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post #1 of 41 Old 06-17-2011, 07:45 PM Thread Starter
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I4 Dual Exhaust Conversion

Hey guys, I'm converting my single I4 exhaust to a dual magnaflow exhaust

parts:
2 Magnaflow #14832 Mufflers (2.25 inlet, 2.5 outlet)
Lawson Industries Y split Pipe (2.25 inlet, dual 2.25 outlet)
Heat Shielding and Misc Parts

I found a mechanic and he quoted me $150, but the thing is he wants to run 2" piping from the Y split back to the two mufflers while I want to run 1.75" piping in order attempt the sustain the same back pressure.

so questions:
Which piping should i use 2" or 1.75"?
Should I keep the Resonator on? (I want a deep sound, not too loud though)

pictures next week after the conversion is done =]


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post #2 of 41 Old 06-17-2011, 08:25 PM
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its gonna look great !


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post #3 of 41 Old 06-17-2011, 08:28 PM
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I would like to see that,,when its done,please post pictures

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post #4 of 41 Old 06-17-2011, 10:50 PM
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He probably wants to run 2" piping because it will be easier to step it down from 2.25" at the y pipe and then back up to 2.25" at the muffler. He should be able to do 1.75" but expect to pay more since he will have to do more labor. I don't think it will make too much of a noticeable difference either way. Back pressure is such an overused, misunderstood concept. What you want to maintain is exhaust velocity which smaller piping will allow. It's your car so you can pick the pipe size you want but labor costs will go up with the smaller price. If it was me I'd probably just go 2". You're already a step ahead of most I4 dual conversion drivers by stepping down to 2".

As far as the resonator I would leave it on. Drive around for a few hundred miles and see if you like the sound. If you want still want to change the sound more after that you can have the resonator replaced with a different resonator or with a piece of straight pipe.

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post #5 of 41 Old 06-18-2011, 05:43 AM
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I'd be concerned with reduced back pressure.

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post #6 of 41 Old 06-18-2011, 06:20 AM Thread Starter
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Quote:
Originally Posted by xJoeMama78 View Post
He probably wants to run 2" piping because it will be easier to step it down from 2.25" at the y pipe and then back up to 2.25" at the muffler. He should be able to do 1.75" but expect to pay more since he will have to do more labor. I don't think it will make too much of a noticeable difference either way. Back pressure is such an overused, misunderstood concept. What you want to maintain is exhaust velocity which smaller piping will allow. It's your car so you can pick the pipe size you want but labor costs will go up with the smaller price. If it was me I'd probably just go 2". You're already a step ahead of most I4 dual conversion drivers by stepping down to 2".

As far as the resonator I would leave it on. Drive around for a few hundred miles and see if you like the sound. If you want still want to change the sound more after that you can have the resonator replaced with a different resonator or with a piece of straight pipe.
I don't mind 2" piping which I assume he already got, but I'm just worried about losing some performance/torque in the low rpm's since if I run 2" piping back I would be at a 63% reduction in exhaust velocity, 1.75" would be an 82% reduction assuming that I simplify the equation to not include thermodynamics and the density of the exhaust gas is constant.

Quote:
Originally Posted by R2D2 View Post
I'd be concerned with reduced back pressure.
I am too lol, but I assume I can regain some of it by intake headers/cai

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post #7 of 41 Old 06-18-2011, 09:30 AM
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Make sure the installer has done a significant number of custom mufflers. Don't want a rookie working on your ride. Lol.
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post #8 of 41 Old 06-18-2011, 09:42 AM Thread Starter
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Quote:
Originally Posted by lui View Post
Make sure the installer has done a significant number of custom mufflers. Don't want a rookie working on your ride. Lol.
Lol, I trust him, atleast the head mechanic/owner of the shop, don't really like one of the guys at the shop, he was talking down to me thinking I didn't know much about cars and didn't know what a cat back was lol. Oh and in a different language I understood lol...

anyway the head mechanic baby's my car xD hes worried about it looking perfect rather than slapping/welding it on, so he went to go get custom bent pipes from another shop free of charge lets just hope they're mandrel bent =]

For the love of jeebus I'm a mechanical engineer I think i know a tiny amount about cars lol

