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post #1 of 14 (permalink) Old 06-06-2016, 08:35 PM Thread Starter
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stock exhaust sizes

I may get yelled at for asking instead of searching a little longer but after looking on my own i cant find the anwser to my questions: whats the diameter of the collector on the stock headers? What are the size are the pipes? Also if i keep the stock headers will i lose power/torque by going to a larger true dual with high flow cats and bullet mufflers? As stupid as it may seem for me to ask the last question, in my research i saw people had written about them losing power/torque after deleting the mufflers and such due to lack of back pressure. If someone could help it would be much appreciated. Thanks in advance
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post #2 of 14 (permalink) Old 06-06-2016, 10:35 PM
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First of all, what car are we talking about?
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post #3 of 14 (permalink) Old 06-07-2016, 05:33 AM Thread Starter
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1996 thunderbird with a 4.6. Sorry for not clarifying
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post #4 of 14 (permalink) Old 06-07-2016, 05:47 AM
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The vehicle in question didn't get headers but log-style exhaust manifolds.

IIRC the pipes are 2" from the manifolds to the front Y-pipe, 2.25" from the Y pipe to the mufflers, and 2" again out of the mufflers.

Ugh, back pressure. Ignore all you read about it except for the part where it's bad. Ever try to drive a vehicle with clogged cats? THAT'S back pressure, and that's why it's bad.

Exhaust upgrades on stock or near-stock cars should be done tastefully as raw flow is not what will provide performance improvements, but rather improvements to scavenging. Scavenging is the process by which the pulsating (high pressure-low pressure pattern) flow of gases through the pipes draws exhaust out of the combustion chamber while the exhaust valve is open due to a low pressure at the valve. If you increase the pipe diameter, you slow down the overall speed of the exhaust gases flowing through the pipes and thereby decrease the scavenging effect at lower RPM when there isn't much air moving, and THAT's where the loss of torque at lower RPM comes from. At higher RPM the flow is less restrictive and the effects of scavenging are returned thereby giving a performance boost. It's got nothing to do with back pressure.

X-pipes and H-pipes allow scavenging to occur between the two sides of the engine in the exhaust stream which will help scavenging from one bank to another. Headers (mid-lengths or long-tubes) give the best scavenging within one bank of cylinders. Scavenging effects are limited with the stock cam profile due to the duration and timing of the exhaust valve events, but having a fantastic exhaust paves the way for more aggressive cams to work properly later on should they be installed.
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post #5 of 14 (permalink) Old 06-07-2016, 11:02 AM Thread Starter
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Originally Posted by theterminator93 View Post
The vehicle in question didn't get headers but log-style exhaust manifolds.

IIRC the pipes are 2" from the manifolds to the front Y-pipe, 2.25" from the Y pipe to the mufflers, and 2" again out of the mufflers.

Ugh, back pressure. Ignore all you read about it except for the part where it's bad. Ever try to drive a vehicle with clogged cats? THAT'S back pressure, and that's why it's bad.

Exhaust upgrades on stock or near-stock cars should be done tastefully as raw flow is not what will provide performance improvements, but rather improvements to scavenging. Scavenging is the process by which the pulsating (high pressure-low pressure pattern) flow of gases through the pipes draws exhaust out of the combustion chamber while the exhaust valve is open due to a low pressure at the valve. If you increase the pipe diameter, you slow down the overall speed of the exhaust gases flowing through the pipes and thereby decrease the scavenging effect at lower RPM when there isn't much air moving, and THAT's where the loss of torque at lower RPM comes from. At higher RPM the flow is less restrictive and the effects of scavenging are returned thereby giving a performance boost. It's got nothing to do with back pressure.

