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Old 09-05-2011, 06:14 PM   #1 (permalink)
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Default WHY does an exhaust leak = hp loss

Explain i'm just curious.

If your running a straight piped 2/12 and you have an exhaust leak on the collector or anywhere else.

(WHY) do you lose power? Velocity escape maybe? Inconsistent flow due to the leak?

Thanks again!

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Old 09-05-2011, 06:20 PM   #2 (permalink)
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It flows more freely. Resulting in you needing a re-tune. It makes your car lean out.
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Old 09-05-2011, 06:46 PM   #3 (permalink)
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The leak works both ways.

It lets gas out, marginally dropping temperature and velocity.

It lets air back in after the positive pulse has passed. This further cools and dampens pulses in the pipe.

The change in temperature and wekened pulse change the time and intensity of the negative pressure pulse that helps scavenge exhaust from the chamber during valve overlap. That means more exhaust and less fresh charge in the cylinder.
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Old 09-05-2011, 07:53 PM   #4 (permalink)
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yep, ruins the collector length, which is critical.
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Old 09-06-2011, 07:22 AM   #5 (permalink)
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Is that also true if the leak is on the exhaust system behind the cat?
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Old 09-06-2011, 07:37 AM   #6 (permalink)
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Is that also true if the leak is on the exhaust system behind the cat?
Yeah, definitely. The cat is pretty close up to the manifold if not on the manifold itself on most Hondas. I would think the further down the pipe, the less of an issue it would be, but it would still be something to have fixed.
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Old 09-06-2011, 08:07 AM   #7 (permalink)
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the flange between my cat and header is leaking. i guess because the 02 sensor is right be4 it. it would think its running lean. cause of the 02 intrusion?
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Old 09-06-2011, 08:25 AM   #8 (permalink)
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Are you throwing a code? I would definitely get that fixed, regardless. Think of it as running a partially open header. You are losing a lot of exhaust velocity with a leak that close to your primaries.
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Old 09-06-2011, 08:38 AM   #9 (permalink)
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the flange between my cat and header is leaking. i guess because the 02 sensor is right be4 it. it would think its running lean. cause of the 02 intrusion?
you think correct. it will running extremely lean. will be irritatingly noisy too. you should get it fixed ASAP.

I had a similar leak once and the narrowband sensor readings used to always hover around 0.05V.
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Old 09-06-2011, 08:40 AM   #10 (permalink)
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you think correct. it will running extremely lean. will be irritatingly noisy too. you should get it fixed ASAP.

I had a similar leak once and the narrowband sensor readings used to always hover around 0.05V.
Yikes. And yeah, the buzzy noise is not cool.
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Old 09-06-2011, 10:09 AM   #11 (permalink)
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the sound is somewhat ghey indeed. u dont hear it when running stationary but it almost sounds like a diesel when accelerating. To fix it ill have to reweld the piping. but i might get away with abit grinding. but seeing we're getting some crappy weather out here for the nexxt comming time i wont be able to fix it just yet

the car doesnt feel like its running crap thou. but ill fix it asap and lets see the results then!
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Old 09-06-2011, 02:07 PM   #12 (permalink)
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If air is leaking back into the exhaust near the oxygen sensor, the engine will not run lean. The O2 sensor will read lean because of the air it sees that did not pass through the engine. This will cause the O2 sensor to signal lean and the ecu to add more fuel which will in fact make you rich.

Each primary or header pipe will have strong pulses occuring in that pipe as the exhaust only flows when the exhaust valve is open, like for about 220 to 260 deg of crank rotation, depending on your cam. That means that there is about 460 to 500 deg of rotation with the valves closed and no exhaust flowing into the pipe.

In a 4 cylinder engine at any point in time, at least some exhaust from at least one cylinder is flowing into the collector and cat. This means the pulses are pretty well cancelld out in the collector and are canceled out a lot more through the cat. A leak at the collector will not likely pull in much air and after the cat is very unlikely to pull in any air. The pulses at that point tend to be from slightly positive to heavily positive pressure. The more it is muffled, the higher the internal pressure and the lower the pulsations.

Serious race engines often use leaky slip joints where the header pipes meet the collector as that is far enough from the cylinder for the suck back through the leak to not reach the chamber and lean out the mixture.
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Old 09-06-2011, 09:59 PM   #13 (permalink)
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@patprimmer, precisely what I was thinking - the engine is not actually running lean but the sensor will read lean.

however, what I dont get is this: when I had said exhaust leak problem I was running with a Tech II connected. that is where I saw the O2 sensor readings. if the ECU was indeed trying to richen the mixture to compensate, I shouldve had bogging / stinking / gas mileage issues, right? I had none of those.
in fact, my exhaust would keep popping all the time when I would take my foot off the gas pedal and upon running for a bit, killing the engine and immediately restarting, the starter would crank very tightly as if the rings were sticking to the cylinder walls. closing the exhaust leak sorted all these things out. no more popping exhaust or tight cranking. why do you reckon that happened?
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Old 09-06-2011, 10:51 PM   #14 (permalink)
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If the O2 sensor sees lean, adds fuel, rich exhaust comes down the pipe, hits air coming in from the leak then burns hence pop. If the system is unstable, ie sees lean, corrects goes rich, sees rich, goes lean in radical swings that will make the pops eratic.

Rich and lean cannot impact on cranking speed appart from the ocasional weak fire might help the starter.

If the exhaust gas condition causes the ECU to advance the timing a lot at cranking speed, (and I don't know that it does) that will make the starter lazy.
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Old 09-06-2011, 10:58 PM   #15 (permalink)
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If the O2 sensor sees lean, adds fuel, rich exhaust comes down the pipe, hits air coming in from the leak then burns hence pop. If the system is unstable, ie sees lean, corrects goes rich, sees rich, goes lean in radical swings that will make the pops eratic.
the system never saw rich Pat. I was monitoring the Tech II all the time and the max the sensor read wad 0.15. it would usually hover around 0.05.

also, popping caused by rich AFR would result in flames being spit out right? and would sound more like sharp crackles than dull pops?
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Old 09-06-2011, 11:07 PM   #16 (permalink)
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You are confusing rich and lean all the way with rich to the leak then rich lean oscilation from that point.
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Old 09-06-2011, 11:11 PM   #17 (permalink)
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What I am trying to say is that the charge in the cylinder is a reasonably stable and homogenous mix, but air leaking into an exhaust pipe in sync with negative pressure pulses in the pipe is not stable or homogeneous. The extra fuel only ignites as the air reaches it. There will only be a little air reaching it so it will only be a little pop.
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Old 09-06-2011, 11:12 PM   #18 (permalink)
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You are confusing rich and lean all the way with rich to the leak then rich lean oscilation from that point.
that makes sense. thanks.
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Old 09-06-2011, 11:41 PM   #19 (permalink)
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nick a straw, now try taking a drink from a cup, same general effect, just different reasons.

pat, sometimes you have to use old 50's and 60's terminology to explain the point.

There are also lean pops out the exhaust due to excessive overall timing. Something Honda corrected with OBD-2a timing maps under decel.
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Old 09-07-2011, 12:48 AM   #20 (permalink)
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007 you really need to get over taking what the Tech II tells you as the gospel truth. Your understandings of how the ECU works is limited.
Just trying to help prevent you from blowing one more engine.
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