ej25 SOHC Heads vs ej25 DOHC Heads?

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 4:15 am

Okay... So which is better to use on an EJ22T block?

I figured that the SOHC Heads would be easier to deal with so you dont have to deal with an interference motor and what not...

But which flows better? I have seen on here, through search, that the DOHC Heads are better... And that the SOHC Heads are better.... So which is it?

Now when it comes to Phase I and Phase II... I'm not sure I understand the differences. Could someone explain that?

Thanks everybody... I'm kinda tryin to create a build list and I'm really curious about different head set-ups.

Oh and what intake manifolds would you use with the different setups? Can you use the ej22t manifold with the SOHC heads? Or do you need the 2.5 SOHC Manifold?

And how do the different 2.5L setups work with the stock ECU? And wiring harnesses? Can you stick with the ej22t wiring harness?

555BCTurbo · Post by **555BCTurbo** » Wed Feb 13, 2008 5:16 am

First thing...just because they are SOHC heads does not mean it will be non-interference.

The SOHC heads are excellent units...with big valves and a roller valvetrain...BUT...

The coolant passages do not match that of a closed-deck EJ block...so they are a poor choice for an EJ22T build.

And...Phase I = DOHC EJ25
Phase II = SOHC EJ25

To use either set, you will need the intake, as it is completely different.

The SOHC EJ25 heads are completely different than the EJ22 heads...

The electrics will work with both

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 5:18 am

Very Cool!! Thanks!

So what would be a good choice? I'd really like to keep the lower-end power...

PhyrraM · Post by **PhyrraM** » Wed Feb 13, 2008 5:42 am

Niether is 'better'.

Beyond simply saying SOHC or DOHC you need to where each one comes from.

SOHC is commonly refered to for both early BC/BF heads such as the stock Ej22t heads and also the heads on later (phase 2) Ej25s.

DOHC is really a bit of a cluster f**k. Ej20g, Ej20k, Ej20h are all DOHC. These are the most commonly discussed here. Ej205, Ej207, Ej255 and Ej257 are all newer (different ECU signals) and basically ignored on this forum. The early (phase 1) non-turbo USDM Ej25s also have DOHC heads.

Ej20g is BC/BF Legacy and early WRX (v1&2). Legacy heads have stub rockers while WRXs have cam-over-buckets. Both of these engines have the 'elbow' turbo inlet like our Ej22ts.

Ej20k is later WRX (v3&4) and is also cam-over-bucket. These engines have the under the manifold turbo inlet. There is also engines in these years Subaru labeled as Ej20g, but should be treated as Ej20ks because electrically and installation-wise that's what they are.

Ej20h is Legacy twin turbo. Electrically these are way out in left field from most other Subarus. But used just for the hardware, not the electrics, it's basically a Ej20k.

USDM phase 1 Ej25 heads are losely based on Ej20k heads. They only need existing bosses drilled and tapped for turbo oil and coolant lines.

The above heads are mostly interchangable because they all accept the same intake manifolds. The 'bolt in/plug-in" deal would be any of the above heads with an Ej20g intake manifold. For our cars the Ej20g intake manifold is prefered becasue it uses comapatable sensors and IAC. It is by all means not the only option.

The Phase 2 SOHC Ej25 head is a very good flowing head, but because it has no provisions for a turbo it seems mostly avoided.

The phase 1 SOHC head is what you have already got on a Ej22t.

Phase 1 and 2 refers to a mild redesign of the block by Subaru about '98. It appears mainly to adrress some problems that cropped up when Subaru 'factory stroked' the Ej20/22 series into the Ej25. Headgasket problems are the most notable. There are numerous improvements but the major ones were a change in the position of the thrust bearing on the crank, the size of the rod journals on Ej25s and structural improvements. As far as heads go there is little to concern yourself with as far as phase1 or 2.

555BCTurbo · Post by **555BCTurbo** » Wed Feb 13, 2008 5:49 am

If you want the torque, keep the SOHC 2.2 heads Spencer...and just cram some air into them.

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 5:53 am

Wow... Great info here!

Basically I have an idea for the short block

22t block
Wiseco Pistons (8.5:1)
Eagle Connecting Rods
2.5L crank
----------------
= 2.34L Stroker

The reason i settled on this recipe is the extra displacement will gice me more low-end power (form what I hear) and theoretically it would be just about bullet-proof (interpret as you will... I mean very strong).

