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Old 01-31-2012, 11:14 PM   #1
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Default 2.75 downpipe 3 inch exhaust

I have searched and have not found the answer. Either I search bad or this is to stupid to ask.

I have the beginning s1 kit. The dp is 2.75. I am looking to buy an exhuast and cat. Seems like I should go 3 inch. But with the 2.75 dp am I just blowing a few extra bucks for the .25 inches that I can not utalize do to the downpipe size.

I find a lot of posts saying go with the three inch but none that asked about the dp size. Maybe most have three inch dp?
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Old 01-31-2012, 11:28 PM   #2
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3" so you can get a real DP later. The more you open up the exhaust, the better.
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Old 01-31-2012, 11:39 PM   #3
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Seems to make sense and it is notmuch more money.
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Old 02-01-2012, 09:38 AM   #4
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Thinking to go with the MeganaFlow 3 inch cat. $109

then go with the enthuza turbo exhaust. $419.

Seems to be the most reasonable prices I can find. Also seems like the posts i have found people have spoke well of both items. The enthuza cat might be nicer but it is 2x more expensive. Everything for me comes down to budget. I do agree going with the 3 now is the way to go. Little sad by not having a real DP.
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Old 02-01-2012, 01:09 PM   #5
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Is the Magnaflow 3 inch cat actually made to fit into the default spot on the miata, or is that just the cost of the cat and you need to pay a shop to make it fit?
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Old 02-01-2012, 01:12 PM   #6
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Quote:
Originally Posted by hustler View Post
3" so you can get a real DP later. The more you open up the exhaust, the better.
Truth.
Even with the 2.75 dp it will already free up more power than a 2.5
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Old 02-01-2012, 01:23 PM   #7
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I'm not sure I follow.

DP is 2.75". great.

Everything should be bigger beyond it. Hell, if anything you want to go to 4" by the time you're at teh muffler.
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Old 02-01-2012, 03:54 PM   #8
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Braineack - That is what i was not undestanding. I was thinking the bottleneck would be the downpipe so there would not be an advantage of going larger after the DP.

I am now understanding that I should not let the limited DP size limit the size of the exhaust past that point.

Oh lordy 4" .. now I have another thing to think about. I do not think I need 4" with the power I plan on doing.

Shuiend - I needed a shop to weld the exhaust together. My assuption was that it would fit. And this is why I make posts because simple things I forget... Like will it actualy fit. I am also going to contact Enthuza to see if they have a more budget set up for a 3" that would work.
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Old 02-01-2012, 03:58 PM   #9
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Quote:
Originally Posted by zeroox View Post
Shuiend - I needed a shop to weld the exhaust together. My assuption was that it would fit. And this is why I make posts because simple things I forget... Like will it actualy fit. I am also going to contact Enthuza to see if they have a more budget set up for a 3" that would work.
I bet after you get done paying a shop to weld in the cat that costs start to get very close to what an Enthuza cat would cost. Jason at Enthuza will prob be able to make one end of the cat the 2.75" to fit the downpipe and the other 3" to fit the rest of the exhaust.
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Old 02-01-2012, 04:24 PM   #10
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Shuiend - I think you might be right on that. I am going to contact him and see what can be done.
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Old 02-01-2012, 04:27 PM   #11
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Quote:
Originally Posted by zeroox View Post
I was thinking the bottleneck would be the downpipe so there would not be an advantage of going larger after the DP.
Take an intro to fluid dynamics class.

2.75" DP to 3" exhaust = better flow.


2.75" DP to 2.75" exhaust = not better flow.
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Old 02-01-2012, 04:38 PM   #12
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Braineack - Why would I have to take an intro class when you just informed of everything I need to know ;o)

I get stuck in the computer world. The way things bottleneck on a network is completly different. Data will only move as fast as the most restrictive point. It is a learning process and I have enjoyed it thus far.

I do appreciate the help that the forum has provided. And I do try to find things on my own and research before asking. I am going to look up fluid dynamics and see I can find on air flow.

Thanks Again
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Old 02-01-2012, 04:52 PM   #13
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nutshell:

high pressure to low pressure.

exhaust gasses cool, slow and expand as they travel through the exhaust. A smaller DP encourages higher flow aft turbo, where the large exhaust allows for less restrictons/bottleneck as the exhaust gasses start to fill out and pack onto each other.
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Old 02-02-2012, 09:05 AM   #14
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Ummm, I was always under the impression that as gases cool, they become more dense and contract. So having a larger diameter as the gases contract makes for a lower pressure and less energy used to push the exhaust gases out.
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Old 02-02-2012, 12:50 PM   #15
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That gasses certainlly don't contract. Gases expand to fill a container. So with that being said, the more volume you have in your exhaust piping, the more exhaust you'll be able to put into it before having to compress it.

So the idea of a smaller diameter DP tube encourages higher velocity...where the turbo has an easiiery job of expelling large quanities of gasses due the the scavenging effect.

If the exhaust tube isn't increased, then the gasses fill up the tubes, and stack up and create backpressure and the turbo must push against it all in order to continue to expell. This is reduces power output.

A step increase, especially after the wastegate dump, is the best way to promote exhaust flow, velocity, and scavenging to reduce the amount of torque waisted pushing out the exhaust and not pushing down the pistons.


Think about how your IM tapers down into a very smaller runner...this increases promotes the increase of velocity into the combustion chambers, allows all the oxygen molecoles to stack up and pile up in as fast as possible...we are doing the opposite on the other side--promoting them to get the hell out.

Last edited by Braineack; 02-02-2012 at 02:54 PM.
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Old 02-02-2012, 01:26 PM   #16
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/\ This post should be stickied. simple and easy to understand and this question gets asked all the time. up until a year or so ago I was under the same impression as OP and thought a bottleneck would basically kill all gains before and after.
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Old 02-02-2012, 02:55 PM   #17
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IIRC my DP starts at 2.25 and ends at 2.75...then my cat and back is 3".
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Old 02-02-2012, 03:00 PM   #18
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You need to go 8" all the way back, Fae style
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Old 02-04-2012, 11:18 AM   #19
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I have it the wrong way round 3 inch t25/t28 elbow to 2.75 inch evo downpipe. It is a good fit though

http://i578.photobucket.com/albums/s...3/P1020570.jpg
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Old 02-04-2012, 12:58 PM   #20
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that's actually not bad since the wastegate dumps into the elbow, gives everything a chance to mingle before being ejected down the dump.
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