turbo_bu
June 14th, 2011, 03:51 AM
After reading Mr. P.'s old school tuning post, it got me thinking about other theories for fuel economy. The examples that I have observed (but not yet had a chance to play with like Mr. P. did), are an 99' K2500 and an 01' K2500 with 6L's and 4L80E's. The main thing that I have observed is that the older 99' OS commands EGR based on MAP while the 01' commands it based on MAF. Not quite sure why GM did this other than maybe it is smoother, or more accurate based on the MAF instead of the older method of MAP.
This observation got me thinking about why use EGR if we do not care about emissions. I know it is being used for reducing cylinder temperatures, thus the reduction in emissions, but could it also be "helping" mileage? The theory that I had goes like this:
In order to use less fuel, we need to use a smaller engine. A smaller engine will burn less fuel since it is using less air. The trade off is that the smaller engine is working harder, thus will have it's throttle body further open which will help to reduce pumping losses. The more it's TB is open, will also tend to drop the MAP value, but the overall airflow will be less since it has less displacement. An example of this is to replace the 6L with a 4.8L. It in theory would get better gas mileage since it has 1.2L's of less displacement. The other examples are engines with DOD. They turn off 4 cylinders so a 6L now turns into a 3L.
Now, if all that thinking is sound, is there any merit to using more EGR to displace the oxygen in the cylinder to thus make the engine look smaller, without actually using a smaller engine. The bad part of this would be that cylinder temps would also go down due to the EGR. Thus, it could result in needing more fuel to offset the incomplete burning of the oxygen and fuel that are being used. I believe that I remember hearing that one of the ways that GM accounts for this is to use more timing. Thus, you kinda get some efficiency back.
Anyone ever done comparisons with / without EGR; with / without added timing? As with most things, I can imagine there are diminishing returns with this theory. And that is probably why most people just turn it off.
This observation got me thinking about why use EGR if we do not care about emissions. I know it is being used for reducing cylinder temperatures, thus the reduction in emissions, but could it also be "helping" mileage? The theory that I had goes like this:
In order to use less fuel, we need to use a smaller engine. A smaller engine will burn less fuel since it is using less air. The trade off is that the smaller engine is working harder, thus will have it's throttle body further open which will help to reduce pumping losses. The more it's TB is open, will also tend to drop the MAP value, but the overall airflow will be less since it has less displacement. An example of this is to replace the 6L with a 4.8L. It in theory would get better gas mileage since it has 1.2L's of less displacement. The other examples are engines with DOD. They turn off 4 cylinders so a 6L now turns into a 3L.
Now, if all that thinking is sound, is there any merit to using more EGR to displace the oxygen in the cylinder to thus make the engine look smaller, without actually using a smaller engine. The bad part of this would be that cylinder temps would also go down due to the EGR. Thus, it could result in needing more fuel to offset the incomplete burning of the oxygen and fuel that are being used. I believe that I remember hearing that one of the ways that GM accounts for this is to use more timing. Thus, you kinda get some efficiency back.
Anyone ever done comparisons with / without EGR; with / without added timing? As with most things, I can imagine there are diminishing returns with this theory. And that is probably why most people just turn it off.