So I've got an idea for the short block... But the heads and intake manifolds... I'm not sure...

I do know that the Phase I SOHC stock heads are pretty restrictive... So I'm not sure where to go from here.

Suggestions?

(And yeah, I know the forged pistons are rather loud when cold... I'll live)

livestrong14 · Post by **livestrong14** » Wed Feb 13, 2008 5:54 am

Right, because it is known throughout the subaru world that DOHC cars cannot produce torque for shit. Just look at what everybody else is doing, and what their future plans are.

There is a reason why they threw out the SOHC Turbo setup back in 94.

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 5:55 am

Will they (stock heads) be sufficient?

555BCTurbo · Post by **555BCTurbo** » Wed Feb 13, 2008 5:56 am

livestrong14 wrote:
There is a reason why they threw out the SOHC Turbo setup back in 94.

It was actually for economic and emissions reasons...not really power

555BCTurbo · Post by **555BCTurbo** » Wed Feb 13, 2008 5:57 am

smh0101 wrote:Will they (stock heads) be sufficient?

Yes...you will be able to make a heap of power with an 2.35 and 2.2t heads

You are pretty limited as far as intake manifolds go...because each type of heads pretty much has its own specific intake.

So, if you are running stock EJ22T heads, you need a 2.2 intake manifold...

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 5:58 am

Oh... Cool...

Then would a good port and polish help them flow better?

555BCTurbo · Post by **555BCTurbo** » Wed Feb 13, 2008 5:59 am

smh0101 wrote:Oh... Cool...

Then would a good port and polish help them flow better?

Yes...a porting and polishing (only on the exhaust side) would help a bit...as well as getting some Delta cams in there

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 6:01 am

555BCTurbo wrote:
smh0101 wrote:Oh... Cool...

Then would a good port and polish help them flow better?
Yes...a porting and polishing (only on the exhaust side) would help a bit...as well as getting some Delta cams in there

Schweet!

Now... the delta cams...

Could you possibly educate me?

livestrong14 · Post by **livestrong14** » Wed Feb 13, 2008 6:03 am

There is also a reason why Subaru didn't intercool our cars. They were vanilla. They were plain, boring, and tried to make the car seem like it wasn't turbocharged at all. We were the only ones to get the non quad cam heads. USDM>JDM is rarely ever said.

IMO Spencer, DOHC will always be better. Torque doesn't have a factor in it at all.

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 6:04 am

Hows the torque on your car?

livestrong14 · Post by **livestrong14** » Wed Feb 13, 2008 6:04 am

You tell me

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 6:05 am

Havent driven it... Only rode in it when ya tookl me home that night...

And it pulled like a raped ape.

PhyrraM · Post by **PhyrraM** » Wed Feb 13, 2008 6:19 am

The choice of an Ej22 and SOHC heads over the (by that time) tried and true Ej20g for us americans was most likely determinded by our driving style.

We like that 'squirt' off the line. We don't like to wait. We want instant gratification. We also don't see high speeds in the grand scheme. The larger engine, smaller turbo and smaller port heads all helped overcome turbo lag to give us what we wanted.

The lack of an intercooler was an economic choice determined by the chosen boost level. It was simply not needed. the fact the BCs/
BFs used a relatively expensive air/water wetup made the choice all that much easier for Subaru.

If you choose a 2.35 build the stock Ej22t SOHC heads *might* be out of breath by 5000 RPM or so. That might be OK with you, might not. Decide how you drive. If your going to be putting money into the SOHC heads carefully consider all the options. SOHC heads can be brought up to respectable numbers, but at what cost? Many DOHC heads are ready to go without additional money past the purchase price. Don't read that as a vote for DOHC, just a vote to do your research.

Either way, you can't have both. Every increase in top end flow is accompanied by a decrease in low end. It's the nature of the beast. That's why almost every manufacturer has developed or licenced some type of variable valve technology. We are actually lucky as we can also use turbo size to fine tune a powerband.

All_talk · Post by **All_talk** » Wed Feb 13, 2008 6:22 am

SOHC small valve (EJ22T), SOHC large valve, and DOHC heads make power in different RPM ranges.

