built motor with 12.5:1 compression?
 

After some crazy knock sound from the crankshaft area, i decided to get a built motor.

Now i know its weird talking about built motors and smog but living in CA, i'll need to eventually smog the car. I'm assuming if i lowered the compression to 10.5:1 I wouldn't be able to pass a smog test? I asked a smog tech about passing smog if i lowered my compression. He said they don't do the sniffer test now for 2000 and up cars. Now he says it won't pass smog.

I want to stay stock compression with the built block. Speaking with my tuner, he strongly recommends the lower compression if i wanted to get to 400whp on pump gas. Asked him to clarify even if i wanted to push the built block with 12.5:1 comp i cant even go to around 350whp safely and efficiently He said 12.5:1 compression will never reach 400whp on pump 91 built or not. I know its called CA poop gas but is 91 that bad? I'm obviously living in fantasy land with those high power numbers. What are the most realistic power numbers that I'll get on 91 with 12.5:1 built motor?

I do have access to E85 but theyre not plentiful where I live. I will usually run some mix of E85 but sometimes I'll only be able to run 91. The power junky in me wants the lower compression and it would seem like a waste on pump gas i could only get 300whp on a built block.

TDLR; Would lowering compression still be able to pass smog? What is the realistic power goal for a turbo setup with a 12.5:1 built motor on pump 91? My tuner said to stay at the lower compression levels for higher power goals.

dont know about smog but
Your tuner is very right, listen to him
he was also optimistic with 91oct power :)

do something like 11.5:1 maybe? to make it better with 91, and plenty of power with e85 mixes
for even bigger targets..listen to your tuner

hmmm. i always have listened to him since getting the car tuned with him.
He sounds confident in it. "10.5:1 would afford 400whp with fair ease and higher is still plenty achievable on ethanol."

but with some wiggle room. "Your power goals won't possibly be so high that they won't still be easily achievable on ethanol, and it helps to know that the car isn't stressed within an inch of its life on 91 to do the same thing. :laughabove:

I guess i will. Just hmm. if the times to smog the car. i wonder what ill do then.
POWER and more MONEY spending here i come! :mad0259::bow:

Compression shouldn't affect smog. A turbo will though. He's right about the compression. Lower compression means more boost.

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I cant tell you about smog since i live in pa but call up IAG. They have both a 12.5:1 and a 10.5:1 option. I suspect the 91 will be the killer

Well, my turbo is carb legal. I could run the oem headers/oem injectors and switch to the carb tune. But I don't want to a swap to an OEM motor if i need to smog. i'm most likely going to go 10.5:1 but i just wanted to see if anyone was running a 12.5:1 built motor on pump 91.



Yea, that's who I chose to get the short block from. I pulled the motor and tomorrow should be able to pull the heads apart and inspect what's salvageable.


No one with 12.5:1 running 91 octane? what power you at? any problems =D

they are currently building my block. but i talked with them and i wanted something that could handled a c38 with the RS pulley since i have the hbp and wanted the headroom in case i upgraded. Around me we have 93 but i will be running flex fuel

Listen to your tuner, they are correct.

Realistic safe power numbers with stock compression on 91 is going to be around 250-300whp. Going higher risks detonation. It is why anyone you see with a stock motor on higher power is running E85.

I would also recommend going 10.5:1 or at least 11.5:1. I went 10.5:1 because E85 in my area is inconsistent and I didn't want to be as power limited on 93. If you know you want to raise power later and don't have access to E85, you are shooting yourself in the foot staying 12.5:1.

Just curious to know how much has it costed you so far to build the block? Planning on building my block in the near future as well

so like do you want how much im paying IAG for a built long block with or without machining, with or without random other parts i need, and with or without the cost to get a donor motor for heads and other parts. A good round number is unless you have 10k you probably shouldnt think about it. That also doesnt include stuff like oil pressure guages, catch cans, rad, oil cooler, fuel pump, injector, oil baffle, headers and exhaust, ethanol kit, tune, or the actual supercharger kit.

Sorry should’ve specified, just the machined long block from IAG

8k which includes few random parts i didnt want to reuse, and since im lazy i had them time it too. This doesnt include new DIs or shipping

Not to mention timing components- you should not re-use your cam gears! They hold plenty of oil and Subaru considers them non serviceable.

May as well get new chains, tensioners, guides.
May as well get a new thermostat
May as well refresh your pads and rotors if they're worn out
May as well consider other annoying supporting mods like a radiator shroud

Do you see where this ends up going? The block is the price of admission. Everything else ends up being the balloon popping games, ride tickets, food, stuffed animals, etc that actually empty your wallet because you weren't expecting them

Not sure if I missed it, but I am assuming you are going big turbo and not SC?
Pointless to drop compression on SC. Always disappointing results.

In either case you will never hit 400whp on 91 octane regardless of compression.
Look at dynos. If anyone has evidence otherwise then link it.

I could possibly see 400whp on the most generous hero run dyno that would probably be 325-350whp on any other dyno in the world. and is running ET/Trap of a 325-350whp car not 400whp.

Keep in mind lower compression requires much more boost to get the same power.
Lower compression results in a less efficient and less powerful off boost engine.
Lower compression with more boost results in more turbo lag.

Don't expect reliable big power out of a 2L without 100+octane or E85 or at least a mix, and even then 450whp is pushing the limits of a built block.

If race gas or E85 isn't easily available, you can safely setup WMI to get equivalent of about 98octane while spraying.

You could also just store E85 and run a mix to only need to make the trip once every 2 months or so.
I have 6 - 5gallons jugs of E85, 1 jug + 5gallons of pump gets you E40+ which is about 100octane. And you could always add in WMI on top of that and you now have the equivalent of full E85, but with better cooling and fuel efficiency.

I would not settle for 91 octane if I was building a block and wanting big numbers, you will be disappointed at the cost to result ratio.