Rear-end weight difference
- Thread starter vynn3
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mopar_nocar
Well-Known Member
i can't tell you poundages, bit it is a LOT!
i could throw the 7.25 around really easily. the 8.25 i put in is a lot heavier. the 8.75 i pulled for my buddy seemed like a ton!
sb
i could throw the 7.25 around really easily. the 8.25 i put in is a lot heavier. the 8.75 i pulled for my buddy seemed like a ton!
sb
That's what I'm afraid of. I'm gathering parts to swap out my 2.76 ratio 7.25" for a 3.23 8.75", but I'm worried that most of what I gain from gearing I'll loose from added weight...
Eventually I plan on swapping my A-904 (100 lbs) for a T-5 (75 lbs), but that's down the road.
Eventually I plan on swapping my A-904 (100 lbs) for a T-5 (75 lbs), but that's down the road.
mopar_nocar
Well-Known Member
not trying to throw water on the fire, but wouldn't be easier/cheaper to find an a-body 8.25 in there. they are heavier than a 7.25 (but not immensely so), can hold up to all the abuse you are likely to throw at it with a slant or 318 and can usually be found just dinking around in boneyards....the a-body 8.75 has become like hen's teeth lately...if you have one, sell it to the highest bidder and that'll pay for your sure grip/set up costs/rear end and maybe still have some $$ left over.
if you have to cut down a b-body 8.75, costs can get high also. there are a few b-body rears that will bolt in just by moving your spring perches and carefully adjusting back spacing on the wheels...65-67 b-body IIRC.
if you are going to stay slanted, even with the T5, i'd go 8.25, but that's just me, not because of weight difference or parasitic loss problems, just cost and ease of finding the 8.25.
sb
if you have to cut down a b-body 8.75, costs can get high also. there are a few b-body rears that will bolt in just by moving your spring perches and carefully adjusting back spacing on the wheels...65-67 b-body IIRC.
if you are going to stay slanted, even with the T5, i'd go 8.25, but that's just me, not because of weight difference or parasitic loss problems, just cost and ease of finding the 8.25.
sb
mopar_nocar said:
I don't want to change to the 4.5" bolt pattern. I've got K-H front disks up front, and I'm happy with wheel choices for the small bolt pattern.
Also, I've already pieced together most of an 8.75" rear. I just need to find a cheap 489 center so I can install a set of 3.23's. I plan to use the 741 case I've got now (2.94's) for 3.73's or 3.91's and a sure-grip when I install the T-5. That'll leave me a "long-distance pig" I can swap out for trips.
Y'know, I seriously considered an 8.25" rear, but looking around, they were REALLY hard to find. They shouldn't have been, but they were for me. None of the local yards had any for an a-body, and car-parts.com listed the closest one to me in east Arkansas (a 6-8 hour drive). I keep hearing how easy they are to find, but I've had better luck finding the elusive 8.75" components.
Thanks for the advice!
70Valiant
Well-Known Member
I saw no decrease in 1/4 mile E.T. going from an 8 1/4 with 3.21's to an 8 3/4 with 3.23's. The rear is a good bit heavier but the gears will way more than make up for it. I went from 17.15 in the 1/4 to 16.90 in the 1/4 by swapping out my 7 1/4 w/ 2.76's for an 8 1/4 w/ 3.21's.
One other note guys. On an engineering stand point. Some rear ends as a result of the pinion to ring gear placement have power loss do to the angle and friction. This is especially true for all those guys running a 9" ford rear end. In fact we get a bad rap, but an 8 3/4 is something like 15-20% more effective than the Fomoco stuff. I am not saying the 9" is not good or not strong, but it does cause some power loss over other grears in the market place. This is true for stock setups. After market is anothre issue for another topic.
That being said, I recall years a go a study that showed an 8 3/4 to have less effort in the torque loss area than a 7 1/4. You would think due to the size of the larger gears etc it would be more, and that the extra I think it is at 60 lbs in weight would slow you down some. But in most cases there is no diff. In fact the extra 60 lbs will if anything give you a little better traction out of the hole. See the weight of the car, the diameter of the tires etc are alos forces applying to the gears to keep it in motion once it is rolling. And the added weight of these larger gears and their design also is a benifit. I have seen the same questions for a dana 60. There is a point you are correct in a lot of weigh will make some diff in performance. But I think you will be fine and the 8 3/4 is so easy to change, swap out the 3rd member for even the do it your selfer that does not setup their own gears.
