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This past weekend I broke down and had the engine replaced in my 98 OBW and the proccess was quite interesting. Turned out it had a cracked head and burnt valves.

 

My certified Subie mechanic took a second gen block, slapped on first gen DOC heads and canabalized parts from a few other 2.5's in the garage. Added electricity and SHE LIVES!!!! And runs better now than she did the first time I brought her back to life.

 

He had mentioned a few weeks before that he had found out that the head of the 1Gen fit the 2Gen block and said it gave more power.

 

Anyone here done this and have more details on performance of such an engine?

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in a way, you didnt really build a frankenmotor.  That involves 2.5L bottom end with 2.2L heads.  

 

What you had done was basically upgraded your block, the Gen 2 N/A block supposedly had better designed rod bearings than the 96-99 DOHC (prone to #3 rod knock, and I say this because I've split the block on over a dozen of them just this year alone, and they were all #3 rod bearing failure) 

 

I honestly don't see you actually "gaining" power, unless you ran thinner head gaskets and raised the compression.  2.5 N/A block is a 2.5 N/A block the only real power making difference would be the shape of the piston tops.  Where the Gen 2 2.5L makes more power would be the 05-07 with Active Valve timing, and even then I I dont think its makin more than a few extra ponies

Edited by torxxx

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in a way, you didnt really build a frankenmotor.  That involves 2.5L bottom end with 2.2L heads.  

 

What you had done was basically upgraded your block, the Gen 2 N/A block supposedly had better designed rod bearings than the 96-99 DOHC (prone to #3 rod knock, and I say this because I've split the block on over a dozen of them just this year alone, and they were all #3 rod bearing failure) 

 

I honestly don't see you actually "gaining" power, unless you ran thinner head gaskets and raised the compression.  2.5 N/A block is a 2.5 N/A block the only real power making difference would be the shape of the piston tops.  Where the Gen 2 2.5L makes more power would be the 05-07 with Active Valve timing, and even then I I dont think its makin more than a few extra ponies

 

 A few is probably right, but the 251 flat tops coupled with the DOHC 25D heads would be a great platform to start throwing power adders at as well. Should be close to 10.2:1 over the stock 9.5:1 from the EJ25D

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Hope he used a thick enough headgasket.  Phase II 2.5 pistons come proud out of the deck just a tad.  Hopefully he used the phase II headgasket.

 

If not, you may have a problem.

 

The reason I know, is that we did this combo, and had to pull the engine for a broken piston top edge after about 5k miles (about 8 months)

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good to know about the EJ251 and EJ253 blocks, I never did bother checking piston height, I just know the ones I built for customers were 00-06 EJ25 small block with Gen 1 long valve 2.5 DOHC cam heads and I always used the thicker gasket just as a safety measure.  So why is it that the EJ251 and 3 headsets come with 2.2L head gaskets in set?  Or are you talking about running EJ22T headgaskets (from what I remember they are the thinnest out of all the 2.2/2.5 gaskets?)

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Hope he used a thick enough headgasket.  Phase II 2.5 pistons come proud out of the deck just a tad.  Hopefully he used the phase II headgasket.

 

If not, you may have a problem.

 

The reason I know, is that we did this combo, and had to pull the engine for a broken piston top edge after about 5k miles (about 8 months)

 

This is incorrect, the Phase 1 pistons protrude from the block about .5mm, while the phase 2 pistons do NOT.

 

Isaac

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not sure about that though, Phase 1 DOHC N/A Pistons are dished.  The 99 2.2L used a domed piston IIRC.  2000-06 2.5 were flat top pistons 

 

 

But that WOULD explain why EJ25D headgaskets are so frigging thick and the 2.2 and EJ251 and EJ253 used the thin gaskets

 

Im intrigued now, heading to work to investigate

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EJ25D pistons have the big square tub in the centers, 99 ej22(single year/roller rocker) pistons are essentially flat, I have a set in the shop.

 

I have built two seperate frankenmotors in the past few years, and several non-franken motors for different projects, I can tell you with the utmost certainty that the phase 1 pistons protrude from the deck. None of the phase 2 blocks have pistons that protrude, hence the much thinner headgaskets.

 

Where the confusion stems from I believe is that several aftermarket piston manufacturers sell pistons that are for phase 1/2 blocks, and as such, do not protrude from a phase 1 deck. I have built motor for people using these and I do not like them (DNJ mostly).

 

The other confusing point is that good ol' SoA used phase 2 blocks to replace bad phase 1 warranty replacement blocks quite often. I have pulled several of these motors apart in the last few years.

 

 

Isaac

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