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Old 02-12-2011, 03:09 PM   #1
GuamSTI
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Default Modifying the 2.5 DOHC NA Motor

Has anybody done any modification to the 98' LGT 2.5 DOHC NA motor? I am not asking about forced induction.
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Old 02-12-2011, 05:10 PM   #2
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Nope, nobody. The 21,000+ threads in this forum are all about people wishing they could modify their N/A engines.
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Old 02-12-2011, 08:33 PM   #3
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Yep, never happened. Not one person on this planet has ever touched their NA motor nor have they posted threads about their builds, mods lists, made a "white paper" thread explaining in detail the mods you can do, or anything. Nope.



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Old 02-12-2011, 09:08 PM   #4
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Old 02-12-2011, 09:39 PM   #5
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For an individual that's been a member of this forum heading 2 years, it's a silly question to ask.

Now he is a STI owner.
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Old 02-13-2011, 12:28 AM   #6
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Originally Posted by Back Road Runner View Post
For an individual that's been a member of this forum heading 2 years, it's a silly question to ask.

Now he is a STI owner.

I just purchased this 98' Legacy and I need to replace the head gasket and since I am at it there might be things that I can do to improve the NA motor. I do not need to make it into an STI, I already have one, it does not need to be an EVo beater because I also have an Evo. It does not need to have a 6 cylinder boxer motor because I already have a Carrera 2. It does not need to be a classic because I have a Classic Mini Cooper. What I want is a nice trouble free, user friendly Legacy.

P.S. No harm in asking right? this is why we have the forum it is a source of information. If nobody has done it here then I will be the first. It's time to do more research in another country.
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Old 02-13-2011, 03:01 AM   #7
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For the DOHC, I wouldn't do much. Regular OE rebuild with the head gaskets, and add a good header and exhaust. Don't do intakes, they'll eff with the MAF sensor. If you really want to mess with the motor, talk to Delta Cams about some regrinds and valvetrain work, but it's around $1000 for that stuff and only minimal gains.
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Old 02-13-2011, 09:19 AM   #8
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Quote:
Originally Posted by GuamSTI View Post
P.S. No harm in asking right? this is why we have the forum it is a source of information. If nobody has done it here then I will be the first.
I think the extremely thick sarcasm in this thread is flying right over your head. A simple search in this forum for "DOHC build" will yield plenty of results, including my own.

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Originally Posted by BAN SUVS View Post
For the DOHC, I wouldn't do much. Regular OE rebuild with the head gaskets, and add a good header and exhaust. If you really want to mess with the motor, talk to Delta Cams about some regrinds and valvetrain work, but it's around $1000 for that stuff and only minimal gains.
While I agree that it's more expensive to do cams on the DOHC engines (simply due to there being twice as many cams), I would hardly say the gains are minimal. When combined with a header and CAI I think the gains were pretty impressive. The difference in top end between the way my car sits now (stock heads and cams with CAI and full exhaust) and the way it was with the Cobb "street" cams and mild head work is huge.

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Don't do intakes, they'll eff with the MAF sensor.
I'm assuming that's a reference to williaty's testing with intakes on the newer cars? If so, I don't think it applies here. I've been running an Injen CAI for about 140-150,000mi and I've never had an issue. I'm hardly the only one running an Injen CAI on the older MAF cars.

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Old 02-13-2011, 10:23 AM   #9
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Nope, nobody. The 21,000+ threads in this forum are all about people wishing they could modify their N/A engines.

