Subaru BRZ engine build - AVO turbo

[14brzboost]

Hey, new to the forum. I Recently bought a 2014 subaru brz with various mods done by the previous owner. Bought it knowing there was a knock coming from the engine on the drivers side. Upon tear down found 3 bent connecting rods (noise was one of the pistons contacting the bottom of the sleeve)

Hoping to get others advice and knowledge on rebuilding it. I will be installing forged rods and new pistons and likely be re-tuning as i'm not sure exactly how it was set up before,

Currently my main question is what would be the best compression ratio pistons to go with 10.5 or stock ratio 12.5? I will most likely be going with JE FSR forged pistons as the piston retains a very similar shape to factory.

Available fuels will be Shell V-power 91 or Petro 94 here in ontario (no e85) with power goals of 260-300whp for a Summer daily and reliable as possible.

Thanks, 14brzboost

[DarkPira7e]

If your power goal is mid 200s, stick with stock compression

[dagle]

lower compression pistons will be more forgiving to your tune should something go wrong (misfire/knock). the horsepower and torque difference for 2 points of compression will be small but noticeable, i would guess on the order of 5-10whp throughout the powerband. the gotcha is that you usually can turn up the boost and advance the timing slightly more with lower compression pistons. for my money i would try to stay close to stock so that i could return to the stock computer if i needed to (emissions, trade-in, warranty/recall work, etc.)

alternatively you could talk to your tuner and ask his opinion on compression ratios and see what he's worked with for good results. there's also no reason you couldn't go somewhere in the middle.

[solidsnake11]

10% hp gain going from 8 to1 to 10 to 1 NA
Vvt could probably make up part of it.
Boost will definitely make it up.

CAR CRAFT Jan 2002 made 4 dyno runs changing the compression only:

Ratio, cranking comp, HP, TQ

8:1, 135, 440, 486
9:1, 163, 460, 506
10:1, 195, 484, 518
11:1, 232, 500, 531

440 mopar with 230, 238 110 cam

so you can see how little power is gained by running race gas in a street car, when going from 10 to 11:1

[14brzboost]

Thanks for the input. I know it will be very tune and fuel quality related but is there a general hp and boost limit on pump gas that would be safe at 10.5:1 and 12.5:1 in case I get the itch for a bit more power? Any downside to using 10.5:1 piston such as off boost response?

[86TOYO2k17]

91 can make 280whp, 94 300whp, E85 350whp easily on stock compression no problem with little to no risk from a compression/octane stand point. Old school train of thought is high comp = bad for boost. Go read the forums from 2012-13 everyone saying RIP boost because 12.5:1 compression, or people saying you’d be crazy running anything over 7psi because of high compression, now people are running 14 psi on pump gas no problem. Tuning software/techniques, the engines ability to monitor and adapt to knock so well and how sensitive it is, and especially direct injection have revised what was previously considered impossible.

If going turbo and being limited to pump gas and wanting 330+whp a mild compression drop would be good. A lot of people running 350whp on pump gas and stock block with no issues so far, but that’s beyond pushing the limits to what i would be comfortable running but if that can be done 300ish should be pretty safe.

The downsides to comp drop are mpg drop, off boost power, throttle response, slower turbo spool/lag, needing more boost to make the same power.

The biggest issues with the FA20 limiting power and causing reliability issues are weak rods and oil starvation.

[dagle]

any particular reason for those threshold values? (280/91oct, 300/94oct, 350/E85), i don't see why you couldn't do 350+ if your pistons/rods support it.

also i don't think it was old school train of thought (i'm from that era), the injectors available weren't nearly as good as they are today. The Bosch EV-14 style injectors weren't readily available yet and RC injectors were the norm. You could expect a 5-10% variance across your injectors and they didn't all start/stop at the same speed either. Combined with the tuning solutions (depending on the car) that didn't support individual injector tuning, lower compression motors were more forgiving. Real world MPG differences were also negligible since most people trimmed fuels as lean as possible without knocking anyways. On a completely cold start you might see a 1MPG difference, but as soon as the engine warms up MPG should be comparable.

I agree on the other points made by 86TOYO2k17 that weak rods and oiling seem to be way bigger hurdles for power output than compression ratio.

[86TOYO2k17]

@dagle
More can be made, but the amount of boost needed to make more power starts putting it into more and more risky territory, more boost = more heat and the lower octane reaches it’s threshold for safety at a much lower boost lvl. Even one instance of pre detonation can raise cylinder pressure far above higher boost/timing lvls when running safely with higher octane. Also only so much timing can be pulled to attempt to make more boost safer eventually you run out and the power gain from adding more boost doesn’t exceed the power loss from timing pulled, and you just increase the risk in the process. Depends on how close to the threshold you are comfortable getting to, casual hwy pulls is a lot less risky and strenuous then tracking so usage can play a role in deciding safety levels. But generally those numbers are pretty close to what dozens of dynos/tuning/setups have concluded. Some people have pushed more and been fine some less and had issues.

Right a lot of advancements have been made even in the last ten years to be able to run higher compression and boost safely. Even 10 years ago 11:1 was considered high compression. boost and 12.5:1 was unheard of. But times are changing, now it’s not so crazy.

Forums been around about 8yrs. At this point there are rarely new questions, only new people asking the same old questions already been discussed dozens of times. All the info is out there if you take the time to look. Fortunately or unfortunately... for me i have way too much time to pretty much have read almost every thread on this entire forum.