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It should live fine behind 380 horsepower. You just don't want to spin the one wheel down the block or you will fry things. I had a Chevy truck with a posi rearend that was basically a glorified Chevy 10-bolt. My son blew several ring and pinion gearsets out of it and I finally found out why. He was spinning the tires up in water puddles and annihilating the rearend when the tires finally caught traction. Abusing the BOP 10-bolt will cause it to fail, but it can take a lot of horsepower if treated properly. My 10-bolt survived behind over 500 horsepower and a 4-speed, but I always rolled the throttle on and never power shifted.
 

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What's the gear ratio? I ask because I had a 2.56 open in my near 400 HP 67 for over 40,ooo miles. It was just fine all that time. From a dead stop and straight take-off, the one tire might or might not spin depending on the season, tire quality, road surface, and if it was brake torqued or not. Overall, a nice set up so long as a good launch was not expected. However, the gas mileages was fantastic for a GTO...at 55 mph she would get 17 or better..AND, I could take it up to 50 plus mph in first and get a good chirp going into 2nd. It was a heck of a good 1/2 mile car! Near 140 mph before it got to floating so bad that, at 18 years old, I had to let off.

Taking off heavy and turning at the same time would yield a single-wheel-peel every time, but without safety track, the car would stay in control compared to the same efforts with a posi when the back end would want to come around. SO, it's kind of a safety feature...UNTIL your in the rain on a corner (at near any speed) and you goose it...then the balance of the car and the forces all align (like evil stars) and both tires spin...then hold on and find out how good your recovery skills are. It can catch you off guard and you end up facing the opposite direction or hitting a Ford LTD square in the nose. Don't ask how I know this. lol

So, if your gears are about 3:1 or less, you don't drive like a nut, and gas mileage is important...you're good. Oh, an open in the snow with stock-ish GTO horse power is just stupid.

I will not be putting an open back in mine. I prefer to get better launches, not concerned with the mileage, and I like knowing that it will come out in the back when pushed every time (rather than only occasionally which is a surprise scenario). An open gear set more than 3:1 would be foolish to my way of thinking. You'd always be rotating your tires to keep that one from disappearing and leaving you with 3 good tires. That was a problem with my original set-up let alone a deep ratio. 380 HP with the expected torque calls for a posi ratio of 3.2 to 3.5, thereabouts IMO.
 

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Cameo Ivory 1967
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thats awesome auto or 4 speed what gear ratio ? can you take off fast from a stop ?tks
It's a Tremek TKX 5 speed, wide ratio, 327 first gear, which is much lower than the stock TH400, and 3.36 gears.

I usually put 100 - 200 miles a week on the car, and they're not easy miles. Yes, I do MANY burnouts, 1st-3rd usually.
 

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Discussion Starter · #9 ·
What's the gear ratio? I ask because I had a 2.56 open in my near 400 HP 67 for over 40,ooo miles. It was just fine all that time. From a dead stop and straight take-off, the one tire might or might not spin depending on the season, tire quality, road surface, and if it was brake torqued or not. Overall, a nice set up so long as a good launch was not expected. However, the gas mileages was fantastic for a GTO...at 55 mph she would get 17 or better..AND, I could take it up to 50 plus mph in first and get a good chirp going into 2nd. It was a heck of a good 1/2 mile car! Near 140 mph before it got to floating so bad that, at 18 years old, I had to let off.

Taking off heavy and turning at the same time would yield a single-wheel-peel every time, but without safety track, the car would stay in control compared to the same efforts with a posi when the back end would want to come around. SO, it's kind of a safety feature...UNTIL your in the rain on a corner (at near any speed) and you goose it...then the balance of the car and the forces all align (like evil stars) and both tires spin...then hold on and find out how good your recovery skills are. It can catch you off guard and you end up facing the opposite direction or hitting a Ford LTD square in the nose. Don't ask how I know this. lol

So, if your gears are about 3:1 or less, you don't drive like a nut, and gas mileage is important...you're good. Oh, an open in the snow with stock-ish GTO horse power is just stupid.

I will not be putting an open back in mine. I prefer to get better launches, not concerned with the mileage, and I like knowing that it will come out in the back when pushed every time (rather than only occasionally which is a surprise scenario). An open gear set more than 3:1 would be foolish to my way of thinking. You'd always be rotating your tires to keep that one from disappearing and leaving you with 3 good tires. That was a problem with my original set-up let alone a deep ratio. 380 HP with the expected torque calls for a posi ratio of 3.2 to 3.5, thereabouts IMO.
 

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I'm running the original 10 bolt in mine behind a 461 that is somewhere around 500hp with a 4 speed. Gear ratio is 3.23 with an open diff. No issues so far. That being said, I've only been running that set up for a bout a year (1300 miles) and I don't drive quite as hard as Amy does. Time will tell how this goes. I figure I'll upgrade the driveline as things break/ wear out.
 

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so if i throw a posi in the 8.2 with hiway gear ,don,t know what i have yet,and with skinny tires you think i be allright ....?
Posi highway gears and skinny tires are a safe way to go. The gears are high...this keeps the tire-spinning on the lesser side, requires less tire traction, if the skinny tires do spin - there is less chance of a hook-up that results in damage, yields better gas mileage, and the posi gives better traction in the rain and gravel. The highway geared rear-ends last longer than the deeper gears since they cannot be abused as much.

