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Old 09-05-2007, 05:44 PM   #1
jvb6806
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Default proper A/F and boost?

i have searched around nasioc and cannot find the answer... if someone knows of a thread for me to read please link me.

that aside...

i have an 02 WRX with a 3" exhaust and am currently running Cobb "stage 2" fuel map.

pulling the map with openECU the target A/F is down around 9.8:1 at redline. i am assuming that they are just playing it safe because the stock map is somewhere around there at full load and redline, but do subaru's really need to run that rich? i tuned for 12:1 at 20psi on my DSM w/ a 16G and supporting mods. i would like to edit the map so i am not pouring raw fuel in. my AEM wideband pegs at 10:1 when i am running out the gears as well.

i figured i would rewrite the map as follows and run something closer to 11:1, because from what i read subaru's need a lot of gas up top. what kind of A/F are you guys looking to hit when tuning?

this is the cobb map (2.09 barA = 16psi)




this what i was thinking about running... (2.24 barA = 18psi)





my second question is it appears from looking at the compressor maps on the 13t that it is still running at ~74% at 18psi at redline.... what kind of boost pressure are you guys running on a stock turbo with a proper tune? i am coming from the DSM world so it was 20psi on a 16g and lean it out till it knocks... trying to play by the subaru rules this time around.
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Last edited by jvb6806; 09-05-2007 at 11:35 PM.
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Old 09-05-2007, 10:46 PM   #2
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I spent months tuning my Stg2 setup on my 04 WRX. I ran 20 psi from 2800 to 5000 and then tapered to 13 at redline. My AFR target was 11.4 which was still somewhat conservative since I didn't have a EGT gauge. Several people run 11.7 or so. Timing also needs to be adjusted so keep that in mind as well.
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Old 09-05-2007, 11:29 PM   #3
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... 7th lagit post...
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Old 09-05-2007, 11:31 PM   #4
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8th legit post... now i can put my damned maps up.
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Old 09-05-2007, 11:31 PM   #5
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i was going to run the cobb timing maps because if those are anything like their A/F maps they are pretty conservative. here is a "link" to the table cobb has set for their timing...



so i hear everyone say that you can't run peak boost at redline but what is the actually reason behind that? all i hear is "no you can't do that" but why? do the injectors max out? fuel pump? not enough fuel pressure? or is it because timing is advanced so much... and for what reason is timing advanced so far?

Last edited by jvb6806; 09-05-2007 at 11:57 PM.
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Old 09-06-2007, 08:11 AM   #6
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You really should learn to read compressor maps and you would know. The turbo runs out of its efficieny range beyond 13 psi or so on the 2.0L engine at redline. Trying to run more boost than that only results in overspinning the turbo, resulting in potential turbo failure and overly heated charge intake air. It has nothing to do with fuel or timing directly.
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Old 09-06-2007, 05:49 PM   #7
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what kind of efficiency range are you looking for?



at 2.0l motor will flow roughly 247 CFM at 7k rpm's.

volume of air (cu ft/min)= engine rpm x engine cid
..............................................(172 8 x 2)

converted into cubic meters per second you are looking at .116

so the flow map would put the efficiency of the 13t at 73-75% percent with 18psi at 7000 rpm's.

am i reading that incorrectly?
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Old 09-06-2007, 06:34 PM   #8
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nope, because CFM goes up with boost, you'll be lucky to hold 14 psi to 7k rpm on the 2.0L with the stock turbo.
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Old 09-06-2007, 06:38 PM   #9
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the Cobb timing map is quite agressive. at full load on the stock turbo I generally tried for 14-15degrees total timing and 24-26 at redline. the Cobb maps put WAY more in than nessecary.
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Old 09-06-2007, 07:07 PM   #10
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Do not make such a big change to your maps all in one go. You can work towards the maps you proposed, but go slowly with it.
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Old 09-06-2007, 07:10 PM   #11
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equations i found were way off... thanks TMessick.

