Change 8.5 rear end ratio from 3.42 to 3.73 or 3.91

I'm in the process of freshening up the 8.5 axle on my GS. Opened it up the other day and confirmed that it has an Eaton posi (which needs servicing badly) and a 3.42 ratio with 41 tooth ring and 12 tooth pinion. If I want to change the ratio to 3.73 or 3.91 do I need a ring and pinion matched set or, just pick up a 45 or 47 tooth ring gear. I plan on picking up the Gear Vendors overdrive unit for my Turbo 400 later on. Haven't decided yet whether to go with 31 spline axles or not.

 

You will need to change both as a matched set. What kind of HP is it? Drag races with slicks? I would only look at upping the spline count if over 450 wheel HP and drag races with sticky tires.

 

Engine is a TSP stroker (stock appearing) with 502 Dyno HP. So, maybe 425+ at the wheels. I run 12.20s at the track and only plan 3 or 4 track visits a year (including BG). I run MT drag radials almost all the time street or track.

 

Good morning Ray,
When I did the rear end on my car I went with
Monzas on the forum. He set me up with everything, and I do mean everything. New bearings, seals, ring and pinion gear, and anything else I would’ve needed. I brought all the parts to my buddy that has a transmission shop, and he’s also a driveline specialist. He did everything for $300. I know, that’s probably the buddy price. I did shop around and Monzas seemed to have the best deal, plus all the parts were exactly what I needed.

 

I'd go higher if you're going to run a gear vendors. 3.73 or even 3.91 is streetable w/o it. Might as well go for the better ET and go to 4.10 or 4.33 depending upon your dyno curve & current trap RPM.

 

the question becomes: what RPM do you want to cross the stripe at? And to figure that our, you need to know how tall your MT drag tires are, and guess at a couple other things. But for me to cross the stripe at 110 at 5500 rpm with a 26.5 inch tire, the gear is 3.73 - for you, I'd recommend maybe a 3.90, depending on your specs..plug your numbers in here:

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

Your current axles and posi are ok to probably a 1.70 60 ft - anything lower than that and I'd go 31 spline, aftermarket axle upgrade..

 

You can get a 3.70 also for the 8.5.....well could pre covid.

Not alot of performance differences but alot more meet in each gear. 3.73 is 41/11. 3.70 is 37/10 gear tooth count. Less teeth =slightly thicker teeth.

I'm no rear end expert, but I have setup 30 or so. My 3.73 is the only 1 to ever break and I cut 5 teeth the ring gear foot braking.

 

My setup is almost identical to yours re: MPH, tire size and RPM. Looks like the 3.91 will work for me. I want just enough to put me into the 11s without having to push the 60 ft. too much. I'm at 12.20s at about 110 now with 60 ft. in the mid 1.8s. Below is my dyno run at 502 HP.

So, if I decide to go with the 31 spline axle upgrade, what should I expect to pay over and above the basic axle servicing and gear change?

 

Ray, something to consider is a 200r instead of the Gear Vendors. That's what is in mine and I love it.

 

If you have an 8.5 it will be 30 spline. When I bought mine years ago the axles with bearings were 700ish from Moser. I would guess a new posi unit would be 350ish.

I run a spool.

Gears and full install kit figure another 300ish.

Then labor to install

 

That would be a nice combo but, for me, it's got to remain stock. My GS has the original NM Turbo 400 and I want to keep it that way. Also, don't want to have to modify the QJ linkage to send info to the 2004R and the additional wiring into the interior. Good suggestion, though.

 

Got ya.

 

Stock 28 spline axles worked for me to high 10s. Not screwing the slicks helps as the tires will turn on rim a little before hurting axles.

 

Ray,
Depends on your ultimate goal, but I would go at least 3.90 to see the performance I think your after. The calculation to make is your cruise rpm at 65 or 70 mph, whatever you like to drive when your on the open road with the gear vendor.. Keep it below 3000 rpm for a comfortable ride... this is much more important than trap rpm for you... if it were just a track car, then we pay more attention to the trap rpm, but Physics dictate that the harder you launch the car from the start (assuming adequate traction), the quicker it will be. So you want to achieve the fat part of the power band as quickly as possible. 4.10 is the standard rear gear for a full weight GS drag car.. of nearly any NA stripe.. The fact that it quits pulling hard before the finish line is not a big deal, the wind resistance at speed from our brick shaped cars takes over long before then.

You just cruise the last 300 feet or so.. your engine max rpm is limited by the hydraulic roller lifters, I have had the Johnsons at 6400 more than a few times on the dyno. Your bottom end is good to 6500+ so don't worry about that.

Those are your guidelines and limitations, so work within them to achieve your goals.

JW

PS... as was stated, if you have a 71 up 8.5, the 28 spline count is plenty.. just upgrade to a performance axle from one of the vendors out there. You just need to step to the sportsman or entry level performance axle, you don't need 31/35 spline max performance stuff. Just an upgrade in material from stock. And if you go with a bigger axle, then your probably buying a new differential case to go with it.. considerably larger expense.

 

I had 3.90's on this car with a 28" tire, was almost exactly the same RPM on the highway as the 3.73's with the 26.5's - Tire size is a big factor here, both your street and racing tires and the rpm you intend to turn in each at various speeds. I dont like turning over 3500-3600 on the highway for long distances, it pushes oil out of the motor pretty good so I have a ceiling at 3.73 for the way I use the car in it's current configuration....

 

I would go straight to 4.11s and not think twice. I was trapping at 120 with the shift light just barely turning on at 6k.

If your racing tires are 28" I would bet you might get in the 11s with a 26" tall tire. 3.42s and a 28" tire is great on the street but lousy at the track. Its way too steep for a decent launch.

 

I also ran stk axles into low 11 high 10s. I wouldn't tell anyone to do that....

I think the 28s cost basically the same as the 30s, its just the different posi that you need for the 30s,

I built a 509ss pickup with 3.73 and gv. Forget tire size...28x12x15 i think......ot ran low 2000s in od


If your for sure on the GV go 4.10s and enjoy the launch.....hope you have so e sticky to play with

 

I would go to 410 gear with a overdrive, launch out of the hole baby!