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Old 07-07-2001, 09:20 AM   #1
Don
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Question Phase I heads on Phase II block?

Hi,

Is it possible to put the DOHC heads from the Phase I EJ25 on the block from the Phase II EJ25? Do the DOHC heads flow better than the SOHC heads? Would one expect any performance gains from this?
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Old 07-07-2001, 10:53 AM   #2
Patrick Olsen
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Quote:
Is it possible to put the DOHC heads from the Phase I EJ25 on the block from the Phase II EJ25?
Yes, it is entirely possible. Some '98 2.5RSs and '99 2.5GTs came that way from the factory as Subaru swapped over production from Phase I block to Phase II.

Quote:
Do the DOHC heads flow better than the SOHC heads? Would one expect any performance gains from this?
Stock vs stock, no, the DOHC heads don't flow any better. Actually, I think they flow worse (just comparing the torque peak and the torque band of the two generations).

If you were building an engine, though, the DOHC heads have more potential. Having separate cams for intake and exhaust just allows you to do things that you can't do with a single cam. I'll be honest, I can't tell you what kind of things you can do, but I remember reading about advantages and disadvantages of the two setups here on the iClub probably a year ago.

Pat Olsen
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Edit: Hello, Cadillac Forums!!!

Last edited by Patrick Olsen; 07-27-2015 at 02:15 PM.
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Old 07-07-2001, 11:35 AM   #3
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if you use a blco with the sohc pistons in it you will raise your compresion ratio by using the dohc heads
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Old 07-07-2001, 06:29 PM   #4
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Question

So why would a DOHC have more potential then a SOHC. Rember both set ups are running four valves. What is it you can do with two cams that you cant do with one??
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Old 07-07-2001, 08:12 PM   #5
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With DOHC you can adjust the cam timing independent of each other (intake timing, exhaust timing) using adjustable cam pulleys.

SOHC in order to adjust exhaust timing independent of the intake timing, you need to re-grind the cam. (extreme example)

Hope this helps.

Last edited by JoeT; 07-07-2001 at 11:50 PM.
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Old 07-07-2001, 11:47 PM   #6
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I understand what your saying, however the potential is the same. To do the samething on the SOHC would just cost more and entail more work.
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Old 07-08-2001, 01:27 AM   #7
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eh.. I was about to say..

go ask Bill Harvey....

but he already appeared..

but yeah, it's possible.
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Old 07-08-2001, 10:11 AM   #8
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Default You might want to double check that

Wolf,

"I understand what your saying, however the potential is the same. To do the samething on the SOHC would just cost more and entail more work."

Here is the thing, say you want to see what advancing the intake by 6 degrees would do:

dohc: adjust the gear
sohc: custom grind a cam, oh yeah, this is presuming you have Subaru blanks and a cam grinder, if not, get your specs to Cobb or something and a couple grand later you can swap out your cam with the one they made.

Then, you figure you would like to back off a little go back to 3 degrees

dohc: adjust the cam
sohc: repeat the incredibly inpractical (and probably unfeasible) procedure above.

Another thing is that dohc you typically have the cam acting more directly on the valve, as opposed to the rocker arms our sohc heads have. They can often rev higher, as in this case where the redline dropped 500 or so rpms when they went sohc.

Car companies don't do stuff without reasons, what are the highest specific output engines in the world, and how many of those are sohc?

cheers,
scotto
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Old 07-08-2001, 10:52 AM   #9
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Cool

Roninscar:
Again the potential is the same, I'm not going to argue the point of feesability and cost. I already conceeded that point in my previous post.

"what are the highest specific output engines in the world, and how many of those are sohc?" Now your mixing apples with oranges because most of those engines are purpose built. First lets look at a top fuel dragster. No OHC's, still using the tried and true over head valve design. Next a typical F1 engine is running DOHC's because of the design of the neumatic valve system. Plus depending on the circuit they are running, a team may opt to use a head configuration with five valves. C.A.R.T. (champ cars) have a ban on certain technologies (i.e variable valve timing) so from a cost point of view it makes more sence to use DOHC's (again this is an argument of feesability and not potential)

Honda's V-TECH (variable valve timing) found in Civics and Accords only uses a SOHC.

