6.9l vs. 7.3l, the debate rages on...
 

I've had both engines, both served me well to be honest. The 7.3l I have now is great, even with 235,000 on it there are no knocks, and it barely sips engine oil. The 6.9l I had was in an '87 dually. It was around 155,000 when I bought it, and oil consumption was unbelievable, around 1 GAL/200 miles! But it started better, and ran more smothly than my current 7.3l, and in spite of oil consumption it is still running today! I prefer the '80-'86 bodystyle Ford trucks, but I have to admit I'm a bit hesitant to relive my experiences with a 6.9l, and I know they are far more well known for oil consumption.

I'm asking for opinions, of all kinds, on which is the "superior" engine. The 7.3l has it's cavitation issues, the 6.9l is known for oil consumption and valve seal failure, yada yada yada... And am I correct to understand that pre-'87 6.9l's didn't have a dual mass flywheel? That may be enough to sway me right there, even though I have a 7.3l auto now, I would never go auto again w/a diesel (I love to shift, it's an addiction :-D ). I know it's a matter of opinion, I guess the main thing I would like to know is what "normal" oil consumption is w/a used 6.9l (I'm not opposed to checking/adding, I just don't like to be stalked by huge wandering clouds of blue smoke), and what the success rate is for lessening this with a re-ring. Plus I've had both, and I loved both, and I guess I want to get one or the other and just keep it till it dies!

my '86 burns about a quart every 500-600 miles.

My 6.9 uses about 1 qt per 800 miles, at a gallon per 200, musta been leaving a blue trail behind you, that's excessive, my old man's old 84 with leaking front and rear mains doesn't go through it quite that fast...

I use about a quart every 200 miles- thats after I fixed all the oil leaks. I too would like to know the success rate of rering kits - I almoast put one in then i didn't and just fixed the leaks

ive had bolth too and i really dont have a favourite...but if i had to chose it say the 6.9....never had a problem with oil loss on the 6.9...even my driver now has 460k on it and it uses like a liter per 3 tanks of fuel..like 2 or 3 liters per change...and my other 2 that had about 280k maybe used 1 per oil change.....but like i said i liked bolth...and it is totaly a prefrance thing..

It was a very expensive truck to drive. I never could figure out what might have caused such a rediculous amount of consumption, because it ran beautifully and had adequate power. Sadly, it had only one very very slow leak, at the rear main, other than that it was ALL going out the tailpipe! It had true dual exhaust, the front pipe (pass. bank) was literally coated with tar! The smoke was unbelieveable, every time you'd let off or when holding 1500-1800 rpms a fog would begin to form. But this was also the truck that started faithfully at sub-zero temps without being plugged in, as long as I kept strait #1 in it, and I always loved that about it. Plus I loved the 4spd, good solid trans. The reason I was afraid it was somewhat normal is I knew a kid who had an '84 6.9l/4spd that was just as bad, it didn't even smell like a diesel when he would drive by! I was just afraid that was how they ended up. I'm glad it's not, I think my next diesel will be 6.9l powered, pre '87 (no dual mass), and 2wd so I can regear and put on an external overdrive, he he he.

Have you serviced the CDR regularly?
A good sign it is the culprit is a lot of oil in the intake plenum.

If the CDR is stuck open, at high RPM the engine will suck to much air out of the engine and the oil comes out with it. Then straight into the intake and combustion chamber.

As far as my opinion on the two engines goes.
The very first production 6.9 engines did have a couple of small issues. Block cracking over the block heater was probably the biggest one. That issue was addressed in the 85 model I believe.

The other big issue with the 6.9 is the 7/16" head bolts that will stretch and cause head gasket failure.

Now with the 7.3 we took a 6.9 block and bored it out 110 thousanths and made it a 7.3.
We also upgraded the 7/16" head bolts to 1/2" and made them much stronger.
But we made the cylinder walls much thinner which magnified the cavatation issue many times over.

