Chassis compatibility question

[Pete Whitstone]

salans7 wrote

Yes, gear ratios need to be the same front and rear, or else the transfer case may develop holes.

The 9" is regarded the stronger of the two, because of the housing design, bolt in axle shafts, and bolted pinion housing. The 8.8 features c-clip retention for the axles, and they are okay for occasional off-road, but hardcore off-roaders and racers with 8.8's despise the c-clips because they tend to break or fall out under harsh loads. Both axles have aftermarket support for disc brake swaps, but neither is budget friendly.

If the gear ratio in your 9" is similar to the 8.8, leave the 9". If the gear ratio in the 9" is not the same, you'll pay a pretty penny to re-gear the 9" (really any axle honestly). So if the 8.8" is in good shape, it would be cheaper to throw that in the truck.

Thanks Shaun. I'm inclined to keep the 9 inch, but I need to figure out what ratio it has. The door tag says the 95 has axle code 19, which appears to be an open diff 3.55:1. I don't really see 3.55:1 in a 9 inch on the Ford factory axle charts that I've looked at, so I'm not sure whether that was a factory ratio or not. It is available after-market in any case.

My existing 9 inch pumpkin is still a mystery, no door tag on the 81. It will need some work anyway, as it leaks and I would like to add a worm-style limited slip. I will see if I can count revolutions and come close to what it is.

I tried that with the 95, with one wheel locked and me rotating the other wheel, I got about half of the 3:55:1, what appeared to be somewhere in the neighborhood of 1:75:1 between wheel revolutions and driveshaft revolutions. I guess this is some effect of the spider gears that I have not wrapped my head around yet?

[Pete Whitstone]

Rembrant wrote

Pete Whitstone wrote

I don't know what ratio's are in either truck...

The 1995 will more than likely have 3.08 or 3.55 gears. By the time the 8.8 diff started being used in 1983, they will almost always have one of those two ratios. There's more variance in the 9" rear ends, especially in the earlier 1980-1982 Bullnoses when they were trying so hard to improve the MPG...2.47, 2.75, 3.00, etc. The 9" rear diff was still used alongside the 8.8 up until at least 1986, maybe later, but it was only in specific trucks...2wd with 351, and 4x4 F150 with 302 and NP435, and maybe a couple others. So in the later 9" diffs, you'll likely see 3.00 or 3.50 as the common ones.

By the way, one issue with using the 1995 F150 drivetrain, you will have nowhere to connect a factory speedometer cable, just FYI. Maybe this was discussed already? The way I'm solving this issue with my 1995 chassis swap is that I bought a transfer case out of a 1990 F150 4x4. It is the exact same BW1356 transfer case, but it still has the speedometer cable drive (1991 was the last year).

You are correct, sir! Door tag says 3:55:1 diff.

I had already considered the speedo drive dilemma, but I didn't know the earlier transfer cases could be used to get around that, that's a great tip, thanks!

My dash is already all aftermarket gauges, so I don't necessarily have to stay with a cable drive speedo. I was thinking about switching to either a GPS speedo, or a speedo that can work with a VSS, or a "speedo drive" that can convert a VSS signal to a cable drive output. None of those are super-cheap options, and I intend to rebuild the transfer case before I put it in the truck, so maybe finding a rebuildable TC core would be the way to go here. I see TC's on FB marketplace for around $200-$250, which would definitely be competitive price-wise with any of the speedo options. Lots to think about here.

[Rembrant]

Pete Whitstone wrote

Thanks Shaun. I'm inclined to keep the 9 inch, but I need to figure out what ratio it has. The door tag says the 95 has axle code 19, which appears to be an open diff 3.55:1. I don't really see 3.55:1 in a 9 inch on the Ford factory axle charts that I've looked at, so I'm not sure whether that was a factory ratio or not. It is available after-market in any case.

My existing 9 inch pumpkin is still a mystery, no door tag on the 81. It will need some work anyway, as it leaks and I would like to add a worm-style limited slip. I will see if I can count revolutions and come close to what it is.

There is a slight difference in the ratios between the 9" and 8.8 rearends. The 9" had a 3.00:1, while comparable 8.8 was 3.08:1, and same with the lower gears...3.50:1 in the 9" became 3.55:1 in the 8.8 differential.

I don't know if you can run, lets say a 3.50:1 with a 3.55:1 on the same truck, but I'm sure it's fine on a loose surface. Dry pavement, maybe not. Converting the D44 to match the 9" rear diff might be the easiest solution (if required), rather than the other way around.

Does your truck still have either of the factory buck tags, either or the firewall or the front right corner of the bed? Might be able to see the differential code on one of them if they are still intact.

[Rembrant]

Pete Whitstone wrote

...or a speedo that can work with a VSS, or a "speedo drive" that can convert a VSS signal to a cable drive output.

The VSS on the 1995 is right on the very top of the 8.8 differential, but you probably knew that already.

[Gary Lewis]

Looking at the FRONT DRIVING AXLE CROSS-REFERENCE GUIDE (Documentation/Driveline/Axles & Differentials and then the Front Axle tab and the Applications List tab) there were only a few options: 3.00; 3.07; 3.50; 3.54; 3.73; 4.09; and 4.10.  Comparing that to the rear axle options that I think were used in the 4wd trucks it looks like the front is usually the same as or slightly higher geared than the rear.  So 3.00 in the rear got 3.00 in the front.  And 3.50 in the rear got 3.50 in the front.  But 3.55 in the rear probably got 3.54 in the front.

In other words, they wanted the front to pull just a bit more than the rear.  So I'm not sure that a 9" with 3.50's in the rear and 3.54's in the front, assuming that's what you have, is what I'd want to do.  I think I'd use the matching 8.8" in the rear.

[Pete Whitstone]

Excellent point, Gary, thanks. I did a bit more research on this and found exactly what you are saying - that the ratios should be equal, or if they are slightly dissimilar, the numerically lower ratio goes in the front.

Apparently the reason for this is that the rear diff is stronger than the front diff, as it carries 100% of the load in 2wd mode. The front only carries roughly 50% of the load in 4wd, so the ring gear can be smaller. Smaller ring gear, different tooth counts, thus different ratios.

Apparently this difference in ratios, along with other factors like uneven tire wear front to back, is why you are not supposed to engage 4wd on dry pavement. It produces a bind you don't want to subject your drivetrain to.

[Gary Lewis]

Yes, that is part of the reason you don't engage 4wd on dry pavement.  Another is that the front and the rear travel a different arc through a turn, so they go different distances.  Pretty soon there's a lot of bind that has to let go somehow.  If you are lucky the tires slip.  If not, the metal slips.

[jeffjee1]

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[Gary Lewis]

This user has been banned until an explanation about why "luxury watches" was discussed in the post.

[salans7]

We were fortunate to not have this problem for awhile, but the more visibility we have, the more likely this is to happen.

[Gary Lewis]

When we migrate newly-registered users will only be able to start a new thread in New Members Start Here. No replies anywhere, like this one, and no new threads anywhere but NMSH until they are promoted to members. And they won't get promoted to members, which allows them to post elsewhere as well as reply until they give us a satisfactory introduction. So at least we will have any spam contained.

And I trust that our members are smart enough to never, EVER click on a link in a first-timer's post in NMSH.