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Discussion Starter · #1 ·
i am debating wether or not to get a full cat back system or to just go the cheaper way and have a shop put 2.25 inch piping in there from the cat back but then it wouldnt be mandrel bent, will this hurt performance at all? will i see any gains from enlarging the piping? i have read all the exhaust articles and its still not clicking, what all would i need to make a great exhaust system, headers, downpipe, cat, cat back, muffler? is this correct? if so would it be the best to buy a cat back first or headers or what u konw what im sayin. thanks in advance.
 

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Why not just get the muffler shop to mandrel bend you some custom piping?
 

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Discussion Starter · #3 ·
i cant find a local shop that does mandrel bent, they all tell me i have to order it. wich is exaclty what i didnt want to do
 

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Discussion Starter · #4 ·
wait, so it says it decreses size about 20% so does this mean i could just go with a lil bit bigger piping or just not worry about it and go with 2.25 and be cool.
 

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If you are concerned with performance get mandrel bent. If not then get whatever is cheapest. A crush-bent 2.25" exhaust will crush down to smaller than your stock exhaust at the bends.
 

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Discussion Starter · #6 ·
so ur saying its pretty much pointless to do that? lol wow thats great to hear, i did this with my last car and thought it helped, but i mean i dont want to spend my money on my car that is not going to do anything, thanks alot. erik
 

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atlimport26 on Sep/23/02 said:
wait, so it says it decreses size about 20% so does this mean i could just go with a lil bit bigger piping or just not worry about it and go with 2.25 and be cool.
No by going w/ bigger crush bent piping you will create an "hour glass effect" and kill your flow velocity.
 

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on a 2.5 in. system, a 2.5 in. crush bent will outdo a 2.25 in. mandrel bent any day....

btw exhausts use tubing not piping. the correct term is tube not pipe which is a common mistake (eg. B pipe) .

a pipe is measured and ordered from the manufacturer
based on its INNER diameter (ID).


a tube is measured and ordered from the manufacturer based on its OUTER diameter (OD).


any muffler or custom exhaust builder will confirm this distinction in terms with you if you ever want to order and make your own custom exhaust.

custom exhausts are ordered based on OD sizing , not ID, and therefore use tubing not piping.
 

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Ok, michael, can you answer this question? Im confused as to whether a compressable gas such as exhaust gasses behave differently when confronted with bottlenecks compared to say a liquid such as water. When an uncompressable liquid hits a bottlenecks in a pipe the flow capacity of the entire pipe is determined by the smallest bottleneck, it doesnt matter how many or how big, the smallest one is what determines how much that system can flow right? My question is if compressable gas behaves differently, since it can be compressed by the motor (backpressure) would the flow capacity of the system by determined by the number and size of all the bottlenecks combined or would it still be limited to the smallest bottleneck? Just trying to understand what you were saying about the 2.5 crush vs 2.25 mandrel, since it would make sense that if the flow capacity would be determined by the number and size of bottlenecks, the reduction in power caused by the backpressure put on the motor as the gas is forced and compressed as it flows through bottlenecks would be less of a power drain than the motor forcing gas through a system which is smooth but of a smaller diameter throughout its legnth.


Thnx,
A beginner to physics and definatly flow dynamics ;-p
 

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you can buy the mandrel bends, i'm sure any exhaust shop wouldn't mind welding them in, and it'll still be cheaper than buying a full out cat-back... just get them to tell you what they need.
mandrel bends for cheap
 

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I ran 2.5" crush bent w/ a apexi N1 muffler and It was fine. I had great top end power(definently noticable). I have no problem with it. I say inless your running forced induction or decently built NA, you should go crush bent. Crush bent w/ apexi N1 muffler 300, apexi N1 muffler with maderal 500+. You do the math. Just my .02 late
 

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when it comes to exhaust flow theory, we all work from the same basic principle of Bernoulli's equation. It assumes exhaust gases and air behaves like a compressible, elastic fluid (the field of physics is called fluid dynamics).

now granted, from the university mechanical engineering graduate's perspective, Bernoulli has several limitations: the biggest one assumes that in laminar flow, all of the parallel layers of a liquid's flow behave the same way. So flow at the center of a conduit (like a tube or pipe) behaves identically to the layer at the periphery near the walls. Therefore, if you sample any single layer, the assumption is that it represents all layers. This is not necessarily true. However, I am not a mechanical engineer and I'm not going derive the better model that represents laminar flow at different subsections of a conduit.

So, when you are talking to me, we are accepting Dr. Bernoulli,... lock, stock, and barrel.

