6.9 or 7.3?

6.9 or 7.3


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35WAI

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Hello,
I am in the planning stages of putting together parts to build my ultimate work truck. I have a 6.7 Cummins to tow with so this will mostly be used for hay, firewood, and light towing if I need to get into a spot I do not want to take my ram (my 4 door lb 2500 is pretty clumsy). 84 F-250 with 97 F-350 axles and springs, S547 ZF, and NV 273 t-case (with manual lever kit)
I have gathered all of the mentioned parts so far but where I am having trouble deciding on a direction to go is the engine. I have 2 engines currently to choose from a b model 6.9 updated to 7.3 rockers and a factory turbo 93 7.3. 6.9 runs but is in a wrecked truck with no radiator and a sketchy fuel system. Do to air it takes a bit to start but seems to run good the short bursts I've ran it, mileage unknown. The 7.3 had almost 400k but had been supposedly overhauled once. I heard it run 10 years ago before it was pulled and replaced with a 12 valve. It seemed to run fine but the IP had been maxed out and I believe it was run fairly hard but seemed to run ok last time I saw it run before it was pulled. I have a banks sidewinder and plan to stud either engine I use. I understand that there are some advantages to the turbo engine like crank, rods, valves, and head bolts. But some other factors seem to favor the 6.9 like block and head cooling. Given this information which engine would you put on an engine stand and start freshening up? Any tips, tricks, or advice would be appreciated.
-Thanks
 

Austin86250

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If your running over 15 psi go 7.3 anything under it doesn’t matter, cavitation is a over inflated issue and the rest of the 6.9 “problems” can easily be swapped like the rockers as mentioned
 

IDIBRONCO

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But some other factors seem to favor the 6.9 like block and head cooling.
I agree with Austin. The cavitation issue is overblow these days. While it can happen, by now, most (if not all) of the blocks with issues have either been fixed or scrapped. Cavitation wasn't even that big of a problem when these engines were in common use. Compared to today, it happened a lot more often, but it wasn't like 1/4 of the blocks got cavitation. As for the cooling, you can modify the 7.3 blocks and heads to cool like a 6.9.
there are some advantages to the turbo engine like crank, rods, valves, and head bolts.
Head bolts? Not really unless you're talking about the 1/2" in the 7.3s vs the 7/16" in the 6.9s. The rest don't matter unless you're wanting to make big power numbers and it doesn't sound like you are.
the IP had been maxed out and I believe it was run fairly hard
As far as I know, maxed out IP doesn't hurt anything. People do that fairly often. That adjustment only affects pretty wide open throttle. At part throttle, it's still getting the same amount of fuel. If I'm wrong on that, I'm sure that someone will correct me. The being run pretty hard part is what would tend to steer me away from the 7.3.
 

35WAI

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I have known of 1 1992 7.3 to cavitate in a friend's truck who bought it new. No maintenance was ever done to the cooling system. I am probably just unlucky to see one in the 10 or so of these engines I have owned or been around so I am worrying more about it than I probably should.
Head bolts? Not really unless you're talking about the 1/2" in the 7.3s vs the 7/16" in the 6.9s. The rest don't matter unless you're wanting to make big power numbers and it doesn't sound like you are.
I was talking about 6.9 vs 7.3 in general.

Thanks for the help so far everyone, I am leaning towards the 7.3. Before the weather turns cold I will get it cleaned up and bring it into the garage and check it out. Is there anything in particular I should be looking for to indicate it was run hard? Cracked pistons? I cannot afford to do a full rebuild so if I find something wrong with the 7.3 the 6.9 will be a good backup plan.
 
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lotzagoodstuff

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Do a compression test and report back on what you get out of each cylinder. I would actually do that on both of the engines you have. In my opinion, if you get pretty even numbers out of all cylinders, that's a really good sign that the long block is in decent condition. If you find super low numbers or uneven cylinders, then you pull the heads and investigate.

Good luck whichever way you go, sounds like a neat farm/work truck project.
 

IDIBRONCO

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I was talking about 6.9 vs 7.3 in general.
That makes sense then. If you're just going with a stock Sidewinder kit and not planning to get much more than stock boost, you'll be fine with either engine with head studs.
Cracked pistons?
For one. Excessive cylinder wear is another. Generally, if you can see piston damage of any kind, you know that it was probably run hard. You won't be able to tell that until you pull the heads.
 

Austin86250

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Pistons in my opinion are the weakest link in an idi, I’ve personally torn down 3 motors and seen them cracked or melted, can be easily avoided with a simple egt gauge
Right now I’m rocking my “temporary” motor which is a 6.9 with an ats 085 I run it 8-10 psi but I did 14psi once and turned the pump back down on head bolts no problems at all
Idi bronco is correct it doesn’t harm the pumps to be maxxed out
The best experience I have with beating an idi was my old 6.9 had it maxxed for 20k miles I towed 14k probably 10 times always floored it massive amounts of coal 7 and 8 had stuck rings due to minor melting but the crd was slinging oil like there was no Tommorow
 

ROCK HARVEY

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This is a tough one. I feel like you’re rolling the dice on either one because you don’t know their true condition. Any chance they will let you pull the heads before buying? If not, I vote for the 7.3 because at least where I live they are more plentiful if I need spare parts. If I need a new set of heads I’ll have an easier time finding a 7.3 set than a 6.9 set.
 

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