JoeT
02-03-2004, 12:24 AM
Maximizing Performance of your EJ25
Now that we've gone out and spent all that money on Headers, Camshafts, Exhausts, high flow cat, what's next.
Before we continue, here's a power history of where we came from:
Stock: 121 WHP (2 WD Dyno)
Cat Back Exhaust: 126 WHP
Header Back with HF Cat: 141 WHP
All of the above + Camshaft: 153 WHP
Not to mention the flatness of the torque curve, with all of the above there was over 32 HP difference between stock and the final product at 6000 rpm. Which meant that you'd hit the rev limiter before the power dropped. A huge improvement in performance.
So yes, all these mods do perform something, now how do I optimize these to make them work more efficiently together.
Before we continue on this road, I would like to mention that everything that I'm going to write here is based on my personal experience from everthing I've learned and is geared towards improving the enjoyment of my chosen motorsport.
Ok, enough of that, now to the nitty gritty. Ever wonder where the mysterious 14.7:1 AF ratio came from? Here's the answer TREE HUGGERS. Yep, 14.7:1 is the theoretical number that is used to describe the "Full" burn off of fuel. Most people call it Stoichometric, or Stoich for short.
The 14.7:1 is used under low load conditions for cruise to save fuel, usually under 3000 rpm, or in the case of the WRX under 4000 rpm or 40kpa load. Ever wonder why there's closed loop and open loop maps in an ECU? We'll get to that soon.
There is no magical number that's used for all engines, all AF ratios are targets. The actual formula for calculating AF ratio's for the most amount of power / torque is determined by Bore / Stroke / Combustion Chamber Design / Cam Timing / Compression Ratios. If any of these are unknown, then it's a guessing game, and this is where the big debate starts, and tuning philosophies start.
Bottom line is, most if not all engines will blow up if you try to attain 14.7:1 AF ratio at high load. More important than AF ratios in generating power is the "Timing Map". You can absolutely mess up your AF ratios and your car may continue running, but if you mess up your timing map, it's a ticking time bomb, your engine life will be shortened.
Now back to Subaru's. On the EJ25, the Bore is HUGE, meaning that it takes time for the spark to ignite the fuel mixture inside the cylinder. That's why the factory timing ranges between 16 degrees BTDC at idle to over 47 degrees BTDC at 5000 rpm. Do that to a long stroke small bore engine and KABOOM.
On Doodiehead's car, with the above modifications + ECU tuning and ported and polished heads he got another 26 more horses out of the NA 2.5rs.
To review: The NA EJ25 went from 121 WHP to 179 WHP, and maxed out his stock injectors at the stock fuel pressure.
That pretty well maxes out the EJ25 in NA mode.
Notes to live by:
Timing = Torque
HP = Torque X RPM / 5252
Rich + Lots of Timing = Lots of torque (to a point)
Lean + Lots of Timing = Scrap Metal
14.7:1 = Complete Burn, to be used at low loads only, or at cruise.
Close Loop Maps:
The closed loop map is used to manage the engine vitals (AF + Timing) using the O2 Sensor + TPS. It automatically adjusts to a preset table maintaining as close to Stoich as possible to reduce emissions during Idle, low rpm crawl, low load cruise.
Open Loop Map:
Static map that ignores all sensor readings except for the Crank and Cam trigger as well as knock sensor. This static map determines the injector cycle and ignition timing based on the TPS input or MAP load. Usually anything beyond 40% TPS or 40% load. If the knock sensor detects the presence of knock it retards the timing by 3 degrees, then another 3 degrees if more is detected.
Hope this helps. Any questions, please feel free to post and I'll elaborate in this thread.
Next Article: How do I turbo my EJ25?
Now that we've gone out and spent all that money on Headers, Camshafts, Exhausts, high flow cat, what's next.
Before we continue, here's a power history of where we came from:
Stock: 121 WHP (2 WD Dyno)
Cat Back Exhaust: 126 WHP
Header Back with HF Cat: 141 WHP
All of the above + Camshaft: 153 WHP
Not to mention the flatness of the torque curve, with all of the above there was over 32 HP difference between stock and the final product at 6000 rpm. Which meant that you'd hit the rev limiter before the power dropped. A huge improvement in performance.
So yes, all these mods do perform something, now how do I optimize these to make them work more efficiently together.
Before we continue on this road, I would like to mention that everything that I'm going to write here is based on my personal experience from everthing I've learned and is geared towards improving the enjoyment of my chosen motorsport.
Ok, enough of that, now to the nitty gritty. Ever wonder where the mysterious 14.7:1 AF ratio came from? Here's the answer TREE HUGGERS. Yep, 14.7:1 is the theoretical number that is used to describe the "Full" burn off of fuel. Most people call it Stoichometric, or Stoich for short.
The 14.7:1 is used under low load conditions for cruise to save fuel, usually under 3000 rpm, or in the case of the WRX under 4000 rpm or 40kpa load. Ever wonder why there's closed loop and open loop maps in an ECU? We'll get to that soon.
There is no magical number that's used for all engines, all AF ratios are targets. The actual formula for calculating AF ratio's for the most amount of power / torque is determined by Bore / Stroke / Combustion Chamber Design / Cam Timing / Compression Ratios. If any of these are unknown, then it's a guessing game, and this is where the big debate starts, and tuning philosophies start.
Bottom line is, most if not all engines will blow up if you try to attain 14.7:1 AF ratio at high load. More important than AF ratios in generating power is the "Timing Map". You can absolutely mess up your AF ratios and your car may continue running, but if you mess up your timing map, it's a ticking time bomb, your engine life will be shortened.
Now back to Subaru's. On the EJ25, the Bore is HUGE, meaning that it takes time for the spark to ignite the fuel mixture inside the cylinder. That's why the factory timing ranges between 16 degrees BTDC at idle to over 47 degrees BTDC at 5000 rpm. Do that to a long stroke small bore engine and KABOOM.
On Doodiehead's car, with the above modifications + ECU tuning and ported and polished heads he got another 26 more horses out of the NA 2.5rs.
To review: The NA EJ25 went from 121 WHP to 179 WHP, and maxed out his stock injectors at the stock fuel pressure.
That pretty well maxes out the EJ25 in NA mode.
Notes to live by:
Timing = Torque
HP = Torque X RPM / 5252
Rich + Lots of Timing = Lots of torque (to a point)
Lean + Lots of Timing = Scrap Metal
14.7:1 = Complete Burn, to be used at low loads only, or at cruise.
Close Loop Maps:
The closed loop map is used to manage the engine vitals (AF + Timing) using the O2 Sensor + TPS. It automatically adjusts to a preset table maintaining as close to Stoich as possible to reduce emissions during Idle, low rpm crawl, low load cruise.
Open Loop Map:
Static map that ignores all sensor readings except for the Crank and Cam trigger as well as knock sensor. This static map determines the injector cycle and ignition timing based on the TPS input or MAP load. Usually anything beyond 40% TPS or 40% load. If the knock sensor detects the presence of knock it retards the timing by 3 degrees, then another 3 degrees if more is detected.
Hope this helps. Any questions, please feel free to post and I'll elaborate in this thread.
Next Article: How do I turbo my EJ25?