Please help! GN 8.5 rear max h.p.???????

[deeloc1]

Hello everyone, i am new to the forum and i have a g-body elcamino. i am performing a LS1 engine swap and i have my eyes set on a GN rearend that i want to buy as opposed to buying a power robbing 12bolt or 9". I plan on making around 700+rwhp with N2O. Will a 8.5 Rear with 33spline axles hold up to that kind of horsepower abuse i'f i am running slicks and stalling at about 3800-4000rpms on the track with a 3500lb race weight car? i have heard that these rearends will take abuse through rumors...but i wanted to get answers from guys who actually have grandnationals who are running low 10 and 9 sec E.T.'s!!!!

i guess i'm really trying to ask how much can i throw at a built 8.5 gn rear end 33spline axles, spool etc...??? Any info will be greatly appreciated...thanks guys.

 

[deeloc1]

Come on guys...i just need a few honest, professional opinions!

 

[TURBOTWIN2]

Many are well into the nines with those upgrades.

 

[GNVAIR]

The can be built to support power, but the G body 8.5 10 bolt still has the same small diameter axle tubes as the 7.5.
The 12 bolt doesnt take anymore power to turn than the 8.5 (they are only .375" difference in ring gear diameter)

 

[HighPSI]

I have gone mid/high 8's with that setup for several years and have a customer who went high 7's.

 

[deeloc1]

The can be built to support power, but the G body 8.5 10 bolt still has the same small diameter axle tubes as the 7.5.
The 12 bolt doesnt take anymore power to turn than the 8.5 (they are only .375" difference in ring gear diameter)

that may be true...but where you loose power with the 12-bolt and 9" over the 10 bolt is weight of the entire rear. i have seen dyno numbers with 10 hp loss and about 8lbft of torque! i'm trying to squeeze every little horse power out of my aluminum ls1 while keeping my car as light as possible. imagine having a g-body around 2800 pounds gutted...with 700+ to the rear wheels with spray! my car doesn't weigh that now...but that's the goal. every pound counts!!!

 

[deeloc1]

what can a gn8.5 rear take with stock axles and upgraded posi and motive gears? the rear end that i am looking at is identical to this setup. i will only go to 33 spline with the spool if the stock axles can't handle 9's (E.T.).

 

[ijames]

On the old gnttype mailing list several posters said that the stock axles were good down into the 1.5 60' range but if you went lower they would give up pretty quickly. I wouldn't even try stock axles with your expected power, and besides, you really need to do some kind of c clip eliminator (either the bolt on ones or the weld on ford 9" ends or weld on gm a body ends) to be safe, anyway (much less nhra legal). The stock gears are great, they hold up better than the housing/axle tubes. Your cheapest newly-built 8.5 is going to be starting with an open rear with stock 3.42 gears, toss the carrier and axle bearings and axles, then do the c clip eliminators, axles, and carrier/posi/spool, in either 30 or 33 spline (33 would be best).

 

[norbs]

If you add it all up, your not saving that much over a complete 9" and sometimes in rare occurences the ring gear breaks, and could destroy your tranny and driveshaft instantly. Which will cost you more in the long run. Build it once and be done with it. A bolt in ford 9" will cost you $2300 approx. complete. How much money will the 8.5 cost you?


housing $400-500
spool $250
welding tubes $150 approx
axles with bearings installed approx $500
install kit $200 with labour to set up gears
gears $175
lpw brace and cover $250 approx

Comes up to about $2000

 

[VadersV6]

If you're open to a 9", you should consider an 8.8" ford rear end. The mustang aftermarket is so huge that theres almost no limit to what you can buy for them. Weld the tubes, throw on a girdle, and auburn diff if thats in the budget, and you'll be able to handle the power just fine as long as you have tough control arm bushings to keep the axle tubes from tweaking on launch, which throws major stress on the diff, the caps and the ring and pinion. But keeping the axle tubes solid is critical on any rear end. Make sure to get a newer model rear (96-04) since they have reinforcement ribs in the center section. The rear in my cobra came out of another cobra where he had switched it in from a GT (cobras have IRS garbage)...and all it had was 31 spline superior axles, A 31 spline diff out of a ford truck, with an upgraded spring and extra clutches, and it handled high 1.5's all day, and the axle tubes werent even welded and didnt have a girdle. This was with a manual transmission and 6500rpm launches. Add the girdle and weld the tubes, with some solid control arms, and it wont break for a LONG time. If you want a rear that can handle the most power with the least upgrades, the mopar rears are the strongest. A Dana 40 is stronger than a GN rear, and the Dana 60 is incredibly strong. But if you're stuck on the GN rears, the common mods will keep it together just fine...weld the tubes, good bearings, C-Clip eliminators, better axles, good diff and boxed lower control arms and upgraded uppers and you can throw anything at it.

