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auto trans question for Cady

autotester on Sun July 23, 2006 2:48 PM User is offline

Year: 74
Make: cad
Model: deville
Engine Size: 472

Hello. I've got a 74 cady with a factory 3 speed automatic in it. It works pretty good but I've heard that a four speed auto trany would work as well plus I might get a little bit better mpg. I've heard that a GM R6 type 4 speed automatic would work. Might have to make some modifications on the overdrive part (might have to add a switch under the dash to shift to fourth). Anyhow, has anyone heard about this or any four speed automatic trany I could use? Thanks.

NickD on Sun July 23, 2006 4:03 PM User is offline

I did some searching for my motorhome using essentially the same TH400 AT and believe that the 4L80 was the choice replacement, but my head was spinning with so many different numbers. Do recall the price tag, a Vss contraption was required to enable the operation of the cable type speedometer plus the computer as a box added someplace. It came to about $3,000.00 with me doing the conversion, AT's are rated by the maximum amount of torque they can handle and was told by transmission experts, the best transmission GM every made was the TH400, any later transmission would be going downhill in terms of reliability. The next big if was would I gain any fuel economy and couldn't get this question answered, with the 0.7 OD feature and the torque lockup, it is very well possible I would get even lower fuel economy with this conversion due to engine overloading. I have to agree with this as with my last C-4 Ford transmission, didn't make a bit of difference if I drove in 2nd or 3rd gear. What really made a huge difference was driving on flat compared to hilly roads, and the wind direction. I am sticking with the TH400.

I did have a 1973 Fleetwood, purchased it barely used during the energy crisis with a nice 75% discount, people were paying 3-4 times that price for cars that would only give about 25% in fuel economy and I had room to haul my kids around. Never did fill up that trunk, and could stretch my legs out in the back seat. But 13.4 mpg was tops for that car, 5,700 pounds with a 472 CID engine. If I remember correctly the rear end ratio was 3.54:1 or could have been 3.78:1, little foggy on that, was years ago. Did some checking on different gears, think back then the dealer wanted 800 bucks and kept on striking out at the wrecking yards so forgot about it. My 78 Fleetwood had a 2.2: 1 rear end, lowered the engine to a 425 and knocked the weight down to 4,400, but that car only got 17-18 mpg. I contribute that more to the weight loss than the rear end ratio, generally your best mileage is to operate an engine in it's peak torque range.

Look at today's pickup and SUV's with all that electronics, overdrive, fuel injection, most are doing worse than my 73 Fleetwood that kind of tells you, if you want fuel economy, look around for a four cycle moped.

CorvairGeek on Sun July 23, 2006 4:49 PM User is offline

Nick is right, the 4L80 is the closest 4 speed relative to the Turbo 400 (called a 3L80 now), but it won't bolt to a Cadillac V8. The most common conversion for a non-Chevy bolt pattern is to use the 200R4 transmission. You would need a control (vacuum switch) for torque converter lock-up, readily available aftermarket. A 200R4 would have to be well built for the Cadillac torque and vehicle weight in this application. You would probably also need a revised spark control/advance for operation in overdrive at lower RPM (lugging).

This is a relatively common type swap now-a-days. Is it worth it??? Hard to say. It won't approach the reliability of a Turbo 400.

-------------------------
Jerry

bohica2xo on Mon July 24, 2006 11:52 AM User is offline

A 200R4 in a 7000 pound car with a 472? LMAO. A week or two in some cities would leave a pile of slag under that Caddy where the trans was....

Stick with the TH400. Not enough room under the car for an AT545, so if you really want another gear, look at a Gear Vendors Overdrive unit.

.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Mon July 24, 2006 1:40 PM User is offline

My 92 DeVille is, or I should say was capable of getting 31-32 mpg, that was when we were getting good $1.50 gallon gasoline, this $3.15 stuff seems to be about 30% air. That was setting the cruise at 53 mph in overdrive on the freeway, but this is only a 3,600 pound car!

