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2005 OBXT Ltd Obsidian Black 5MT
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Discussion Starter · #1 ·
Let's say a turbo EJ255 engine was built, but instead of the 8.2:1 ratio the ECU was expecting it's inadvertently built to a different ratio, whether higher or lower.
What would the ECU do in response without a tune? What might logs look like?
 

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Fresh Out of Outbacks!
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If it was too high, I expect you'd see a heck of a lot more detonation, and all the programmed responses to such- pulled timing, reduced boost. Honestly wouldn't expect the pistons to last long if this were the case.

Too low? I dunno... I guess I'd expect to see signs of it running rich at high load, and normalling out closer to idle but I admit that's one I haven't thought too much about.
 

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2005 18psi supercharged U5 Outback w/207k+ miles
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I was wondering about this because I had been thinking about (some day) a EJ255/EJ253 hybrid build using my tuned EJ253 ECU.
 

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Certainly depends how far from stock you end up, 0.1 certainly won't make a dent.

But in general doesn't the ECU doesn't "know" or "care" how the air gets into the car - it just meters it and reacts accordingly?

They have MAP sensors, wouldn't the ECU just run it at whatever metered rate it sees? that's what happens when people go the opposite way and build a Subaru turbo block and install it in a stock non-turbo subaru, the ECU just meters whatever air it sees and it runs just fine, albeit with lower performance due to lower compression pistons.

In the end you'd just adjust it accordingly like people do when they build 25+ psi motors.
 

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If you change the core hardware just tune it. It's not smart enough to know better and the learning stuff could go all wonky over time. **** its barely smart enough to properly deal with exhaust changes and intake leaks. Stuff isn't AI even though it 'learns' a little bit. Even 'learning' on these ECUs is just crude adjustments.
 

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Discussion Starter · #6 ·
If you change the core hardware just tune it. It's not smart enough to know better and the learning stuff could go all wonky over time. **** its barely smart enough to properly deal with exhaust changes and intake leaks. Stuff isn't AI even though it 'learns' a little bit. Even 'learning' on these ECUs is just crude adjustments.
The key phrasing here is 'inadvertently' with regard to changing the compression ratio. I was asking what would be the signs if that were to happen.

The reason I've asked, my tuner asked my compression ratio and I had to admit I didn't know and that got me going down this path of questioning.
When I purchased the brand new shortblock, I only confirmed that the shortblock was a match for the VIN on my car, not for the heads that I was putting on. I did not measure for compression ratio when I put the heads on. The orignal engine is an EJ255 with B25 heads. The replacement heads are B25, which should be correct, the heads are from an '06 SpecB, which was an EJ255. I know there is a difference between the B25 and D25 with regard to compression chamber size.

I don't think the compression ratio is wrong, but I'm not 100% sure it's right. And with all the other problems I'm having, I can't rule it out.
 

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Discussion Starter · #7 ·
But in general doesn't the ECU doesn't "know" or "care" how the air gets into the car - it just meters it and reacts accordingly?
That's my thinking for part of it, it's just suck, compress, bang, blow. (sounds like an old girlfriend of mine 0:) ) The ECU just changes when those things happen.

I'm just trying to see what the logging reaction would be.
 

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Outback 2.0XT, Audi TTq, Ducati M750
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Logie :29: http://www.healey6.com/Technical/Nominal%20compression%20pressure.pdf
Compression Ratio
Estimated Cylinder Pressure @ TDC given 60% volumetric efficiency at cranking speed
Percent Change from 8:1 CR NominalDynamic*

PTDC= 1atm · VE · nominal CR

1.3[corrected air-gasoline specific heat factor] = for 10:1 CR & 60% VE, then
14.7 psi · 0.6 · 25.119 = 176 psi – 14.7 psi = 161 psi gauge reading

7:1
4.2:1
96 psi gauge reading
-18%

8:1 (baseline of data, also almost exactly stock CR)
4.8:1
117 psi gauge reading
0%

9:1
5.4:1
139 psi gauge reading
19%

10:1
6.0:1
161 psi gauge reading
38%

11:1
6.6:1
184 psi gauge reading
58%

12:1
7.2:1
208 psi gauge reading
78%

If you change the core hardware just tune it. It's not smart enough to know better and the learning stuff could go all wonky over time. **** its barely smart enough to properly deal with exhaust changes and intake leaks. Stuff isn't AI even though it 'learns' a little bit. Even 'learning' on these ECUs is just crude adjustments.
Totally true, after my experience with the TT's ECU I am disappointed in how basic Subaru's adaptability is. I threw all kinds of parts at the TT before a proper tune and it handled them perfectly fine, but if I so much as install an intake or downpipe the car needs a tune or it might just blow up. A real tune is always better obviously (XRT soon for me), but the lack of actual learning/compensation bothers me to the point I'm considering a 91oct tune just to protect the engine on roadtrips.

Bosch ME7.5 for life!
 

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Discussion Starter · #9 ·
Awesome. Thank you for that scale. I had read through a lot of other forums and posts about how to test using a compression test and figuring it out that way but there was a lot of ambiguity to the formula for calculation for some reason. I even bought a compression test kit. Now to just get around to getting it done this weekend.

I agree that they are programmed to operate in way to narrow of a tolerance range. They don't know what to do if something is just outside of that range. Every other car I've owned was very forgiving of moderate upgrades with no adjustments.

I've got Ed at XRT standing by while I figure out what the **** is going on with mine. I really want to get it done.
 
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