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

This is a discussion on So what's best 2.5 or 3 inch exhaust?? within the Veloster Performance forums, part of the Veloster Turbo Garage category; 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 ...

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

  1. #1
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    So what's best 2.5 or 3 inch exhaust??

    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.

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    Time to google. See how lower displacement engines gain or lose power in certain areas.

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    Looking at Honda forums the majority of 1.6l turbo cars are running 2.5 inch exhausts. Not many 3 inch exhausts.

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    i tried explaining how exhaust velocity =/= backpressure to some people here before it didn't go over to well and that 3" isn't necessarily the way to go. I'm glad someone sees this lol

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    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.

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    Quote Originally Posted by The Mackies View Post
    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.
    theres more then one?

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

    I would like to see dyno sheets of both 2.5" and 3.0" so that there is concrete numbers and no hypothesizing.....

    2013 Veloster Turbo 6spd MT - Ultimate edition

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    there was the one here and I heard another on youtube, but thinking it may of just been the same one now I seen the 2nd video that was posted OOPS.

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

    Quote Originally Posted by jeremy View Post
    I would like to see dyno sheets of both 2.5" and 3.0" so that there is concrete numbers and no hypothesizing.....

    What Jeremy said, plus I want to be able to compare the curves.

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    For a 1.6 Turbo engine 2.5" might already be too wide. The engine needs backpressure in the exhaust system.
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