My own BSFC chart using software from road dyno

[ever_green]

Few months back I posted a thread about modifying my Drive By Wire maps to improve fuel efficiency. Now that I had the Drive By Wire problem solved I started exploring oppurtunities to use the Tactrix adapter (For Subaru and Mitsubishi only) to find my engine's efficiency sweetspot. This was really time consuming and I had to log for hours and hours, anylze the logs and calculate BSFC based on calculated fuel consumption and engine power all estimated by a software which takes into account factors like acceleration, ignition timing, manifold pressure, MAF, AFR and dozen other values. Basically I used 3 different softwares, one that calculates power, one that calculates fuel consumption (based on AFR, MAF and other variables) and one that helps me graph my calculations.

So far I only have had time to explore the 1400-2800 RPM range which is where I spent most my time in day to day drivings. Also excuse the roughness of the illustration. I have only just begun my work.

Here is the final rough copy of chart:


First off i'm not sure how accurate the numbers are. This is in no way a scientific test but usually the road dyno software does a good job on estimating power compared to traditional chassis dyno and can even convert that number to power at the crank. It estimated that my car makes 162hp at full throttle near 5600rpm vs. the advertised 170hp at 6000rpm . So i'm not sure if subaru over-rated the engine or the software is just in-accurate. Anyway I was disappointed with 0.48 lb/hp/hr raing at full throttle at 2400rpm (which is the best BSFC at WOT). I was expecting 0.4 lb/hp/hr. This graph also surprised me because the sweetspot occurs fairly high up where little manifold vacuum is present. For you scangauge users the sweetspot translates to about just under 90% load near 2000-2200 RPM (about 50hp). Also from this experiment, it was obvious to me that fuel enrichment at WOT loads (even though it was only minor enrichment) has negative effects on BSFC.

*Note: I simplified the drawings by truncating the values (not rounding) when illustrating a BSFC contour. So for example a 0.480 island can include 0.48 to 0.489.

**Engine specs: 2.5L SOHC H4 with variable valve lift, 170hp @6000rpm, 170ft-lbs @4400rpm, 10:1 compression with retarded valve timings (to simulate Atkinson cycle?), cooled exhaust gas recirculation.

[MetroMPG]

Very cool project! Thanks for sharing.

Has your rough chart caused you to change the way you drive at all? Are you aiming for the BSFC sweet spot now to pulse & coast?

[gone-ot]

How are the "MPG numbers" you get from driving by that map, ie: what MPG (better, worse or same) at about 2100 rpm and 80-85% of full engine load? Does it seem to coincide with your MPG sweet-spot?

[ever_green]

No I have not started following this chart yet. I still have to do more logging and confirmation of the numbers. I find that stuff like temperature and humidity play big role in this. For example the ECU disables EGR at freezing temperatures. This increases combustion efficiency considerably.

With EGR enabled though lower loads are generally more helpful to help 'evacuate' the dirty gasses in the combustion chamber.

So this chart is for winter use and I am still logging and constantly changing the above chart. The changes are minor though. For example a test run last night indicated that on a fully stock impreza the BSFC sweetspot is "taller". What i mean is that the BSFC sweetspot is anywhere between 85-99% load at ~2000rpm with stock ecu but with a modified ECU above 90% load fuel enrichment causes a dip in mileage (open loop delay is removed). This open loop delay was removed to increase engine reliability and prevent knock. Knock actually further depreciates engine efficiency since factory tune is very aggressive timing wise and whenever knock is detected engine cuts considerable amounts of timing thereby killing volumetric efficiency across the powerband. But because the tune solves the knock and ignition retard problem my overall BSFC is better with a tune at 0.45 vs 0.47, just the sweetspot island seems smaller.

My findings basically indicates that no matter what bells and whistles your engine has, vacuum is #1 enemy to BSFC. Then it's fuel enrichment followed by ignition timing and engine speed. So i just accelerate now with highest possible load without going open loop, which is around 85-90% engine load. I try to keep RPM as low as possible which is usually 2000rpm. Lower RPM not because of just friction but because Open loop is entered more easily at higher engine speeds. I only accelerate at lower loads if i Know I will come to stop soon (redlights, stop signs, traffic). So yes it has improved my mileage by fighting vacuum when appropriate (ie merging or pulse and gliding on a long highway stretch).