A note on swapping SOHC EJ25 heads onto the Ej22T...

[Matt Monson]

I know that on more than one occassion I have said that this would still be a good combination in direct contradiction to Larry Witherspoon's tech article on the coolant passages.

While I do still think this is a totally viable and good set up, I want to add a piece of information that I have known about for sometime, but only very recently recalled it. If you use the SOHC Ej25 heads on a factory turbo car, with the factory plumbing the up-pipe hits the back end of the heads. This is something to take into consideration if considering this set up. You will want to get a custom up-pipe made to make this work. There are a couple of guys on NASIOC who have ground a portion off of the head to get clearance, but that seems questionable to me...

[-K-]

A new up pipe would seem to be a better idea.....

[Matt Monson]

my thoughts exactly. Plus if you are at that point of a build, you are likely going to be buying all new exhaust components, and maybe even getting custom pipes for a garrett turbo, or something along those lines. I just wanted the info out there because those heads are being brought up a lot on NASIOC, and might be mentioned here in the near future...

[206er]

how do these heads flow as opposed to DOHC ej25 heads or ej20g heads? Ive heard that the ej25's are pretty similar in flow but one has to be better right? the SOHC must be pretty special if people are going to the trouble to grind them down for clearance.

[-K-]

SOHC would be my first choice of a 2.5 head.

[IronMonkeyL255]

Supposedly, the SOHC heads are the best flowing subaru heads yet.

There is a graph floating around that shows that the stock SOHC EJ25 heads can outflow even Chevy LS1 heads.

[legacy92ej22t]

Damn, really? I knew they flowed well but I didn't know they were the best Subaru had to offer.

[Matt Monson]

Damn, really? I knew they flowed well but I didn't know they were the best Subaru had to offer.

They aren't! The graph referenced is from Cobb's website with their benchflow on them. They also benchflowed the DOHC EJ25 heads and they flow about 5 cfm better. And keep in mind that the US DOHC EJ25's use the same basic core casting as the EJ20K JDM heads.

And even though I haven't bench flowed Ej20G heads, since they measure larger ports (and I am talking standard ones, not my RS-RA ones) I would expect the Ej20G heads to flow a little bit better.

Furthermore, if you do a search on NASIOC, you will find that modern factory v6 & v7 STI RA and Spec C heads outflow all the others by something like 30-50cfm. IIRC they are around 280cfm to the SOHC EJ25's 236cfm!!!

Now to answer the question of why people are using these heads? It is twofold. The easy answer is that many of the guys dropping an EJ22T bottom end in their car have MY99-MY01 2.5RS's. So they already own the heads.

But the other part of it is the design of the heads themselves. The roller rocker design of the SOHC heads allows for very quick transitions from open to close of the valves. Further more, there is reduced interia in the heads because of the use of a single cam. Even though they have the rockers, whille the DOHC are direct actuation, they are still offer a lighter overall valvetrain.

This advantage is so great that recently the Rigolis, in Australia, have switched to the SOHC heads over the STi v8 heads they have been using for the last year or two. For those not familiar with the Rigolis, they are generally credited with the creation of the high powered EJ22t w/EJ20 heads formula and have 8 and 9 second drag cars to back that up. And they were doing that years ago. Tony Rigoli personally helped set up Ali's ESX car before it went tearing around our country setting records stateside.

[IronMonkeyL255]

And also they are more easily available.

Thank for setting me straight.

I wish I could have visited Rigoli's shop while down in Australia.....

[BAC5.2]

Matt - What do you think about EJ205 heads with shimless buckets as an overall head choice? I'm still pretty hung up on the fact that Phil, when he worked at TurboXS, made 498whp with EJ205 heads on a 2.5 block and JUN prototype cams.

The infamous east-coast-legend Dug-E-Fresh is running V7 Spec-C heads. Made 4xx on pump gas, and is in the 10's now IIRC.

[scuzzy]

what about slapping the EJ22T cams in the SOHC EJ25 heads? better flow? more lift? can it even be done?

[Project_Legacy]

sounds like a good idea. since wouldnt the sohc 2.5 heads have cams that would overlap?

someone was also talking about MY97 Legacy GT 2.5 heads. is that the sohc ones??

[scottzg]

i dont have the specs (hell, i dont know crap about ej tuning) but generally stock na cams have overlap and a taller profile than turbo cams. I doubt swapping to stock turbo cams will net any on-boost gains.

