8 1/4 rear....what HP limts

how much power will an 8 1/4 rear take...3000 lb car with 582 HP

582 hp is fine so long as it never gets any traction, BUT if it does then that is more than enough to destroy it.

You need at least an 8-3/4 axle with that amount of power.

I destroyed an 8 1/4 with a small cam in a 318 with an auto trans in about 6 months in my 35 Dodge street rod. The 3:55 Sure Grip, 235*60*15 tires, and traction proved too much for the carrier bearing bolts. In low gear, standing start, wot, and BOOM!

The 8 1/4 came out of a 73 340 auto Duster with 55K miles on it that I converted to a dirt track car. We had an 8 3/4 set up for racing we used in the Duster. Gene

I agree. Unless you're slicks or similar and just driving on street tires and smoking them once in awhile you'll be fine. Currently I have one with close to 500hp and over 500tq and it's been fine for quite awhile. 3.21 gears. I've had three HP 360's with 8.25 rears and street tires with sure-grip and they held up fine for thousands of miles.

Have a 8 1/4" rear that has survived fine on the street between a somewhat stupid 340 with a 3000+stall converter and a set of hoosier quicktimes. It may have lived on borrowed time but might also have alot of life left in it.

Maybe one of those cast covers with the bolts that apply pressure to the back of the caps would make it live longer. I have a 8-1/4 with posi and 430 gears that I've been waiting on a car to put in, maybe something will turn up.

I agree also. Mine is behind a 470" 4spd(500+ HP) It's a street toy running 245-60/15 BFGs. They don't stand a chance but the diff has been fine for a few years. If I don't put sticky tires on it I suspect it'll live a while. 3.73 gears with a sure-grip.

I run one in my Dart. Do not know HP but I run 11.5 on slicks, 3500 stall. No problems at all. 4.10 gears. I do have the aluminum cover with the bolts for the carrier bearing caps. 3000 lb

Depends on how much maintenance you're willing to do on it. Dunno if it's still true but there used to be MANY NHRA stockers that ran the 8 1/4 because they were the lightest and those guys were able to make them live. They had a specific need to run that rear - if you do not have to run that rear, there are cheaper alternatives with less maintenance.

Billet caps and longer bolts will go a long way towards making many rear ends stronger.

Do you have a source for those covers with the bolts to support the carrier caps? I did a quick search but didn't stumble across any for the 8 1/4.

I found these thanks to that post

http://g2axle.com/pages/view/torque

Until i read this thread i didnt have a clue those covers where available and nice to see that they are made in the USA:D
One of those are on my to buy list now!

This is the cover I use. Nice quality. Highly recommend.

or this one
http://www.jegs.com/i/LPW/619/301-10C/10002/-1

Dudes, the original question is so bogus! The only HORSEPOWER limit on a rear end would be the horsepower that would generate enough BTUs to cause the rear end to overheat. Example: NASCAR, where the Ford 9" they used required an oil cooler of its own. I don't know what they are doing nowadays.

What the OP should have been asking is how much TORQUE the rear end can tolerate. That's how axle manufacturers rate their axles, torque capacity.

Shock loads are also important and I think there's usually a factor for shock loads included in the calculations.

Why not use horsepower? Because an engine with rated horsepower at 5,000rpm will produce roughly 3 times the torque as an engine producing the same max hp at 15,000rpm.

R.

Thanks!! I did stumble on that one....agree,love the fact it's USA made! It's now in my ebay cart. Thanks guys!!

Great, something else I've just got to spend money on!!

However that 15000rpm peak hp car would probably use 3 times as much gear ratio and at that point just as much torque would be trown at the axleshafts anyway and at that point you are back to squareone again.

You're overanalyzing my example.

My point was simply to illustrate that you cannot tell an engine's torque output, from its horsepower rating.

Anyway, we weren't talking about axle shafts. The weak point in most hypoid gear axles is the intersection between pinion and ring gear teeth.

As the torque applied increases, the force trying to push the gear teeth apart from each other increases. The gear teeth then become loaded closer to the tooth's tip which increases the moment on the tooth's attachment to the gear. Result is the tooth breaks off. The reason the Ford 9" can take so much torque is its pinion support. There is another smaller bearing surface behind the gear teeth and thus the pinion is supported on both ends, lending stability to the pinion. It's harder for the pinion to move away from the ring gear.

I saw a picture recently that showed the pinion sheared off right behind the inner bearing. While this looks at first glance to be the engine twisting the pinion shaft too hard. My theory is that it's a fatigue crack that was stated by the pinion moving away from the ring gear through whole revolutions of the pinion. The metal fatigued and eventually snapped with a brittle fracture. This is the same idea used to fatigue test metals.