Last edited by jvb6806; 09-06-2007 at 07:53 PM.
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Old 09-06-2007, 07:42 PM   #12
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Quote:
Originally Posted by jvb6806 View Post
what variable do i need to add to that equation to be able to read that map correctly?
Compressor map is in standard mass flow rate (i.e. flow rate at standard atmospheric pressure). As such, the BIG thing you're missing is pressure ratio (manifold pressure over standard atmospheric). You're also missing VE, which will be less than 1.0 and tend to reduce the flow...

Anyhow, from your eq. before:
volume of air (cu ft/min)= (rpm x cid x VE) * (Manifold Pres + 14.7)/14.7
OR (in SI):
flow rate (m^3/s) ~= (RPM * disp * VE) * (MAP/101.7) *(1/120)

disp in m^3
VE in %
MAP in kPa

at 18psi of boost, MAP = 225kPa
m/s = (7000 * 0.002 * 0.85) * (225/101.7) * (1/120) = 0.219, which putts comp. eff off the charts....
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Old 09-06-2007, 07:56 PM   #13
jvb6806
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Quote:
Originally Posted by TMessick View Post
Compressor map is in standard mass flow rate (i.e. flow rate at standard atmospheric pressure). As such, the BIG thing you're missing is pressure ratio (manifold pressure over standard atmospheric). You're also missing VE, which will be less than 1.0 and tend to reduce the flow...

Anyhow, from your eq. before:
volume of air (cu ft/min)= (rpm x cid x VE) * (Manifold Pres + 14.7)/14.7
OR (in SI):
flow rate (m^3/s) ~= (RPM * disp * VE) * (MAP/101.7) *(1/120)

disp in m^3
VE in %
MAP in kPa

at 18psi of boost, MAP = 225kPa
m/s = (7000 * 0.002 * 0.85) * (225/101.7) * (1/120) = 0.219, which putts comp. eff off the charts....

thank you for explaining your answer... time to do some more math now that i have the correct equation.
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Old 09-06-2007, 09:04 PM   #14
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lol cobb stage 2 lol
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Old 09-06-2007, 10:27 PM   #15
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... rant removed ...

Last edited by jvb6806; 09-06-2007 at 11:29 PM.
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Old 09-07-2007, 03:24 PM   #16
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I found that my stock turbo would not run any more than about 13-14 psi above 6k rpm no matter what I did, including switching to a GM BCS and setting WGDC at ridiculously high numbers. It just plain runs out of breath at 5k rpm and drops like a rock after that - 16 psi at 5200, 15 at 5600, etc.

Also that boost map is really aggressive in the low TPS columns - you don't need to ask for that much boost at anything under full throttle. If you want more boost, give it more throttle is my tuning philosophy. Sure, the mid-throttle boost will make the car FEEL faster but it won't be, and it will just waste gas.
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Old 09-07-2007, 03:28 PM   #17
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And tuning a subaru motor is similar to a 4g63t - inch up to a target boost, watch your KC and total timing for signs of pulling timing and adjust to reduce or eliminate pulled timing. Then lean fuel and repeat the timing tweaks.

The only difference is that a 4g63t will take a heck of a lot more knock before permanant damage. The EJ205 is more fragile.
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Old 09-08-2007, 10:22 AM   #18
jvb6806
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thanks rob.

i plug used the equation given above and have rewritten my map and i am going to give it a shot.

and ironically i had the same thought as you about boost limit at partial throttle about 20mins before i read your post and had already changed the map accordingly. i raised the boost in the midrange and only above 88% throttle.



thank you for your intelligent response!

i was trying to DL enginuity to give the logging feature on there a try but i am waiting for the confirmation email.

if i get the software before the 18th i will be heading up to drag strip, laptop in hand, to see what happens... i am also going to shoot for 11:1 A/F and watch the knock.
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Old 09-17-2007, 12:38 PM   #19
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Sounds like a good plan. PM me for my stage II map if you want to see one that works pretty good with 3" exhaust, GMBCS and IC for 91 octane.
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