After saying this you now are telling me to check a thread that you have a build in there. Thanks anyway

I got the point on cams. Any ideas on machining? has anybody shaved the cylinder head?
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Old 02-13-2011, 11:00 AM   #10
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Wow. Again, I think the extremely thick sarcasm in this thread is flying right over your head.
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Old 02-13-2011, 12:06 PM   #11
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Not to mention the totally modest and humble unsollicited listing of your garage's content
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Old 02-13-2011, 04:04 PM   #12
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Originally Posted by GuamSTI View Post
I got the point on cams. Any ideas on machining? has anybody shaved the cylinder head?
Probably, but I thought you said you just wanted a reliable runaround? At MOST, I would do 1000 grind cams from Delta, a TWE header, and a nice exhaust behind it. Just do the rebuild with some basic stuff. If you go much farther than that you need EM, which is hard to do on an older OBDII Subaru.
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Old 02-13-2011, 04:07 PM   #13
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Quote:
Originally Posted by Patrick Olsen View Post
I'm assuming that's a reference to williaty's testing with intakes on the newer cars? If so, I don't think it applies here. I've been running an Injen CAI for about 140-150,000mi and I've never had an issue. I'm hardly the only one running an Injen CAI on the older MAF cars.

Pat Olsen
If you've replaced the MAF housing (which I believe the Injens do) then you are most certainly effing with the MAF. The question is, do the benefits outweigh the risks? I really have no idea, but I'm firmly in the don't-change-your-intake-if-you-can't-rescale-your-MAF crowd. OP says he just wants a very reliable DD and has many other toys, so I wouldn't recommend it.
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Old 02-13-2011, 05:27 PM   #14
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Thanks for keeping your comments about my original inquiry. I will take your advice look into the cams and keep it simple no CAI. Do you have a link to the cam naufacturer
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Old 02-13-2011, 06:19 PM   #15
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Originally Posted by BAN SUVS View Post
If you've replaced the MAF housing (which I believe the Injens do) then you are most certainly effing with the MAF.
The MAF on the older (pre-02, I think) cars does not have a removable element. So the Injen CAI for the late 90s cars uses the stock MAF housing.

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Any ideas on machining? has anybody shaved the cylinder head?
Not sure what your desired result is with machining the heads. It can be done, but (1) the pistons of the EJ25D protrude from the deck at TDC so there's not much you can do, (2) shaving the heads will result in cam timing changes, with the effect being different for each head, (3) your stock tensioner may not be able to put enough tension on the belt, and (4) it would be a lot easier to just use a thinner head gasket (since the EJ25D HG is the thickest factory gasket).
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Old 02-13-2011, 08:58 PM   #16
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Quote:
Originally Posted by Patrick Olsen View Post
The MAF on the older (pre-02, I think) cars does not have a removable element. So the Injen CAI for the late 90s cars uses the stock MAF housing.
Okay, I take it all back then. My refusal to have much to do with DOHC 2.5s makes me not want to dig deeper.
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Old 02-13-2011, 10:18 PM   #17
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Quote:
Originally Posted by GuamSTI View Post
I just purchased this 98' Legacy and I need to replace the head gasket and since I am at it there might be things that I can do to improve the NA motor. I do not need to make it into an STI, I already have one, it does not need to be an EVo beater because I also have an Evo. It does not need to have a 6 cylinder boxer motor because I already have a Carrera 2. It does not need to be a classic because I have a Classic Mini Cooper. What I want is a nice trouble free, user friendly Legacy.

P.S. No harm in asking right? this is why we have the forum it is a source of information. If nobody has done it here then I will be the first. It's time to do more research in another country.
Quite a collection of cars, although it makes me wonder why you'd buy an old Legacy over a cushy full-sized cruising sedan.

Unfortunately, the motor head gasket issue is often repeating. You may find yourself doing another in 50,000 miles. You might have the block and heads checked to make sure they're true and if not shaved flat. From an old service bulletin, one part of the head gasket issue as Subaru describes it is due to manufacturing variation (manufacturing tolerances). Making sure everything's good on the motor is simply a check. There are also better head gaskets than the oem ones that came on the car. Subaru has improved their own gasket, but Cometic also makes a gasket, and at least for the SOHC folks, the oem STI gasket was also a better choice.

The motors actually last quite long when driven for normal daily use. It's just the head gasket thing is a pesky annoyance that was all too common.

