Hey I was wondering what kind of improvement in gas mileage the 4.1 V8 gave the big body Devilles and Fleetwoods over the V-8-6-4 6.0 V8?
What was the 4.1 rated at? I wanna say 16/25.
What about the V-8-6-4? I wanna say its like 14/22

Well, personally I hear both engines are to be avoided but I would think the V8-6-4 would be the better choice since with that engine you'll be able to actually move the car haha.

Around town, I'd bet anything the 8-6-4 whomps the 4.1. On the highway, I bet the 4.1 does pretty well.

I have driven a 1981 Fleetwood Brougham Coupe with the V-8-6-4 and a 1990 Brougham D'elegance with the 307. They seemed like they had the same amount of power. Both were slow, but I'd imagine the 307 would be a little quicker due to the 4th gear.

That's true. I do love that 4th gear, makes a hell of a difference. Not too many people may agree with me here but the 307 is a good choice for a Fleetwood Brougham engine. The reliability is through the roof....

What I remember of my mom's 85 4.1L was around 21-22 highway. We were all pretty happy when we could make it 500 miles on one tank on a trip (just started to show "L" on the FDC when we got there). None of our cars until then could do that. The car the 85 replaced was a 84 98 307. Which came very very close. Next was our 76 Delta 88 Royale 350. It had a larger tank and only got 400-450 miles per tank highway.

Around town, I'd bet anything the 8-6-4 whomps the 4.1. On the highway, I bet the 4.1 does pretty well.

And my mom is a lead foot, so we stayed around 75-80 on the highway.

Hello,

Chad, who started this post, drove my 1981 Fleetwood Brougham Coupe with the V8-6-4. The car has 236k miles and just had considerable service work performed. The power is not what it should be since the tune-up was done and I'm guessing the timing is off and once the problem is corrected if anyone is interested I will report back if the performance is better. Mileage is poor at the moment as a result.

The EPA 1985 Gas Mileage Guide lists the HT4100 fuel economy as 16/22 in the full-size rear-drive Fleetwood Brougham.

I can't locate any information on the V8-6-4 although I will try to find some. The 1979 Cadillac with 425 V8 was rated as 14/20 with 425 4-Barrel and 12/18 with 425 EFI.

EPA lists 1989 Brougham with 5.0 V8 as 17/24.

I am always amazed that people are so interested in the V8-6-4. It is a mechanical nightmare that I would not ever consider buying again.

Have fun!

Dave

Dave

Poor 8/6/4. Always the target.
They get just about 20 MPG on the highway on a steady 60 MPH roll in 80 degree weather with the AC on.

Dunno about around town.

I never owned an HT-4100 RWD car that ran
:histeric:

My 85 Fleetwood 75 limo with HT-4100 got 24-27 MPG on the highway.

Take an 8/6/4 and marry it to the same transmission as the 307 cars and you would have something.

That is what I was thinking. A V864 on a THM200 4R with 3.08 gears would be ideal.

The LT1's are rated 17/26 (2.56) 17/25 (2.93) and 17/23 (3.42). Overall they gained power and lost nothing in mpg. We have come somewhere....

I still would like to see a good EFI on a 307. What could it have done in fuel economy if it had a good solid SPFI like the LT1 did. Can anyone adapt a TPI chevy intake to one as a project?

I don't see how OD makes the car faster, it is just used when cruising on the highway..... well, yeah you can then have a shorter gear in the rear end, but still.

My '79 dosn't have OD, and has 2.28 gears, at 70mph it is right at 2,000RPM..... overdrive on this car, with the 425 is not needed.

Oh yes, thanks to dave for letting me drive that monster :D
It was world apart from my '92!

The power is not what it should be since the tune-up was done and I'm guessing the timing is off and once the problem is corrected if anyone is interested I will report back if the performance is better. Mileage is poor at the moment as a result.


Oh yeah, what does that average for mileage? The FDC said like 11-12 mpg average when I was driving it. Is that true?

Very cool car :cool2:

Faster, well not really. Quicker? Yes by all means. 2.74 first gear ratio and 3.08's is a heck of a lot more gear and starting torque than 2.28's and 2.48's. With 425's torque it should be on par with an LT1 car off the line. Overall drivability much improved by the no so lazy gear. 2.74*3.08=8.4392 2.28*2.48=5.6544. My LT1 FWB is 5.5746 if I start off in SECOND gear (which does ok mind you, but having 1st is a lot more fun when you punch it). Mine is 10.652 in first. That is a lot of extra oooommpppphhh to get the car moving. And allows you to do it with less gas pedal to accomplish the same acceleration performance that you would with the 2.28's and THM400's 2.48 first.

2000 rpm is still a gas waster. If you can get it down to 1500 rpm you will save on gas due less friction and 25% less combustion events per minute. That alone is a gas saver. You can likely go lower (Like GM did with 2.56's and OD), but you really need a small cam and high velocity heads to make that work well.

Not needed? Well, yes, but OD and shorter gears is much welcomed.

I don't see how OD makes the car faster, it is just used when cruising on the highway..... well, yeah you can then have a shorter gear in the rear end, but still.

