Have you ever noticed what those exhaust silencers do to your AFR curve?
02-18-2004, 08:30 AM
#1
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Have you ever noticed what those exhaust silencers do to your AFR curve?
I borrowed a single exhaust from a friend of mine a few weeks ago. I think it is J's Racing, but I can't remember. Anyway, I did some fuel tuning to get a nice smooth, flat AFR curve from 3K to redline at about 13.5. After capturing several AFR logs with the silencer removed, I stuck the silencer in there and did some more AFR logging. I can definitely feel a loss in power, but it is only above VTEC. Down low, it feels just the same (only quiet). After viewing the AFR logs, I can see why it felt so weak in VTEC. What used to be my leanest point (13.6) of the RPM range, is now my richest point (11.13). This occurs at 6500 RPMs. Above that the AFRs squigle up and down between 12 and 13.5. It isn't a fluke either, since I captured many logs, and they were all identical. Any ideas why it would do this? I figured it would get progressively more rich as the RPMs increased in a near linear manner.
02-18-2004, 08:46 AM
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Yep, installing silencers will definitely affect your air fuel curve. You will *always* have to retune when you have silencers installed. Since these create backpressure and turbulent exhaust flow, they effectively change the volumetric efficiency of your engine. While they don't affect your VE as much as radical cams or a turbo would, they definitely require a retune to make your car run optimally again. Good data!
02-18-2004, 02:32 PM
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Doesn't that pretty much make silencers useless? If I were ever to use a silencer, it would be for daily driving, then remove it for competition. At the same time, I would want my tuning to be for competition as well, so that means that I would have to drive around with a crappy tune on the street.
The other thing that I was thinking about later today is that the ECU will probably adjust its long and short term fuel trims (and maybe timing) based on the characteristics of the silencer. Removing the silencer for an autocross or track event might cause the engine to run dangerously lean until it adjusts to the new characteristics again. Of course, you could reset the ECU every time you add or remove the silencer, but then you have to deal with the crap idle and possibly poor throttle response (as reported by some owners).
The other thing that I was thinking about later today is that the ECU will probably adjust its long and short term fuel trims (and maybe timing) based on the characteristics of the silencer. Removing the silencer for an autocross or track event might cause the engine to run dangerously lean until it adjusts to the new characteristics again. Of course, you could reset the ECU every time you add or remove the silencer, but then you have to deal with the crap idle and possibly poor throttle response (as reported by some owners).
02-18-2004, 02:47 PM
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It all comes down to how picky you are about your tune. If you are very picky, then you're definitely going to want two tunes - one for the silencers installed and one for them uninstalled. Running the same tune with and without the silencers won't create an engine harming condition unless your tune is on the ragged edge to begin with.
For me, it's simple to create two tunes since we have a dyno here at the shop. Everyone needs a dyno in their garage...then there wouldn't be a problem
For me, it's simple to create two tunes since we have a dyno here at the shop. Everyone needs a dyno in their garage...then there wouldn't be a problem
02-18-2004, 04:03 PM
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Originally posted by TorqueFreaks
Running the same tune with and without the silencers won't create an engine harming condition unless your tune is on the ragged edge to begin with.
Running the same tune with and without the silencers won't create an engine harming condition unless your tune is on the ragged edge to begin with.
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02-19-2004, 04:11 PM
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I am missing something here. Are you guys saying that a simple exhaust back of the cat will dramatically affect A/F ratios? 2 points or more? Or is that for dyno-tuned ECM's only. It would seem to me that after a bit of "learning", the adaptive elemtents of the engine management system would work towards its optimum A/F maps, using the feedback data from the primary HO2S - it tries to be stoichiometric at 14.7, but of course it varies. That 12.5 seems pretty rich, esp ina car with a knock sensor.
When I used the DiabloSport predator to remap my Cobra, i picked up a bunch of power by taking the pig rich WOT above 4K at WOT and leaning it to about 13.6-13.8. Since I have access to 93 octane, that was not worrison=me, as my EGT's never got scary - and this car remeber has no knock sensor or timing retard dynamic capability - it tries to work around it by backing off timing when it gets hot, which is why these SC'd Cobras love to be cool.
When I used the DiabloSport predator to remap my Cobra, i picked up a bunch of power by taking the pig rich WOT above 4K at WOT and leaning it to about 13.6-13.8. Since I have access to 93 octane, that was not worrison=me, as my EGT's never got scary - and this car remeber has no knock sensor or timing retard dynamic capability - it tries to work around it by backing off timing when it gets hot, which is why these SC'd Cobras love to be cool.
02-19-2004, 05:03 PM
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You are supercharged and you are running 13.8:1? wow, gutsy man.
AFR after the cat is pretty pointless to read, if you want to measure the real AFR, you gotta do it before the cat. Why? because one of the cat's primary jobs is removing oxygen from the exhaust to make CO into CO2. If you are doing this, how can you really know what your A/F ratio is?
Get a device from http://www.plxdevices.com to measure a/f or weld on a bung for an external o2 sensor.
AFR after the cat is pretty pointless to read, if you want to measure the real AFR, you gotta do it before the cat. Why? because one of the cat's primary jobs is removing oxygen from the exhaust to make CO into CO2. If you are doing this, how can you really know what your A/F ratio is?
Get a device from http://www.plxdevices.com to measure a/f or weld on a bung for an external o2 sensor.
02-19-2004, 06:24 PM
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[QUOTE]Originally posted by Road Rage
I am missing something here. Are you guys saying that a simple exhaust back of the cat will dramatically affect A/F ratios? 2 points or more? Or is that for dyno-tuned ECM's only. It would seem to me that after a bit of "learning", the adaptive elemtents of the engine management system would work towards its optimum A/F maps, using the feedback data from the primary HO2S - it tries to be stoichiometric at 14.7, but of course it varies. That 12.5 seems pretty rich, esp ina car with a knock sensor.
I am missing something here. Are you guys saying that a simple exhaust back of the cat will dramatically affect A/F ratios? 2 points or more? Or is that for dyno-tuned ECM's only. It would seem to me that after a bit of "learning", the adaptive elemtents of the engine management system would work towards its optimum A/F maps, using the feedback data from the primary HO2S - it tries to be stoichiometric at 14.7, but of course it varies. That 12.5 seems pretty rich, esp ina car with a knock sensor.