Veloster Turbo Forum banner

So what's best 2.5 or 3 inch exhaust??

31K views 69 replies 30 participants last post by  CatfishSoupFTW 
#1 ·
With these exhausts coming out for our VT what are your thought about this article?




Since it is a time of giving, I will try to offer some easy to understand information on turbo exhaust flow that I have learned over the years.
The first thing to look at is how a turbo works……..
A turbo is driven by heat and pressure differential ……….The higher the pressure and heat before the turbine wheel, and the lower the pressure and heat after the turbine wheel, The faster the turbo will spool.
One of the reasons tubular manifolds work better on turbo cars is because the tubes are thin material and heat quickly adding to the heat differential causing the turbos to spool faster.
A simple way to see how heat looks for cool air and drives a turbo is by looking at a turbine vent on a roof top.....
When not a breath of wind is in the air, But it is a warm day. You can see the turbine on the building spinning like crazy......This is because the attic is very hot and the hot air is looking for the cool air outside and driving the turbine.

A turbo engine also does not need any extra back pressure because the turbine wheel creates more then enough back pressure for the engine to operate correctly before the turbo spools……..Any additional back pressure in the exhaust system will simply cause the turbo to spool more slowly.
Turbo engines do not have a problem with blow through and NA engines normally only have problems with blow through at idle or VERY low speed when the cams have excessive overlap.
From what I have learned Exhaust sizing is a science (Even on Turbo Cars).
The proper exhaust tube size is part of the equation of tuning the car.

The bigger is better is old school and has its place on a race car that only runs at wide open throttle and has a very short exhaust……..
Because you want the exhaust as cool as possible and you want no back pressure it is easy to see where the idea came from about bigger being better :p
On a race car the exhaust is normally about 6 inches to two feet…..Because the exhaust is so short, velocity is not a problem.
The problem on a street car is that it runs at many different loads and has a fairly long exhaust system, Many more factors come in to play that cause backpressure besides the restriction in size alone…….
On a street car like a Z, The exhaust is about 10 feet long on each side and when the exhaust is this long and you start putting bends in the exhaust system, exhaust speed or velocity starts to become very important.
Backpressure in a exhaust can be caused by turbulence and slow exit speed, Just as easy as in can by size restriction.
If the exhaust gas remains in the pipe without exiting from the pipe size being too large in a long exhaust system, it causes back pressure from the slow exhaust flow.
I have been told by NASA engineers that one 90 degree bend has the same exhaust restriction as adding 25 feet of straight pipe…..
This means if you have two 90 degree bends in each side of your exhaust it has the same effect as adding 100 feet of straight pipe to your Z!
Turbulence can cause a large amount of backpressure……Some of the things that cause turbulence are bends, Sudden pipe size changes, rough pipe ends, Ect…….
Something that use to be common was gutting cats…….This gained top end power and higher peak numbers on a dyno, But at a huge sacrifice in low end power due to the turbulence caused by the sudden inner pipe size change. The reason this hurts low and mid range power is because the turbulence causes the velocity of the exhaust gas to slow and even though the area is larger it causes backpressure from the turbulent slow exhaust speed and thus the turbo spools up slower. At higher RPM’s the exhaust speed is higher and less restriction from the cats being gutted still net a gain on top, Although it is possible to have a net loss in power because it can hurt the low end enough.
I remember back in the 80's working on a single turbo 280zx and gutting the cat....It really killed the low end power on the car.....What was interesting was that after the low end power went away we decided to try a cheap test.....
We welded a smaller pipe to the inside of the cat to see what would happen......To our surprise at the time the car made better overall power with a smaller pipe inside the cat then it did gutted or with the cat in tact.
We later made it a test pipe the same size as the exhaust and pick up even more power.

Several years ago we did quite a bit of testing on the Z32's……..At that time most of the Z cars were making between 350 and 500 RWHP and that is the area we tested the exhaust components in.
At up to 500 RWHP we found that 2.5” pipe offered the best overall power and sound level. We found that 3” pipe hurt low end and mid range power because of the slow exhaust velocity with the larger pipe. We also found that the 3” pipe at the 500 RWHP level did not net any measurable gains at the top end.
To be fair the 3 inch down pipes that were available back then were very poor and could have added to the low end performance problems.
One of the reasons the newer long split down pipes worked better at low end is because the exhaust coming off the turbo wheel is very turbulent and it takes about 12 inches for the flow to gain velocity……By making the down pipe tube about 12 inches long before bringing the wastegate pipe back in the velocity is higher allowing better turbo spool up.
Some of the reasons we have been able to use 2.5” pipe at fairly high power levels is because we pay attention to detail…….By making the bends as minimal as possible and by keeping from having rough pipe edges by welding on the inside and grinding smooth, We have kept the velocity very high and caused as little turbulence as possible.


