6.9 and 7.3 Ford Diesel, Indirect injection.
The 6.9 is a V8 design, and is a very good engine. It sports a Stanadyne injection pump, that has proven reliable with veggie oil. The pump has a Normal life cycle of about 80K , but I have seen them go much longer. ( I had one that had 160K on veggie.) They are not as strong as an inline injection pump, but they are good, and relatively cheap to replace. $450 vs $2000 for an inline pump.
You can expect about 15 MPG with this engine, give or take a few.
The 7.3 is almost identical to look at, and is the same design with the same injection system on it. Just a bit more displacement, and a bit more power.
Both are excellent choices for conversion.
The unique thing about the Ford trucks up to 97, was that they almost always had 2 tanks from the factory, so they lend themselves to a Golden Fuel Systems main tank conversion quite handily.
The early 94’ 7.3L, pre- Powerstroke came with a factory turbo and is an exceptional engine. It was the last year of a long production run, and they had all the bugs worked out. The Turbo makes a difference. 16 to 18 MPG is not uncommon.
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