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I'm curious about this as I've never read a thread on any board or heard of anyone doing it. Do many people swap out their straight sixes for a V8? It doesn't seem like it's done very often.
Is it difficult to do? Are straight sixes rare? Or do people that have straight sixes just love them so much that they just don't do it?
Just curious as I've never heard of a six to eight swap.

Your thoughts...
 

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AbandonedBronco said:
I'm curious about this as I've never read a thread on any board or heard of anyone doing it. Do many people swap out their straight sixes for a V8?
It's definitely do-able, and shouldn't be all that tough in most cases. Easier if you're carbureted, of course.

AbandonedBronco said:
It doesn't seem like it's done very often.
Is it difficult to do? Are straight sixes rare? Or do people that have straight sixes just love them so much that they just don't do it?...
Let's see:
Not particularly, definitely not, and yes, this third question probably explains it. You'll find a lot of the guys on here love their sixes, and for good reason. :thumbup

Edit: Looking at your info, I see you've got the TOD (toploader overdrive, the OD four speed). You can swap in an NP435 non-overdrive four speed pretty easily, and they're a LOT beefier. The combination of your 2.73's and that tranny are why your truck (I'm assuming, based on your question) feels so slow -- by ditching the overdrive and adding a much taller first and second gear, your straight six will be much more capable, and probably a lot more satisfying. I have 2.73's in my Mustang on little 26" tires in a lightweight car ... I can't even imagine trying to drive a Bronco on 30" tires with them. An NP435 will put you at about 1800rpm instead of 1300rpm for 60mph cruise in top gear, which should still net you good gas mileage and get you there a lot more enjoyably.
 

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Discussion Starter #4
I love my straight six and it's been a long time since I've thought about swapping it for a V8. I did when I first got it, but now am a proud six owner. I was curious why I've just never heard of anyone doing it and was wondering if it was for the same reasons I haven't. :) They're kick butt motors and even though they can't always keep up with the V8, you just can't stop them.

As for my gearing, ugg...yes, the OD with the 2.73 rear end is very sluggish. My axle code on my door is a 14, which, if my research is right, is a 3.00, so someone actually switched it out for the 2.73. Not sure why. I usually find myself going about 35 - 45 mph up some of the steeper grades. (When I first got it and hadn't done any work on the engine, I'd be going 25 with my hazards on). I have been looking for information on just swapping out the final drive to maybe a 3.5, but haven't found anything. I do like my 20 - 23 mpg on the highway though. :)
 

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Discussion Starter #5
No interest in changing to a V8. :) Was more of a curiosity as to why no one does it, and got some great answers which amplified why I like my 6 even more.

Question though, what is the OD gear ratio for the 4 speed overdrive in 81? Some sources say 0.71 and some say 0.81. If it's 0.81 then I have a 2.73 final drive, if it's 0.71, then I have a 3.00.


Where's a good place to find these axles? Especially with limited slip? I'm not even sure where to begin to look and there aren't any junk yards for miles and miles. Will the 8.8" swap in with the 9"? If I remember right, they didn't start using the 8.8 until like '83 or something. Also, what's the differences between the two? (strength, etc...)
 

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I felt a big difference from a i6 to the 429 that I put in. I would have kept the i6 in it but I had this motor sitting around and I blow up the i6 in a clay field. I like the i6 but it lack alittle power for me in the mud.
 

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My 81' went from a I-6 to 302 v8 in a weekend it's not bad at all. I've got the pics, and i'll post them tommorrow when i get the chance. You can Pm me for specific info:thumbup
 

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You would be mutch better off regearing than you would by puting another motor in there.

The 300 is probably puting out more torque where you are using it. It sounds like with your crappy gearing, you are lugging the motor in a low rpm range very often. A 300 will do better than a 302 in this aspect. The 351 would probably be *slightly* stronger down low, and better mid and high.

Steve83 is steering you in the right direction. 3.55 rear ends are probably the most common gearing to be found, and if you don't want to mess with the gears you can just swap in a whole axle. The front end would be a noodly mess, but you can just put in the third member much easier.
 

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Steve83 said:
You can swap a whole 8.8" axle in with whatever ratio you can find; it's a bolt-in. 3.55 is pretty common, even with a limited slip. My spare axle is an open 3.55.

There are a few people who have swapped 6->8, but it's not common for the reasons already mentioned. But it's pretty easy. You need the complete engine, radiator, hoses, engine mounts & frame perches, flexplate/flywheel, Y-pipe, and the starter. After that, it's just some wiring. If you're gonna do it, you might as well upgrade to '90-95 EFI.

Any particular reason to start with a '90 instead of an '86/'87?
 

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AbandonedBronco said:
...Question though, what is the OD gear ratio for the 4 speed overdrive in 81? Some sources say 0.71 and some say 0.81. If it's 0.81 then I have a 2.73 final drive, if it's 0.71, then I have a 3.00...
I don't know offhand which is correct, but I can vouch for the fact that the 2.75's exist, we just put a set in a non-OD F-150 that was being set up as a highway vehicle.

If you're basing off of highway speed VS. rpm using a gear calculator -- check in third gear, since you know that's 1:1 anyway. You'll be certain to get the right number. :thumbup

Steve83 said:
...The 8.8" is slightly stronger than the 9", but not by enough to justify a swap for strength alone. Aftermarket support for the 8.8" is ALMOST as good as for the 9".
I still think this is a matter of opinion. I've seen more blown up 8.8's than 9's ... and honestly, having now dealt with both, there's no way in bloody heck I'd pull a 9" out in favor of an 8.8. ;) Between the thinwalled axletubes and the lack of pinion nose support ... no way. I also disagree with the statement made a while back that the new axle must be stronger because it replaced the older axle ... we may just have to agree to disagree on this one, now that I've had a chance to look at both a bit harder. :toothless

Abandoned, all the information in here is fundamentally pretty good, though. One advantage of your 9" axle is that you can very easily change the gear ratios without having to set up the gears, if you have no experience doing it. Unlike the 8.8, the 9 has the gears set up in a removable assembly, so changing the gearing can be as simple as finding a good assembly (called a third member) in another truck with the ratio you want, and swapping them. You take out four bolts at the end of each axletube, pull the axles out, and then take off the 10 nuts holding the third member in and swap it. Don't get me wrong, the third member is heavy and can be a royal pain in the butt to get pulled out when it's been there for 25 years, but with so many Ford trucks that used that axle, the gears can be found.

Main thing you need to watch for is the axle size ... certain very light duty F-100's, early Broncos (as in pre-78), and the cars got smaller 28 spline axles. Take your good axleshaft to the yard with you and check fit, that's the simplest way to make sure you're getting the right thing if you don't have much experience with it otherwise. :thumbup
 

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Steve83 said:
Yeah, with the 8.8", that assembly is called an "axle" ;), and there were even more 8.8" trucks built than 9", so they're even cheaper & easier to find than 9"s. :toothless And they're usually in MUCH better condition.
No argument there! Pulling a third member that's been in for 25 years is not fun sometimes. :banghead
 
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