Why does my hot cam make my lt1 run rich
05-03-2004, 06:22 PM
#1
Burning Brakes
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Why does my hot cam make my lt1 run rich
I have the lt-4 hot cam and 1.6 RR's. Why does it run rich now? Anything I can do until my custom Prom gets here?. Will this clog my cats? Ski-down-it will get the chip as soon as I get the car back from the 3.73 gear install. I heard he was very good. Any of you tuners want to break it down to me in layman terms? 
05-03-2004, 10:54 PM
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Re: Why does my hot cam make my lt1 run rich (RUBYREDVETTECARD)
If you are running the stock chip for one thing the idle speed is not too low. Also if you are running a 93 and earlier LT1 your control scenario is speed density based control. So now with the new LT4 cam installed the VE maps has to be changed to reflect the cam changes that you have done by installing the LT4 hot cam.
Before the cam change the engine was probably idling at around 35 to 50 Kpa and now its idling probably at around 55 to 65 so that means the ECM thinks there more load on the engine so its adding fuel to match that load. However the once it goes into close loop the O2 sensors will tell the ECM that its too rich so it then tries to compensate but there are limits to the amount it can compensate or to how it trims the fuel based on what it sees from the O2 sensor. So thats more than likely the cause of the rich condition. You probably won't hurt your cats unless you spend all your time idling around. Once you are cruising the VE maps are still reasonably close where the ECM can compensate OK.
Before the cam change the engine was probably idling at around 35 to 50 Kpa and now its idling probably at around 55 to 65 so that means the ECM thinks there more load on the engine so its adding fuel to match that load. However the once it goes into close loop the O2 sensors will tell the ECM that its too rich so it then tries to compensate but there are limits to the amount it can compensate or to how it trims the fuel based on what it sees from the O2 sensor. So thats more than likely the cause of the rich condition. You probably won't hurt your cats unless you spend all your time idling around. Once you are cruising the VE maps are still reasonably close where the ECM can compensate OK.
05-04-2004, 01:38 PM
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Burning Brakes
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Re: Why does my hot cam make my lt1 run rich (tjwong)
That's what I am talkin bout. Someone who understands computers. I just work on engines. I can tell it's running rich because of the soot out & on the garage door when I rev it up from idle and the strong smell of gasoline. Kinda like the ol lawnmower starting-up smell. (not very scientific eh?). I am curious where the computer picks up the info and why it increases the injector pulse to create a rich condition. I understand the duration of the cam is greater than stock. Then why wouldn't the car run lean with a stock injector pulse? What tells it to run waaay rich. You may have answered me already, Ha Ha. I am a little slow with this sorry. 
Oh yeah what is kpa and ve?
[Modified by RUBYREDVETTECARD, 6:39 PM 5/4/2004]
Oh yeah what is kpa and ve?[Modified by RUBYREDVETTECARD, 6:39 PM 5/4/2004]
05-04-2004, 02:38 PM
#6
Burning Brakes
Re: Why does my hot cam make my lt1 run rich (RUBYREDVETTECARD)
you have to consider that the HOT cam has considerably more overlap at idle. a larger percentage of the unburned air/fuel mixture is being pumped out through the exhaust. this is why i asked how you were determining whether or not it's rich, since the symptoms you describe also fit my car, which happens to be running stoicheometric air/fuel ratios at idle. of course, i don't have catalytic converters, and the converters can hide a lot of the unburnt hydrocarbons.
the ecm in your car doesn't know how much air is flowing through the engine. it calculates what it thinks is flowing by looking at certain sensors, one of them being intake manifold pressure. since the bigger cam/more overlap pulls less vacuum in the intake manifold at idle, as tjwong explained, the ecm thinks the "load" is higher. it thinks the engine is struggling against more load than there really is, so it's assuming more air is pumping, so it adds enough fuel to compensate.
except you're not flowing the amount of air it thinks, so it's too much fuel.
once the O2 sensors warm up, they are giving the ecm feedback on the air/fuel ratio, so it will try to lean things back out. it probably can't compensate quite enough to get back to stoich, which is why you need the chip. the chip will likely have different values for volumetric efficiency in the range where it's idling (and everywhere, actually), so it knows less air is flowing at 900rpm and 50kPa manifold pressure, for instance.
kPa is kilopascals, btw. it's a unit of absolute pressure, so it works in the reverse if you're used to seeing inches of vacuum. full vacuum would be 0kPa, and atmospheric pressure is ~102kPa. when the throttle is wide open, the manifold pressure sensor will be telling the ecm it sees 102kPa of pressure, so the ecm knows which cells of volumetric efficiency to look at.
VE = volumetric efficiency. 350 cubic inches of air pump, if the ecm says at 800rpm and 50kPa the engine is flowing 60% of its capacity, it injects enough fuel for the a/f mixture it's trying to achieve. but if in reality the amount of intake charge being trapped and burned in the combustion chamber is really only 44%, then it will be running far too richly.
-michael
the ecm in your car doesn't know how much air is flowing through the engine. it calculates what it thinks is flowing by looking at certain sensors, one of them being intake manifold pressure. since the bigger cam/more overlap pulls less vacuum in the intake manifold at idle, as tjwong explained, the ecm thinks the "load" is higher. it thinks the engine is struggling against more load than there really is, so it's assuming more air is pumping, so it adds enough fuel to compensate.
except you're not flowing the amount of air it thinks, so it's too much fuel.
once the O2 sensors warm up, they are giving the ecm feedback on the air/fuel ratio, so it will try to lean things back out. it probably can't compensate quite enough to get back to stoich, which is why you need the chip. the chip will likely have different values for volumetric efficiency in the range where it's idling (and everywhere, actually), so it knows less air is flowing at 900rpm and 50kPa manifold pressure, for instance.
kPa is kilopascals, btw. it's a unit of absolute pressure, so it works in the reverse if you're used to seeing inches of vacuum. full vacuum would be 0kPa, and atmospheric pressure is ~102kPa. when the throttle is wide open, the manifold pressure sensor will be telling the ecm it sees 102kPa of pressure, so the ecm knows which cells of volumetric efficiency to look at.
VE = volumetric efficiency. 350 cubic inches of air pump, if the ecm says at 800rpm and 50kPa the engine is flowing 60% of its capacity, it injects enough fuel for the a/f mixture it's trying to achieve. but if in reality the amount of intake charge being trapped and burned in the combustion chamber is really only 44%, then it will be running far too richly.
-michael
05-04-2004, 03:24 PM
#7
Burning Brakes
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Re: Why does my hot cam make my lt1 run rich (MSR)
I applaud you.......................
Thank you for explaining this. Now, I understand what will change with the new chip. Wow you are one intelligent dude. :bb I need to go a few books.
BTW, Is it safe to say that there will be a dramatic increase in efficiency and Slight gain in HP with the new chip???
[Modified by RUBYREDVETTECARD, 8:41 PM 5/4/2004]
Thank you for explaining this. Now, I understand what will change with the new chip. Wow you are one intelligent dude. :bb I need to go a few books.
BTW, Is it safe to say that there will be a dramatic increase in efficiency and Slight gain in HP with the new chip???[Modified by RUBYREDVETTECARD, 8:41 PM 5/4/2004]
