7.5 diff. Will it hold up?

Hi I joined this forum about 2 years ago, but didn't post anything at the time because it seemed like a bit of a ghost town. I got an email from the administrator of the forum a couple of days ago and noticed that there has actually been some activity in the last couple of days, so I decided to post a topic. Here it goes.

I bought a 1978 Caprice coupe with a friend to get back into drag racing on a budget. I chose this car because from the time they came out in 1977, I always thought they were pretty cool and always wanted one. It has a 350 with a turbo 350, and had a 2.41 gear in a 7.5" 10 bolt diff.  I found a complete 3.73 posi 8.5 for it for $250, but one of my friends blew his diff and needed it more, so I sold it to him. I couldn't  find a deal like that again, so I bought a 7.5 posi carrier with a 4.11 gear set for $200 and put it in. Works great, even quiet, but how easy will it be for me to blow it up?
Engine is completely stock now but has a very strange power curve like it has a cam in it. Doesn't start pulling until 2500 and then pulls really hard, so I changed the carb with my freinds Qjet from his Malibu, and no change. Have to date run a best of 15.9 at 87 mph. Not bad considering it still has a y pipe. I want to put my carb/intake/cam/header setup from my old race car, so the power curve isn't going to get any lower. To remedy this I want to put a 3000 stahl converter (I have kicking around) so it will launch. I run slicks at the track. So again, here's the question, will that 7.5 hold up to the setup I just described in a 3800 lb Caprice? What have your experiences been?

I had a 145hp 305 that I put an edelbrock performer intake and 1405 carb on.....  and a 150hp nitrous plate kit.  
I ran a whole 10lb bottle through it, lol!  My open 7.5 with a 2-something gear handled it with no problem.

In a different car I put about 425hp to the rear and used to break the rear u-joint on every 2nd full throttle upshift to second gear. (turbo 350) I used to carry spare drive shafts w/new u-joints with me. And again with tall gearing.

They're fairly tough.

Well it's good to hear you didn't blow it up with nitrous. Was this at the track, and if so what et and mph did the Malibu go? It would give an indication of what hp was going through that diff. I'm running slicks and probably 300-400 more lbs.
It's a shame they put those 7.5's in the B and even C body cars after 1976. My 6 cyl 74 nova had a 8.5 in it.

Sorry, I just re-read your reply, and nowhere in it does it say Malibu. Was it an Impala or Caprice? And the other car you mentioned, did it also have a 7.5?

I had the nitrous on a 79 Impala and only ran it on the street.  

The other one was a 77(?) Olds Omega street/strip car.  It ran consistent 13.2s @103mph.  

Thank you. This was the kind of real world information I was hoping to get. I think I will go forward with my mods and not drive my self nuts trying to find another diff. I'll post my results here.

If you put lower gears in the 7.5 w/posi. it will be a little better.

Now find the 28 spline axles and 28 spline positraction case to go with it.

Also use the 7.625 gears that came in Camaros. Stronger.

I built a "G" body axle and put the heavier setup to withhold the power.

terry cronk wrote:If you put lower gears in the 7.5 w/posi. it will be a little better.

Now find the 28 spline axles and 28 spline positraction case to go with it.

Also use the 7.625 gears that came in Camaros.  Stronger.

I built a "G" body axle and put the heavier setup to withhold the power.

When you say "lower gears", do you mean Higher numeric, like 4.11 vs 2.41? There has been a discussion at the track regarding this very topic. I favor the low (like 4.11) for less stress on the carrier and gear set. However with the increased torque multiplication, the stress on the axles must increase, but this does not seem to be the week point of the 7.5, as I always see people breaking gear sets and carriers.
My friend (who I borrowed the Q jet from) replaced the sb 400 in his 78 Malibu with a built up 350, and promptly smashed his 7.5. This incident might back up the low gear theory, as he still had the stock 2.29's in his diff.

On the street tires most rears usually hold up fine. The moment you put slicks and over 300hp to them they start to break.

It's usually slicks that cause rears to break although I have broken a few 8 1/4 Chrysler rears on street tires

but I abused them and they had high miles on them.

