So which Sti heads are preferred to match the USDM 2.5L block and how do you work around the AVCS?

 

One would assume the USDM STi head would be the best match (don't know the combustion chamber volume of the JDM vs. USDM heads) but heck they're having detonation problems on bad fuel BONE stock with STOCK boost!

I'd have to say they were your best bets for 100% compatibility right now though.

You could disable the AVCS or get a USDM computer...

To do it 100% it's going to be an expensive swap!
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well, not to say I know better than someone that is building motors, .... but I think they're half-full-of-sh*t. I mean, they ARE out to make money, And they are doing it the 'full' right way (if that makes sense). I probably would do the same thing if I were them.
What is their "ideal" compression ratio?? And a higher compression ratio than 8:1 or 8.5:1 isn't a bad thing if you are Tuning you car, so long as the components can take it.
And using a Stock WRX head is less than idea, ... I think we already covered that. Head work has always been something that would need to be done with the 2.5 block.
If you can take your block to Snyder's or if Ed can do it, check the valve clearance, and try to figure out the compression ratio. That is something you should be doing anyway. There Also are other options, you can buy a thicker metal head gasket (like a lot of people do), that's not a big deal.

my $0.0000002
Sean

 

Since I'm not a mechanic, but I play one in my mind, I can't tell you how to clay pistons.....




But I can tell you how to play with clistons all day long.


I know. Weak attempt.

 

http://www.fordmuscle.com/fundamenta...ve/index.shtml

This is for an OHV motor, but the fundamentals remain the same.

Static compression ratio is something you REALLY want to get right. It will hamper your power making potential.

A 2.5l motor that detonates at the drop of a hat or can only run limited boost or must run race gas is NOT something you want to end up with. Just IMHO. Been there, done that.
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Sean's advice re: the head gasket is another possibility. It's the "poor man's" combustion chamber volume tuning agent. I've done that before as well.

What that doesn't take into account is anything other than chamber volume (such as peak and valley interaction between piston dome and chamber roof) which can affect detonation characteristics of a motor.
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For the lazy: "Claying" a piston is just a quick way of measuring the clearance (if any) between the valves and crown/dome of the piston at the closest point between the two.

1. Take cylinder head off
2. Put clay on piston near valve reliefs (if present) or highest point
3. Bolt head back on the car (including necessary valvetrain components)
4. Run engine through a couple of complete revolutions *SLOWLY* (remember, there may already be clearance problems)
5. Remove valvetrain components and cylinder heads
6. Inspect impressions in the clay (the valve should have opened enough to leave a depression in the clay).
7. Measure depression with calipers (take clay pad off of piston, cut in half and measure with instrument)
8. Specifications will be up to your motor builder/factory manual
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How will your cam timing change significantly?
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I've never been accused of being anial before!

But you're right. Cam timing will be changed a certain amount due to running a thicker head gasket. Who is to say what is significant? Not me, because nowadays I'd change the pistons or combustion chamber volume to get the desired compression ratio and not band-aid it. I've done it in the past with no ill effects.

However, I'm also not the one doing any work to my Suby right now.

Anyway... Are you performing this swap? Do you have any helpful information for us if you are?
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Note that if you read my post I never said this.
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Also if this were important enough to you (and you didn't want to change pistons) you could find adjustable cam gears, but that's beside the point...
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