X-pipes and H-pipes allow scavenging to occur between the two sides of the engine in the exhaust stream which will help scavenging from one bank to another. Headers (mid-lengths or long-tubes) give the best scavenging within one bank of cylinders. Scavenging effects are limited with the stock cam profile due to the duration and timing of the exhaust valve events, but having a fantastic exhaust paves the way for more aggressive cams to work properly later on should they be installed.
Awesome, thank you for the info. Since my car is stock, what would be the best exhaust size? 2 1/4 or 2 1/2? And the exhaust i am planing would have an x pipe
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post #6 of 14 (permalink) Old 06-07-2016, 03:36 PM
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Depends what your plans are and what your ultimate goals end up. Generally speaking the 2-1/2 dual setup from Trubendz is a good place to start, leaving you with plenty of room on the table down the road.

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post #7 of 14 (permalink) Old 06-07-2016, 08:53 PM
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Quote:
Originally Posted by Wile E. Coyote Jr. View Post
I may get yelled at for asking instead of searching a little longer but after looking on my own i cant find the anwser to my questions: whats the diameter of the collector on the stock headers? What are the size are the pipes? Also if i keep the stock headers will i lose power/torque by going to a larger true dual with high flow cats and bullet mufflers? As stupid as it may seem for me to ask the last question, in my research i saw people had written about them losing power/torque after deleting the mufflers and such due to lack of back pressure. If someone could help it would be much appreciated. Thanks in advance
Here is a very good essay on exhaust: Exhaust Function 101 - Automotive Forums .com Car Chat Exhaust Function 101

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post #8 of 14 (permalink) Old 06-14-2016, 09:26 AM
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I would get the 2 1/4" Trubend system, and add an H-pipe. More than big enough for a stock or slightly modified 4.6.

H-pipe will help low-end torque, and give the exhaust a deeper sound.

Al

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post #9 of 14 (permalink) Old 06-14-2016, 10:05 AM
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Originally Posted by T6Rocket View Post
I would get the 2 1/4" Trubend system, and add an H-pipe. More than big enough for a stock or slightly modified 4.6.

H-pipe will help low-end torque, and give the exhaust a deeper sound.

Al

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post #10 of 14 (permalink) Old 06-14-2016, 10:40 AM
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Ugh, he lost credibility when he started spouting about less back pressure being bad.

-Brandon
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post #11 of 14 (permalink) Old 06-14-2016, 04:10 PM
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Originally Posted by theterminator93 View Post
Ugh, he lost credibility when he started spouting about less back pressure being bad.
The value of the video is primarily in its explanation of scavenging, and the difference in the two types of hardware.

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post #12 of 14 (permalink) Old 06-15-2016, 02:04 PM
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Originally Posted by theterminator93 View Post
Ugh, he lost credibility when he started spouting about less back pressure being bad.
It is .. cutting off your mufflers completely will make a noticeable difference in low end torque. If you unbolt the cat back portion of the exhaust and run it wide open after the resonator, you will feel a much greater lag off the line. I had a dual 2" exhaust setup at one point in time, when I installed a factory exhaust it was actually an improvement. So yes, less back pressure is bad.
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post #13 of 14 (permalink) Old 06-15-2016, 02:21 PM
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Wow, I thought a true 2" dual system would be the best for a stock vehicle. That exact system (plus H-pipe) is factory on our CVPI, and it has excellent low end torque, but it is a 250 HP motor.

SCTBird1994: When you had the duals, was there an H-Pipe in there?

Al

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post #14 of 14 (permalink) Old 06-15-2016, 05:43 PM
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Consider this tidbit from the article I linked to:

*MYTH KILLER – “You need back pressure in an exhaust system”*
I’ve heard this several times over the years but it simply isn’t true. This theory probably started when someone lost an exhaust pipe at or near where it connected to a factory exhaust manifold and their engine ran worse without the exhaust system attached. A factory system is engineered to create a scavenging effect that is needed to maintain exhaust flow and pull the exhaust gases out of an engine. The above scenario wouldn’t happen on an engine equipped with tuned headers, which both explains what happened in the lost pipe story and why this doesn’t happen on a race car that has no exhaust system.

http://www.automotiveforums.com/vbul....php?t=1135176

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