R.

If you concerned about the strength of an 8 1/4" rear, step up to an 8 3/4" because you'll be spending extra to strengthen it. Same philosophy when concerned about an 8 3/4" - move up to a Dana 60.

I know of some 8 1/4 rears that have been used in 3,600+ lb cars running mid-high 11's in the Stock class. But they spent a lot on them and at least one did eventually upgrade to a Dana 60.

Weight and gear ratio are also big factors in durability. The lower the ratio (higher numerically), the weaker the teeth because they are smaller. Between an 8 3/4" and Dana 60 - it's the gears that are the weak link in the 8 3/4. In many cases they both use 35 spline axles. Not only is the same ratio in a Dana 60 stronger, because it's bigger, but it's also more efficient because of the pinion position in relationship to the ring gear.

I would think that the ring gear would be the weak link in an 8 1/4" rear, but the caps wouldn't be far behind. Same with an 8 3/4'. I've broken some 8 3/4" ring rears over the years, but no caps (knock on wood). I am looking at a Dana 60 for peace of mind.

Even if it's built "right", it's not just how much power/torque the rear will handle, but for how long. And if you're not sure what the weak link is in the rear you choose, it can eventually show you.

Geez , I just wanted to know how much.MOTOR- POWER- TORQUE-SNOT -an 8 1/4 rear would take....Sorry I used the wrong word ...guess I should have said torque, but I thought all you gear heads out there would get the message..Car is 3000 lbs. motor is a 493 stroker.never been dyno tested..moved a 3600 lb car with 3;55 gears. with poor tracion .. 1.67 60 ft. to 11.35 at 119.5..now that motor is going in a different car that has 8 1.4 rear..foot brake 3000 rpm stall.. just wanted an opinion on rear life.. thanks everyone for your input..

The simple answer is a 493 will kill an 8-1/4 in a short time, especially if it dead hooks, or spins then hooks.

Hope this helps, my 64 Plymouth with an almost dead stock LC 440 (509 cam) and a 3800 vert, 14-32 Firestones 4.88 gear ripped the teeth off an 8-3/4( all of them) with Strange axles and spool in 25 1/8 mile runs. I seriously doubt an 8-1/4 would've lasted that long. There are many factors in the equation, but tires, weight and RPM launch are the main ones you need to look at. Run it till it breaks (it will) then step up to the Dana and never think about it again. J

Spend all you can on a 8.25 and it will not be as strong as a stock 8.75. Break it and take the trans out also.

I agree

Don't waste your money trying to make either an 8 1/4 or 8 3/4 live. Even a stock Dana 60 will out last them and probably for less money in the long run. Break a rear end and replace your slush box.

But if you already have an 8.25 like some of us, and don't plan on racing it with tons of grip, it will last, so why spend on big bucks on the initial purchase of an 8.75 and doing the swap?

The reason I jumped all over you for saying horsepower is probably because in so many cases the "hp rating" method is used when it isn't horsepower that matters. Case in point: Connecting rods, where it's the rpm and the piston mass that cause failures not traceable to detonation. Transmissions and rear ends transmit torque and that's how they should be rated. So it's a pet peeve of mine, hence the reaction.

Now on to the question that it seems likely you wanted an answer to: Will an 8 1/4 live behind a 493? not very likely. Chrysler put the 8 3/4 behind 340s, telling me they didn't trust the 8 1/4 for that torque level.
The only stock similarities I can find is the mid '70s Cadillacs with their 500 inch motor. The 472 came out in '68 and I think they were using an 8.5 rear although it could have been an 8.2".
By 1970 they were using the GMC 8 7/8 truck differential. My conclusion is that there was enough trouble with the axles to justify a switch. Those engines had gobs of torque and a 5000lb car to accelerate.

I think you built the 493 to use, and you will press down hard on the gas pedal from time to time. It'll just take one time for the rear tires to hook solidly and the rear will be broken.

IMHO a bigblock stroker deserves a Dana 60.

R.

One other thing, you break an axle and the tire-drum might pass you going down the road, nothing to keep it under the car except your quarter panel when it breaks.

A 8.25 rear end will last for long time with up to 300 rear wheel horsepower The torque of the 493 you have will kill it in short order if running slicks or 11.5 inch drag TA's like I did but up until then I had close o 800 runs including two trans explosions that took out trans and drive shaft split in two my 8.25 survived all that and broke the spider gears Open rear end Fixed it in parking lot and drove it home