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Old 04-25-2001, 09:24 PM   #1
Stubaru
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Question N/A Mods Vs larger exhaust

I understand the dynamics behind having a wider diameter exhaust pipe for a turboed car, BUT if someone were to modify their NA engine would/could they create the need for the same diameter pipe that a turbo needs? For example if I put on all the goodies I could find, effectively increasing my HP to 220 or so. Would I have altered the breathing ability of my car, so that it could benefit from the larger plumbing?

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Stu
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Old 04-25-2001, 09:38 PM   #2
Blindeye_03
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No because youre still not pushing out what a turbo car puts out.
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Old 04-26-2001, 03:38 AM   #3
twend
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If you open the intake,port and polish, more aggressive cams, bigger valves,roller rods, header, pipe, etc then the larger muffler will complete the package allowing the engine to breath more(exhale what it is now capable of in haling)Though the turbo pipe may still be to big.

[This message has been edited by twend (edited April 26, 2001).]
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Old 04-26-2001, 06:09 AM   #4
NebScoob
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Just my opion here, but here goes.

The reasoning behind a smaller NA exhaust has to do with back preasure and scavaging, and even exhaust temp. Exhaust doesn't flow evenly from an NA motor, rather in pulses... Those pulses create waves and the right size exhaust actually "tunes" the waves to help pull out exhaust gases. The right size will help a NA car, through out the powerband, where as big will ONLY help at the top, where exhaust resistance becomes an issue.


On a turbo car, there is really two sets of
exhaust. Before the turbine and after the turbine. Before the turbine, you acctually want smaller piping for quick turbo spool up. After the turbo you just want very little resistance. Backpreasure and scavenging are not a factor becuase the turbo is spitting out air constantly, and the bigger, more free flowing the out side, the faster it can spin and dump exhaust gases into the exhaust.

Also, I'm thinking that, and I'm not quite sure, but since the turbo motor is force fed, it sees a lot more air at mid rpms, meaning that any exhaust restrictions play a role not only at the high rpms like on an NA car, but at low to mid rpms as well.
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Old 04-26-2001, 08:30 PM   #5
rbehny
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so your saying a 3" exhaust like the scoobysport would not be a wise idea for a na motor?
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Old 04-26-2001, 09:54 PM   #6
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Since I don't own a sube yet I can't comment if a 3" pipe will help you on yours, but I can speak from my own experiences in my Jeep.

I rebuilt my engine last year, along with upgrading the intake (12" K&N filter), stroking the engine from 4.0L to 4.8L, upped the CR, adding a high perf cam, etc, just about everything you can do. The thing just pulls, its a torque monster, but everybody who was experienced in doing these kinds of mods said to avoid a large exhaust like 3", because of backpressure. You will actually lose power where you need it with a large exhaust. I put a Borla header to a 2.5" flowmaster exhaust and that did the trick.

I find it kind of funny seeing NA cars (hondas in particular) with such huge exhaust piping (beyond the fart muffler) - they're losing whatever little precious torque they have...

Oh well

-N
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Old 04-27-2001, 06:47 AM   #7
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According to SCC, it isn't backpressure that kills the low end on a N/A car, it's low exhaust gas velocity. If you use piping that is too big for a given size engine, the velocity of the exhaust gas is lower reducing effective scavaging. The ideal setup would be to have a very high flowing muffer (straight through design) which has minimal backpressure while using the correct size piping for maximum exhaust gas velocity. According to two separate sources (SSC and Modern Engine Tuning by A. Graham Bell) the optimal diameter piping for an N/A 2.5 liter engine is 2.25".
whatever it's worth
-Shawn


[This message has been edited by S.Damery (edited April 27, 2001).]
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Old 04-27-2001, 07:05 AM   #8
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Kinda like wiring (guage and resistance) ...well, sortof...
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Old 04-27-2001, 09:30 AM   #9
gavin
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Yes, Shawn is correct. In theory, zero backpressure is optimal since there is absolutely no resistance on the exhaust leaving the cylinder, but in practice, you lose all your low-end power.

What maintains low-end torque, on a small engine is maintaining exhaust speed. There is a period in the piston stroke in which the intake and exhaust valves are both open. As the exhaust exits, it creates a low pressure vacuum around the valve, which helps draw more air/fuel into the cylinder for the next detonation. Too large an exhaust eliminates this vacuum, because the exhaust isn't moving fast enough, and your low-end power goes to hell.

[This message has been edited by gavin (edited April 27, 2001).]
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Old 04-27-2001, 09:40 AM   #10
remarcable
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I know of somebody running a straight-pipe (no muffler) on a '95 Impreza L.

He sez it gives him 15hpz!

I just had to laugh when I heard that.
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Old 04-27-2001, 10:14 AM   #11
J Colman
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I wouldn't go above 2.5 inches on N/A
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Old 04-27-2001, 11:40 AM   #12
inpreza kid
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i have a 2.5" exaust system on my car. i haven't noticed any low end loss so far but if i was staying N/A i would have gone with 2.25"
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Old 04-27-2001, 11:56 PM   #13
Kevin Thomas
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Question

Shawn or anyone else, I am hoping you can answer this question.

What is the formula for finding out what exhaust diameter is needed for how many liters your car have? If there is no formula, then what is the maximum exhaust diameter for a Subaru Justy and SVX? I believe they are 1.3 and 3.3ltrs respectively. I'm at work and can't surf the web real quick. Gotta go!
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Old 04-28-2001, 07:28 AM   #14
S.Damery
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Kevin,
I don't know the formula but I assume that there is one, probably based on the cross-sectional area of the pipe diameter vs. displacement etc. I can relay the guidelines for suggested exhaust piping diameter for a N/A engine from SCC though.

1500-2000cc-2.0"
2100-2500cc-2.25"
2600-3000cc-2.50"

One could argue that based on this info that we are on the upper end of the range for 2.25". This is interesting because I think that I used 2.25" on my 1.6 Civic Si which was thought at the time to be optimal. Oh well, live and learn. Trey says the the stock exhaust system (I think is 1-7/8") was generically designed by Subaru with the 2.2 engine in mind. They also mention that 2.5" would be the minimum for smaller turbo engines and 3.0" for 2000cc+ turbo engines. I left my "Modern Engine Tuning" book at work, I can give you those numbers next week sometime if you'd like. Hope this helps.
-Shawn
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