If you rebuild a 7.3 the only option is to sleeve the cylinders to restore them. Yes, I have actually seen 20, 30, 40 and 60 over 7.3 pistons, but that option would be like playing Russian Roulette with only one bullet removed from a 6 shooter because of cavitation. The 60 over would be more like Russian Roulette with an automatic, you will lose first time out.

With a 6.9 the same pistons are available and it can actually be bored out and still have thicker cylinder walls than a 7.3 does stock.
Also head studs are available that remove the head bolt stretching problem.

I am putting my money on an 85 6.9 block, boring it 30 over, milling the piston crowns down 40 thousandths, head studs, 5000 RPM balanced, ATS turbo, 3" plus straight exhaust, custom injectors and IP, a bit of porting and polishing, some turbo tricks and we will see what I come up with. I am planning 25+ PSI boost numbers and enough HP and torque to make the Power Stroke and Dodge boys blush if I can keep the driveline under it. Time will tell.

I killed three 7.3 engines that had been sleeved in under 30 thousand miles (2700, 9700, 17000), so my opinion of sleeves is not to high. That is my reasoning for going back to a 6.9 block.

Dave,


I had an 86 with a 6.9 + ATS (I loved it)


with all of the below in mind,


I have an offer of an E4OD out of a 7.3 Super duty for a core price. (I'll have it rebuilt with all the SB's and upgrades)

I am thinking about putting it in my 1955 F-600 + a 6.9 + ATS.

You're saying that the 6.9 might be a better choice over the 7.3L if built right? If you're going to pump more fuel and air into it are you going to install an intercooler and a larger radiator(Will the water pump have enough flow rate?)?

Also does the piston crown milling REDUCE the compression ratio?

It seems that if you're going to pump up the boost you'll need to drop the comp ratio to say 17-19:1 to keep the exhaust temps in check.


Regards,

Rick

In my opinion after all the trouble I had out of reman 7.3 blocks, they are great till they need bored to straighten them out for new rings. Since the cylinder wall is already thin, boring without installing sleeves is not an option. Granted all my reman engines were from the same place. The sleeves dropping down in the bore farther and cracking the block was what killed all three engines. The sleeves that they used had no top shoulder, the fire ring in the head gasket rested entirely on the sleeve. I personally do not think much of that design for the sleeve. The 2700, 9700 and 17000 miles were how long each sleeved motor lasted in my truck. Never again will I install a sleeved motor. After 5 motor swaps in 2.5 years I am getting good at it though. I can pull one and swap the parts that need swapped including the turbo and have it running again in under 10 hours.

An after cooler or water injection is in my future, I would like to go the after cooler route. Problem is my snow plow bracket is right in the way and it is 1/2" thick steel plate from frame horn to frame horn right where the air flow needs to be for the aftercooler.

I have resleeved several big Cummins motors, they use wet sleeves that have shoulders at the top of the sleeve and o rings at the bottom to seal the cooing jacket from the crankcase. You then use shim rings to set the projection above the deck of the block to the speced height. That is a good system.

40 thousandths will drop the compression ratio to about 20 to 1 with Sealed Power hypereutectic pistons. I still have to crunch some numbers to check the compression ratio at several boost levels after I recalculate everything I am having done at the machine shop.

Milling the pistons is to reduce the pressure on the head gasket, not reduce the EGT.

The big exhaust is to reduce the EGT.

I already have the biggest radiator I could get, never have the temp raise much at all even with big loads here in the mountains.

I'm surprised they don't use the dry sleeves like in the IH ag tractors, they have the shoulder you speak of, but then, are also factory issue...

Most IH tractors have a cummins in them - that would explain the sleeves. If designed for sleeves the engines are much better in my mind.

These were definitiely not Cummins motors, it was a 656 diesel that I refer to, the 856 motors are the same, well before Tenneco bought IH ag. It was all IH design. They also have wet sleeves. The 5.9 Cummins is not sleeved either, it is straight bore. I can't answer for the Case IH tractors, but the true IH tractors did not have Cummins motors...

