7.3 or 6.9?

[79ford7.3idi]

ok, i have owned a 85 6.9 idi before and loved the thing. do the 7.3 idis run about the same? my 85 had 4:10 gears and a 4 speed, and it ran like a ***** ape. i cant believe i sold the thing. i now have a 1979 ford f250 with the 300 six cylinder...its killing me not havin the power of that diesel. so i have came up with a plan, put a idi in the 79. so i guess im askin how it would be to put one in there. and which would be easier to go in? and also could i run a dual turbo system on them? runnin a turbo off of each bank.

thanks alot for any help.

 

[rhkcommander]

outside they are virtually the same... inside the bore has changed. unless you find a turbo idi 7.3 circa 94 which has some internal differences. Also the head bolts are bigger on a 7.3 so heads wont interchange without drilling or using sleeves

In case any nitpickers come at me, the glow plug systems can differ, fuel filters, etc.

 

[BleednBlue68]

There is a guy on here doin a 77 build with an idi. I think its a tight fit with the turbo stuff but can be made to work. I just saw the thread a few days ago.

 

[Brianedwardss]

If you're gonna go with a rebuild, a 6.9l with head-studs may be a better choice than a 7.3L. The reason being is 7.3L's have relatively thin cylinder walls, and once bored a bit for a rebuild (after they've already lived a full life), you'll be at a good risk of something called cavitation. Read up on it for more info. 6.9L's have 1/4" thick cylinder walls, much better. The 'BUT' on the 6.9L's is their 7/16" head bolts, which are easily stretched with a turbo putting out more than 10-12psi. But there's a remedy, ARP Head Studs. The 7.3L's have 1/2" head bolts, and can generally get away with using factory bolts on turbo app's. 6.9L's also cool better due to all the coolant passages being unrestricted. 7.3L's have one of their coolant passages plugged from the factory, IH's genious idea of trying to prevent coolant leaking from the head gasket areas on high mileage motors.

 

[Agnem]

I'll take a 6.9 over a 7.3 for these reasons.

Thicker cylinders
Better cooling
Better starting

A stronger block is nice, but you can mod the 6.9 with internal parts choices that will beef it up a little. For daily driving and longevity, you need the 3 things I just mentioned more than a stronger block (and that extra strength is a debateable topic).

The thicker cylinder thing has been well discussed. What hasn't, and what you may not be aware of, is that the 6.9 was known to leak coolant out of the lower corner (rear) of the head gasket area when the head bolts got tired. There were at least 3 revisions to the head gasket over the years to try and correct this, culminating in an O-ring being fitted into the gasket around the water coolant passages. Head studs and a modern gasket make this a non-issue. So what did IH and Ford do to fix this problem in the 7.3? The REMOVED the water passages. Consequently the 7.3 has less water flow through the heads, and if you check all the overheating threads you will find 99% of them are all 7.3's.

6.9's start easier, because the injector squirts right on the glow plug tip. For emissions reasons, they offset the glow plug in the 7.3, and as a result they can be harder to start in cold weather.

 

[79ford7.3idi]

o ok. thanks alot. i have kinda been leaning towards the 6.9 just because that is what i had before. also another question that came to mind is...did the 6.9s come with a 5 speed? and did it come with an aluminum bell housing? the 4 speed i had shot the clutch fork right out the side of the thing and it was a pain finding another one.

 

[Agnem]

Only the 6.9's offered in 87' had the 5 speed. However, it's no big deal to put a 5 speed behind a 6.9 that came with a C6 or T-19. You just need to get all the parts from the crank flange back.

 

[icanfixall]

I agree with the above posting. I have been doing some research on the 7.3 heads and the 6.9 heads. The only change in the gaskets I can find is the closed coolant passages in the front and back lower area on the 7.3. I compared both 6.9 and 7.3 gaskets. I looked and took many measurements and found they are the same gasket... The fire rings are the same size... The size and locations of all cooling ports are the same too. Its just not open on the 7.3 gaskets so... I feel we can use the 6.9 gaskets on a 7.3 if we remove the coolant plugs in the front and back lower sides of the heads. If a 6.9 inproved heds gsket doesn'y lek then I find no reson why they would leak on a 7.3. I relize this is lep of faith to use a 6.9 head gasket on a 7.3 but chances need to be taken sometimes. In the end we wont know till someone takes that leap and posts back here.

 

[Agnem]

That's good to know Gary. I've never measured the two side by side. Might be an interesting solution for a chronic over-heater.