 

[GNVAIR]

that may be true...but where you loose power with the 12-bolt and 9" over the 10 bolt is weight of the entire rear. i have seen dyno numbers with 10 hp loss and about 8lbft of torque! i'm trying to squeeze every little horse power out of my aluminum ls1 while keeping my car as light as possible. imagine having a g-body around 2800 pounds gutted...with 700+ to the rear wheels with spray! my car doesn't weigh that now...but that's the goal. every pound counts!!!

The entire weight of the rear is not going to affect the horse power to the wheels. No way, no how. I work for a shop with a dyno and 12 bolts have the same efficiency as a 8.5 10 bolt. The weight difference between the 2 is barely nothing as they are both integral rears with cast iron centers and steel tubes and weigh between 180 and 190 lbs. The 9" will require about 3.5% more power to turn because its hypoid style gear set which has the pinion located off center (or to the bottom) of the ring gear. The plus side of this is almost zero deflection thanks to 3 pinion bearings. Most integral rears use over hung mounted pinions with 2 pinion bearings. The over hung style rear is lighter and more efficient, but suffers for pinion deflection since it lacks the 3rd bearing behind the pinion gear. The 9" also weighs approximated 230-240 lbs with a cast iron center. Power wise, the 12 bolt will require zilch compared to the 8.5. The 8.5 and the 12 bolt both have the same diameter pinion stems, same basic bearing diameters. Only difference as I previously stated was the neglible difference in ring gear diameters and the axle tube diameters.

 

[GNVAIR]

If you're open to a 9", you should consider an 8.8" ford rear end. The mustang aftermarket is so huge that theres almost no limit to what you can buy for them. Weld the tubes, throw on a girdle, and auburn diff if thats in the budget, and you'll be able to handle the power just fine as long as you have tough control arm bushings to keep the axle tubes from tweaking on launch, which throws major stress on the diff, the caps and the ring and pinion. But keeping the axle tubes solid is critical on any rear end. Make sure to get a newer model rear (96-04) since they have reinforcement ribs in the center section. The rear in my cobra came out of another cobra where he had switched it in from a GT (cobras have IRS garbage)...and all it had was 31 spline superior axles, A 31 spline diff out of a ford truck, with an upgraded spring and extra clutches, and it handled high 1.5's all day, and the axle tubes werent even welded and didnt have a girdle. This was with a manual transmission and 6500rpm launches. Add the girdle and weld the tubes, with some solid control arms, and it wont break for a LONG time. If you want a rear that can handle the most power with the least upgrades, the mopar rears are the strongest. A Dana 40 is stronger than a GN rear, and the Dana 60 is incredibly strong. But if you're stuck on the GN rears, the common mods will keep it together just fine...weld the tubes, good bearings, C-Clip eliminators, better axles, good diff and boxed lower control arms and upgraded uppers and you can throw anything at it.

The 8.8 is internally almost a dead on copy of the 65-72 Chevrolet passenger car 12 bolt. I have even used 12 bolt shims on 8.8's when setting them up.
I have seen some local guys down into the 7's with them. Usually they will add housing braces as well as a quality rear cover like an LPW which has provisions for a rear brace. The 86-95 8.8's also have the webbing on the front where the damper weight is bolted. Heim jointed ends bolt up to these points and then tabs are welded to the tubs inside of the brake backing plates where the other end bolts. We have a local chassis guy that retubes the housings and adds the 9" housing flanges.

 

[norbs]

My moser rear weighs 155 lbs minus brakes. The aluminum center and locker are lighter than a posi. I almost think the 8.5 is heavier , but i never weighed it. I agree on the 3% power issue, but i can live with that for the trade of durability on 20 psi transbrake launches.

 

[GNeric]

The 12 bolt is the most efficient passenger car diff that GM put out. Look at the centerline of the pinion to the ring gear. The pinion gear doesn't have to "climb" the ring gear to transfer power. All other GM rears have the pinion lower and do rob a little hp compared to the 12 bolt. Hope this helps.

 

[Ormand]

Hypoid gears are very efficient. It's true the higher pinion in the 12 bolt makes it more efficient, but you're talking about something like 98% efficient vs 97.5%, not a huge diff. It looks big if you compare "inefficiency" or losses- then its 2% vs 2.5%, or a 20% decrease in inefficiency. But if you look at the total loss, it's NBD. Besides, if you really want that last hp, you can get a Ford center section cast to use the higher pinion. There are a couple of vendors that make them, and that would give the strength and convenience of the Ford, with the efficiency of the 12 bolt GM.

 

[bison]

8.5 with 30 spline Mosers and Eaton LS with a girdle has gone low 9's and low 1.30's without breakage. Race Jace ran low 1.40's consistently at BG this week and a best of 1.33 at another strip. His car is over 3500 lbs. Car runs 137+mph. Looks like about 680 RWHP. Jack cotton ran the 30 spline Mosers for a long time a few years back running low 1.30's on them.