[Matt Monson]

what about slapping the EJ22T cams in the SOHC EJ25 heads? better flow? more lift? can it even be done?

can't be done. Totally different head designs.

[Matt Monson]

sounds like a good idea. since wouldnt the sohc 2.5 heads have cams that would overlap?

someone was also talking about MY97 Legacy GT 2.5 heads. is that the sohc ones??

Those are the DOHC EJ25 heads. Same cores as '97 and '98 WRX heads...

And overlap is relative. Cars with really big turbos run more overlap than stock NA cams.

[greg donovan]

ok im a bit lost. it appears some of you are talking about the SOHC 2.5 heads on the USDM RS coupe and some are talking about the SOHC 2.5 heads on the USDM STi and the forester XT.

who is talking about what.

[THAWA]

I don't think anyone is talking about SOHC 2.5L Heads from the STi or XT.

[Impregacy]

with the 2.5RS heads what compression ratio do people end up with? I heard it was like 7.1:1 and is the engine still non-interference? does the turbo coolent return adapter bolt on to the back of the right side one?

[Matt Monson]

with the 2.5RS heads what compression ratio do people end up with? I heard it was like 7.1:1 and is the engine still non-interference? does the turbo coolent return adapter bolt on to the back of the right side one?

The compression ratio is more like 7.3:1 Check the stick compilation thread to find the exact numbers with the math. But really, who cares the exact number. Anything in the low 7's is great for a ton of boost

It does make it an interference motor. But again, who cares? Don't mess up your timing and you don't mess up your motor.

And you need to find a new place to put both your coolant and oil returns from the turbo. This sort of head swap is not for the faint of heart. It is only for folks playing in the big numbers arena for power, and big numbers of $$$ to set it up correctly. It isn't just a swap in some heads to make more power sort of solution...

[-K-]

If you want cheap I'd just throw 20psi at the stock heads, they will flow...

[greg donovan]

I don't think anyone is talking about SOHC 2.5L Heads from the STi or XT.

d'oh.

[greg donovan]

I know that on more than one occassion I have said that this would still be a good combination in direct contradiction to Larry Witherspoon's tech article on the coolant passages.

While I do still think this is a totally viable and good set up, I want to add a piece of information that I have known about for sometime, but only very recently recalled it. If you use the SOHC Ej25 heads on a factory turbo car, with the factory plumbing the up-pipe hits the back end of the heads. This is something to take into consideration if considering this set up. You will want to get a custom up-pipe made to make this work. There are a couple of guys on NASIOC who have ground a portion off of the head to get clearance, but that seems questionable to me...

i mistakenly read this as from a factory turbo car.

[Matt Monson]

I know that on more than one occassion I have said that this would still be a good combination in direct contradiction to Larry Witherspoon's tech article on the coolant passages.

While I do still think this is a totally viable and good set up, I want to add a piece of information that I have known about for sometime, but only very recently recalled it. If you use the SOHC Ej25 heads on a factory turbo car, with the factory plumbing the up-pipe hits the back end of the heads. This is something to take into consideration if considering this set up. You will want to get a custom up-pipe made to make this work. There are a couple of guys on NASIOC who have ground a portion off of the head to get clearance, but that seems questionable to me...

i mistakenly read this as from a factory turbo car.

No worries. I probably should have been more clear. Since this is a BC/BF board, I meant only the factory turbo Legacies. Though is would still apply if one were to put SOHC Ej25 heads onto a WRX swap...

[sammydafish]

ok... time to drag up an old thread.....

I've got an EJ25 SOHC engine out of an 03 (I think) outback. I'm wondering if it would be worth while to steel the heads off it (engine is junk) What else would I need to swap. It has the new intake mani with the top feed injectors. I wonder if it would be worth while to swap that too?

What's the deal with the coolant ports, I assume they don't match up perfect? Is there a good place to get more info on this?

The exhaust ports look the same, so I assume that'll just bolt up. This is going on an EJ22e also (N/A motor).

I have a spare set of 1990 EJ22e heads that I was going to port out and splap on, but maybe this will be a better choice?

[sammydafish]

so... since there's like 10 threads on head swaps right now.... anyone know the answer to my questions?


BTW, the EJ25 is out of a 2000 outback.