Something that you may find interesting is a test we did on my car at the drag strip……..
With the automatic I run fairly consistent, So it is easy to see small changes and test what works in the real world…….
We wondered if the car would go faster without some of the exhaust components……
We made several runs and established what the car would run….The 60 foot made the small differences in the ET within a tenth of a second and the mile per hour was always the same. We then removed the muffler section (our mufflers with minimal bends in the pipes) and we made a couple more runs and the car ran exactly the same MPH. We then removed our X-Pipe and the car dropped 3 MPH……We made a couple runs and they were the same each run. We then bolted the X-Pipe back on and the MPH came right back!
It was obvious that blending the exhaust from both sides of the engines made a significant difference.

Now that many Z cars are in the 600 plus RWHP area I do believe we need to retest and make sure we are not giving up any HP by staying
With 2.5” exhaust.
The way we will test besides the dyno and track is by taking pressure readings in the down pipe to see if any back pressure is present at the higher power levels.
We are currently prototyping some new exhaust products to test with. It is possible by expanding slowly to a larger size we could see some gains. We will also test to see if the whole system needs to be larger or if just a small section near the down pipe needs to grow and shrink like a expansion chamber on a motorcycle.
Because the exhaust expands from heat we might find that going bigger for a short time then using a cone to neck back down after it cools and the exhaust contracts we keep the highest exhaust velocities without causing restriction.
I have had some ideas I have wanted to try for some time and time did not permit to get the prototypes and testing done.
I have hired a sales manager at Specialty-Z to help free up my time for more Research and development.
The new parts sales Manager is Dean Delevie and I believe you will find him a pleasure to work with!

I hope that helps and I look forward to providing you all with more data in the near future.
 
See less See more
#5 ·
I used to run a twin 3inch system on my heavily worked 13B in my series 1 RX7 that was hard work as it felt like I was losing torque down low but needed it once the revs got around the 11,000-12,000RPM range. I get the feeling this little motor would be the same but 3 inch is probably a good idea after you have done a signifigant amount of mods to the car. but each to their own and the 3 inch systems I have heard do sound beastly.
 
#13 ·
again very car forum i come across has this debate, i for one believe on a turbo or high displacement engine no back pressure is needed but on a NA 4 cylinder there probably needs to be some back pressure just my .02
 
#14 ·
Any other comments regarding this? I went straight pipes from the cat and can't seem to notice a power loss or gain. I'm not looking for either or as I just went for sound but I continued the same size pipes all the way back. Should I have gone bigger?
 
#17 · (Edited)
Ok CB,..lots of guys jump on here and say your full of it. I have to throw the BS card.

The only truth in what you have spout out here is that JUN has a dyno that shows an increase over the ridiculously restrictive OEM CAT,....and the ridiculous turbo elbow. Anyone that has actually seen theirs out of the car would shudder. Talk about a constricted right angle. Oh damm!

On its face,..bigger is better.
Less restriction is always better.
If that weren't the case TORK would NOT make the downpipe he makes.
Now,..you mixed and mis-quoted several theories of air-flow and aerodynamics. The most significant is the idea that "turbulence" slows down flow. Not only is that completely wrong, what your suggesting is very mis-leading.
The best example of how "turbulence" increases the speed of air is a golf ball. If a golf ball was smooth it would NOT travel as far. The irregularities and patterns on a golf ball CREATE a specific kind of turbulence. This increases air speed, hence increased travel of the struck golf ball.
Intakes and exhaust are the same.

As I have increased the diameter of my system only improvements in capacity and performance have been noted. (I'm not talking about the sound either).
The minute my VT fired up with the new full 3 in downpipe it was evident across the entire band. (I'm not even tuned for it yet,..wait till TORK adds this one to his mix).

***Jeff from Solo recommended that I use the resonator with this same air flow principle in mind. That is the resonator is filled with directional 3 dimensional "fish scales" as it were. As the exhaust travels over these shapes it:
1. Picks up exhaust velocity.
2. Sound is softened and not as edgy.

So try again my brother. It does not add up.

So in summation,...gentle female like curve of said down-pipe right at turbo into 3 in no cat dumps to resonator picks up speed and exits out full 3 inch piping with NO MUFFLER to slow her down = BADASS.
 
