Economy & Performance - carby & diff

[stiebs]

For my '74 Monaco. Here's the specs.
Fitted with a 440 taken from a donor '74 New Yorker.
Holley 750 into a Torker.
As far as I'm aware, stock as a rock block.
Diff is a 8.75 fitted with 2.7:1 gearing.

This is a car that's being set up for a once-a-year HARD drive. Ie, 3000 miles, 10 day trip on under-maintained dirt roads in outback Australia.

The diff ratio makes it sit nicely at 60mph+, but it lacks any get-up-n-go, so considering in switching out for a 3.3. Any opinions?

It's also heavy on the juice, and blows plenty of black out the exhaust, so running way richer than it needs to be. Not looking for performance, just want to get it set right. What are my options with the carby - tune the 750 (different jets?), drop down to smaller holley? Switch out for something else?

Keeping in mind that I'm in Australia, so would prefer a carby that is reasonably easy to service and maintain here.

And possibly needs a new thread, but how/where can I figure out what axles this uses, and how do I go about getting a spare? (Are both sides the same, or are left and right different lengths)

Thanks!
Mark.

www.bluesmobileaus.org

 

[stiebs]

.. on the diff, and considering my absolute lack of any Mopar knowledge .. I'm led to believe that the diff is an 8.75, but looking on rockauto under '74 Dodge Monaco, it's only showing 8.25 and 9.25 diff parts..

 

[halifaxhops]

Check that both advances (mechanical and vacuum) are working in the distributor also makes a world of difference in performance and gas efficiency

 

[CBODY67]

I would suspect a 9.25, all things considered. 2.71 is the stock ratio with 3.21 optional.

IF it's blowing dark smoke at WOT, then things are waayyy too rich. The 4160 750cfm has no secondary metering jets, just a metering plate that would need to be changed. The 4150 has a secondary metering block as the primary side has, which has jets in it. I believe that the 780cfm is a 4150, which will also need a dual-feed fuel line? Have to check to make sure what the differences are between the 750 and 3310-style 780cfm carbs. Check to make sure that the air bleeds for the secondaries, just inside the air cleaner "ring" on the top of the carb are open and not reduced-size with accumulated deposits. When those air bleeds clog or get diminished in size, it makes that circuit on each side go full rich, which might be the particular issue.

That Holley is usually calibrated on the rich side to begin with. With the stock engine and single exhaust, might need to downsize the jets a bit, so it operates more efficiently.

On the surface of things, 300 miles/day would not be that hard to do on normal USA highways, but with your terrain down there, it would mean the average speeds would probably be less in some areas. With the 2.71 gear ratio and P235/75R-15 factory tire size, that would mean that 90mph would be right at 3000rpm. That "passing gear" would run out to about 80mph+ or so, too. With the 3.21 gear ratio, 3000rpm would be more like 78mph, which might work better with your terrain at lower speeds.

I think the main thing would be to get the car on a chassis dyno and get the carb tuned to run cleaner at cruise and at WOT. I'd suspect that some of the larger speed shops or tune-up shops have a chassis dyno, but not all might be fitted with the necessary things to do WOT testing. But I'd suspect something more like a part failure that's causing the over-rich WOT situation, which a good carb rebuild/verification might uncover.

The way the Holley secondary system works, the secondaries only open as much as is needed, even at WOT. There are some different springs for the secondary diaphragm which can be swapped to get them to open more, if needed, which could be verified when the carb is disassembled. So, the 750 carb could be only flowing 600cfm, as that's all the engine needs, for example.

Just some thoughts,
CBODY67

 

[stiebs]

Thanks for your thoughts CBODY. I haven't done anything with the carby, so I guess first step would be to see what condition it's in.

What's the best way to identify the diff?

 

[Turboomni]

I agree in getting your carb dialed in as best you can and see where you stand first. 750 cfm is pretty big for a stock 440 especially if you are searching for low/mid throttle torque.
I have a stock 440 and run a 600 cfm I thought would be too small. Turns out on a stock 440 the carb becomes a restriction from 6200 rpms up. I never go there so it's fine as I prefer the torque of the engine and not WOT HP. Definitely check your base timing and vacuum/ mechanical advance works correctly. I found mine car's vacuum advance was not hooked up correctly and had a bad diagram causing a leak.Base timing was wrong as a result etc. Don't know much about Holly carbs . I run a 1406 Edelbrock carb which I find very easy to adjust just the way you want and I would think the smaller cfm gives better metering of air/fuel in the throttle range you will be using it in,which I guess would not be WOT. Depending how bad you carb, timing and advance is setup ,with it all running properly your torque could go way up.