I have tralked to several rebuilders about the sleeve issue and the install process.
Some use heat and cold to fit the sleeves like you do the pins in a dozer track.
Most use a Locktite like product that glues them in after they are pressed in.
I also understand there is a sleeve made that has the shoulder at the top of ther sleeve like the big Cummins wet sleeves.

When I tore down all three of the engines I cracked the block in I found the sleeves had dropped from where they were when they were installed. You could see where the block had been surfaced after they were installed and the surfacing marks on the block continued across the top of the sleeve. But the sleeve top was 7 or 8 thousanths below the block deck after it had been run. Since the fire ring on the head gasket rests on the sleeve, this allowed the torque on the head bolts to be lower than spec called for. The ones that were installed in my blocks were the glued in type sleeves.

Bottom line for me is sleeves have left a very bad taste in my mouth.
I may be full of something that does not smell to good going the way I am, but I like the option better than another stab at getting a properly sleeved motor from another place. I had hoped that I could have gotten by with boring less than 30 over, but this block was already 20 over.

The shop doing my engine machine work do not install sleeves, they also had bad experiences with them.
They are good friends of mine, I fixed several of their basements that were leaking water in when it rained.
Initially they did not want to do the work till I explained what I wanted, why I was going with a 6.9 block and that I did not like the sleeve option either.
When I explained my reasoning to them, it made enough sense to them that they agreed to give it a try.

So far they have been plesently surprised at how well it has all turned out.
I did drop their jaws when I asked for the piston crowns to be milled down, but after I explained the boost levels I want to run and how much it would reduce the pressure on the head gasket at boost, they understood what I was after.

If I get this right, I will be a happy camper for myself. I will also have converted a machine shop from thinking only bad things about the IDI diesel as far as rebuilds go to realizing that it can be done if you go at it the right way.

Since they agreed to do the machine work, I think I have done a good job of convincing them so far as to the soundness of my logic.

All I have to do after the engine is running is take a picture of the truck with the black smoke rolling out the stacks to put on the wall with all the mud trucks and drag cars doing their thing. I thought that was a fair request.

Dave,

It appears that you would suggest to use the 6.9 then. AND if possible try to get one from a truck that is on a first run(never been rebuilt etc).

Dropping the comp ratio is what Peninsular Diesel has done to the GM 6.5L engine to get more power and reliability out of them. They use pistons supplied by Mahle I think. They are 18:1 comp ratio. Then they are able to crank up the HP. They are selling Marine engines and are getting up to 340hp out of a 6.5L engine. http://www.peninsularengine.com/marine400tai.html They're able to do this since they have a whole lake for a heat sink!

There's no reason that one couldn't do a similar thing with the 6.9/7.3 engines but the compression ratio must be lowered to keep the EGT's in check at the higher boost levels. Are lower comp-ratio pistons available or must you machine stock pistons?

Those kind of (continuous) HP ratings aren't possible in a truck application since we're limited by radiator size (you might be able to get a larger radiator in a Ford though!). They were getting 300hp (continuous) out of a 6.5L engine in a truck with the largest radiator(and 15-20psi boost + aftercooling) they could get in there. The only downside I think was a little harder starting with the slightly lower compression ratios. http://www.thedieselpage.com/finale.htm
With block heat and good continuous duty glow plugs it wasn't a problem.


While I don't need 300hp etc....I think the 6.9 could benefit from using lower comp pistons and produce somewhat more HP safely. It appears that from your experiences that getting a 7.3L engine might not be good unless you know you don't have to bore it.

I believe the 6.9 + a rebuilt E4OD will perform good in my old F-600 and give it a pleasant sound. With the 5.83:1 rear axle and my 40" tires I should have a pretty low cruise rpm on the highway too.
(around 2100)


regards,

Rick

From my calculations 40" tires with a 5.83 gear and E4OD in overdrive at 2100 RPM you should be going 60 MPH.

Original 6.9, 18 years and 300,000 miles with no problems at all.
3- 7.3 remans in 1.5 years and 30,000 miles.
1 salvage yard 6.9 with no idea how many miles in 5 months and 8,000 miles.