6.9 vs 7.3 which is better all around

[Brett1989]

Which is a better all around engine the 6.9 or 7.3? Which gets better mpg? which pulls the best when stock? Which reacts best to a turbo?
Brett

[David85]

All round motor is a little subjective because it depends on what you want to do with it. My vote is for a 1985.5-1987 6.9L.

Advantages of the 7.3L:

Stronger head bolts
Better glow plug system
Better water separator
Better valves
Better rockers
Better oil control (can be a disadvantage-I'll explain later)
Available with overdrive transmissions (ZF5 speed manual or E4OD automatic) for better fuel economy in stock form
More torque
More power




Disadvantages:

Thin cylinder wall; this makes the block more expencive to overhaul because reboring is not safe and cavatation is a problem
Head bolts are good but if you want aftermarket the cost is twice as much as for the 6.9L
The increaced oil control on the valve guides that eliminate oil consumption can lead to premature wear of the guides



6.9L Advantages:

Can be safely rebored, several times if desired. Stock cylinder wall is over 1/4" thick
Upgrade head studs are cheaper
Valve guides are designed to leak small amounts of oil. Sounds bad, but actually prevents a lot of wear and tear in the top end
Stock transmissions were usually a 4 speed stick, or 3 speed auto, both of which are bullet proof, rare 1987s could have the 5 speed


6.9L disadvantages:

Weak stock head bolts, 10 psi of boost is the highest for reliability, the 7.3 can take 15 reliably
Uses oil by design
Not usually available with an overdrive transmission, so MPGs will not be as good on average
glow plug systems from 1983-1986 are not as good, 1987 has the same glow plug system as the 7.3
Water separator is not as good and prone to leaking, 1987 also has the better separator
1983-1985.5 is prone to cracking the block at the block heater (avoid)
1983 heads are prone to craking (avoid)
Less power
Less torque


Summary:

The cavatation problem is enough reason form me to favor the 6.9L since there is no way to detect it. Both engines have the same potential for turbocharging, but the 7.3 can go higher without having to get head studs. Changes to the prechamber were made to the 7.3 for emissions, its not known if this is any better for performance, but the fuel spray is not directed as much at the glow plug on the 7.3. Prahaps this could mean harder cold starting but better fuel burn? Not sure but my 6.9 has always fired up easily even with a few dead glow plugs.

Performance is similar between the two engines, even if the 7.3 has the slight advantage.

I'm sure I missed a lot, but others will be sure to add more

[Dodge/Cummins]

I have a 7.3L turbo as I LOVE TURBOS!
I agree with everything stated here, my Dad owns a machine shop and he's relined a bunch of 7.3Ls. My pick would be a LINED, rebuilt, head studded 7.3L turbo. ...BUT, when my motor goes T.U. I am probably going to rebuild a 6.9L so I don't have to sleeve it, I don't want the expense of sleeving it and I don't even have to pay for the installation.

Stock- I'd say '87 6.9L with aftermarket turbo. I'd get a 5-spd. but I'm an admitted manual transmission nut when hooked to a diesel. Guys who know more than me about the autos seem to know how to tinker with theirs and love them. I don't so I stick with 5-spds. as I want overdrive.

[FORD MEGA CAB!!!]

If mine goes, Im going with the 6.9 if i can find one cheap. It seems like more people favor that engine over the 7.3 and Like everyone else, the thin cylinder walls prone to cavitation are a big minus.

[Dave Sponaugle]

If you take a 6.9 and bore it out .110 you can run 7.3 pistons.
The only thing that would be different is the 6.9 has 7/16" head bolts and the 7.3 has 1/2" head bolts.

You can swap most of the parts between the engines, more will go from a 7.3 into a 6.9 that the other way around.
Most of the reason for that is because of metal upgrades, not because they won't fit.

I am running 7.3 valves, valve springs, cam, lifters, rockers, oil pump, water pump, injectors, injection pump, and other parts I can't remember right now in my 6.9.

The cavitation issue and the sleeve issue when rebuilding a 7.3, give me a 6.9 any day.

And yes, you can make a 6.9 run with the best of them if you don't mind feeding it.

[FORD MEGA CAB!!!]

So when you boar a 6.9 into a 7.3, are the cylinder walls still thicker than on a 7.3 and by about how much? Also with them smaller head studs on the 6.9, do you have to watch your boost pressure or do they tend to hold up just as well.

Dave S, I need your address so I can send The 6.9 so you upgrade it!

[Dave Sponaugle]

When you bore a 6.9 to a 7.3, it is a 7.3 with small head bolts.

Cylinder walls are just as thin as the are on a 7.3.

ARP head studs and milled pistons, I am running mid 20's for boost.

Send bag of money with engine.....how fast you want to go?
Faster = bigger bag