How much power can an 8" really take ?

ultrastang said:

The saying I always heard was; someone (insert name) could tear up un anvil with a Q-tip.

In it's stock form, the 8-inch rear end might take 350-400 hp, but it wasn't designed to do so. You may get away with it for some time, but sooner or later at those levels, it's time will be up.

If the factory 8-inch rears were capable of handling 400 hp, then why didn't Ford use them in the K-code 289 Mustangs or the Shelbys? The K-code was only putting out 271 hp (129 hp less than 400) and the Shelbys were only generating 306 hp (94 hp less than 400). The reason is because the Ford engineers knew the 8-inch wasn't up to the task of reliably handling the power from even a 271 or 306 hp engine.


The installation of some aftermarket 8-inch rear end parts may beef things up, but it will most likely only put the 8-inch to the level of a stock, run-of-the-mill 9-inch.

Many people want to increase the power of their engines, and usually the first line of attack is to start putting performance parts on/in their engine. Maybe I'm backwards, but I think the first place to start hopping a vehicle up should be with the brakes, suspsension, and drivetrain --then the engine.

Most early Mustang stock brakes and suspension are marginal at best with the factory-installed engines they came with. Add more power to the engine, and you are making a bad situation potentially worse. Going fast is fun. But, all the go-fast-in-the-world it isn't worth anything if you can't control the vehicle, or more importantly --STOP IT!

In the end, the arguments on the subject of 8" vs. 9" will never end. I've seen it going on for years now on the forums. Personally, since I never plan to run a factory-stock small-block, I'll continue to use 9-inch rears, because I know that it will handle any power level that I would ever attain with the engine, and not only will it handle it, it was engineered to do so.

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That is the absolute best response to this issue I have ever read!

I suppose I am one that can break an anvil with a Qtip!
I sure ain't one of the lucky ba$tards! I went to a casino last night, first time ever, and lost my a$$.

I never thought about the whole K-code analogy...
Not only did Ford NOT put an 8" behind the 271 fwhp engine (roughly 215 rwhp), but they took the time and expense to custom build a 9" that was not in inventory. They made them by grafting a 9" center into an 8" rear as a stop gap measure. Car companies just don't spend that kind of effort for no reason.

Great post.
Dave

my opinion is that the early pre-67 8" rears are junk and the later 67-up 8" rears are ok for up to and no more than 400hp at the flywheel behind an automatic closer to around 350 with a stick. if you don't plan on ever exceeding those limits then an 8" rear will be fine, though a 9" would be better. i would never even consider using an 8.8" in a vintage car period, i absolutely hate C-clips axles and that's one of my main reasons for disliking gm products as much as i do but not the only reason of course.

the versialles 9" is heavier than a regular 9" mostly becuase of the huge over engineered caliper mounting brackets but it also has the stronger tapered housing like what was used in trucks rather than the weaker mustang and midsize car housing. all of the aftermarket companies that build 9" rears use the same style housing as the versailles rear. yes, the versailles discs are a little outdated but they are still very functional when properly maintained and are better than the stock mustang drums any day. i sold the one i built for my car a few years back for $900 bucks and that was a completely detailed rearend with 3.70 gears, new rotors, rebuilt calipers and all new bearings and seals. i really wish i hadn't needed the cash and had not been forced to sell it becuase it would have made a great rearend for my car but such is life.

my next car is either going to have one of ultrastangs rear disc kits or one of the vintage venom rear kits with the vented crown vic rotors with the internal drum parking brake. i still have not made up my mind yet on which way i'm going with this one. i do plan on pulling a small boat behind it on occassion so i'll probably go with the CV style kit for the better parking brake but it's still open really.

anyway, my thoughts are if you're going run big power, big block or stroker small block with slicks or a manual trans go with the 9" rear, if you are not planning on huge power and just want something a little better than stock, power wise, a well built 8" will suffice provided you use the stronger 67-later pumpkin.

ratio411 said:

That is the absolute best response to this issue I have ever read!

I suppose I am one that can break an anvil with a Qtip!
I sure ain't one of the lucky ba$tards! I went to a casino last night, first time ever, and lost my a$$.

I never thought about the whole K-code analogy...
Not only did Ford NOT put an 8" behind the 271 fwhp engine (roughly 215 rwhp), but they took the time and expense to custom build a 9" that was not in inventory. They made them by grafting a 9" center into an 8" rear as a stop gap measure. Car companies just don't spend that kind of effort for no reason.

Great post.
Dave

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Thanks, Dave. I'm just trying to look at this logically and calling it like I see it. Nothing more, nothing less.

The argument will be that the revised '67-up 8-inch cases are far superior to the weaker '62-'66 model 8-inch cases --and they are, but my reasoning again goes right back to the factory small-block Hi Po engines. The 271 hp Hi Po 289 was still available in the '67 model Mustangs, and Ford still did not use the (new-for-'67) 8-inch with the revised 8-inch cases in them. --They came with 9-inch rears.

The '68 small-block Shelbys came with enhanced 302s, and again, no revised-cased 8-inch rears were used in them. They came stock with 9-inch rears. If the '67-up revised-cased 8-inch rears were engineered to reliably handle up to 400 hp, why didn't Ford use them instead of the 9-inch? It goes back to what something can be subjected to vs. what something can be subjected to reliably/consistantly.

