I spent a good chunk of money this week dyno tuning my Nat Asp 2.5L Subaru. Most of the funds were spent in part throttle tuning. I wanted to find the best BSFC at part and full throttle to improve my acceleration efficiency on the street. I already posted a graph in BSFC sticky but I don't think that's very helpful. I'm not good with graphs so I will just post the RAW data which the computer recorded in ascii files here. Aside from using an inductive dyno to calculate power at all wheels, the dyno used sensors on the vehicle to calculate other information such as BSFC. These sensors included injector pulse width reported by ECU, AFR at tailpipe, mass air intake and etc. The ASCII data files were huge and I used them to come up with the following information file (also attached):
I basically sorted data based on throttle position and found set of RPM's with its respective BSFC, vacuum and power for that engine speed and throttle. The best BSFC for each RPM is highlighted in bold as to simulate best method of acceleration for efficiency.
The information gathered from this dyno indicate that at higher engine speeds (+2.3k RPM) wide open throttle is required to achieve best BSFC despite fuel enrichment (λ0.92). Absolute best BSFC was observed at 1800 RPM with 3-4"Hg of vacuum. Vacuum at higher RPMs tremendously degrades BSFC. Leaner mixtures and advanced ignition timing does not make up for vacuum pumping losses but it does improve BSFC. In other words vacuum plays a bigger role than fuel mixture, timing and engine speed to a certain extent for BSFC. If these dyno numbers are to be trusted, high vacuum and engine speeds do not go hand in hand for best BSFC (nothing new here). This further supports longer gear ratios for lower RPM operations at highway speed. Eliminate or minimize vacuum and you are certainly operating at best BSFC so long as fuel mixture is not too rich and timing is not too retarded.
Notes:
- I could not dyno past certain engine speeds at lower TPS due to lack of power
- All runs were done in top gear (overddrive)
- WOT and 40% run were in unlocked torque converter mode
- Depending on vacuum the air fuel mixture ranged from λ1.0 to λ0.92
- Ignition Timing varied with load and RPM