BUT,, it all depends on your goals. The above benefits of a highway geared posi is not in my goals, but could be great for someone with different driving habits. My old highway set-up was a lot of fun to drive, great at 45 to 85 mph, but not so fun off the line. I'm changing up and going to 3.2 to 3.5:1...eventually.

The only trouble I had with my old 2.56:1 was with brake torquing to get some tire spin (showing off)...brake torquing is NOT good for anything, but breaking things. With deeper gears, the urge to spin the tires come naturally without brake torquing.
 

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Cameo Ivory 1967
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so if i throw a posi in the 8.2 with hiway gear ,don,t know what i have yet,and with skinny tires you think i be allright ....?
There are too many variables for me to say "if you'll be alright". I don't know how you drive or what the condition of your car is...

But I can tell you this; I have a 67 GTO with 3.36 gears and and Auburn Posi. I built the rear myself in my driveway with the car on jasckstands. I have my rebuilt 400 making at least 400 hp, and I do burnouts all over the damn place. I have 235/60/15 tires... Which arent exactly skinny... Standing still, roll-on's, donuts, drifting... I roast the tires, but the car also hooks and goes!
 

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Discussion Starter · #14 ·
thats awesome auto or 4 speed what gear ratio ? can you take off fast from a stop ?tks
There are too many variables for me to say "if you'll be alright". I don't know how you drive or what the condition of your car is...

But I can tell you this; I have a 67 GTO with 3.36 gears and and Auburn Posi. I built the rear myself in my driveway with the car on jasckstands. I have my rebuilt 400 making at least 400 hp, and I do burnouts all over the damn place. I have 235/60/15 tires... Which arent exactly skinny... Standing still, roll-on's, donuts, drifting... I roast the tires, but the car also hooks and goes!
 

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Cameo Ivory 1967
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i do think the 67 400 335hp came stander with 3.36 i could be wrong ,it mite be a good gear to have..?
3.36 is considered the best compromise gear. It's not terrible on gas, and not terrible on performance... but it's also not great at either.

I was in a bad spot, because I do a lot of parkway cruising at 80 MPH, and I was taching 3500-3800 rpms with them... so I couldnt put anything more aggressive in there.

ANDDDDDDDD at the same time, it wasnt a rocket off the line either, but I did have a few unsorted performance issues.

Yes, 3.36 was standard without air. Yes, Im glad I kept them!

With the wide ratio TKX, I have a 1st gear of 11, and with the overdrive, I now cruise at 2200 rpms/ 80 mph.

The car is a blast at any rpm in any gear.

Knowing what I know now, if I had to do it all again and keep the TH400 Hurst, I would still use the 3.36's... Just need to shift manually, as intended, and know when to shift! I had my rev limiter set too low.
 

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When I factory ordered my GTO back in 1967 the standard gear with the wide ratio Muncie was 3.55:1. I was in the military and was driving considerable distances so opted for a 3.36:1 on the order sheet. In town the 3.55's would have been better stoplight to stoplight and 3.23's would have been nice for the open road. So as said above, the 3.36's were the best compromise.

In 1986 I purchased another GTO and it came with A/C and a standard open 2.93:1 rearend. I eventually replaced the open rear with a 3.55:1 limited slip rear, but it always felt like I needed to shift again.
 

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it always felt like I needed to shift again.
This is where the TKX has shined. Im really amazed that somehow, I can cruise at 25 mph in 5th gear, and also go 120 mph in 5th gear.

I think the 336's really help to keep it balanced.

With the wide ratio, 355's would be way too much in 1st gear. It's already like a dump truck
 

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Great info on this thread, and I've had the same results. Not as long as Lust4Speed has, but going on 45 years of driving these cars. I have blown up a couple of 8.2 rear ends. Both time with less than 425 HP, both times with 4 speed transmissions, and both times by running sticky tires and abusing the hell out of them. Last one was in 1990, my '65 GTO, and when it cost me $940 then to have it rebuilt, I haven't hurt one since. Diven with the right technique, no fat slicks, no side-stepping the clutch at 5,000 rpm, and minimal power shifting, you should be ok. When I blew the 3.55's in the '65, I had a 3.36 ring and pinion fitted, as I drive the car and the speed limit was no longer 55 mph. 3.55's suck at any cruise speed over 60 mph, IMO. I removed the 3.36 one-legger out of my '67 GTO with a TH400 and installed a 2.56 posi because it was free. Would rather have had it be a 2.93, my favorite street gear for a TH400 GTO. The 2.56 will burn both tires off from a stop easily, and the 2-3 shift at WOT comes in at 105-107 MPH. Top end is limited to about 135-ish as the front end starts lifting and the steering gets scary. MPG is 21 to 21.5 at a steady 75-80 mph. It has gotten 23 mpg at a steady 65. Not bad for a 55 year old 3800 pound car. It cruises 90 mph at the same RPM my '65 cruises at 75. 8.2 rear ends, especially the 2-pinion ones (most are) are not strong. An 8.5 out of a later car is much better if you plan on abusing it. Driven responsibly, however, they can hold up. The one in my '67 has never been apart and has a zillion miles on it, and the one in my '65 has been fine in the 32 years since it was rebuilt. Becoming an old man has made my parts bills drop a whole bunch.
 
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