Actually to use the SOHC would have other benefits. less rotaing mass (which would more than offset the added weight of the extra rocker arms). Less friction ( no beaging surfaces needed for the second cam. So why did Subaru go from the DOHC design to a SOHC, surely it wasn' a corporate decision to revert back to some retro technology.

White Wolf

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Old 07-09-2001, 06:48 PM   #10
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Question

What would the compression ratio be on a SOHC smallblock engine with DOHC heads? My DOHC 2.5L is being replaced with a SOHC smallblock. DOHC compression was what, 9.7:1? How much will it be raised by?

-Chris

Last edited by cakagan; 07-09-2001 at 06:52 PM.
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Old 07-09-2001, 08:58 PM   #11
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Subaru small block? Where's the Subaru big block?

Interesting notes... first of all we have a twin cam 16 valve engine even though we have a SOHC design. Second of all, redline for the DOHC was 6500, while redline for the SOHC was 6250. Power delivery changed significantly. Third, the DOHC heads flow worse than the SOHC heads. Cobb already tested it, and if you look on his website, I believe he has the charts still up to prove it. Fourth, I believe that the change from DOHC to SOHC allowed Subaru to make the intake manifold longer. There are other reasons-several good posts have been made about it.
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Old 07-10-2001, 10:03 AM   #12
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I have the Factory installed DOHC Heads with the Phase II block. Compared head to head with a full Phase II engine with similar mods my car has faster throttle response and better top end power. Cobb had an article about the difference between the Subaru SOHC and the DOHC heads. If I recall correctly the DOHC heads had a much better intake flow. Otherwise most everything else was the same.

I would go for it. I wouldn't pay a lot for it but I would do it.
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Old 07-10-2001, 10:14 AM   #13
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White Wolf - I beg to differ with you on the 1.6L SOHC VTEC D16Z engines as well as the 2.2/2.3L SOHC VTEC F22/F23 engines. Coming from a very extensive Honda background, the so called "SOHC VTEC" is not really a true "VTEC" system. It's only encorportated into the Intake lobs of the cam and not the exhaust side. This in effect can only adjust for the inflow but not the outflow of gasses from the reciprocating engine. This had a few advantages vs. NON-VTEC, one being higher intake velocity and the other being able to become more aggressive under hard acceleration. The true VTEC system is DOHC which alters both intake and exhaust cams to achieve maximum overlap thus creating a high specific N/A output.

DOHC has more flexibility vs. SOHC period. Yes, DOHC has more reciprocating mass as well as more complex parts, but the reason why Subaru changed over from DOHC - > SOHC for the EJ25 us b/c it decrease the weight of the engine, decrease the # of parts etc... hence, all in all adds up to a savings in the pocket for Subaru.... those "bean counters" decided on this.

If it was the "perfect" world, all cars would be DOHC, high revving with variable valve timing of some sort to provide maximal efficiencies in producing power. (IE : BMW's Double VANOS, Porches' VVT, Mitsu's MivTEC, Nissan's VVT, Toyota's VVT-i, Honda VTEC just to name a few).

Last edited by GTBGUY; 07-10-2001 at 11:24 AM.
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Old 07-10-2001, 11:20 AM   #14
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Default I beg to differ

Wolf,
You have misrepresented several items in your post.

1. The rules book is what keeps top fuel dragsters using pushrods.

2. Please explain where in a pneumatic actuation system the cams fit in? I knew that there was a lot of money in France going into pneumatic actuation of valves, and the advantage of this is it removes the conventional valvetrain all together. The valves are opened via compressed air, electronically, removing the rotating mass and friction of, oh yeah, cams.

3. The five valve head design was pioneered by Yamaha, (I had an 89 FZR1000, brilliant engine) and, oh yeah, it uses DOHC.

4.Honda's lame version of vtec used on non-si civics (don't know about the accord stuff, I thought they were dohc) is sohc, but it is the cost-effective solution and produces nowhere near the power of its dohc brethren.

5. Did you actually try to make a case that moving the ej25 from dohc to sohc was done for performance reasons? Dude, the performance standard bearer for Subaru is dohc, living in the engine bay of a lucky WRX owner near you. The sohc ej25 is a corporate engine for Foresters and legacys, and happened to be the most powerful non-turbo engine they had available to make the 2.5RS model.