Myloth wrote:<snip> they had high miles on them.

I think you may have hit on something there. I've pushed the 7.5 hard and haven't broke one but they've been fairly low mileage.

Yes, my reference is to a HIGHER numerical gear.

Running street tires on the street and on occasion at the track you could survive awhile.

Adding slicks just changes the game.

I will add that I installed the studs in the 7.5 case bearings on the "G" body rear.

There is also an aluminum cover that you can get to add more support to the bearings with bolts that press against them.

All in all I have my 8.5 w/3.73 Auburn posi.. I dismantled it when torn down. Washed out the case and checked for damage.

So far its good w/synthetic gear lube. The 383 build will put it to the test.

Good luck!

More good stories and information.
If I was driving this car on the street on street tires I wouldn't be worried, because the tires will give, and you can feather the throttle. I would not have posted this topic if I wasn't running slicks, but at my track you can't hook without slicks. Myloft's rule of thumb of 300 hp and slicks agrees with what I have seen and heard at my track. I think you can break a 7.5 with less than 300hp if you shock the diff, that's why I plan to load my converter rather than stab the pedal. My 77 Malibu 454 would do 1.70's 60 ft times by stabbing the pedal off the line and really flaring that converter, but that was an 8.5 and they will take a beating. Terry Cronk I think your 8.5 will fare well with your 383.

For $200-300 that it costs for a 8.5 Posi from a Late B-Body to me it was a no-brainer. One of the first things I did was

get RoadMaster 8.5 Posi for $200. The install is easy my neighbor and myself did it in under 3 hours. I wanted the Posi

also for Winter nothing worse than a RWD car with an open rearend in the snow. 7.5s are fine for stock motor but if you

plan like me for big hp lose the 7.5. Another thing is those big 11" drums are really not bad if you can't find a Disc rear

and they are vastly better than the stock 9.5" drums. I stop better with stock 11" discs (good oversized pads) and 11"

drums than my significant other's Honda with all disc and abs. (And out weigh the Honda by 250lbs). Point is there is all

kinds reasons to swap rears and none to keep the 7.5 (especially with miles over 100k) just the $200-300 it costs.

Adding to that 8.5 rear, rebuild the drum brakes.

Then purchase the lighter 11 inch drums from Autozone.

Their only 17 lbs. versus the 23 lbs. stock. And a good replacement as per salvaging the heavy ones.

You will have to get different rims as the bolt pattern is 5x5.

When I rebuilt the one(92 Roadmaster diff.) I purchased, I went all the way with the brakes.

New springs, shoes, brake cylinders, drums, axles and lines. My original axle was the 8.2 10 bolt.

Myloth wrote:For $200-300 that it costs for a 8.5 Posi from a Late B-Body to me it was a no-brainer. One of the first things I did was

get RoadMaster 8.5 Posi for $200. The install is easy my neighbor and myself did it in under 3 hours. I wanted the Posi

also for Winter nothing worse than a RWD car with an open rearend in the snow. 7.5s are fine for stock motor but if you

plan like me for big hp lose the 7.5. Another thing is those big 11" drums are really not bad if you can't find a Disc rear

and they are vastly better than the  stock 9.5" drums. I stop better with stock 11" discs (good oversized pads) and 11"

drums than my significant other's Honda with all disc and abs. (And out weigh the Honda by 250lbs). Point is there is all

kinds reasons to swap rears and none to keep the 7.5  (especially with miles over 100k) just the $200-300 it costs.

The reason for keeping the 7.5 in there, is because it's in there. If I could get a 8.5 diff for $200-300 I would, but the B body station wagons have disappeared from our wrecking yards. The bigger brakes are unimportant to me because our track has a generous shut down area.

Those large drums have not a lot about slowing if you have a mile to stop.

Its called more inertia to spin. And weight your shocks have to handle also.

In this case less weight is better than heavier. And the same for stopping ability.

I didn't even have to change the master cylinder when switching from 7.5 to 8.5 drum brakes.

Good Luck!