Thread: Ask a tuner...
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Old 03-29-2010, 08:16 PM   #27
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2014 P3 Import Auto
AWD Dyno, Mods, etc


So it was brought to my attention that some Cobb tunes for 90-91 octane run the injectors at 100% . I just got done doing some checking on this and also looking at some logs from when I tried one of these tunes in one of my cars.

It does seem in fact that specifically Stage 2 90-91 octane pre-2009 STI tunes do run the injectors to 99% or more. So is this really bad you may wonder, well let me walk you through the steps of what happens when there is trouble.

So when the car is completly happy your running 18lbs or so of boost and maxxing the injectors out at or close to redline. In the event of severe enough knock or very cold conditions the car will switch to different fuel maps or add fuel. Seems like a no brainer right? You must need bigger injectors! Well let see...

A big piece of Subarus "fail safe" technology is based on something called IAM (ignition adavance multiplier) or the same DAM (dynamic advance multiplier). On an STI when all is happy and there is no knock this number is at 1.0 got it? 1.0 is best and 0.1 is worst.

Now when there is a knock event for any reason if its bad enough the car will drop the IAM down a certain amount depending on various factors. So lets just say the IAM has dropped, here is a chart for what happens on an 05 STI with Cobb Stage 2 90-91 octane:

1.00 Very happy car.
0.75 Global timing reduced.
0.50 Global timing reduced.
0.35 Switch to failsafe fuel maps, Global timing reduced
0.20 Boost control shut down (min boost), Global timing reduced

So when we switch to failsafe maps we see the car wants to add in some fuel with a failsafe map, it adds as much as 5% more fuel for an STI. But here is the catch.

The Cobb tune doesn't max out injectors until close to redline. The failsafe tables only add fuel in the peak tq regions and look the same as the normal fuel maps towards redline. There is JUST enough room to add the fuel needed.

The other part is the fuel tables are calling for 10.6:1 at redline, the reality we see on the wideband is close to 10:1 which is VERY rich at redline. You actually don't need near that amount of fuel past peak TQ. Peak TQ ranges is where the knock tends to be not at peak HP range. So leaning out at redline as long as it doesn't exceed 11.5:1 *should* be ok.

More catch, if the knock is bad enough and your using the factory boost control then the car also pulls the boost out which means......Yep less fuel needed.

So if I have completly confused you now the original question was does my STI need bigger injectors to be safe?

Yes and no:

No -- If you run stock boost control, don't go below sea level and redline it, will never run over 18.5PSI, and feel your car will never blow a hose or loose boost control.

Yes -- If you run after market boost control, and if you want to be absolutely sure to be safe. The possibility for spikes in air flow due to boost control failure or other condition are real!

All it takes is one good pull in a lean condition and your piston is done in most cases due to severe knock. Some injector overhead would be nice to have but won't solve all problems, at least it would be one thing less to worry about.
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