Compression ratio

[nicklmort]

So i ordered 9.0:1 wiseco pistons. I did this without realizing my heads have been milled 30 thousanths. i plan to run 17 pounds oin a td04. is this a safe compression ratio for 91 octane and stock headgaskets? what about for more boost in the future. I still have time to change the order should i do so?

[kimokalihi]

You should change the order. The WRX compression is like 8.5:1. I would not run 9:1.

[nicklmort]

The head chamber in a stock 22t is 49cc what i should have asked is what is it with .030 taken off of the head

[nicklmort]

Off the shelf pistons wiseco makes for sti's are 8.9:1 please explain your reasoning. Most turbo street builds ive seen run 8.5:1 - 9.0:1 because theres better off boost power and drivability on bigger turbos.

[Legacy777]

As was mentioned in your other thread, the TD04 is really just going to be making hot air at that pressure and not really move much volume.

For the CR you're planning to run, turbo, and fuel, I think you're going to have to lower your desired boost. Remember, boost pressure is not a critical item, the air flow through the turbo and engine is. I can take a small turbo and run 17 psi and get x amount of air through it. I can then take a slightly larger turbo and run 13 psi, and probably get more air through it. The larger turbo is going to be the better option assuming the smaller turbo is on the outskirts of its efficiency islands.

Have you looked at a turbo map for the TD04?

[nicklmort]

No i have not. What are they efficient to?

[93forestpearl]

It took me 19 seconds to find the map via google images.

From the looks of the map, the max pressure ratio you would want to run is 2.2, meaning 17.5 psi BEFORE losses such as intercoolers and other plumbing. Considering what intercoolers most people here run, I would chop 2 psi off of that. Basically, 15-16 psi should be all the further you push a TD04. Granted, this is just mere guessing by looking at the compressor map without doing any calculations, which would show what you want to shoot for.


I suggest you read Turbo Tech 101, 102, and 103 at www.turbobygarrett.com. These articles will shed a ton of light on the dynamics of turbocharging.

[BoostedSubie]

Not to jack your thread, but i also had a compression question. I have a spare turbo block i am rebuilding. I was going to buy the wiseco off the shelf 8.5 forged pistons that are .40 over, with a phase 2 2.5 crank to bring it to the 2.35l stroker. I also planned on doing the dohc 2.5 swap in the process. When normally doing the dohc swap, the compression is lowered from the stock 8:1 to 7.5:1 unless using the thinner headgasket. My question is using the higher 8.5 wiseco pistons, dohc heads, and ej22t headgasket, wouldn't the compression be at 8:1?

[nicklmort]

You did jack my thread. And search I've seen a thread on here where someone took the time to calculate almost all ej22/head setups with stock and 8.5:1 pistons.


And I understand the efficency of turbos that was not my question. I do not have the money for a bigger turbo right now. I understand I will make more power on less boost with a bigger turbo. My question was what kind of power can I make with the td04. I simply said 17 pounds because I was not EXACTLY sure what it was efficient to. So 15 to 16 pounds depending on intake temps with the 06 07 wrx top mount.

[evolutionmovement]

If you don't have the money for a bigger turbo, how are you going to afford to rebuild the engine after running ridiculously high CR with an inefficient head design and a turbo at the high end of the efficiency range especially where you presumably can't afford the expensive controls that would be the only slight hope of keeping this thing from burning up? The "Lego" analogy holds to the point of interchangeability between Subaru models, but the physics involved is much more complicated. You're going to have to do some thinking for yourself since nobody here has tried what you intend, but as a starting point, you've chosen pistons with too high a CR.

Here's a good starting point: Engines An Introduction by John Lumley.

[93forestpearl]

And I understand the efficency of turbos that was not my question. I do not have the money for a bigger turbo right now. I understand I will make more power on less boost with a bigger turbo. My question was what kind of power can I make with the td04. I simply said 17 pounds because I was not EXACTLY sure what it was efficient to. So 15 to 16 pounds depending on intake temps with the 06 07 wrx top mount.

Fair enough.


All you can really do is tune it out to what you can safely run. Speculating at this point is of really no use.


If you have time to change your piston order, I would. Honestly, the EJ22T longblock with the 8.5:1 pistons, fresh valve job and all that with the 220 grind cams was plenty snappy off boost. I was never dissappointed with how much grunt that engine had off the line at low RPM's. Couple that with any of the smaller framed turbos and you will have a plenty streetable setup.

[nicklmort]

I actually changed the order to 8.5:1 before anyone posted a reply after doing some research. And evolution, my car spun a bearing on a td04, 06/07 top mount, apexi neo, tuned by pre on 11psi at 210k miles. Ive always planned on a 2.35 with wiseco pistons and dohc. I wasnt going to do it this soon but i do not want to spend the money to rebuild my motor back to stock and then tear it down again in the future. So i am building the shortblock now and keepeing the rest as is. Your all treating me like am a f***ing retard and answering questions i never asked.

[nicklmort]

Fair enough.


All you can really do is tune it out to what you can safely run. Speculating at this point is of really no use.


If you have time to change your piston order, I would. Honestly, the EJ22T longblock with the 8.5:1 pistons, fresh valve job and all that with the 220 grind cams was plenty snappy off boost. I was never dissappointed with how much grunt that engine had off the line at low RPM's. Couple that with any of the smaller framed turbos and you will have a plenty streetable setup.

Hey someone finally answered my question THANK YOU.