Now how does a mechanical engineer actually measure and calculate flow?

Well, there are 2 devices:

1. a pressure manometer

2. a pitot static tube

a pressure manometer which is a pressure plate attached to a spring in which the amount that the spring is compressed is proportional to the amount of a calibrated known pressure value.

The second device is a Pitot static tube. This device looks like a glass tube in the shape of an upside down capital "L". This device measures flow speed at a very specific point.




theory for measuring flow using pitot tubes

so we can calculate bulk flow of a compressible elastic fluid by an equation:

bulk flow = Pressure + ( [density of air/2] x flow velocity squared] )


They take 2 measurements along the length of the tube and obtain the pressure manometer readings at these 2 points. The density of air is a constant for a given temperature. You can sample in different layers (because they are supposed to be all identical) or you can sample in the same layer at 2 different points along the length of the tube. We can do this on a crushed bent tube and on a mandrel bent one and then compare after we calculate out the value for the equation.


The other way to calculate flow is by taking a pitot tube measurement and calculating the total area.

Flow(cfm) = Flow Velocity(ft/min) x Average Path Area (ft^2)

So when it comes to crush bent vs mandrel bent, we must see the effect of a change in the cross-sectional area of the tube for a mandrel bent tube vs a crust bent tube. If we say a perfect circle has an area of Pi x radius squared, then it is the square of the reduced amount of radius that affects the amount of flow. It's NOT a straight linear 1:1 relationship.


If Magnaflow says the equivalent diameter of a crushed bent 2.5 in. tube is 2 in., then I am incorrect in my statement. How they arrived at that value, I do not know.

If it's true then, the 2.25 in. mandrel bent tube would flow 27% better.
 

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Unlike most message boards, when I come on here I actually feel smarter by reading the posts, not like I just dropped a few brain cells :). Thnx for the info mike.
 

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ICC96INTEGRA on Sep/23/02 said:
I ran 2.5" crush bent w/ a apexi N1 muffler and It was fine. I had great top end power(definently noticable). I have no problem with it. I say inless your running forced induction or decently built NA, you should go crush bent. Crush bent w/ apexi N1 muffler 300, apexi N1 muffler with maderal 500+. You do the math. Just my .02 late
Um its really not that expensive lol. I dont know what kinda crazy shops there are around you. I did my full 2.5 Mandrel for a little under 300 and that included my 2.5 muffler. The only thing that i need is a donut gasket for the end of the header because the one i have on there now is leaking. As for the part that the shop did...i had them mandrel bend everything from the cat back, put hangers on there, and weld the muffler on there and it was only like $120. If that is more than crush bent then i think i would rather pay a little more for good quality exaust.
 

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I was going on Magnaflow's diagram that there is a 20% pipe reduction size in a crush bend. Since they are talking about cross-sectional area I figured this:

Cross sectional area for 2.5" mandrel bent pipe:
(1.25^2)pi = 4.91 sq. in.

Reduce cross-sectional area by 20%
4.91*0.80 = 3.93

Find equivilent pipe diameter.
(r^2)pi = 3.93
r^2 = 1.25
r = 1.12
d= 2.24

So according to Magnaflow a 2.5" crush-bent pipe would give the equivilent cross-sectional area of a 2.24" pipe at the bends.
 

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To calculate the crush bent tubing needed for the same area as a 2.5 in. mandrel bent tubing, we just need to multiply the area on the 2.5 in. mandrel bend by 1.25 (i.e. 1 / 0.20 = 1.25 or the inverse of 20% loss in area from crush bending).

Cross-Sectional Area for 2.5 in. mandrel bent tubing (using pi x radius squared) = 4.91 sq. in.

Proper Adjusted Cross-Sectional Area for Crush Bent Tube = 4.91 x 1.25 = 6.14 sq in.

area = PI x radius squared = 6.14 sq in.

radius = SQRT [ 6.14 / PI ] = SQRT [1.95] = 1.40 in.


where SQRT = square root

Therefore,

Diameter Needed For Crush Bent Exhaust to have the same Area as the 2.5 in. OD Mandrel Bent Exhaust is :

2 x 1.40 = 2.80 in.


So if you are going to get a crush bent exhaust made for a 2.5 in. system, use a 2.80 in. exhaust Bpipe. You must weigh the size of the diameter in the straight portions to see if it's still within your power range without going to large though.

thanks SurferX.
 

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I need to stop saying pipe. It's really a hard habit to break. Measurements are on the OD, so tubing is the correct word as MD has been calling it.
 
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