#20 ·
Ok CB,..lots of guys jump on here and say your full of it. I have to throw the BS card.

The only truth in what you have spout out here is that JUN has a dyno that shows an increase over the ridiculously restrictive OEM CAT,....and the ridiculous turbo elbow. Anyone that has actually seen theirs out of the car would shudder. Talk about a constricted right angle. Oh damm!
The JUN dyno chart was on the catback version, which is what I have.
 
#18 · (Edited)
I did HVAC for a while and when designing duct work, as the run got longer and had more bends you had to reduce the duct size in order to keep the correct cfm. I wonder of this has been tested and cars and if it would help
 
  • Like
Reactions: -X-
#21 ·
Gaijin, I really appreciate how you push the envelope and while I agree with some of your points (the BS one especially LOL), the golf ball analogy is not entirely correct. The dimples do not increase air speed, they reduce drag.

I have copied this from an article: " 2.5" may flow enough for 300-350 h.p. without being a restriction. 3" is usually capable of flowing 500-600 h.p. before becoming a restriction. This is assuming that you have designed the rest of the system up to par.". If you are interested, look at this exhaust design article.
 
#24 · (Edited)
Gaijin, I really appreciate how you push the envelope and while I agree with some of your points (the BS one especially LOL), the golf ball analogy is not entirely correct. The dimples do not increase air speed, they reduce drag.


Understood,..........but the principle works whether its is the object itself propelled through air, or as in our case , airs behavior enclosed within a constrained system. In other words the "drag" is from the surface itself, as the air moves through it, versus the object moving through air.

If you notice the two specific scenarios I listed when encouraging the 3 in were more robust future platforms. Don't limit yourself. The reality is, if you want to keep your VT growing, get the three.

CERMA John actually encouraged me to come up with a intake where I had lined the interior of the piping with dimpling like a golf ball.
(***OFF TOPIC, but related, very interesting)
***To further how CB got it wrong, He gave me the plans for a true RAM air. What it involves is the "swirling air" that CB alluded to does not slow air down.
By placing a restriction just inside the intake after the air filter, it causes the air to squeeze by the "restriction" but it then "swirls" behind the restirction causing LOW PRESSURE. This in turn makes suction. It is a cascading effect which only increases, forcing more air into your turbine.
The best example of this is the SR-71 Blackbird's engines. The engines suck their way through the atmosphere at their WOT. 80% of their propulsion is from the suckage, the efflux of the jet accounts for less than 20%.

I'm not an engineer, but CERMA john is. Electrical/chemical/mechanical.
 
#22 ·
im going to have stacks out my active hood vents!
 
  • Like
Reactions: Magneticmuffin
#23 ·
I got the answers I needed, thanks guys! I'm glad I was able to invoke more argument over a theme that might not ever disappear lol
 
#26 ·
The only real proof is a dyno test, and I have seen after-market exhausts dyno tested that didn't do much and sometimes even degraded power (more noise doesn't automatically equal more power). I have a dyno test on mine showing a modest increase in power and torque right through the rev range, and that was good enough for me.

I also have a dyno test on the after-market exhaust and matching tune on my motorcycle, and that was good enough for me too. My motorcycle doesn't have wide-band lambda sensors so it needs a tune to match any modifications, or else it would run lean.

I never said that a larger diameter exhaust WOULD mean less power but everyone knows it COULD, unless it's been tested. Unless it's been tested then everything is just guesswork.
 
#27 ·
On another platform I have ran 550-600 bhp/600-650btq through a 3" dp to 2.25" exhaust with an electric cutout and 38mm wg dumped to atmosphere. Flow wise this equal to roughly a 2.75" exhaust...... I would say if you running stock or stuffed turbo 2.5" will be plenty. 3" should not be needed unless you talking built motor and larger turbo running over 350-400bhp. Take this fwiw.....
 
  • Like
Reactions: -X-
#28 ·
The article is a good read, but really got my attention at the statement:

"Unless you are making over 500-600 h.p. anything over 3" is a case of diminishing returns and in most cases has no advantage."

The last truck I owned was a crew cab 2009 Silverado 1500 with a 5.3L engine. The stock dyno numbers for that truck are around 260/275 (rwhp/rqtq). I replaced the entire stock exhaust with a set of long Pacesetter tube headers (1 7/8" primaries, 3" outlet), an off-road y-pipe (3" inlets single 3" outlet), a 3" to 4" adapter, and a 4" exhaust from Diamond Eye (no muffler). I put down 305/322 (rwhp/rwtq) on the dyno, the exhaust system being my only mod. The tuning shop compared those numbers to other trucks with the exact same headers and y-pipe, no other mods, and found that with a 4" exhaust system massive gains were made across the entire RPM range compared to a 3" exhaust system.