As for building up the motor, I'm not sure why you'd do it. I'd understand why you'd do it if this was your only car and you planned to say do some auto-x and rally-x events with it. Then you'd want to maximize what you had. Even then you might lean more towards a racing motor build, not something so mild. A mild build can get a little more pep out of the car but to what benefit and is the cost worth it? Like you said, you have other cars for sport, so what's the point. Generally upgrades are done as a need to gain something more from the car to fit intended goals. If there are no goals for the car, the need isn't there.
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Old 02-14-2011, 09:40 AM   #18
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Originally Posted by Back Road Runner View Post
Quite a collection of cars, although it makes me wonder why you'd buy an old Legacy over a cushy full-sized cruising sedan.

Unfortunately, the motor head gasket issue is often repeating. You may find yourself doing another in 50,000 miles. You might have the block and heads checked to make sure they're true and if not shaved flat. From an old service bulletin, one part of the head gasket issue as Subaru describes it is due to manufacturing variation (manufacturing tolerances). Making sure everything's good on the motor is simply a check. There are also better head gaskets than the oem ones that came on the car. Subaru has improved their own gasket, but Cometic also makes a gasket, and at least for the SOHC folks, the oem STI gasket was also a better choice.

The motors actually last quite long when driven for normal daily use. It's just the head gasket thing is a pesky annoyance that was all too common.

As for building up the motor, I'm not sure why you'd do it. I'd understand why you'd do it if this was your only car and you planned to say do some auto-x and rally-x events with it. Then you'd want to maximize what you had. Even then you might lean more towards a racing motor build, not something so mild. A mild build can get a little more pep out of the car but to what benefit and is the cost worth it? Like you said, you have other cars for sport, so what's the point. Generally upgrades are done as a need to gain something more from the car to fit intended goals. If there are no goals for the car, the need isn't there.

Thank you for the input I guess what I wanted to hear was is there a way to get the head gasket issue a bit more mileage. When I bought this car it has 139K miles on it. Maybe what I was trying also to find out is what causes the head gasket to give? You mentioned machining tolerances, are there any other day to day driving activities that could cause this to happen?

As for the cushy full size cruising sedan, Guam is a small island and drives are no longer than 1 hr at the most. I might if I find an old 59' Cadillac or a 63' Lincoln.

This Legacy is one of it's kind on Guam.

Anyway thanks for the input. I think based on everybody's comment I get the idea that trying to squeeze out a small amount of extra power and reliability from the NA motor is a waste of time and resources. So I will just refreshen the motor and make it aesthetically appealing.
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Old 02-14-2011, 04:26 PM   #19
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Well, for the older cars it was also the quality of the gasket. Subaru has since improved the design from whatever they used back then. There are also aftermarket options.

The only issue with looking at power gains is trying to actually start from the best base platform. While the DOHC is geared for high rpm use, the SOHC heads are superior, and this makes the 2.5L SOHC engine a better starting point. However for power, it would transition to forced induction and starting with the STI block, and yada yada we end up with a built STI motor which ends up kind of pointless relative to the cars you already have. An alternative route would be to convert the Legacy to a hauler. There were some old manual gear boxes with a low range unit, and you could upgrade all the diffs to lockable types. You could jack up the car a little and make it decently off-road capable. An H6 could be tossed in for torque gains, and the old EG33 motor can be found quite cheap and makes good torque. If you wanted to evolve the car into a unique tool in relation to what you already have, that's the kind of thing you could step to if you really had the modding bug.

For basic reliability though, you're back to tossing in a new, better designed head gasket and simply making sure the head and block surfaces are reasonable flat.
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Old 02-14-2011, 05:38 PM   #20
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Quote:
Originally Posted by Back Road Runner View Post
Well, for the older cars it was also the quality of the gasket. Subaru has since improved the design from whatever they used back then. There are also aftermarket options.

For basic reliability though, you're back to tossing in a new, better designed head gasket and simply making sure the head and block surfaces are reasonable flat.
I got the idea is there a particular head gasket that works best? Thanks again
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