Not trying to flame, but you are using fallacious arguments, misstating and misrepresenting facts. Not good form.

Cheers,
Scotto
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Old 07-10-2001, 03:13 PM   #15
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so like why did they do it?

price. a SOHC engine is much cheaper do build. and since they managed to do it with very little performance loss, it makes even more sense.
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Old 07-10-2001, 10:03 PM   #16
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Have a Nice Day?

The only misrepresented things is your interpritation of my post.

1)In your post you said "Car companies don't do stuff without reasons, what are the highest specific output engines in the world, and how many of those are sohc?" You made no refrence to exclude engines that are are bound by rules. Knowing this I stated that it was like comparing apples to oranges and even made mention to the rules that govern C.A.R.T.

2) For more information on neumatic valves and thier actuation please do a search on Formula One Valves, Engine Design etc.. There is plenty of reading explaining the technical workings of neumatic valves being actuated by CAM SHAFTS

3) I could care less who pioneered the five valve head (How did I mesrepresent this??

4) Being a mechanic for over 20 years I have come to appreciate the engineering and design of Honda. Lame is not a word I tend to associate with Honda engineering. Sounds as if you have a grudge against anything that doesn't have the word Subaru on it. You can check out the different engine configurations on Honda's home page.

Again the only case I was ever making is that there is the same potential in SOHC vs DOHC. Please go to the COBB site to see the notes on the superior flow of the SOHC vs DOHC head desigh

Before you decide to offer constructive critisism to someone please be cognizant of the complete thought process.

White Wolf




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Old 07-10-2001, 11:35 PM   #17
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GTBGUY:
According to www.honda.com (specifications) the 1.6 I4 Civic EX, 2.3 I4 and 3.0 V6 are VTEC SOHC. Why would Honda or any other car mfg for that point choose to eliminate a camshaft from a head with out getting a gain in return. Cost savings is one reason to eliminate it, however car mfgs are out to sell cars and if the compitition comes to the arena with a better product people will buy it. So if Honda can make better with less they will and have. If you feel it that necessary to get bent out of shape because Honda took out one of those coveted cam shafts, please feel free to do so. I wouldnt care if the used tooth pics and gum to hold thier cars together.

Ask 12 people about what thier perfect engine would be and you would get 24 answers. You like DOHC's NASCAR folk want V-8s with a carb (DISCLAIMER...I don't agree with the NASCAR folks, thats why I drive a Subaru).

White Wolf

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Old 07-11-2001, 06:16 AM   #18
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Why did they change the head from DOHC to SOHC?
Ans: To cut cost. In Japan, the Legacy Lancaster uses the EJ25 BUT with the DOHC. The Phase II block is definitely stronger but changing to the SOHC head is not to gain performance but rather for cost saving in North America models.
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Old 07-11-2001, 09:48 AM   #19
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Does Subaru have an SOHC in their current lineup in Japan?
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Old 07-11-2001, 11:43 AM   #20
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Default Let me elaborate

Wolf,

1) I thought it self-evident that if dohc were prohibited by the rules, that it wasn't a question of making the best technical choice. Sorry for not clearing that up.

2) I stand corrected, after doing some searching you are correct, pneumatic actuation only replaces the springs, F1 cars are still using camshafts. However, dohc isn't necessary in pneumatic actuation. It is not a design requirement, it is a better way of doing it.

3) You threw out 5 valve like it was something different, it is dohc.

4) So replace lame with pedestrian, they have nowhere near the performance of their dohc counterparts. Since you are a mechanic, with over 20 years experience, you should know how engines work. There is not the same ultimate potential with sohc and dohc.

An engine is an air and fuel pump. Normally aspirated engines can produce more power by flowing more fuel and air. The primary way to flow more fuel and air in N/A is to increase rpms. The direct actuation of a dohc design allows you to achieve higher rpm levels than with a more indirect sohc design.

Hence, there is more potential with dohc on a N/A engine.

Again, and this time let's limit it so you won't try to confuse the issue with apples or oranges. What are the highest specific output engines in production cars?

N/A
S2000 240 hp 2 liters
ITR 195 hp 1.8 liters
M3 330 hp 3.2 liters

One thing all these cars have in common is dohc.

