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Several people have talked about the possibility to use the 3.4 DOHC heads on the 3x00 engines. Some for performance, and some for replacment, where getting a 3.4 DOHC block might be difficult, but getting a 3400 (or even a 3100) block might be easier to come by.
For these test fittings, I used a 1994 3.4 DOHC BLOCK, a 1994 3.4 DOHC REAR head, a 1999 3100 SFI BLOCK, and a 1996 3100 SFI head.
This article is broken into 3 sections.
This is the BASIC COMPARISONS between the 1994 3.4 DOHC, and the 1999 3100 SFI.</p>
</p> This is a bare 3.4 DOHC block</p> 
</p> And this is a bare 3100 SFI block</p> One of the biggest NOTICEABLE differences is the OIL VALLEY. The 3.4 DOHC does not have lifter holes, and it has a cover on it. When the engine is ASSEMBELED, the valley is actually OPEN to the air, and not sealed off like the 3100. The 3.4 DOHC has that cover on it to keep anything from getting in there.</p> 
</p> Here is the cover on a 3.4 DOHC</p> 
</p> This is me holding the cover in place on a 3100 block
For these test fittings, I used a 1994 3.4 DOHC BLOCK, a 1994 3.4 DOHC REAR head, a 1999 3100 SFI BLOCK, and a 1996 3100 SFI head.
This article is broken into 3 sections.
- The first section is BASIC COMPARISONS where I compare the BLOCK of the 1994 3.4 DOHC, and the 1999 3100 SFI.</p>
- The second section is a 1994 3.4 DOHC head on a 1999 3100 SFI block.The third section is a 1996 3100 SFI head on a 1994 3.4 DOHC block.
This is the BASIC COMPARISONS between the 1994 3.4 DOHC, and the 1999 3100 SFI.</p>
</p> This is a bare 3.4 DOHC block</p> 
</p> And this is a bare 3100 SFI block</p> One of the biggest NOTICEABLE differences is the OIL VALLEY. The 3.4 DOHC does not have lifter holes, and it has a cover on it. When the engine is ASSEMBELED, the valley is actually OPEN to the air, and not sealed off like the 3100. The 3.4 DOHC has that cover on it to keep anything from getting in there.</p> 
</p> Here is the cover on a 3.4 DOHC</p> 
</p> This is me holding the cover in place on a 3100 block
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