I have a 76 F100 that was fixed up but now needs engine/tranny. I have two potential donors. The first a 390/T18 combo... I've spent a lot on the top end of this motor and really want to do shortblock rebuild on it as I love the torque that my 302's don't have. The second is the truck I'm driving now w/ 302 and Rug OD 3+od manual tranny. I like the ability to have overdrive and better fuel economy. I don't go off road and don't tow. My aim is a peppy daily driver that cruises on the Freeway. I persued an Idea of getting a Tremec 3550 for 390, was told I could make that work, was also told my RUG od behind my 302 would go behind 390 as well. I know that bellhousing patterns are different between smallblock and FE... but what about the depth of bell, and the tranny to bellhousing bolt pattern? I'm about to pull the engine/tranny out of truck that has 390. Are there differences in the input shaft dimensions? I know I've seen some 78-79 2wd's w/ Rug od's and 351M/400... What do I need to know about putting an OD manual behind 390? How much can the tremec 3550 be had for?..... should I be considering using the GearVendors under/overdrive as a cost effective option to Tremec 3550? I will say this... I like the way the T18 shifts better than the Rug Od but I can live w/ the Rug for the Overdrive. What do I need to consider as far as measuring things?

BBT

 

BBT,
Will the lighter trans handle the torque of the 390?
Other than comparing them on the ground I wouldn't know what else to tell you to do. The clutches might be different. The length of the driveshaft(s) may also differ. I swapped out a 3 spd that was behind my 390 for a Ford 4 spd once. I don't even know what the original make model was for the 4 spd but it was a direct bolt in.

Good Luck

Trail Rider

 

You don't really need an OD. What you need to think about is the overall top gear ratio. If your T-18 has a 1:1 4th and your RUG has an OD ratio somewhere around 0.7-0.8 (just a guess here, if someone knows the real number please jump in) then you could get fuel economy AND strength by changing your final drive ratio by the same percentage.

Suppose you've got 3.55 gears in the truck now. That makes 1st gear in the T-18 a creeper and you're pulling too many revs at highway speeds. If you change the rear end ratio to a 2.73 or even a 2.47 you'll cut the highway RPM accordingly. Your creeper 1st gear would then be usable, actually it would be necessary, to get the truck moving. The major fly in the ointment is that the T-18 has a non-synch 1st, so you cannot easily downshift into 1st unless you're stopped.

2 things to consider: first, the RUG probably isn't strong enough to live behind the 390. Second, because of the way they work, OD trannies are actually significantly less efficient in top gear than 1:1 trannies. For the same number of gears and same overall ratio you're going to get slightly better highway fuel economy from a 4 speed w/1:1 top gear and a tall rear end ratio than from a 3+OD and a short ratio.

If you're thinking 5-speed, the ZF offers both creeper and overdrive but I don't think it can be easily fitted to an older FE series engine.

 

OK..... just how usable will my 1st gear become w/ 2.75 and 31" tires? Right now I only use it while trying to muscle old truck w/ manual steering around sharp drive through turn. I can live w/ non-synchro 1st... I virtually never downshift into 1st anyway. My other concern about going w/ tall gears is: what gear and RPM's am I going to be at in city street driving (30-40)? I do have a 3.00 31spline that I don't have axles for. If the yard I got it from has 28spline in ratio I want, they'll trade it back... but so far they haven't had the 3.00 or 3.25 I thought I wanted... they may have a 2.75 though. As far a the RUG... I turn 1800 in OD at 60 w/ 31" tires and 3.50 rear. I believe thats' a 1:.78 ratio. I've seen this tranny behind a 400 before. and so far noone has any expirience w/ the Tremec 3550 tranny?

BBT

 

The numbers you gave me for the RUG work out to be a 0.78:1 OD ratio. To get your T-18 to give you the same highway revs, the 2.75 also works out. I don't know which T-18 you have, but the most probable has ratios of 6.32/3.09/1.69/1.00. At those ratios 3500 RPM in each gear will be approx:
1st 18 MPH
2nd 37 MPH
3rd 67 MPH
4th 113 MPH (like you care)
This worries me a little bit because 2nd is now too tall to pull well from low revs (1000 RPM is approx 11 MPH) and you can't drop into 1st real easily. If you think the RUG can live behind the 390, well maybe??

 

I'm about to put in a 3.25 nine inch from the 3.50 I had. I am running 31" tires. What math formula are you using for your figures? I'm wondering what my R's will be like after shifting up. How much will I be lugging in 2nd, 3rd gears. The 302 powered truck I'm driving currently (w/ the RUG)has a really wide gap between 2nd-3rd. It drops about 1000 Rpm.... so going up my hill (fairly steep) that I live on top of, I have to run the truck in 2nd up to 2500+ Rpm and then it's at 1500 w/ the load of the hill into 3rd (1:1... 3.50). I know the 302 will pull adequately (not spectacularly) at 1500rpm. The 390 should have more grunt for that kind of pulling. There will be another difference when I change motor/transmission from my 69 to my 76. The 69 has a Dana limited slip differential (not full floating.. its a half ton) and the 76 has a 9" type. Should I consider swapping in the Dana rear axle assembly? If I do, what about the spring pads, will they likely be in the same spot? And brakes, should I or can I use the 76's rear brakes, or will the drums on the 69 by the same as far as the proportioner valve is concerned? I have so many options I'm considering... help!

BBT