GN 8.5 w/discs?

[Bonnewagon]

I'm thinking of going to rear discs and I have 7.5 rear now but an 8.5 is available to me. I was wondering if since the GN 8.5 rear is very similar to a 2nd gen "F" body rear, could you swap on a set of rear discs from like a 79'-81' TA? Or does the GN rear have that Chevy style bolt pattern with 2 bolts and the big spring clip? I haven't seen many and can't remember. The TA caliper mount needs the 4 bolt type flange I think. I remember Hot Rod mag did an article on a"C" clip eliminator kit and they drilled the 2 other bolt holes, so if that's possible then I figure I could drill holes for the caliper mount. I'm not sure if the axles are the same length either, but I know the disc axles are cut down to fit the rotor and the drum axles won't work unless cut also. Does anyone have any knowledge of this swap or am I barking up the wrong tree with this? Or does any other GM rear disc bolt up? Or do any other rear discs fit the 7.5 rear, say from 3rd or 4th gen "F" cars?

 

[pontiacgp]

You use the same brake backing plate on a 7.5 and 8.5....you can do a conversion to set up the backing plates for the disk brakes by cutting of the top part to make it sorta square and there is enough meat to drill two other holes for the top holes on the backing plate. If you want to install the parking brake, and I suspect you will since your running a stick, you have to move the shock mount to make room.

here's a good link that'll show ya better than I can explain it..

http://www.thirdgen.org/ls1reardisc

 

[turbo78]

You use the same brake backing plate on a 7.5 and 8.5....you can do a conversion to set up the backing plates for the disk brakes by cutting of the top part to make it sorta square and there is enough meat to drill two other holes for the top holes on the backing plate. If you want to install the parking brake, and I suspect you will since your running a stick, you have to move the shock mount to make room.

here's a good link that'll show ya better than I can explain it..

http://www.thirdgen.org/ls1reardisc

good useful info seeing that im also thinking about doing the rear disc conversion on my car so i can get more clamping power for stalling on the line.... thanks pontiacgp 😀

 

[pontiacgp]

if you want to do a quick cheap upgrade to the drum set up you can put S10 non power brake cylinders. I was told by a shop that does alot with GN's that they can get the boost up to 8 lbs before the back wheels would break loose with that set up. With the stock set up they said the wheels would break free at 4 lbs. You can also use two primary shoes on each side to improve the braking...but that being said disk are still better.

 

[Bonnewagon]

Thank's for that link GP, very clean swap it looks like. Yes, I want discs, I'm tired of dealing with drums. So I can stay with the 7.5 that makes life easier too. That guy used 4th gen discs, I am familiar with those, my kid's car has them and I was impressed with how easy they were serviced.

 

[turbo78]

if you want to do a quick cheap upgrade to the drum set up you can put S10 non power brake cylinders. I was told by a shop that does alot with GN's that they can get the boost up to 8 lbs before the back wheels would break loose with that set up. With the stock set up they said the wheels would break free at 4 lbs. You can also use two primary shoes on each side to improve the braking...but that being said disk are still better.

i dont know about the GN guys brake boost problems, but i could brake mine to 10 and the tires would just explode with an instant smoke show while pushing through the lights. i tightened up the rear brakes so much they drug and it slowed the car down a tenth. so i figured a pinion brake would work but nhra has something against them(dont know why) so i figured disc conversion would work best.

 

[Bonnewagon]

Don't you guys use trans brakes? My friend has one in his T-350 and he said it engages first and reverse at the same time so the car can't move, then he hits a button and releases reverse and off she goes.

 

[turbo78]

no, i dont use a transmission brake. im a footbraker, and i dont want to buy a ton of really expensive parts to would use a t-brake. but without one, beings that i am boosted, parts have a much longer lifespan, and traction isnt such an issue when you launch soft and it rips your head off at about 80 feet out.

 

[axisg]

I always figured that disks don't have the clamping force that drums do. For "holding" a car in place drum brakes I would figure are better as they contact more of the surface area ( hence the use of S10 wheel cylinders to get even more clamping force for the foot brake race guys ). As proof the parking brake on modern rear-disk cars use a drum-in-hat setup so they can use the drum portion just for the parking brake as it holds better than disks.

The downfall of drum brakes is they cannot displace the heat as well as disk brakes and overheat quickly leading to brake fade.

 

[pontiacgp]

I never thought about it that the static braking of a drum brake would be superior to a disk brake set up but it does make sense...