The 8 1/4 is a good choice too, but you probably want to keep the 4" pattern, and if you have an 8 3/4 I would go for it! :headbang:
Lastly, we could speak for hrs. back on the pinjion and ring combos. Did you guys ever wonder why a GM and or Ford is a 4.11:1 and we have 4.10:1 ? We have more teeth and also an odd count. What is good about that is, that it is know as a hunting gear pattern. This allows the pinion to not land in the exact same ring tooth all the time. Thus the gears wil llast longer, and are more of the time quieter.
Hope this helps you some..
PS I didi an 8 3/4 years ago even when my 273 was a stock 4b motor. Went to a 3.23 dog leg, and replaced the I think it was 3.21 7 1/4. I lost no time at all in the 1/4. Car did seem to track a little better. I have played with gears from 4.86, 4,10, 3.91 etc and even had a 2.76 for circle track. These are great rear ends.
good luck!
:glasses6:
That being said, I recall years a go a study that showed an 8 3/4 to have less effort in the torque loss area than a 7 1/4. You would think due to the size of the larger gears etc it would be more, and that the extra I think it is at 60 lbs in weight would slow you down some. But in most cases there is no diff. In fact the extra 60 lbs will if anything give you a little better traction out of the hole. See the weight of the car, the diameter of the tires etc are alos forces applying to the gears to keep it in motion once it is rolling. And the added weight of these larger gears and their design also is a benifit. I have seen the same questions for a dana 60. There is a point you are correct in a lot of weigh will make some diff in performance. But I think you will be fine and the 8 3/4 is so easy to change, swap out the 3rd member for even the do it your selfer that does not setup their own gears.
The 8 1/4 is a good choice too, but you probably want to keep the 4" pattern, and if you have an 8 3/4 I would go for it! :headbang:
Lastly, we could speak for hrs. back on the pinjion and ring combos. Did you guys ever wonder why a GM and or Ford is a 4.11:1 and we have 4.10:1 ? We have more teeth and also an odd count. What is good about that is, that it is know as a hunting gear pattern. This allows the pinion to not land in the exact same ring tooth all the time. Thus the gears wil llast longer, and are more of the time quieter.
Hope this helps you some..
PS I didi an 8 3/4 years ago even when my 273 was a stock 4b motor. Went to a 3.23 dog leg, and replaced the I think it was 3.21 7 1/4. I lost no time at all in the 1/4. Car did seem to track a little better. I have played with gears from 4.86, 4,10, 3.91 etc and even had a 2.76 for circle track. These are great rear ends.
good luck!
:glasses6:
Thanks, guys! So more 60 lbs., huh? Ewww.
I'll trust you more experienced guys that it won't hurt perfomance much, but I'm looking at gas mileage, too (and hoping the the benefits to city mileage even-up the highway loss with the 3.23's). Any other ideas where I could make up some of that weight? I know the T-5 weighs 75 lbs, but that doesn't include the custom bellhousing (maybe 10 lbs? haven't weighed it), and I've heard my 904 is around 102 lbs.
An aluminum center section is cost prohibitive. How many pounds could I save by going to an aluminum driveshaft? I'll need a new one (or get mine shortened) with the rear swap anyway...
Probably worrying about this too much. Thanks for your patience!
I'll trust you more experienced guys that it won't hurt perfomance much, but I'm looking at gas mileage, too (and hoping the the benefits to city mileage even-up the highway loss with the 3.23's). Any other ideas where I could make up some of that weight? I know the T-5 weighs 75 lbs, but that doesn't include the custom bellhousing (maybe 10 lbs? haven't weighed it), and I've heard my 904 is around 102 lbs.
An aluminum center section is cost prohibitive. How many pounds could I save by going to an aluminum driveshaft? I'll need a new one (or get mine shortened) with the rear swap anyway...
Probably worrying about this too much. Thanks for your patience!
70Valiant
Well-Known Member
I used to get 20-21mpg highway with the 2.76's and now get 18mpg highway with the 3.23's
70Valiant
Well-Known Member
City driving didn't change I always got 16mpg around town until I put the 4bbl on. Now I only get 14.5 around town but it is a lot of fun.
Johnny Dart
Well-Known Member
vynn3 said:
Just wondering what is wrong with your 7.25.
Did you break it?
Lots of guys talk about going to a 8.75 in there slants,at the same time never have had a problem with there 7.25
The 7.25 rear is a much better rear then many give it credit for.(for a slant).
and will hold up to a hot little slant,if built correctly.