My '79 dosn't have OD, and has 2.28 gears, at 70mph it is right at 2,000RPM..... overdrive on this car, with the 425 is not needed.

Rick, in your 79, where does 1st gear end?

It should somewhere around 20-30 MPH if my memory serves me correctly from driving a 79 Coupe. I've thought of the idea of installing the TBI from a 1990 307 onto my 89 but I dunno how well it would work. The carb can sometimes be troublesome and you can't compare to the convenience of FI.

Yeah thats one thing that '81 had. I could start the car with out pumping the gas 3 times like on that '90 Brougham with the 307.

I'm guessing 75 mph is around redline.... And that is a conservative Cadillac redline....

A friend of mine ran 2.29's and a THM350 with a 400 Pontiac to the tune of 106 down the quarter, and that was second gear..... Top speed? 165-170 is what the tach was calculating to...

Rick, in your 79, where does 1st gear end?

Hi Chad,

The Fuel Data Center reading changes constantly. I played with the average fuel economy button and it didn't change from the instant so I don't know. I have to admit I've never checked the mileage.

I got just about 16 MPG on my trip from Wisconsin to Florida and back to Wisconsin in the '79 Sedan deVille. I am leaving back for Florida again on Tuesday if all goes well. The carb is being adjusted and hopefully the fuel economy will increase.

I would love to remove the 8-6-4 stuff from my 1981 since the car runs great in 8 cylinders. I also sometimes wish my '79 425 was fuel injected. Not a perfect world.

Dave

It should somewhere around 20-30 MPH if my memory serves me correctly from driving a 79 Coupe. I've thought of the idea of installing the TBI from a 1990 307 onto my 89 but I dunno how well it would work. The carb can sometimes be troublesome and you can't compare to the convenience of FI.

No can do, my friend. The 307 was still carbureted in 1990. When they switched to the 305 in '91 it became TBI, and 175 horse.

Look at the Oldsmobile Custom Cruiser's 307. In 1991, it got TBI.

Dave,
I wonder how your '81 FWB coupe compares to my '92 SDV for mileage in the kind of driving I do. About 60 % city & 40% highway driving at somewhat heavy throttle. I average about 16mpg in summer. I bet in your car, it would be something like 11-12.

1991 sales brochure shows 1991 Custom Cruiser with FI 307. 1992 Custom Cruiser went to 305 w/FI.

was the 350 optional in 1991? I know it was in 1992.

All I know is with my car running the way it is, it gets about 10mpg. I need to replace the ecu. Runs better in warm weather like when I first got it the milage meter said average ranging from 13 up to 18mpg. I don't know how accurate it was, it got stolen.

My '81 averaged 16-17 MPG in mixed driving.I took it out on a 300 mile round trip 2 years ago and was getting 23 MPG on the freeway (or so the MPG sentinel said).That was with the cruise control off and having a case of leadfoot.Also, the modulated displacement was disabled,so the engine was operating in 8-cyl mode all the time.

I thought this was interesting...

1986-90 307 V8- 4bbl carb, 140hp & 255 lb/ft of torque
1981-83 368 V8-6-4, EFI, 140hp & 265 lb/ft of torque

Interesting to notice how little difference that Fuel Injection and that extra liter of displacement make...

That 307 doesnt look so bad after all.

I'd be willing to hazard a guess that the 368 was subject to underrating to some extent, like the 500s that made "190hp and 380ft.lbs" but pull over 300/500 on an engine dyno today.

I dont know about that, the 307 actually felt more powerful.

The timing on the 368 I drove was off though, which probably hurt it, and it had 236k miles!! :bonkers:
The 1990 Brougham D'Elegance with the 307 I drove had 130k miles, but it had a sticky choke.

Neither were quick, but it felt as though the 307 had better throttle responce (which is very odd considering its carbed) and the fact that it had one more gear probably made it a little quicker also.

the v864 is definately underreted my coupe deville w/ 62000mi feels more powerful than the 307 broughams that I have driven, and not to diagree with you Rick but even my deville could use OD it would probably improve mileage by 5mpg or better and a probably w/ a "freed up"(newer exhaust etc.) v864 in 4 cyl mode and in 4th gear would be over 30 mpg easy.

My 83 DeVille has the HT4100 and gets surprisingly good mileage. Around town it's 17 mpg and on the highway I have no problems hitting over 26 mpg on long trips doing 5-10 mph over the limit.

The car is all stock except timing is 2 degrees advanced over stock and the stock cat is replaced with a CatCo hi-flow replacement. The CatCo made a noticable difference. Best highway mileage with the stock cat was 22 mpg. I think it was pretty plugged up because the little HT4100 felt like it had twice the power with the new one.

I own an '81 DeVille with 115k on it. I've owned it for three and a half years and amazingly have never once had problems with the engine. Anyway, here is the gas-mileage. 14 city / 18 hwy. I drive pretty conservatively too, trying to keep it on 4 cylinders on the highway.

hmmm that sounds about right for that car. Thats the same thing that the EPA rates the F-150 with the 5.4 and the 4x4 at IIRC.