1976 Fleetwood driveability

[HevansAbove]

Hello,
I am getting my 76 Fleetwood back on the road and there are two issues:

1. The transmission is just a 3 speed, and is designed for 55mph speed limits. I will be driving on the highway sometimes - can I change the rear axle gearing to allow for more relaxed cruising at 70? At the moment, it is out of breath at 70. I don't mind slower acceleration, but I would prefer a more comfortable, lower RPM at highway speed. I would rather not change the transmission if possible.

2. My mechanic wants to replace the original quadrajet with something different rather than rebuild it. Has anyone replaced their carb, what with, and is it better or worse?

Many thanks

[71 Fleetwood]

I found a calculator in a different thread at this site:    1976 Fleetwood Rear differential and Front Suspension Questions  (Read 4862 times) 
https://www.crawlpedia.com/rpm_gear_calculator.htm

As to the carb, my Rochester is making me very happy.  The small primaries are perfect for the big block. 

When new, your 1976 gave 10.9 miles per gallon.  If you go to Holley, for example, you'll need a new intake manifold or adapter plate to handle four equal sized intake ports.  I'd expect even lower miles per gallon, but that's just a guess.

[76Caddy]

My 1976 Fleetwood has excellent acceleration and cruises effortlessly at 70.  Actually it's sweet spot is around 75.
Sounds like  you may have other issues other than the rear end.

Just my 2 cents.

Tim

[HevansAbove]

Hello,
I don't think there are other issues. I simply want to gear it so that I'm at or under 2000 rpm at 70.  Does anyone know what the transmission ratio is?
It may have the optional 3.10:1 rear end, in which case it should be an easy switch.

[wheikkila]

I think you need a new mechanic. Your car should have no problem running 70. On your carb. There was just a forum on great places to send your carb to have it rebuilt properly. Replacing your carb with something else is not a straight exchange. You will also have to spend 400 to 500 just to purchase a different carb. Then you will have added expense making the mods to fit a different carb. I would rethink what your are thing about doing. I would start with contacting some of the company's for estimates on price and time line to have your carb rebuilt. This forum is full of good advice and information. Please keep let us know what you plan on doing.
Thanks Wayne   

[Cadillac Fleetwood]

Standard rear end ratio (except for Series 75) was 2:73 to 1.  Optionally available was 3:15 to 1. The higher numerical ratio was standard on Series 75 models.

Charles Fares

[35-709]

Have your Q-Jet rebuilt, they were made for your engine and are the right carb for your engine.  Maybe your "mechanic" wants to do a switch because he doesn't know where to have it rebuilt, but there are several carb specialists nationwide that can do a Q-Jet with no problem.  One well-known and respected rebuilder is Daytona Parts in New Smyrna Beach, FL., no doubt you can find one closer to you but Daytona Parts is one of the best.

https://www.daytonaparts.com

To add:  I would not let just anyone rebuild that carb --- that would definitely include your mechanic. 

[savemy67]

Hello Harry,

Welcome to the forum.

In order to determine the desired rear axle ratio, a few more bits of info are needed.

The transmission output shaft ratio in top gear (3rd) is 1:1.  That is for every engine RPM, the transmission output shaft turns at the same RPM.  What is the diameter of your tires?  Larger tire diameter will exhibit decreased engine RPM at a given road speed.  This is theoretical as the tire diameter decreases at the tire's point of contact with the road due to the weight of the car.

The factory shop manual will tell you how to identify the axle ratio in your car.  Using the calculator supplied by 71 Fleetwood, you can see that in order to obtain engine RPMs under 2000 at a road speed of 70 MPH, you would need to have relatively large diameter tires, and or a numerically low rear axle ratio - lower than I think Cadillac made.

The transmission in your car was not designed for 55 MPH speed limits.  The engines were "de-tuned" to meet EPA mileage standards.  If your car is "out of breath" at 70 MPH, it has nothing to do with the transmission or rear axle, unless a previous owner made some significant modifications to the transmission/rear axle.

You could swap the TH400 for a 700R4 or 4l60E that have overdive ratios of about .7:1, but the cost of the swap may not be worth it as your '76 would probably require several modifications.

To get the RPM's down, find the numerically lowest Cadillac rear end, and couple it with the largest tire diameter that will fit under your fenders.  You still may not achieve 2000 RPM at 70 MPH.

Regarding the carb, if your mechanic suggests replacing the Rochester, I would question his/her recommendation.  Earlier Cadillacs used Edelbrock carburetors as a replacement for some models of the Rochester carbs, but the Edelbrocks were similar to the Carter carbs which were sometimes used as factory original equipment.  If your mechanic suggests a Holley as a replacement, it is time to find a new mechanic.  While Holley makes a spread bore carb, it flows about 100 CFM less than the Quadrajet used on Cadillacs.  It is designed for a 300 - 350 Cubic inch engine.  It might make your car seem like it was "out of breath".

While you may not do your own repair work on your car, I recommend that you get the shop manual so you have a good understanding of how your car works.  Good luck with getting your Fleetwood on the road.

