for how it relates to turbo cars… Turbocharged cars have significantly higher temperature exhaust gases, and it is that temperature that runs the turbine, however after the turbine, there’s still tons of heat left. Heat does a couple of things. It lowers the viscosity and it raises the speed of sound of the medium. Both these things make flow a hell of a lot better, and make it easier to get the air out of your exhaust with a good velocity. Also, these cars make a lot more massflow than their normally aspirated counterparts. With those specific circumstances it becomes a lot better to have that big kitten eating exhaust.
^^^^
is there a coles notes version of that?
been up since 5:30 and still at school. reading is the last thing im doing right now.
smaller diameter pipes suffer less from bends than larger ones, the reason being all the air within the tube is of a higher density further towards the sides, so the fall-off is steeper and will less likely cushion further incoming air from deflecting around a corner
and if you do increase in diameter you end up with the possibility of some air ramming through the outside layer if you can call it that to smack into the walls of the bend creating pressure reflections (like a resonator), therefore the larger the pipe the more you have to avoid bends, and any bends should be as large a radius as possible…
another element to velocity and how it relates to the flow is you can get away with minor diameter changes which will lower pressure and velocity slightly without causing any reversion depending on starting temperature/density, a downpipe for example that starts at 2.5" and increases diameter over the next foot to say 3" will reduce velocity more than a 2.5" to 2.75" downpipe leading into a 3" cat flange
but anything any of us say will not always apply, a normal engine isn’t running at a constant state and therefore the flow characteristics will change based on ambient temperature, the resonance frequency of certain rpm in the pipes, even the hydrocarbon levels present in the exhaust gas will affect it
basically you will find that the car will have different behaviour at different throttles/loads during a drive, and your foot will have to relearn the power band to make the most of it
and I’m talking from first hand experience, I’ve done lots of exhaust experiments on the same cars
thought i would weigh in on this…
the best exhaust is no exhaust…or conversely…the largest straightest diameter that you can fit…
you can make huge gains in power on turbo cars from larger downpipe and exhaust…for example from before and after testing on my turbo sohc…all things equal except a change from a full 2.5 to a full 3 inch diameter system, I gained 23rwhp and 32ftlbs torque…thats not peanuts
here is a question…lets say using a smaller diamter pipe is good for power becuase it magically increases velocity without increasing backpressure…
if that is the case…why do top fuel dragsters just have open zoomi headers?
most race series have noise limits…so cars must run full exhausts…if there are no rules… you will see race cars running open headers 100%
Because N/A cars also exist 
For turbo cars, yea, running open downpipe would probably make power.
a turbo is enough of a restriction as is, and a turbo will never ever spool faster with restriction behind it. perhaps some sort of aerodyanamic suction would help turbo spool. hmm how could we do this
anyway back on track. sometimes diffusers work inside exhausts but they need to be sized right otherwise they will just choke the exhaust flow.
for n/a cars open header always sounds the best
and since sound > power
open headers for life
I don’t think what I said came out quite right.
By a restriction, I meant having 99% of your exhaust 3" but at a point beyond peak velocity. Like the end of the downpipe, but then bellowsing out to 3" again afterwards.
This is again, just hearsay. But race cars and street cars is a bad comparison. In a race car, no-compromise power makes sense.
In a street car, you are not at WOT all the time. Exhaust velocity changes from cruising RPM to redline.
Power vs Driveability.
Yes, you will gain more power with a straight exhaust on a turbo car, especially one making close to 400 hp to begin with.
On a street car, you’ll hit a point of diminishing returns. Yes, on a stock SR, if you throw a huge 4" exhaust on it, you might pick up 3-4 hp. But the 8-10 ft-lbs you give up, plus the 30db you pick up, are likely not worth it. And, you may find the same power on a 3" exhaust.
If you spend most of your seat time in the torque band, you may not want to give any up for a few unnoticeable horsepower in the range your car rarely sees.
Too many people throw a 3" exhaust on a car, hate it, add two resonators, a cat, a mid muffler and a can with a silencer to make it livable. And they’re back to near stock levels of flow.
You want a mindfuck? Google the Chrysler Sonoramic Commando Engine exhaust setup from the 60s. Dynamic sound-tuned exhaust, which used a combination of sound frequency and fluid dynamics to produce an exhaust that essentially generated boost backwards. It caused negative backpressure (essentially vacuum) in the intake stream as a result. The exhaust left at such a velocity that the vacuum in the combustion chamber pulled the awaiting air in faster than it could at regular atmosphere. As a result, this was the first NA motor to run over 100% VE.
That is one case (proven mind you) that an exhaust system can produce more power and more velocity than an open header.
Hot cars at the time came from the factory with headers, straight pipes and chrome tips.