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Old 12-01-2012, 12:11 AM   #11 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
stock systems do a pretty good job, but long tube headers with a crossover pipe can help, depending on the engine in question
They might pick up a little bit, but headers are expensive and technically not smog legal since they are "emissions equipment".

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Old 12-01-2012, 12:29 AM   #12 (permalink)
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In Mazda's promotional pages for their Skyactiv engines they talk about the advantages of a long 4-2-1 header in reducing/preventing knock in very high compression engines:


Quote:
In order to lower the temperature at compression TDC, reducing the amount of hot exhaust gas remaining inside the combustion chamber is effective. For example, with a compression ratio of 10:1, a residual gas temperature of 750 deg. C, and an intake air temperature of 25 deg. C, if 10% of the exhaust gas remains, the temperature inside the cylinder before compression increases by roughly 70 deg. C, and the temperature at compression TDC is calculated to increase by roughly 160 deg. C. Therefore, it can be easily inferred that the amount of residual gas has an major impact on knocking.

These calculations are summarized in Fig.2, and as indicated, if the amount of residual gas is halved from 8% to 4%, the temperature at compression TDC is calculated to remain the same even when the compression ratio is increased from 11:1 to 14:1.

This reduction of residual gas was focused on for SKYACTIV-G, enabling the realization of a high compression ratio gasoline engine.

One option to significantly reduce residual gas is the adoption of a 4-2-1 exhaust system. As shown in Fig.3, when the exhaust manifold is short, the high pressure wave from the gas emerging immediately after cylinder No. 3’s exhaust valves open, for example, arrives at cylinder No.1 as it finishes its exhaust stroke and enters its intake stroke. As a result, exhaust gas which has just moved out of the cylinder is forced back inside the combustion chamber, increasing the amount of hot residual gas. With a short exhaust manifold, the high pressure wave arrives at the next cylinder within a short amount of time, causing this adverse effect to continue from low to high engine speeds. However, with a long 4-2-1 exhaust system, since it takes time for the high pressure wave to reach the next cylinder, the effect mentioned is limited to extra-low engine speeds, making the reduction of residual gas at almost all engine speeds possible.
Source= MAZDA: SKYACTIV-G | ENGINE | SKYACTIV TECHNOLOGY

They go on to describe challenges with keeping the Cat warm caused by having a long header.
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Old 12-01-2012, 10:50 AM   #13 (permalink)
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Where even the long tube headers improve low end torque, it's still up near torque peak RPM. Hypermiling has us keeping revs far lower than that where benefits will be negligible and not worth the hassle/money.
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Old 12-01-2012, 02:24 PM   #14 (permalink)
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I have always knocked out the honeycomb portion of the cat after it fails and triggers a MIL code. That has resulted in about a 5% mpg increase in my experience. It also makes the engine noise a bit more pronounced. No smog checks in most of Oregon. Heck, smog doesn't exist in Oregon, so the checks they do have are silly.
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Old 12-01-2012, 02:47 PM   #15 (permalink)
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Remember though that "knocking out the honeycomb" of a failed cat may not have the same results as replacing a functioning cat. What caused the cat you knocked out to fail? If excessive missfires has resulted in a cat overtemp and partially melted the matrix, the cat would have become restrictive.

On the other hand, replacing a functioning cat with a pipe seems to have been tested to be non-productive (according to earlier posts). I haven't tested this, so I'm not arguing one way or the other, just pointing out that for this thread to be meaningful, we need to compare apples to apples, eg replacing good cats with pipes.
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Old 12-01-2012, 02:57 PM   #16 (permalink)
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Quote:
Originally Posted by serialk11r View Post
They might pick up a little bit, but headers are expensive and technically not smog legal since they are "emissions equipment".
Not true mine were carb approved.
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Old 12-01-2012, 03:08 PM   #17 (permalink)
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Quote:
Originally Posted by StellaTheZJ View Post
Remember though that "knocking out the honeycomb" of a failed cat may not have the same results as replacing a functioning cat. What caused the cat you knocked out to fail? If excessive missfires has resulted in a cat overtemp and partially melted the matrix, the cat would have become restrictive.

On the other hand, replacing a functioning cat with a pipe seems to have been tested to be non-productive (according to earlier posts). I haven't tested this, so I'm not arguing one way or the other, just pointing out that for this thread to be meaningful, we need to compare apples to apples, eg replacing good cats with pipes.
Very good points.

One of the cats I knocked out had failed mechanically on my GFs S-10. The honeycomb disconnected from the rest of the cat housing and would rattle loudly.

2 others had failed likely due to their age and heavy use (15years - 200,000+ miles on the cars). Visually there were no abnormal restrictions in the honeycomb when I knocked it out, but that doesn't mean they flowed as efficiently as intended.
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Old 12-01-2012, 09:20 PM   #18 (permalink)
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+1 to the above issues already noted.

And also remember the cars OEM computer is programmed to intentionally leave a small amount of unburned fuel in the exhaust in order to be used by the CAT ... unless you change the computer system as well ( much more involved ) ... the car will send this unburned fuel into the exhaust weather there is a CAT or not... at least with a CAT in place that little bit of ICE 'extra' fuel goes to good use by reducing smog emissions like NOx and such ... without the CAT but with the OEM computer system ... that fuel sent for the CAT is just thrown away.

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