IDI Dilema of my life!!!!!!!!

[tractorman86]

so here is my dilema: i have a 1991 250 i am rebuilding, i have two suposedly running 7.3's or a bare 6.9 i had $2,700 worth of machining done to it(sleaved to std., crank and rods all magnafluxed ballenced ect. i am also going to drill&tap heads and block to slap on 7.3 studs for extra strength:Sly ). i let a so called friend of mine do my bidding and was told the motor was a 7.3 but was infact a 6.9 . is there any reason i shouldnt bore out this motor and run 7.3 pistons in it? (going to get whichever ones i use ceramic coated via ken :Sly ) it is only .11" after all. if u havnt noticed i am planning on adding a lot of power adders; turbo(possibly a 6-71 instead), water/**** inj, propaineinj/nos, intercooler and so on and so forth. well i guess i've rambled enough, should i put the 6.9 togather and sell it while doing the same work to the 7.3 or just run it since i am already this close? any ideas yall?
any help i can get is GREATLY APPRECIATED!!!!!!

 

[Freight_Train]

I would vote selling the 6.9L and building up a 7.3L.For one,you can't drill and tap the head bolt holes.There isn't enough meat on each side.You would be Cross cutting the threads since I believe there is 1TPI difference between them(I think 6.9 is 7/16x12TPI and 7.3 I know is 1/2x13TPI).Only way you could do this is have someone drill the holes out,weld them back up,and start over from scratch.Another,you can get Mahle pistons from Ken Coated which are Forged from the factory.You can use the big pin Turbo rods if you get turbo spec pistons.The Crank and cam are the same between the two so you wouldn't need a new one.

 

[The Warden]

Personally...in the interest of strength and minimizing the risk of cavitation, I'd go with the best of both worlds. Take a 7.3l block and resleeve it, with sleeves thick enough to use 6.9l pistons. IIRC turbo69 did this with his engine block with success.

Considering that there was very little power difference between the 6.9l and the 7.3l (175 vs 185 horsepower IIRC), I don't think you'd even notice the difference...and, certainly, once you get the turbo on there it won't matter and this way, the coolant has that much more metal to go through before cavitating the block.

Just my $.02...I don't know if Mahle pistons exist for the 6.9l or not, but turbo-spec conrods are REAL hard to find, so I wouldn't count on finding them unless they're already in your posession. Freight's right regarding the difference between 6.9l and 7.3l head bolts.

Good luck!!

 

[Agnem]

If you can find a way to take a 7.3, and make a 6.9 out of it, then you'd have the best of both worlds. However, the money you have in the 6.9 already will be difficult to recoup. You may be in a financial pickle.

 

[RKOCH]

There is no replacement for displacement. I am a 7.3L Man. My old 6.9L's never seemed to run as good as my 7.3L's.

 

[f-two-fiddy]

Just get the studs for the 6.9. You can get any piston custom made, that you want. For the money you have invested, and the power your looking for. $500 for custom pistons is a drop in the bucket.

 

[icanfixall]

Installing studs will help the 6.9 because they break at 190,000 psi. The 7.3 studs break at 270,000 psi and are only make by Ken. Stock 7.3 head bolts new tested to failure break at 153.000 psi so the stock 6.9 will be somewhat lower than that. Taking a 7.3 and sleeving it back to 6.9 will really be the best of both worlds but there really is no replacement for displacement too. Sure, the 5.9 cummins is only 360 cubic inches and the new 6.0 fords are small to but a straight 6 always makes more torque than a v-8 will.

 

[RKOCH]

I get head studs and all other engine studs for 6.9 and 7.3 from ARP. They have a kit fro each. My motors looked like a porcipine on the stand.

 

[Freight_Train]

?!?!?!ARP doesn't make a 7.3L IDI stud kit.They said they would but it would cost well over a grand for one set and still be the weak 190K breaking point.They make a kit for a 7.3L PSD but never seen an IDI kit.

 

[The Warden]

There is no replacement for displacement. I am a 7.3L Man. My old 6.9L's never seemed to run as good as my 7.3L's.

Okay, then, why is there only a 10 horsepower difference between a stock 6.9l and a stock 7.3l? I tend to agree with the "there is no replacement for displacement" argument, but when the numbers are as close as they are with the 6.9l and the 7.3l (both in terms of cubic inches and horsepower), and you run that much less of a risk of cavitation with a 6.9l, it seems to me that it's worth it from a longevity standpoint. Yes, you can take steps to try and prevent cavitation, but some people have had failures even with using SCA.

I get head studs and all other engine studs for 6.9 and 7.3 from ARP. They have a kit fro each. My motors looked like a porcipine on the stand.

Oh? Based on what you said in this thread, I had been under the impression that you hadn't done any modifications whatsoever to your long blocks (i.e. still using head bolts, etc), and that nothing had ever even been opened up. What did I miss?

 

[tractorman86]

well like mel said i think i am going to have to build the 6.9er. i will use the best that i can find for the old girl. frieght train- mahle makes forged pistons for the 6.9 or just the 7.3 ? yea, i am going to have to do some digging to put this one togather!!! thanks for all the help guys and keep the good ideas flowing!

 

[82fordtruck]

I agree that that would be the best option - who needs that extra few HP anyway?

It's only about 10% difference between the two engine, displacement wise - which is why it's only about 10 HP.

 

[Freight_Train]

I do believe they also make 6.9L pistons but not with the factory Turbo large pins.Has anyone ever had a Sleeved 7.3 recavitate?With a 6.9L your still stuck with the small head bolts and small wrist pins.But you can sleeve down a 7.3L to a 6.9 and have the large bolts.This is one of those Damned if you do Damned if you don't deals.

6.9L-small studs,no factory turbo rods(unless you spend big bucks for custom pistons),less worry about cavitation(6.9s can still Cavitate!)
7.3L-Large studs,Factory turbo rods,Worry about cavitation
Hybrid 6.9L/7.3L big studs,small rod ends.....very little cavitation worries

 

[f-two-fiddy]

When someone has an ARP Stud break, give a shout. I've got to think that 190K PSI is plenty to hold the head on a lower compression 6.9.

As for pistons, I remember guys ordering custom pistons for high HP V6's. If I remember right, Ceramic coated and teflon skirts were right around 4 bills.

If it were me, I'd spec out a little lower compression, with the larger wrist pin. IF you have, or can get turbo con rods.

Speaking of wrist pins, has any one had one fail? I can't remember ever hearing of a total failure.

 

[Freight_Train]

Yep.I have!Well,Travis actually.Sucker was knocking real bad when I got the truck.But they also make stock lower compression pistons for the 7.3L with large pins.They are made for blocks that have had the deck cut down a ways but Mahle makes the pistons with a lowered crown.Not sure how much they lower it but I remember someone talking about around 19.5-20/1 compression over on STD.Oh yea,In the hay day of IDIs(before the Powder choke,Maxipad,and 24 valve) Ford had a problem when people would drop a Non-wastegated turbo on a N/A engine with the wrist pins failing.