66 Fury III rear end upgrade advice?

[coach2ma]

Hi, all....newbie here.
I have a 66 Fury III (original 318 LA car) 2 door hardtop that I recently upgraded to a 383 stroker with mild+ cam and Torqueflite auto with upgraded spline. Just put in all new replacement drum brakes in back (complete) and front disc conversion was done successfully before I got the car. I have the original 8 3/4 rear with the "41" case (low horsepower/medium weight application, as per the original specs on the car) and VIN id says it would be the 2.76 ratio. My new engine tested fine on the dyno, at somewhere in the 450 hp/465 torque range at 4000 rpm. It will be a daily driver car, not going to be put on a line anywhere....my question is (and general concerns from us working on the car) is that the existing 8 3/4 41 case with that ratio will not handle the new drivetrain. (Springs are shot, too, they are being replaced). Anyone have (reasonably affordable) rear end upgrade advice for a car that I really just want to be street, but a bit evil when it wants to be?

 

[commando1]

Before your question can be answered, I would like to know about your engine. You say you have 450 hp from a 383 and a "mild+" cam.
Was this engine dyno'd or are you going by the info told to you by a previous owner.

 

[coach2ma]

Before your question can be answered, I would like to know about your engine. You say you have 450 hp from a 383 and a "mild+" cam.
Was this engine dyno'd or are you going by the info told to you by a previous owner.

No, it was Dyno'ed, I was there when it was done. The project began from scratch from a raw block (1970 cast 383). 440 Source Stroker kit (aluminum "Stealth" heads)...Hughes BB cam (nothing too crazy, just what they consider be their street, but with a little growl)....the car is going to have a complete TTI exhaust, 1 7/8 headers to tips. I just need to be sure that the car doesn't twist into a pretzel if I take 50 mph turns (which there are a lot of where I drive).

 

[coach2ma]

Screenshot of dyno

 

[cbarge]

The gear ratio should compliment the cam and torque convertor choices.
Another important factor is tire size outer diameter.
For example, you can run 3.55 gears with a 29.5 inch tall tire. Or 3.23 gears with a 28 inch tall tire.
Either gear will still be fun for stoplight action and highway driving.
3.91 and 4.10 will haul *** down the dragstrip but not fun on highway with engine buzzing away.zt

 

[coach2ma]

Thanks! My torque converter is the "Dacco" 764 (I think that's 2300-2500 rpm stall?). 3.55 sounds like the right zone...any recommendations for manufacturers for the gear swapping? And also...because I am new to this (I am the "apprentice", not the master builder), I assume that the case ("41") needs to be changed as well (to "42" for heavier duty?) Is that correct?

 

[gyknot]

In my opinion, the 741 case will hold up just fine as long as you don't bolt on a set of slicks and start doing really hard launches. You say it is for a street car and occassional fun. That is what I am judging this from.
The 2.76 gears probably won't make you happy though for any acceleration at all, and I don't think you can even get gears for a 741 setup anymore unless they are used.
Probably the easiest way I can think of, is to just locate an entire pumpkin with 3.23 gears and either the 742 case or the tapered pinion that I cannot think of the number for right now.
You can swap out pumpkins fairly quickly.

 

[coach2ma]

In my opinion, the 741 case will hold up just fine as long as you don't bolt on a set of slicks and start doing really hard launches. You say it is for a street car and occassional fun. That is what I am judging this from.
The 2.76 gears probably won't make you happy though for any acceleration at all, and I don't think you can even get gears for a 741 setup anymore unless they are used.
Probably the easiest way I can think of, is to just locate an entire pumpkin with 3.23 gears and either the 742 case or the tapered pinion that I cannot think of the number for right now.
You can swap out pumpkins fairly quickly.

Seems reasonable to me...I don't plan on dragging the car, but I also don't want it to be a dog...which, considering the way it tested, and what's already in it...for a street car, it definitely won't be. Safety is more the concern, I guess, because people are giving me horror stories about the car being a nightmare because of what I am putting under the hood given the current rear end config.

 

[fury fan]

For years everybody steered clear of the 'weak' -741 centersection.
Then some folks were willing to admit on the internet that they had used it with higher-power engines with no issues. (as @gyknot mentioned)
Seems like, if you're not drag racing or getting complete traction, that a -741 can hold up.
But with that said, gear availability will be the next obstacle. But even if you can find gears, you'll need a limited-slip, then.

If you find a decent used/complete -741 with 3.23 that could be a stop-gap fix.
Otherwise, a -742 or -489 in your future, and musclecar guys help keep those prices propped up. A new/reman unit is likely $1250, and that drives the cost of used ones.
The -742 SureGrip uses a $100 serviceable friction-clutch pack, while the -489 is not serviceable unless you do some machining of the metal cones it uses. Or get an aftermarket limited-slip.

... I just need to be sure that the car doesn't twist into a pretzel if I take 50 mph turns (which there are a lot of where I drive).