 

[1978 NYB]

What was the original engine?

Anything but a 440 would have a 8.25 axle.

A 440 would have 9.25 axle.

 

[stiebs]

Original engine was a 400. I have a couple of pics of the diff, bit don't know enough about them to know if it's possible to tell them apart visually, but according to the previous owner he switched out the diff. Not sure if that means he just switched the ratios or the whole diff though

 

[Turboomni]

Send a pic for sure.

 

[stiebs]

 

[stiebs]

And here is the RH axle. 27 spline

 

[stiebs]

Not very often it runs WOT.. more concerned about tuning for fuel efficiency at cruising speed. Will definitely take a look at the carby and suss out vacuum advance. Wouldn't surprise me if it's not hooked up properly

 

[Turboomni]

Not very often it runs WOT.. more concerned about tuning for fuel efficiency at cruising speed. Will definitely take a look at the carby and suss out vacuum advance. Wouldn't surprise me if it's not hooked up properly

I got 4 + more mpg and much more mid range power after I tuned the timing and the carb. I rarely floor it. But all that torque comes in handy pushing my 69 Fury III effortlessly.

 

[Polara_500]

View attachment 213309

Kinda looks like an 8.25 rear. Do you have any more distant shots with the cover on?

 

[furious70]

picture of the holley is helpful too to know what model you have - squarebore vs spreadbore, double pumper vs vacuum secondaries, etc. Like said above, if a vac secondary it's only going to open as much as the engine needs with the right spring in it, so it'd be fine.
Unless you're trying to do rock crawling with it, I'd think either rear end would hold up. Looks like you've got the 8.25 there.
Torker isn't a terrible street car manifold but and eddy performer or performer rpm would help the low end a bit.

 

[CBODY67]

In the realm of Edelbrock single-plane intakes . . . The Tarantula came first in the late '60s. First aftermarket intake that sought equal mixture amounts to each cylinder. Mopar Perf Race Manual further refined that distribution issue with specific instructions using epoxy and popsicle sticks on the plenum floor.

Torker was originally a discarded prototype for Tarantula, but didn't give the dyno numbers they were seeking. An employee pulled it out of the scrap heap, allegedly, and put it on his ElCamino (with a PowerGlide trans) and discovered that it had the low-mid-range torque that engine/trans combination needed. With his drag strip activities, they then took it back and refined it into "Torker". Still, single plane and lower-rpm oriented, with orientations toward more consistent mixture distribution among the cylinders.

Factory intakes are dual-plane and more torque oriented, with wider mixture variations among the cylinders. With proper sizing of the ports and plenum styles, they've worked well over the decades! One advantage of the normal dual plane is that as #5 and #7 fire in sequence, with a normal single plane, #7 theoretically gets a smaller piece of the total mixture pie, as their ports are side by side in the intake manifold. The Holley 28-Z intake addresses that situation -- single plane with a full plenum divider and a port connecting #7 and #8 cylinders so #7 can pull from the other side of the plenum divider also. That manifold is now very "out of production", might fine one for a small block Chevy, but probably not for a Chrysler V-8 or any type. BTAM

With a stock or slightly built 440, needing to get to over 6000rpm, or even past 5500rpm should not really be necessary with the right combination of components to effectively use the inherent torque of the 440 rather than seek ultimate top rpm horsepower instead, for a generally-street driven car, to me. Torque builds tire smoke, high rpm horsepower happens later.

Respectfully,
CBODY67

 

[Mr C]

"Stock as a rock block" 74 440 would have a TQ, not a Holley, so it's been mucked about with already. Your Diff is a 8.25, from the pic you posted.

 

[stiebs]

"Stock as a rock block" 74 440 would have a TQ, not a Holley, so it's been mucked about with already.

Yep, realise they the Holley is not stock, but I was referring to the engine internals. Seems that it's been put together for external bling factor rather than having components selected for performance. Either that or it was based on what was easily obtainable. In Aus big blocks re only generally seen on the drag scene and boat racing.

 

[stiebs]

Wow, overwhelmed with all the info! Will take me some time to digest! Pretty sure it's a vacuum secondary but will post pics after I've cleaned out the dust from the engine bay after its last trip that we just finished!

 

[stiebs]

Back again after finally getting a chance to get to where the car is being stored at the moment. Haven't had a chance to clean out the engine bay yet, so excuse it's rather dust-ridden condition.. but here are photos of the carby, which will hopefully help to ID it..