Optimizer 6.5 upgrades, including the aftermarket turbo. A 700R4 transmission from TCI, set up for towing/ diesel applications. Secondary transmission cooler, the big one. OR try to find a General Products engine. Here is whats from their website .
Created by General Engine Products, a subsidiary of AM General, the Optimizer 6500 is a high-performance, American-made engine that greatly improved on the power and reliability of the General Motors engine originally offered in production Hummer® H1s. The Optimizer incorporates many improvements in design, technology, and metallurgy to redefine the abilities of the original engine.
GEPP takes the power and reliability of this fine engine and steps it up to the next level, adding a custom turbo charger and other performance enhancements to create the finest engine available for critical military, racing, and extreme off-road uses. GEPP's engines have been used by virtually every H1 off-road race team and by many military groups worldwide.
I have A 6.5 General Products 6.5 in my 1987 Federal Suburban. I replaced the dead 6.2 with the 6.5 , then put in a heavy duty 700R4 from TCI. Now I got luckly , I got my engine by accident. I bought a M1009 that a fire company had and in the back sitting on a pallet was the 6.5. Apparently they thought the engine had a knock and was bad and got the engine from federal surplus to fix it but never did. Turned out their 6.2 only had a cracked fly wheel. So I had a new clean 6.5 sitting here. I like it, its got some pep over the older GM version of the 6.5