There is no way a 8.75 has less drag than a 7.25,period.Everything about the 8.75 is heavier,making it harder to turn.
I weighed my 7.25 and 8 3/4. there was about a 70 lbs. difference.
dgc333
Well-Known Member
I read somewhere and don't know if it's true but what i read suggested the 8.25 rear end is heavier than the 8.75. Looking at them side by side it does not seem out of the question.
Johnny Dart said:
Of all my years of research, you're the first person I've ever heard say that. I know that if you can find one of the rare-as-hen's-teeth 7 1/4" sure-grips, it can help with durability, but even slanters regularly blow out their 7 1/4"s when they start to add power (according the folks over at .org). My long-rod 225 is already making a lot more power than stock. I figure the lower gears, the planned T-5, and further upgrades will eventually toast the 7 1/4" in my Swinger, and I'd rather build in some insurance and add a lower ratio.
My only personal failure experience is with my 273 2bbl '67 Dart, totally stock. The current 7 1/4" rear howls like a banshee. Regardless, I'll likely swap the Swinger's rear into the '67 when I install the 8 3/4" in the Swinger, just because it's free and, thus far, quiet.
dgc333
Well-Known Member
The 73 Dart Swinger I picked up for a parts car from a co-worker had almost 300k miles on it and the orginal 7.25 was still under the car. This was a 318/904 car.
Considering that the vast majority of A-bodies were \6 or 318 cars, most all of these came with 7.25 rear ends and the majority lasted the life of the car, the 7.25 can't be as bad as folks make it out to be. They were still putting 7.25 under Dakota pick-ups into the 90's.
Considering that the vast majority of A-bodies were \6 or 318 cars, most all of these came with 7.25 rear ends and the majority lasted the life of the car, the 7.25 can't be as bad as folks make it out to be. They were still putting 7.25 under Dakota pick-ups into the 90's.
Johnny Dart
Well-Known Member
vynn3 said:
I will ask you again.
Did you break it?
You say with your upgrades you will eventually toast it,but you still havent broke one yet.
Like another said they regullary put these in 318 cars(with 230hp,good luck achieving that with your slant)and they hold up.
7 1/4 howls like a banshee......well after some 30 yrs of service you would howell to,and that still isnt failure.Any rear will howell after time,and need to be rebuilt.
You are right though,I have no clue on what I am talking about. :sad:
Johnny Dart said:
Not trying to disrespect you in any way. Your comments are just the first positive comments I've EVER heard or read about the 7.25 rear in nearly ten years of research.
"Did I break it?" Well no. I realize I didn't mention it, but unlike lots of the "toys" that other Mopar owners drive on weekends and to shows, my Swinger is a daily driver. By that I mean I drive it DAILY, seven days week. Along with having some fun with it, I always try to make sure sure that it's absolutely dependable, which means overbuilding it in some areas. I can't afford to have my rear break on the way to work. So consider it insurance.
I guess I should clearly define what I call "broken". To me, a howling rear end need to be fixed. Therefore, although the rear can still technically perform its duty, it IS "broken". The '67 Dart is unpleasant and embarrassing to drive because of the noise, and thus, it's seldom driven.
I LOVE your car. You obviously know what you're doing, and did not mean to imply otherwise. I acknowledge that you've had good luck with the smaller rear over the years, and I've taken note that. But I've read literally HUNDREDS of posts over the years from hot rodders, restorers, to slanters that this rear will not hold up to abuse. And I DO plan to abuse it. I can't rationalize or gamble with putting several hundred dollars installing a R&P in a 7.25, only to have it possibly fail a few years later. Also, I anticipate the need/desire for a sure-grip in the coming years, something I can't easily get in a 7.25.
So I'm sorry if I've offended you, the lone voice in support of the 7.25 as opposed to hundreds of folks with WAY more experience than mine. For what it's worth, I don't think that 7.25's are worthless, just a gamble I'm not willing to take. I'M the one who'll have to deal with the consequences if it blows up. Why is that so insulting?
Johnny Dart said:
Thanks! WOW! 230 whole horsepower? Geez, it's around 200 at the crank now. What are the chances that a turbo will add 30 h.p.? <gasp> What if it adds 35? Or 40? Is it worth it to upgrade the rear then?
