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Subaru ej25 Timing V.S. Boost

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I have a 2005 Subaru Outback xt with an ej255. Forged short block, BC 272 Cams AVCS, stock bolt-ons with the exception of a catless up pipe and a 3-inch catless downpipe, with stock catback. Running California 91 Octane, street tuning.

I made a tune, that I've been running on for about 1500 miles, and I like to think its a good and safe tune, but I have a couple of questions.

My max boost is at 18 psi on a VF40 which is about the most you can push out of this turbo, but most likely out of its efficiency range. In order to achieve this boost without knock I needed to decrease the timing by about 8-10 degrees in the WOT areas from the stock timing table at 11.5 psi. I can do a 3000RPM to Redline pull without knocking with this tune, but if I go out and drive it hard in the canyons i get some Feedback knock which is caused by hard shifting, but ill get some fine learning knock in the 4000 - 5800 range above 1.60 g/rev its usually only a degree or so pulled.

Is this normal or should I be looking at eliminating fine learning knock completely?

Part of me feels like I should lower the boost to 16 psi, so I can increase the timing a bit, but Im also upgrading my intercooler very soon, as well as switching to e70 fuel. I think the intercooler may fix my issues on 91 which makes me think I should just leave it until I put the new intercooler on because it will be able to make the same boost at lower temps. Switching to e70 will also fix my dilemma, but I want a good 91 tune for when I need to switch back.

Should i just leave it the way it is, or should i lower the boost a little so i can increase the timing?

I never got a chance to see what a stock good condition engine runs like on a day to day basis so i have no baseline to go off to know if this is normal.

Thanks in advanced

for a start pat yourself on the back,

you're asking all the right questions because it's your own car, and it's a cool thing to do I would try every different way of tuning the more you do the more you learn

the intercooler and e70 will make a huge difference but you may as well keep learning

Regards Ross

Thank you for the advise, ill give it a try and see what the results are. My natural instinct with working on cars is if it ain't broke dont fix it, so I didn't really think about just trying to see what happens lol.

On the small turbos it is often turns out that lower boost with increased ignition timing results in better power outcome since higher boost levels result in hotter intake air temperature that significantly increases knock likelihood. Knock occurs with compressed air-fuel mixture temperature is around 280-300 degrees C and the intake air temperature raises in power of 1.4 when compressed. So even 10 degrees of air intake temperature will make huge difference on knock occurance when being compressed.

So I finally found time to work on my low boost tune. I used my existing target boost table and lowed the target boost in the WOT area, and had to tinker with targets at low rpm because I was getting some integral windup, and over boosting. I still need to do a little more tinkering with it, and I haven't touched the timing yet so it's the same as the 18psi tune.

I went to the 1/8 mile drag strip and my previous times were 9.33 pretty consistent. I have two-step now which I didn't have before and I ran a 9:30 on the 18psi tune and then flashed the low boost tune and made another pass and ran a 9.22. It only got better after that I ended up getting an 8.92 and beat my buddy's 3 valve mustang gt who ran a 8.99.

The car seems a lot happier and a lot less knock prone, and I have actual data to show how much of a difference it made (a lot more than I thought). I still haven't got my intercooler yet, but I may just stick with this 16 psi tune because it seems to be working.

Thanks all for your advice, ill update when I smooth things out a bit and hopefully get my intercooler.

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