Not sure what your concern is here, but there's nothing in the rear axle pertaining to that at your performance level. You will sometimes see an axle brace on an 8-3/4 axle, but it's not needed 99% of the time.

But be aware - the rear axle/wheel bearings take grease like the front bearings - they are not lubed by gear lube. So make sure your rear wheel bearings get a cleaning/inspection/re-packing.

 

[coach2ma]

For years everybody steered clear of the 'weak' -741 centersection.
Then some folks were willing to admit on the internet that they had used it with higher-power engines with no issues. (as @gyknot mentioned)
Seems like, if you're not drag racing or getting complete traction, that a -741 can hold up.
But with that said, gear availability will be the next obstacle. But even if you can find gears, you'll need a limited-slip, and that will likely *NOT* be findable for a -741 except for used.

If you find a decent used/complete -741 with 3.23 that could be a stop-gap fix.
Otherwise, a -742 or -489 in your future, and musclecar guys help keep those prices propped up. A new/reman unit is likely $1250, and that drives the cost of used ones.
The -742 SureGrip uses a $100 serviceable friction-clutch pack, while the -489 is not serviceable unless you do some machining of the metal cones it uses. Or get an aftermarket limited-slip.


Not sure what your concern is here, but there's nothing in the rear axle pertaining to that at your performance level. You will sometimes see an axle brace on an 8-3/4 axle, but it's not needed 99% of the time.

But be aware - the rear axle/wheel bearings take grease like the front bearings - they are not lubed by gear lube. So make sure your rear wheel bearings get a cleaning/inspection/re-packing.

Thank you! If safety or twisting isn't a concern, then I think I have my answer that I can, at the very least, enjoy running the car and then put the gear swap on next year's project list.

 

[fury fan]

I'm not a dyno-chart expert by any stretch, but I noticed your BSFC was in the high .3 area (.45-.55 is the range I see published a lot) and then noticed the AF is rich on the lower end, and has a lean spot around 4000-4500 rpm (where you have the torque peak). The lean area would not be unheard of for an engine at part-throttle cruise, but not what I would expect to see running at FT. Again, not an expert.
*320
Did they dyno the engine as-is or do a lot of tuning to maximize it?

 

[coach2ma]

I'm not a dyno-chart expert by any stretch, but I noticed your BSFC was in the high .3 area (.45-.55 is the range I see published a lot) and then noticed the AF is rich on the lower end, and has a lean spot around 4000-4500 rpm (where you have the torque peak). The lean area would not be unheard of for an engine at part-throttle cruise, but not what I would expect to see running at FT. Again, not an expert.
*320
Did they dyno the engine as-is or do a lot of tuning to maximize it?


View attachment 548107

Now you are beginning to speak in an area for which I am out of my depth on. (Again, I was the "apprentice", not the master builder). But because I was there, I can tell you that it was dyno'd as is, but using the racing shop's carb and headers, (not mine). But other than that, nothing at all was tuned or done to the engine other than hooking it up to the machine. Not to sound like an idiot (but admittedly, there is still so much I need to learn)...I know what the A/F ratio is, but I do not know what the BFSC is. Is the phenomenon you describe a good thing, or a bad thing here? I do know that when we were done, the shop owners and the master builder all agreed that they were very pleased with how the engine tested, and we were happy as hell that we built a project engine from scratch that worked and it did not blow up when we tested it.

 

[gyknot]

I was unaware that the positraction unit from a 741 and 742 will not interchange? I always thought they did.
Am I wrong?
In fact I was under the impression, that all 8 3/4 posi units could interchange between the different cases. Including the 489 case.

 

[fury fan]

In fact I was under the impression, that all 8 3/4 posi units could interchange between the different cases. Including the 489 case.

Oops. I believe you are correct, I was mentally envisioning someone selling the a SG as coming specifically from a 741, and thinking how unusual it would be to find such a listing. I'll correct that.
I have never considered whether an older friction-type SG could be installed in a -489, all the discussions I've seen were related to what was in the unit and how to fix it.

 

[gyknot]

Cool. I've only ever dealt with 741 and 742 cases so don't know for sure on the 489 myself.

 

[fury fan]

Now you are beginning to speak in an area for which I am out of my depth on. (Again, I was the "apprentice", not the master builder). But because I was there, I can tell you that it was dyno'd as is, but using the racing shop's carb and headers, (not mine). But other than that, nothing at all was tuned or done to the engine other than hooking it up to the machine. Not to sound like an idiot (but admittedly, there is still so much I need to learn)...I know what the A/F ratio is, but I do not know what the BFSC is. Is the phenomenon you describe a good thing, or a bad thing here?

Ah, using the dyno carb explains most of it. Shop headers are often used for hook-up convenience, but a carb could go either way (shop or customer). Sometimes you'll see/hear of jetting changes happening between dyno pulls, or even carb swap or other changes. But that takes time, so it costs $$ also. So the shop carb got thrown on and jetted in the ballpark.