While the air resistance increases proportionately to the speed of the vehicle the horsepower required to get that speed is proportional to the cube of that speed. A major difference between my 92 compared to my 73 Cadillacs is the frontal area, would guess the frontal area was reduced by 60% in the 92. So there are two very noticeable differences, drastically reduced weight and frontal area. Not to mention riding in the back seat of my 92, like riding on an airliner with my knees jammed into my chin.

Also running the engine in it's maximum efficiency range, gas mileage really drops fast when driving below 53 mph, positively disgusting when having to drive at 35 mph, cuts down to 22 mpg. Lucky to get 16 mpg when driving around town, but if in a hurry, that can drop to 12 mpg. Not nearly as bad as driving at 72 mph, more in the 27 mpg range. My 65 Buick was geared for the best economy at 75 mph where I consistently would get 20 mpg, but when they dropped the speed limit to 55 mph, was lucky to get 17 mpg. So much for governmental regulations on saving fuel.

Didn't seem to make much difference on my 73 Fleetwood, was 13 mpg regardless as to how fast I was driving, but avoided town as could only get about 7 mpg with that car. My wife coming from a different country with rotten roads and no speed limits doesn't comprehend our speed limits with long distances to travel and silk smooth roads. I guess I don't comprehend it either.

But do not feel that changing the gearing is going to improve fuel economy and neither has all this electronic control with 500 buck fuel injectors in place of a ten buck carb rebuilt kit or a 1,100 buck computer on some cars or how about a 500 buck direct ignitor system module? Still the same basic piston engine burning carbon based fuels with the same problems. A recent report on new vehicles shows lower average fuel economy figures than back in 1985, think it was 10% higher today. So where is this industry going?

When thinking about conserving fuel, want to get a moped, but when thinking about surviving with all the maniacs on the road, a Mack tractor isn't big enough.

mhamilton on Mon July 24, 2006 7:49 PM User is offlineView users profile

Don't forget that the '73 Cad had pretty low compression by comparison. As Detroit scrambled to meet the new emission regulations, the bottom dropped out on compression ratios. I think 1975/1976 was the lowest compression since before the war on a Cad, and they bloated out to 500ci to make 220hp.

The modern fuel injection systems are good for a few extra mpg, but so was the short lived electronic carburetor. And FI systems are usually good for some extra power with the right engine tuning.

I agree, though, the aerodynamic "styling" and light weight is a major factor in economy for new cars, although it doesn't leave much substance in front of the driver.

NickD on Mon July 24, 2006 8:42 PM User is offline

Hot Rod Magazine was excellent in the 50's and early 60's, loaded with technical articles, use to read those from cover to cover. A favorite series is where they would get in a brand new Detroit whiz bang of an engine and slap in on a dyno with an advertised BHP of say 300 only to read about 120. Just by using a distributor tester and correcting the spark advance curves to factory, gapping the plugs, using a fuel air ratio meter to set the cruise AF to 14.7, yes, they knew about 14.7 back then, the power ratio to 12:1 the horse power jumped to 200! More elaborate stuff included putting a degree wheel on the crank and correcting the valve timing to spec, but that was usually pretty close.

I took these articles to heart and got all the equipment except my dyno was 0-60 times and keeping close track of the fuel economy, when the fuel economy started to drop, was time to open the hood. The distributor was easy, carb more difficult in basically guessing at the correct jet size, would braze them shut and redrill and carefully adjust the tapered rod for the power settings. Made a huge difference in both HP and economy, the two work hand in hand.

Also learned back then we were shipping the same engines to Canada, except you couldn't buy premium leaded gas in Canada and the OE's were supplying a thicker head gasket. When the price of premium skyrocketed to over twice the price of regular, was time to do something about that. Got the gaskets reducing the CR from about 10.5:1 down to slightly below 9:1 and also was able to peak up the spark advance a bit without pre-ignition. Didn't notice a bit of difference in economy and maybe a tad less in HP, but was more interested in economy, but sure did notice a huge difference in my gas bills.

So when I am comparing what they are doing today to what could be done back then by tweaking, really not that much difference. Big difference between today and back then, people back then could get there with four spark plug wires missing, where today with one barely fouled plug, can only top 40 mph on the freeway with a red light blinding your eyes.

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