Just before the 4th of July I went out and did a few test runs with various spark timings to try and find an optimal cruise timing for max efficiency. The testing and results are discussed below.
To conduct this test I went to a road south of town which has been built across a marshy area. Since the road was raised for construction, it offers about 2 miles of flat surface with a posted speed limit of 65mph. To make a test run I made sure the car was fully warmed up, uploaded a set of test parameters to the ECU. With the parameters uploaded I started data logging, set the cruise at 65 mph and made a pass across the flat road, then turned around at the far end, set cruise at 65mph and made a pass back in the opposite direction to eliminate any effect of grade or wind. To compile the data, I cut out any acceleration, stopping, turning data and just used the steady state cruise. A typical datalog is shown below...the gray areas weren't used for calculation - only the white steady state cruise zones:
Using this method, I left all parameters the same but varied the ignition advance in three degree increments from 31 to 47 degrees BTDC. I left the car in closed loop mode and let the O2 sensor adjust the injectors to maintain an AFR of 14.7:1. From the datalogs, I calculated the average injector pulse width for each complete 'down-n-back' run, then plotted the pulse width versus the ignition advance:
The graph strongly indicated that there was an optimal timing of ~37-38 degrees BTDC at 65mph (~2900rpm) for my particular engine. The shape of the curve also suggested that being slightly more advanced was preferable to being slightly less advanced
Several months ago, I found that a tuning magazine did some dyno testing of an Acura RSX similar to mine, except running gasoline. They looked at the effect of ignition advance, cam advance and AFR on overall engine efficiency - with the car running at a simulated 65 mph. The article is online at:
http://www.hondatuningmagazine.com/tech/0510ht_fuel_economy_tuning/index.htmlOf particular interest were their findings of the ignition advance versus efficiency. Their plot is displayed below:
Their test suggested an optimal timing of ~42-43 degrees BTDC and again showed that slightly more advance is preferable to too little advance. At first, there may seem to be a little difference between my E85 results which found maximum efficiency around 37-38 deg BTDC and Honda Tuning which found optimal timing of 42-43 deg BTDC. But we also have to take into account the flame speed of E85 versys gasoline:
Considering ethanol and likewise E85 has a faster flame speed than gasoline, the peak pressure in the cylinder would build faster and it stands to reason that all else being equal, E85 would like the spark to occur slightly closer to TDC (ie less advance) when compared to gasoline. The results showed about 5 degrees less advance in this case was optimal.
In summary, this test showed that E85 does indeed like slightly less timing for optimal efficiency when compared to gasoline. Additionally, in either case (E85 or gasoline) it is better to have slightly more advance than optimal compared to slightly less advance. Efficiency is lost rather rapidly with too little advance and affected only slightly with too much advance. Although efficiency gains can be realized by slightly reducing the timing advance when running E85, the effect is likely to be rather small - probably a gain of <0.5% and most surely <1%. In my specific case, given an optimal gasoline advance of ~43 deg BTDC to an optimal E85 advance of ~37 deg BTDC would net approximately 0.63% efficiency gain (ie 25mpg to 25.16mpg)
