Max power 8 3/4 rearend

[gitrdone73]

Does anyone have an idea of how much power an 8 3/4 rearend can handle? The reason I am asking is because I want to get the most power out of my car without expensive upgrades.

[deputycrawford]

     Put those words into a search of this site. You will find your answer if you read up on old posts. Then buy a Manciniracing.com rear end brace, have it welded on and don't worry about it. It should handle around 500 to 600 hp even if it dead hooks. Just make sure to ditch the crush sleeve in the pig and use a solid spacer with shims.

[firefighter3931]

The 83/4 will live behind big power as long as it doesn't dead hook. Even with slicks behind a 600hp combo it will live for awhile but will eventually break....if you're planning to race the car with slicks then it's Dana time. If it's gonna be running street tires then you're fine with the 83/4. 


Ron

[aifilaw]

drive line and mechanical components are measured in torque.

The 8-3/4 is rated at 750 ft lbs of torque. The vast majority of mechanical components are rated at 100% of 150%. Which means an 8-3/4 in good condition should be able to withstand 1,200 ft lbs of torque.

Which usually begs the question from someone who has put a 500 HP engine in front of a 727 with a 4000 stall converter and broken an 8-3/4... well.... my engine only produced 450 ft lbs of torque.. why did it break?

a torque converter has the added benefit of a multiplier, which means that for a short time period (0.1-1 second usually) of stalling it up and them launching you are multiplying the current torque your engine is producing at that rpm. This is also referred to as the STR rating.
Typical high end expensive and well-built torque converters will get up to 3. End result being, if they launch a 4k converter at 4,300 rpm, which is their max torque from the engine at 450, consider the loss through the transmission and driveshaft that's 1350 minus 10% is right above 1200, there's why its breaking.

the tires dead-hooking is another added factor that breaks axles as well

[deputycrawford]

Great. You mean my braced rear end and solid spacered pig will probably puke no matter what? I did not want to hear that. I will have to remember to hold off on the adivse for awhile and learn a few more things. I have seen no problems with the braced rear ends yet but I guess its only time. Thanks for setting me straight.

[aifilaw]

I made no mention of after market additions to increase the strength of a differential. So I have no idea if you are being facetious with the response or not. I am simply transferring information. An 8-3/4 rear differential should be capable of handling 400 ft lbs of torque in front of a 727 with a high torque converter on a dead hook over and over and over again. It should be able to handle an 833 launch with 500 ft lbs of torque just as easily. Beyond that, and I would upgrade.

[firefighter3931]

Great. You mean my braced rear end and solid spacered pig will probably puke no matter what? I did not want to hear that. I will have to remember to hold off on the adivse for awhile and learn a few more things. I have seen no problems with the braced rear ends yet but I guess its only time. Thanks for setting me straight.

I wouldn't worry about your 83/4 it'll be fine. The problems start when you run a transbrake or 4 spd....then the shock load gets intense. It'll take a long, long time and many passes to break an 83/4 behind an auto trans car with anything less than a 500ci stroker. Heck, the old max wedge cars ran the hated small pinion 741 case center with excellent results....that's all that was available back in the day. 

A friend bracket races a heavy (3800lb) Cuda running a stout 440, slicks, Caltracs, 3800 stall and 4.10's. The car runs low 12's high 11's and has probably near 1000 passes on the original housing, centersection, axles. The ring and pinion and axle bearings and SG unit were changed and it has a solid spacer. He did break the factory 7260 (small) yoke and replaced it but otherwise the rear is "as delivered". The car 60ft's in the 1.50's and will occassionally lift the driver side tire off the ground a few inches....it hooks hard. 

If the tires spin, even behind 800hp....it won't break.....good traction +big power+heavy weight is the deal breaker. 



Ron

[gitrdone73]

Ok thanks guys you've answered my question and then some. So yeah I all I want is to have about 500 hp and to use the car as a street cruiser.

[firefighter3931]

Ok thanks guys you've answered my question and then some. So yeah I all I want is to have about 500 hp and to use the car as a street cruiser.

The 83/4 is fine for that application....no worries there ! 



Ron

[SeattleCharger]

drive line and mechanical components are measured in torque.

The 8-3/4 is rated at 750 ft lbs of torque. The vast majority of mechanical components are rated at 100% of 150%. Which means an 8-3/4 in good condition should be able to withstand 1,200 ft lbs of torque.

Which usually begs the question from someone who has put a 500 HP engine in front of a 727 with a 4000 stall converter and broken an 8-3/4... well.... my engine only produced 450 ft lbs of torque.. why did it break?

a torque converter has the added benefit of a multiplier, which means that for a short time period (0.1-1 second usually) of stalling it up and them launching you are multiplying the current torque your engine is producing at that rpm. This is also referred to as the STR rating.
Typical high end expensive and well-built torque converters will get up to 3. End result being, if they launch a 4k converter at 4,300 rpm, which is their max torque from the engine at 450, consider the loss through the transmission and driveshaft that's 1350 minus 10% is right above 1200, there's why its breaking.

the tires dead-hooking is another added factor that breaks axles as well

Ya, that's what I was gonna say.   

[deputycrawford]

Thanks firefighter. Thats conforting. I have some cars run many runs with the 8 3/4. If I find the breaking point then you can  . Oh, and Seattlecharger, too funny.