Here is a truck with the same engine and exhaust setup as mine:



I'm not saying this article is wrong, but I have NEVER experienced what they do in "most cases". I did nothing but gain horsepower and torque throughout the entire RPM range and the biggest gains seen between the previously dyno'd 3" systems and my 4" system were at the bottom end. The baseline numbers in this dyno video were from the stock exhaust (roughly 2 3/4"), but our final numbers were very similar. As you can see, this truck doesn't make power in the "500-600 h.p." range, but saw massive gains from the increase in exhaust size. Many people, including myself at one point, would think that bigger isn't better, but I've seen the difference a large diameter exhaust can make first hand on a stock, <300 whp pickup truck. I've also seen it, though not to such an extreme, first hand on my 70 Oldsmobile, 02 Honda and 13 Accent. These experiences are what is driving my decision to having a custom 3" exhaust system built for my car.
 
#39 ·
I want to preface by saying do not take offence to what I am about to say.....

Congrats on the power gained in your example on the 5.3 with exhaust mods...very respectful gains. Do not however jump to conclusions that in that case the 4" gained any over the 3". I know you said same mods however it was on another vehicle and not sure if even on same dyno, same conditions, ect thus can not accurately be compared. Also unclear if stock dyno numbers were from your truck or another one so total gains may be off here as well.

All this plays a big part when you start throwing numbers around and can get confusing. And when referring to same mods on any vehicle and one making more than other people automatically start worrying why theirs does not make same ect but rarely consider all the variables that may still be different.

Also keep in mind the author who wrote article may be speaking in general terms and/or based on totally different setup than anything we are discussing here. This is why I tried to clarify in the post I made on power, pipe sizes, ect. I would like to add that setup I mentioned both pipe sizes together had same cross section as 2.75" however flowed more than a full 2.75" setup as both 38mm wg and 2.25" dump were hot gasses upstream in system. Likely flowed like 3"+ full exhaust. As the gasses cool velosity slows as well as flow rate. Length of system, bends, mufflers, and heat lost through system to name a few play a part as well.

I share this info based on actual proper dyno testing various platforms with repeatable results. Not comparing a golden delicious apple with a granny smith apple!
 
#32 ·
Thanks for the example VooDoo.

This explicitly demonstrates the engineering truth about what I alluded to with the Golf Ball. It shows that the principle works whether its the "object through the air" OR "air/exhaust through a fixed object".

******James,..he's not attributing the heads application to our car. More to the point about the dimpling or course surface causing increased speed of flow in a given system,..i.e. intake,manifold,exhaust.

CB,.I know you like your stock VT, and your pre-produced products. Be for real your VT and mine arent even in the same GALAXY.
My exhaust is the colaboration of input from a Hyundai FE, SOLO exhaust pro's, and CERMA John. Not my ideas,..their directions.

For those that want specifics.

1. A True downpipe of 3.25 squeezed onto the exhaust side of the turbo. ZERO restriction.
2. NO CAT 3inch into an exhaust of 3 in. Met with the SOLO resonator which is aerodynamically scaled, causing the exhaust flow to SPEED up.
3. Exits out a full 3inch pipe with NO MUFFLER to slow her down.
 
#40 ·
I have spent some time myself and with others comparing flow of smooth/polished, various levels of scoring/roughed up, and dimpled with the winner in most cases a scored/roughed up surface.....talking like 120-200 grit sand paper..... the more saturated the air the more flow edges towards dimpled. Less saturated air edges toward smooth/polished. Scored/roughed flows more in most conditions. Think of moisture as in humidity, fuel, water ect through a pipe.....

Also that exhaust should sound pretty mean! When/IF you dyno it see if you loose or gain anything by dumping from dp or before the resonator..........
 
This post has been deleted
#36 ·
They are golf balling all kinds of stuff now. Ive heard of it on race and aircraft engines but those are meticulously maintained. I would think that particles in either the exhaust or intake would just gather in those dimples, build up and require frequent cleaning to remain productive.
 
#46 ·
hmmmmm, after having straight pipes, from the cat back, I definitely notice a decrease in low end torque. How can I fix this without losing too much sound.... Any suggestion would help as I am no where near being mechanically inclined to understand some of this stuff :question:
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top