Again, not trying to flame you, so I apologize if you are taking any of this personally, but the choice of companies making performance motors is dohc.

Regards,
Scott
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Old 07-11-2001, 11:59 AM   #21
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White Wolf - Yes I'm clearly aware those "quasi-VTEC" engines are in those cars. I myself have a '97 Honda Accord EX-R myself which came with the F22 engine (2.2L SOHC VTEC). I can tell you from my experience that by no means L per L for Honda engines that a SOHC is equal to or better than that of the DOHC. Case and point are the H22A vs. F22A engines, both have same displacement, both are Honda engines. Differences : (1) H22A = DOHC VTEC and F22A = SOHC "VTEC". I've personally put my F22A to the limits of SOHC tuning short of going forced induction, given the same amount of money, going with a swap to the H22A yielded WAY more gains vs. the SOHC.

Believe me, I'm a hardcore car enthusiast, and given the time and resources, DOHC is the only way I'd go with any vehicle that is intended for performance use.

Tell me, which Honda "sports" cars use SOHC? Not one of them (unless you consider the pathetic Civic EX coupe or Accord EX coupe "sports" cars). ALL high-output Honda engines are DOHC. Why you ask? B/C of unlimited tuning options as well as finite tuning vs. a SOHC engine.

Honda only went with SOHC here in North America (for those cars) b/c it was more of a cost savings measure as well as for emissions sake. When you have 1 cam, overlap stays the same no matter how you advanced the cam or retard the cam. With DOHC they are ofcourse independant of each other, and due to certain regulations, if the cam timing was adjusted, emissions would be over that of the allowable limit.

No matter for which manufacturer, given the same displacement. A DOHC WILL out peform the SOHC by a far margin with the same modifications.
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Old 07-11-2001, 01:49 PM   #22
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Moderators: I know that this post may seem a bit heated, but please don't lock this... I find the views being expressed here (albeit opposing) are very interesting.

Thanks.

A question for those involved... are there any production SOHC turbo charged cars around?
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Old 07-11-2001, 02:25 PM   #23
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2yLiTe - Older Porches ran SOHC flat-6's. Turbo'd Porsches usually just upped their boost... The 911's moved from a SOHC to the now DOHC design on all of the cars.

a Hi-Pro car I can think of that was SOHC was the Ferrari Tesstarossa, which utilized a SOHC flat-12 engine. The Koenig Compeition bolted on a S/C and two turbo set-up to this beast. It was fast and it's an Ultra-exotic car.
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Old 07-11-2001, 08:35 PM   #24
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GTBGuy:
Please clarify your statment on adjusting the cam timing. Are you talking about a manual adjustment or how the vtec system works??

White Wolf
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Old 07-11-2001, 09:35 PM   #25
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Default I think what he means is

Wolf,
VTEC is a system where you switch lobes at a given point in the rpm range, and these new lobes typically will have higher lift and duration. If they are changing duration, then they are also going to change when the valve opens and closes relative to it's exhaust counterpart. I'm not aware of any Honda sohc engines that change the lobe for both intake and exhaust valves, I believe they are intake changes only. But here is the thing, if you were to make a really complicated head, with vtec for both intake and exhaust, no matter what you changed they would never be able to change relative to each other when "on the lobe".
There are basically 2 kinds of variable valve technology, the first is vtec, the other is varying the timing without changing the lobe of the cam. This is what is used in BMWs Porches, I think they call it Variocam and Vanos, anyway, it advances and retards the valve timing by rotating the cam a few degrees forward and back, while the cam is rotating.

If you have dohc, and a system like this you can change when during the exhaust and intake strokes all the valves are opening and closing in case you want to increase overlap between the two, which can help increase high rpm power in N/A motors.

You will never be able to increase this overlap on the fly in a sohc engine because the lobes are on the same shaft.

The new vtec, and I think the vvti in Toyotas have both technologies built in. So not only do you get a bigger cam lobe at switch over, but they can change valve timing to give you a nice smooth idle and a hot built overlap at the high end, with everything in between.

This coupled with it's greater robustness and strength in high rpm applications, remember for those rocker arms that the forces are increasing geometrically as revs rise, and dohc continues to provide more potential.

Hope this helps,
Scotto
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