If you don't think a slant can make 230 (or more) horsepower (1 h.p. per cubic inch in my case, BTW), let me point you BACK to slantsix.org. There are several guys there you should talk to.
And I realize I didn't detail all my future plans for the engine/car, but neither did you ask. Thanks for the sarcasm.
Just a quick note, it is mostly shock and or heavy loads that break rear ends. If you do high rpm clutch popping, trans netraul drops, or pull very heavy loads, towing etc is most common.
But it you do install slicks and really get on it hard you could tear up the 7 1/4. But most driving on a strong /6 it should be ok.
But it you do install slicks and really get on it hard you could tear up the 7 1/4. But most driving on a strong /6 it should be ok.
Johnny Dart
Well-Known Member
vynn3 said:
Johnny Dart
Well-Known Member
Odviously a 8.75" is a very strong rear.I actually have one sitting on the side of the house.I personelly havent had any reason to use it yet.My slant car is a factory 4spd car,and I hammer it all the time.I built the 7.25" rear with a 3.55 gear.I replaced all the internal bolts with new grade 8 bolts.No problems whatsoever.
I have been on .org and .com for years.My point is everybody talks about going to a 8.25" or 8.75" and you ask them why and the only answer is it is stronger.They have never had a problem with there 7.25".
You have mentioned you are building a daily driver.The 7.25" has held up to daily drivers for years.
Charlie S is a 7.25" supporter.Including a 13sec /6 7.25" car.
I guess that makes 2 of us.
I have been on .org and .com for years.My point is everybody talks about going to a 8.25" or 8.75" and you ask them why and the only answer is it is stronger.They have never had a problem with there 7.25".
You have mentioned you are building a daily driver.The 7.25" has held up to daily drivers for years.
Charlie S is a 7.25" supporter.Including a 13sec /6 7.25" car.
I guess that makes 2 of us.
Johnny Dart said:
I've followed his build from the start. He A) doesn't know what he's doing, B) THINKS he knows more than he actually does, C) doesn't listen to those more experienced than he is, and D) hears what he wants to hear from his incompetent machine shop/engine builder. His failure to meet his performance goals has nothing to do his engine being a slant, it has to do with him. Oh, and despite the lower-than-expected horsepower, it supposedly makes nearly 300 ft. lbs. of torque, IIRC.
Why not cite one of the several competent and accomplished slanters, like Doug Dutra, Lou Madsen, Slantzilla, or our own 70Valiant, not to mention the guys running turbos and FI. THESE guys know what they're doing.
Johnny Dart said:
Could'a fooled me. Although there are some exceptions, this is the least "slant-friendly" slant six forum I've ever visited. I've owned small block Mopars all my life, but prefer the slant over all of them. It's not always about going as fast as possible, something many folks just don't seem to understand. To each his own.
70Valiant
Well-Known Member
Most people dont swap in an 8 3/4 because it is stronger they do it because of the ease of swapping gears. I run 3.23's on the street and 3.91's on the strip and can swap them out in under 40 minutes with the help of my 11 year old son. Plus I got a screaming good deal on my 8 3/4, If you had the chance to pick up a complete drum to drum 8 3/4 w/ 3.23's and a sure grip for $250 wouldn't you jump on it?
Charlie S is running a 7 1/4 with 3.91's and a sure grip, his 66 Barracuda with 170cid /6 on nitrus puts out 204 rear wheel HP and 355 foot pounds of torque. Your avarage 7 1/4 would blow appart on the first launch with that kind of power, his sure grip is where all of the strength comes from in his situation.
Charlie S is running a 7 1/4 with 3.91's and a sure grip, his 66 Barracuda with 170cid /6 on nitrus puts out 204 rear wheel HP and 355 foot pounds of torque. Your avarage 7 1/4 would blow appart on the first launch with that kind of power, his sure grip is where all of the strength comes from in his situation.
Charrlie_S
Well-Known Member
70Valiant said:
I did swap out the 7 1/4 rear for a 8 3/4 in my Cuda. Low 13's in a 3250 lb car, is asking a bit much from the 7 1/4.
HOWEVER: it is now residing in my 66 Valiant. The first weekend out, it went a 13.928 at 91.91 mph at a weight of 2680 lbs.
This 7 1/4 is now in it's 5th race/street car, since 1974
I also have a 5.13 SG 7 1/4 and the parts to build another one with 4.56 SG.
PS: I would not run a 7 1/4 behind a hot slant with a stick. The 904 trans is a lot easier on the rear.
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