Respectfully submitted,

Christopher Winter

[wrefakis]

had 20 of them back to 76 cruise 75 all day never had any feeling of engine stress same with 70 on radials

[Jeff Rosansky CLC #28373]

Respectfully, it sounds like you are going off some bad information. Why do you want to gear it down? Fuel? Say you increase by 40% and drive 1500 miles/year will save you about 45 gallons/year or say $150/year. It will take a while to make that back if you do any heavy mods to get there. Longevity? People on here have 2-300k+ on their engines. In a classic car that will last you a lifetime.
Admittedly many of us are purists here but the 1 thing to remember is that back in the day these cars were the best at what they were. Most of the time you have to work to keep the speed down.
So, again respectfully here, I would suggest you recheck the source of your information, your reasoning for the changes you want to make, but most of all it sound like you need another mechanic.
Now there are those on here who have modified their vehicles and are VERY knowledgeable. If you are wanting to make the modifications for reasons other than normal driving then please post what you are wanting to do.
Bottom line here is that you have the PERFECT car to cruise at 75 all day long without blinking an eye.
Best of luck---- And post pictures!!
Jeff R

[wheikkila]

I would have your mechanic check the cat converter. Your car should have the old stile with the BB's in it. These converter were know to stop up. My dad had a 76 Eldorado with the 500 cid. He purchased it with very low miles in the late 80's. It ran fine till you got up to around 70 to 80 Then it flattened out. I removed the converter and installed a test pipe. It woke that car up. Just after that we drove that car from Michigan to California to Nevada and back to Michigan. We ran 70 most of the time. It was one of the best road cars I ever took a road trip in. It was one of the main factor in my purchasing a LaSalle. It isn't the trip car I envisioned as of yet. But it will be one day.
Thanks Wayne         

[71 Fleetwood]

The cat, great idea wheikkila.  That would rob a bunch of power.

Yes, my mid-70's small blocks would smell like rotten eggs before the cat died.  It was like being in a gassy elevator while everybody feigns innocence. 

[wheikkila]

They would bring tears to your eyes. Not in a good way.
Thanks Wayne

[bcroe]

With pretty decent oversize tires and a (late 70s) 2.41:1 axle,
I manage 2000 rpm at 60 mph.  That is about as good as it
gets, unless you want to use a (rather delicate) 4 speed
overdrive trans with a lockup torque converter.  Practically
speaking what you can do with tire dia is extremely limited. 

For that size car, I would stick with the 2.73:1 axle, but
make sure the original timing chain set has been replaced,
install a really low back pressure exhaust, and make sure
everything else is in good order.  good luck, Bruce Roe

[mario]

I changed my t 400 to a 200 4r with a lock up converter.  77 mph is 1770 rpms. 28" tires and 2.73:1 gear ratio.
The trans has all the billet parts. Not cheap but it will take the 500ci torque with no problem . Also mileage increased from 10 mpg to 12.5 mpg. Funny thing is it mounted in the same space just changed the yolk on the drivshaft. By the way, it is a custom install in my 49 sedanette. The t 400 was a short tail shaft, but the mount was in the same spot.
Ciao,
Mario Caimotto

https://gearstar.com/200-4r-transmission-the-holy-grail-of-power-fuel-efficiency/

[TJ Hopland]

I too think something is wrong.  Ya you get used to driving a 6+ speed modern car going to a 3 speed is a bit strange but 70 mph should be no problem and as pointed out by others you are unlikely to break even on gas savings anytime soon unless you are in the 1000 miles per week range of driving. 

Exhaust is a good thought.   Cat being plugged is a good possibility but don't overlook the muffler either.   Many mufflers from this era were packed with a fiberglass like material that was held in with a wire mesh.  With age the wire mesh deteriorates and no longer keeps the fiberglass against the walls where its supposed to be and it can end up blocking the main outlet while still looking new on the outside.   

Any exhaust restriction just kills the power but they will still start up and run nice and smooth, they just run hot and don't make power.   If you have somewhere really dark you can drive to drive there and open the hood and or look under the car.   If there is a decent restriction you will likely see the exhaust manifolds and cat glowing orange.     

[wheikkila]

The best thing to use is a  gun to check for temperature deafferents.
Thanks Wayne

[hornetball]

The best thing to use is a  gun to check for temperature deafferents.
Thanks Wayne

This.  Quick, easy and foolproof.  If there is a restriction, you'll see a big temperature drop across the restriction.  Note that a catalytic converter that is operating properly will actually show a temperature rise from inlet to outlet.

I had a MB 380SL with clogged cats.  It was a different car once that was fixed.

[V63]

I would have your mechanic check the cat converter. Your car should have the old stile with the BB's in it. These converter were know to stop up. My dad had a 76 Eldorado with the 500 cid. He purchased it with very low miles in the late 80's. It ran fine till you got up to around 70 to 80 Then it flattened out. I removed the converter and installed a test pipe. It woke that car up. Just after that we drove that car from Michigan to California to Nevada and back to Michigan. We ran 70 most of the time. It was one of the best road cars I ever took a road trip in. It was one of the main factor in my purchasing a LaSalle. It isn't the trip car I envisioned as of yet. But it will be one day.
Thanks Wayne       

I replace those old cats as SOP any more, huge difference in performance each time. Those old units weighed a TON too! Significant weight savings.

[Jeff Rosansky CLC #28373]

I replace those old cats as SOP any more, huge difference in performance each time. Those old units weighed a TON too! Significant weight savings.

Just wondering--What about just removing the pellets on those that have them?
Thanks
Jeff R