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Guys, what are the major differences between P400 and GEP engines?


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On another form this has just come up and I am wondering what the differences are in the heads and the long blocks and the short blocks between GEP blocks and P400 blocks. Are there alloy differences? Are there water jacket differences? Are there differences between the pushrods? I believe I know some of the answers but I would rather speak more intelligently before I weigh in.

( FYI, I have a new to me military Humvee ECV with a GEP and I also have a P400 awaiting install for my civilian Hummer H1.)
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So Chris from TSP probably knows more than anyone. The differences I know of are bottom girdle, forged crank, different rods and pistons, metallurgy I believe is similar to GEP. Heads are some different as well, Chris sacrificed one and cut it up to see the differences. Pics are on here somewhere
@ak diesel driver pretty much covered it. Bottom end girdle & rotating assembly were the best configuration you could get for a 6.5

The connecting rods are fracture broke unlike the GEP rods

Heads are very different too, something about the P400 heads being hardened between the valves, and the valves are slightly different/ not interchangeable if memory serves correctly

I'll tag @Twisted Steel Performance to see if he can add anything with regards to metallurgy/ alloy makeup of the two engines. Chris has also developed his own upgraded configuration of valve springs, slow bleed lifters and pushrods, so if you're wanting to improve an already stout platform he's the guy to work with


There are a few other things as well, rod & main bearings have different part #, me thinks they are a different makeup... pistons, rods, crank,oil pump is different, the bypass spring, because the P400 has much different oil passages, and other significant things I don't make public...