BSFC is a fuel usage parameter (calculated) based on amount of fuel used to make horsepower. Lower numbers mean less fuel used, .45-.55 is a range that is commonly published for automotive gasoline engines, with .45 being naturally-aspirated and .55 toward forced-induction engines. The BSFC goes down as the AF ratio goes up. This is happening in the middle of the range, where the torque peak is, where an engine is making the most torque per fuel used, so I would anticipate could be leaner there. But being that lean (AF of 15+) at full-throttle seems like some power was left on the table?

I do know that when we were done, the shop owners and the master builder all agreed that they were very pleased with how the engine tested, and we were happy as hell that we built a project engine from scratch that worked and it did not blow up when we tested it.

Presumably there was some joking going on there - I'd never want a Master Builder to be happy as hell his engine didn't blow up durign testing!

 

[coach2ma]

Cool. I've only ever dealt with 741 and 742 cases so don't know for sure on the 489 myself.

Ah, using the dyno carb explains most of it. Shop headers are often used for hook-up convenience, but a carb could go either way (shop or customer). Sometimes you'll see/hear of jetting changes happening between dyno pulls, or even carb swap or other changes. But that takes time, so it costs $$ also. So the shop carb got thrown on and jetted in the ballpark.

BSFC is a fuel usage parameter (calculated) based on amount of fuel used to make horsepower. Lower numbers mean less fuel used, .45-.55 is a range that is commonly published for automotive gasoline engines, with .45 being naturally-aspirated and .55 toward forced-induction engines. The BSFC goes down as the AF ratio goes up. This is happening in the middle of the range, where the torque peak is, where an engine is making the most torque per fuel used, so I would anticipate could be leaner there. But being that lean (AF of 15+) at full-throttle seems like some power was left on the table?



Presumably there was some joking going on there - I'd never want a Master Builder to be happy as hell his engine didn't blow up durign testing!

Ha, yes, some joking going on there, yes...we kept joking about those Dyno Disaster videos, so that's where that came from...I personally was nervous as hell during the test, because it was the first build I've ever been a part of....thanks for the BSFC explanation! I do remember the shop guy choosing a certain carb (off of his shelf of at least a dozen) for the test...can't remember which it was. (My application will be an out-of-the-box Holley 750 cfm). So if understand you correctly, (and if I get my carb dialed in and mixtures correct), I could anticipate a tick or two up in power? That wouldn't be terrible.

 

[fury fan]

Cannot make guarantees, but yes, would expect some more power in lean areas - but that's only 3900-4500rpm.
I would focus on leaning it a bit from 2700-3000, too - you'll spend more time in that range.
But that's only possible if you get a wideband AF gauge and try to hold some steady-state conditions while you're driving.

 

[1970FuryConv]

Ha, yes, some joking going on there, yes...we kept joking about those Dyno Disaster videos, so that's where that came from...I personally was nervous as hell during the test, because it was the first build I've ever been a part of....thanks for the BSFC explanation! I do remember the shop guy choosing a certain carb (off of his shelf of at least a dozen) for the test...can't remember which it was. (My application will be an out-of-the-box Holley 750 cfm). So if understand you correctly, (and if I get my carb dialed in and mixtures correct), I could anticipate a tick or two up in power? That wouldn't be terrible.

FWIW, I think a 741 case is fine for a street car. Have a quality shop upgrade your 741 to 3.55 gears and go have some fun.

(Be sure to adjust the axle tapered bearings to a tiny amount of right side axle play after reinstall of the pumpkin.)

 

[CBODY67]

I suspect you need to head over to the Nick's Garage YouTube videos and watch their dyno runs. Lots of interesting information, generally, plus some neat camera angles during the runs.

Consider that how the engine acts on a "full-load" dyno might not fully reflect how it will act in the "partial-load" vehicle. No dyno issues with part-throttle acceleration or off-idle driveability when cold. SO . . . as close as you might get with the carb cfm, you will STILL NEED to massage and tweak the carb for best general results. That means main jetting, accel pump tuning, when the secondaries open, PLUS making sure the ignition system is all it needs to be. No real "plug 'n play" items here, as a general rule. They might all work decently well together as it, but would not really be the best they might be, by observation.

So, once you get everything in the car and driveable, seek out a speed shop that has a chassis dyno where you can do "road load testing" for the carb mixture and such. Do not worry about horsepower, you are just seeking to make sure the air/fuel ratio is where it needs to be and NOT too rich at normal cruise rpms on the highway.

As to the "duty level" info you came up with, consider that those who made those designations might have some definite torque capacity levels THEY are thinking about. Which might not mesh what what "enthusiasts" are thinking about. The issue of "high traction" can be in the mix, too.

Probably need to make a motor mount torque strap for the lh side, too!

Learn how to "drive" the car, too!

Enjoy responsibly,
CBODY67