Comparing heads - TT shim over to sti shim under (v3/4)

So I'm doing my research and can't find any concrete answers on the differences between these heads? I'm talking phase one inline bolt pattern heads.

Obviously the STI has shim under bucket and isn't prone to the shims falling out at higher RPM. From what I've read the bare heads are the same and the difference is in the cam duration? Or is there a difference between the porting from TT to STI?

What's the difference between cams? From what I read the cams are almost identical but the sti has a wider came lobe for longer duration or something.

And which model legacy engine has the more aggressive cams? Which injectors did they come with? What's the engine code to look for here?

My story

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Blown head gasket on bc5 legacy. Was planning on using CDB and sti shim under heads but I can't really find any sti heads so I'm considering using shim over bucket heads. Not looking for massive HP but 200-240 kw is what my td05 18g is limited to so it would be good to get somewhere in that range. Thinking as a good temp fix I could source a TT long engine and block up the lines on the second head and run that in place of the ej20g.I would retain the ej20g sensors and manifold.

EJ20R out of a bg5b/bd5b Legacy. Last of the phase 1 engines?

They are pretty much identical to the v3/4 sti heads, apart from the location the shims and ran the same yellow injectors.

Not sure if there is any minor difference in the cams (from what I've read they have the same spare parts listing but that just means that Subaru don't want/need to carry two near identical cams in their spares inventory) and the head castings can be as bad as each other.

In the real world, I'd bet you'd struggle to find a performance difference. Shim under is preferable if you want to go to bigger cams in the future. But if leaving the heads stock standard, I'd happily use either.

yea lol trying to get some v3 sti heads is a mission

What sort of limitations do the shim over have? Can they still run high boost say 25psi ?

Rpm's is the limitation. Boost won't have an effect.

I was using the gtb heads and making around 270kw @25psi with a dodgy exhaust manifold setup that was hurting performance.

Another limitation the shim overs have is you can't fit as high lift cams as you can in the shim unders, if you were upgrade cams at some stage.

I've only swapped from the gtb heads as I tested three pairs of heads and the sti ones i had were the hardest (had marginally overheated the gtb ones when i blew headgasket).

 Koom said:

Rpm's is the limitation. Boost won't have an effect.

I was using the gtb heads and making around 270kw @25psi with a dodgy exhaust manifold setup that was hurting performance.

Another limitation the shim overs have is you can't fit as high lift cams as you can in the shim unders, if you were upgrade cams at some stage.

I've only swapped from the gtb heads as I tested three pairs of heads and the sti ones i had were the hardest (had marginally overheated the gtb ones when i blew headgasket).

What was the setup for 270@25?

Standard gtb heads, closed deck block with standard compression wiseco's and eagle rods.

Standard intake and exhaust manifolds. Modified single scroll up-pipe feeding a twin scroll holset hx35 (don't recommend doing that, boost threshold was up around the rev limiter).

740cc injectors and a Possumlink. Drive alright off boost, albeit a tad rich, boost came on like a light switch at 5500rpm and it'd pull too 8k redline. Was fun but slow as a road car as I've also got a short ratio gearbox so it didn't really make boost until somewhere round the region of 120kph in third. So was basically a non-turbo on the road haha.

A proper twin scroll manifold and 2.1L should help the cause.

Was that on a dyno or injector duty?

Road dyno spreadsheet off data logs. Correlated with dyno on original setup and injector duty. So it's a ballpark figure but I've narrowed down the ballpark

ok so new question.. Is there any easy ways to tell a gtb head from a wrx head?

And are there any ways to tell an ej20h from an ej20r ?

That's a tough one. Don't the wrx heads have hydraulic lifters (hard to tell without pulling the cams out).

Not sure if the valve covers are different between them?

v3-4 wrx should be shim above buckets? I was wondering does the ej20r use coil on plug and wrx/sti uses coil pack?

This is supposedly an ej20r photo (could be wrong though)

Looks to be an ej20r from the top photo to me and the wrx v3-4 ive seen are shim over bucket, and your rite ej20r use coil on packs apposed to the wrx sti

Hydraulic vs solid is dictated by year not model with the good ol phase 1 blocks, with the one and only exception of the V2 RA's.

everything up to 96 was hydraulic, this can be identified by lines on the rocker cover

everything after 96 was solid, with a smooth rocker cover.

The exception being the V2 RA's (I do not know if this includes WRX RA's too, but subaru's fast system indicates it DOES, but again, proof that is NOT). The V2 RA's had the lines on the rocker cover, and are the older style engine, last of the 20G's, but did have solid lifters, and revved to 8K, just like the 20K's after it.

Concerning later solid heads, telling a smooth 20H apart from a 20R nowdays is practically impossible. The yellow injectors, larger intake flexi pipe, 20R on the cambelt covers can all be swapped. And with the 20R's offing themselves for so many years now, it's impossible to say for sure the engine being pulled out of a manual BG GTB is going to be its original. you can almost guarantee if it has been swapped, there's actually a 20H chilling out in there. I've seen GTB's running grey top injectors before! All one can realistically do is make a best guess, find out if the engine is original, see if all the tell tail parts bolted to it actually match up, and go from there.

But are 20H's and 20R's really that different head wise? And is it going to make that much of a difference to ones build?