Head choice should consider, drivability, RPM limit, power level, turbo choice, compression ratio, boost limit, and just about everything else.

It’s a system and all parts need to work together. The best place to start is figure out how you will use the car, and be realistic, building a track car to drive to work everyday could be miserable.

To over simplify (and answer the direct question)… bigger valves and more cams tend to produce power at higher RPMs.

For my 2.35 street car build I'm going with more displacement, higher compression (8.5:1 or so) and smallish turbo (VF24) to bring the power on quick and 2.5 DOHC heads for better breathing in the upper RPMs at lower boost levels. Its all about linear power and a flat torque curve.

Build thread here (currently kinda stalled wating on machine work and funding)...

http://bbs.legacycentral.org/viewtopic. ... highlight=

Gary

Arctic Assassian · Post by **Arctic Assassian** » Wed Feb 13, 2008 6:23 am

Dont the 2.5 SOHC heads require a chunk out of them?

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 6:25 am

PhyrraM wrote:The choice of an Ej22 and SOHC heads over the (by that time) tried and true Ej20g for us americans was most likely determinded by our driving style.

We like that 'squirt' off the line. We don't like to wait. We want instant gratification. We also don't see high speeds in the grand scheme. The larger engine, smaller turbo and smaller port heads all helped overcome turbo lag to give us what we wanted.

The lack of an intercooler was an economic choice determined by the chosen boost level. It was simply not needed. the fact the GCs/GFs used a relatively expensive air/water wetup made the choice all that much easier for Subaru.

If you choose a 2.35 build the stock Ej22t SOHC heads *might* be out of breath by 5000 RPM or so. That might be OK with you, might not. Decide how you drive. If your going to be putting money into the SOHC heads carefully consider all the options. SOHC heads can be brought up to respectable numbers, but at what cost? Many DOHC heads are ready to go without additional money past the purchase price. Don't read that as a vote for DOHC, just a vote to do your research.

Either way, you can't have both. Every increase in top end flow is accompanied by a decrease in low end. It's the nature of the beast. That's why almost every manufacturer has developed or licenced some type of variable valve technology. We are actually lucky as we can also use turbo size to fine tune a powerband.

First... Very True... Americans are very instant-gratificationistic (is that a word? If not... It ought to be)

And I suppose I didnt think about the trade off... I knew it... Just didnt think about it...

BTW for a turbo I was thinking about like a VF-12 or somthing in that range.

I must admit... I love the low-end grunt... A lot...

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 6:28 am

All_talk wrote:SOHC small valve (EJ22T), SOHC large valve, and DOHC heads make power in different RPM ranges.

Head choice should consider, drivability, RPM limit, power level, turbo choice, compression ratio, boost limit, and just about everything else. It’s a system and all parts need to work together. The best place to start is figure out how you will use the car, and be realistic, building a track car to drive to work everyday could be miserable.

To over simplify (and answer the direct question)… bigger valves and more cams tend to produce power at higher RPMs.

For my 2.35 street car build I'm going with more displacement, higher compression (8.5:1 or so) and smallish turbo (VF24) to bring the power on quick and 2.5 DOHC heads for better breathing in the upper RPMs at lower boost levels. Its all about linear power and a flat torque curve.

Build thread here (currently kinda stalled wating on machine work and funding)...

http://bbs.legacycentral.org/viewtopic. ... highlight=

Gary

EXACTLY!!!

Thats what my goal is... A semi-flat torque curve. and I was planning on 8.5:1 compression ratio...

555BCTurbo · Post by **555BCTurbo** » Wed Feb 13, 2008 7:54 am

smh0101 wrote: BTW for a turbo I was thinking about like a VF-12 or somthing in that range.

I must admit... I love the low-end grunt... A lot...

A VF12 will be kinda laggy

smh0101 · Post by **smh0101** » Wed Feb 13, 2008 8:23 am

Even with a 2.35? I thought the extra displacement helped spool it a little faster as well?

Would a vf-8 be better? A td04 is still a little on the small side... I think...

What would be a good compromise?

Arctic Assassian · Post by **Arctic Assassian** » Wed Feb 13, 2008 9:05 am

td04hl

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ej25 SOHC Heads vs ej25 DOHC Heads?

ej25 SOHC Heads vs ej25 DOHC Heads?