I'm also confused by the reasoning of building a hopped up small-block, or more specifically a stroker, as long as the 8-inch in the rear isn't preceeded by a manual transmission or with a set of sticky tires/slicks(??). In the case of a stroker, they are not cheap to build. Why would someone go through the expense of building a powerful engine like that only to reason that it will be ok as long as the tires are free to spin, and as long as the tires don't hookup, the rear end will survive (??).

If it were me, and I was going to go to the trouble and expense of building a stroker, I'm for sure going to spend the money to upgrade the suspension and tires where the car WILL hookup and allow me to get the benefit out of all the expense I put into the engine. If all I wanted the tires to do is spin, I could just leave a stone-stock small-block in it, without drainging my pocketbook in building a powerful engine that can't get the power to the ground.

ultrastang said:

Thanks, Dave. I'm just trying to look at this logically and calling it like I see it. Nothing more, nothing less.

The argument will be that the revised '67-up 8-inch cases are far superior to the weaker '62-'66 model 8-inch cases --and they are, but my reasoning again goes right back to the factory small-block Hi Po engines. The 271 hp Hi Po 289 was still available in the '67 model Mustangs, and Ford still did not use the (new-for-'67) 8-inch with the revised 8-inch cases in them. --They came with 9-inch rears.

The '68 small-block Shelbys came with enhanced 302s, and again, no revised-cased 8-inch rears were used in them. They came stock with 9-inch rears. If the '67-up revised-cased 8-inch rears were engineered to reliably handle up to 400 hp, why didn't Ford use them instead of the 9-inch? It goes back to what something can be subjected to vs. what something can be subjected to reliably/consistantly.

I'm also confused by the reasoning of building a hopped up small-block, or more specifically a stroker, as long as the 8-inch in the rear isn't preceeded by a manual transmission or with a set of sticky tires/slicks(??). In the case of a stroker, they are not cheap to build. Why would someone go through the expense of building a powerful engine like that only to reason that it will be ok as long as the tires are free to spin, and as long as the tires don't hookup, the rear end will survive (??).

If it were me, and I was going to go to the trouble and expense of building a stroker, I'm for sure going to spend the money to upgrade the suspension and tires where the car WILL hookup and allow me to get the benefit out of all the expense I put into the engine. If all I wanted the tires to do is spin, I could just leave a stone-stock small-block in it, without drainging my pocketbook in building a powerful engine that can't get the power to the ground.

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specifically, i said that unless you build a stroker which makes gobs or torque an 8" will be ok, if you have a stroker motor then DON'T use the 8" rearend, you'll grenade it in a heartbeat.

i don't believe ford offered a traction lok diff for the 8" in the years you mentioned for the shelby and hi-po cars which probably had more to do with why they chose the 9" rear as opposed to the 8" for those cars, not so much as the rear wasn't stronh enough to handle it

bnickel said:

specifically, i said that unless you build a stroker which makes gobs or torque an 8" will be ok, if you have a stroker motor then DON'T use the 8" rearend, you'll grenade it in a heartbeat.

i don't believe ford offered a traction lok diff for the 8" in the years you mentioned for the shelby and hi-po cars which probably had more to do with why they chose the 9" rear as opposed to the 8" for those cars, not so much as the rear wasn't stronh enough to handle it

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Brian,

I don't want you to think I was trying to attack you (or anyone else) or anything like that. There was a notice sent directly to my email when Ratio411 posted, and I made a reply to Dave's message before I even read your post. I was just simply stating my take on the applications the 8-inch rears are being used in. It was just my opinion, and had nothing to do directly with you or any other person.

In regards to the differentials, the Ford 2-pinion Limited-Slip differential was available at that time, and used in both 8- & 9-inch rear ends in both small or big-block applications. In 1969, Ford revised the differential with some different internal parts and changed the name to Traction-Lok. The Traction-Lok was available in both 2- & (stronger) 4-pinion arrangements.

no problem, i didn't think you were meaning me specifically i just wanted to clarify what i said.

i did not realize the equa-lok was available on the 8" rear but even so it was a weak POS. never been much of a fan of the the equa-lok diff. i really should have said the detroit locker wasn't available on the 8" (to my knowledge) and that was really considered bad form back in those days to offer a performance car with no option for a locker like the detroit locker.

the 8" rear is pretty weak in stock pre-67 form but with the better 4 pinion traction lok diff or even better a detroit locker it will handle quite a bit of abuse. the 8" is very similar to a gm 10 bolt in terms or torque capacity though the 10 bolt did have a slightly larger ring gear (either 8.25" or 8.5" depending on year and brand line) and the 10 bolt can take a lot as well but it's still not as good as a 12 bolt.

i guess what i'm really trying to say is that for most mild street buildups the 8" can work just fine but if you really want to run it hard and have lot's of power you'll need a 9"

8" here with Powertrax No-Slip Locker and 3.55's...

Motor mods in sig...

Lots of hard driving...

So far it's held up...



*Knocks on wood*


Funny thing is that I have a 9" housing and 3rd member in the garage, but after finding out it had a cracked diff casing, and pricing out getting it modified to fit my car with the new brakes it turned out to be a rediculous amount of money.

I just wanted gears and better traction, and I got it. So far so good.