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post #9 of 41 Old 06-18-2011, 10:03 AM
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Quote:
Originally Posted by Danni3 View Post
Hey guys, I'm converting my single I4 exhaust to a dual magnaflow exhaust

parts:
2 Magnaflow #14832 Mufflers (2.25 inlet, 2.5 outlet)
Lawson Industries Y split Pipe (2.25 inlet, dual 2.25 outlet)
Heat Shielding and Misc Parts

I found a mechanic and he quoted me $150, but the thing is he wants to run 2" piping from the Y split back to the two mufflers while I want to run 1.75" piping in order attempt the sustain the same back pressure.

so questions:
Which piping should i use 2" or 1.75"?
Should I keep the Resonator on? (I want a deep sound, not too loud though)

pictures next week after the conversion is done =]
I am absolutely not an expert on this subject, but I remember reading that back pressure becomes less important the farther downstream in the exhaust system you go. As mentioned above, backpressure is related to exhaust gas velocity. (As opposed to sticking a potato in the exhaust pipe.) The thinner the tube, the faster the air moves through it. That is what creates the back pressure. But it also means the exhaust gas will travel a greater distance per unit time.

Apparently the concept of moving downstream quickly is REALLY important at the beginning of the exhaust system. It regards valve overlap. For a split second during each intake stroke, the exhaust valves remain open allowing the piston to draw exhaust gas back into the cylinder. At high rpm, when the piston speed is high and shoots the exhaust gas out at a high velocity, the spent gas moves away from the exhaust valves quickly. As a result, the gas is too far away to get drawn back into the cylinder during the intake stoke. But at low rpm, the exhaust gas moves slowly and remains close to the exhaust valves. During the intake stroke, some of the spent exhaust gas gets drawn back into the cylinder. The spent exhaust gas has almost no oxygen in it and starves the cylinder. THAT is what hurts low end power, a lack of oxygen.

The challenge is to design the exhaust manifold and the first few feet of the exhaust system to be thin enough to promote sufficient velocity at low rpm, but not so thin that it prevents the exhaust gas from flowing easily at high rpm. The OEM design is pretty good for that. Once the exhaust gas is beyond the catalytic converter, backpressure has less and less impact on what happens close to the exhaust valves. As long as you use the OEM exhaust manifold and catalytic converters, there should be very little difference between a 2.00" or 1.75" inch piping after the Y pipe.

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post #10 of 41 Old 06-18-2011, 01:46 PM Thread Starter
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I guess the best way to determine my performance loss is to dyno it after the exhaust job is done lol, hopefully its not too much that i can't regain from a cai + tbs

can't decide if I want springs, cold air intake, or minor mods (black out headlight housing, angel eyes, vled switchback, fog lights) next

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post #11 of 41 Old 06-18-2011, 02:37 PM
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For the love of jeebus I'm a mechanical engineer I think i know a tiny amount about cars lol
Your profile says your 19. You already finished college?
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post #12 of 41 Old 06-18-2011, 04:37 PM
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Don't worry about the loss in back pressure and torque. I have the same setup that your going for, 2.25 inch piping without the pre-muffler, connected to MagnaFlow 14832's and I have suffered zero losses in low rpm torque. Car feels the same down low, but sounds excellent!


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post #13 of 41 Old 06-18-2011, 05:16 PM Thread Starter
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Your profile says your 19. You already finished college?
I'm a semester ahead, going into my junior year this fall, lol, studied dynamics and enough about machine design to know a tiny bit about cars but its a learning process =P

Quote:
Originally Posted by k24coupe View Post
Don't worry about the loss in back pressure and torque. I have the same setup that your going for, 2.25 inch piping without the pre-muffler, connected to MagnaFlow 14832's and I have suffered zero losses in low rpm torque. Car feels the same down low, but sounds excellent!
thats good to hear, figure I go with 2" just to be safe and its convenient=P

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post #14 of 41 Old 09-22-2011, 08:22 PM
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conversion

so how did it end up coming out? can i get a pic? i would DIE for a sound clip ahhaha, i'm on the verge of doing this myself

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post #15 of 41 Old 09-22-2011, 08:33 PM
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i bought the full catback for the v6 and had it installed on my i4 ran me $850 (canadian) with install, love the sound its deep.

i dont know to much about the piping and the downsizing your looking at but i kept mine at the 2.25" that comes stock and with what the magnaflow comes with.


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