It was sugested to me to install a 307 in my 1973 olds 98 for fuel economy reasons.just tossing it out there to get opinions.I still have the 455 it needs to be bored .030 and have a sleeve installed.

 

I wouldn't. I think that engine in that car the milage won't be that much better. Also your car is going to be slow. That moter won't have enough hp to get you going. IMO
Larry

 

you can build the 307 to suit your needs, but if you're looking for fuel economy, maybe look at your options for a transmission instead. 1973 didn't offer overdrive. You should be able to bolt up a 200 or 700 R to a 455 or 350.

I had a 307 powered Regency Brougham in 83. It was ok for around town, and cruised ok on the highway. The trans and rear end is what gave the car it's mileage (15 city/27highway). BTW, I know the 73 is a heavy car, but my 83 weighed in at around 5300 lbs so it's not a real lightweight either.

Your 73 455 would have 225 hp and a TH400. A 307 only has 140 unless you get a HO 180 hp version.

 

Foolish economy. That 73 would work that 307 to death and gas mileage would still be horrible- no better than the 455 and possibly worse. Keep the 455 and consider installing a 200-4R built to live behind it.

 

X2. The weight of the car is much more a factor than the size of the engine. If the 455 is in good shape, an OD trans is by far your best choice.

 

Thanks for the advice.Back to plan "A" rebuild the rocket.

 

IF worse comes to worse for economy reasons, I would use a stock 68-72 350 and bolt up a properly built 200R4.
I have a high mileage '72 350 with TH350 and a low mileage '86 307 with a 200R4.
Similar rear end ratios.
The 350 setup averages 16mpg and the 307 18 for a combo of city and highway cruising. Not much difference there and the 350 has 200hp vs 140hp for the 307.
You can easily guess which one is more fun!

 

Having had both, I'd think the 1st gear ratio of a 700 would be of greater benefit with that kind of weight.
A little more to install, but I think it'd be more fun to drive, and pay for itsef in the long run - they're in BB trucks, afterall!
83-84 'burbans only need a 12 volt source and a TV cable - solenoids inside, even for the O.D.!!
My small block 84 got a best of 17MPG with a 3:73 @ 140k miles.

 

I'll have to do some research on trans,I understand the 200-4R will not take the torque of a 455,I have a very light foot and don't care for squealing wheels or jack rabbit starts,so I'm thinking that a 200-4r might be ok,but I'll check it out some more.I still have a ton of things to do before I need to make that decision.thanks again one and al

 

If you have it rebuilt correctly, the 200R4 will easily be able to handle your "light foot" plus an occasional burnout now and then.
If this will be a car that gets many miles a year, then the change to OD will be worth it.

 

I can attest to the "no 307 in large car". I had another 69' 98 same color and year as the one i have now ( the reason I had to get this one) and sold it to a guy that kept nagging me for it. Long story short, he did not want to pay what i was asking for the car with all the work done to the 455 and insisted i put in a 307 i had laying around with a better cam (like it mattered, lol). I reluctantly agreed, and when i took it for a test run after the swap, it was sad and funny at the same time, you had that initial kick then it just fell flat on it's face. I would assume the mileage would be crap too due that little motor being worked so hard.

 

If he was just driving around town with the car, the 307 would be just fine. I agree with Robb though, the 307 should be with the 200R4. That will improve the mileage.
That said, a big car like the 69 Ninety Eight just screams to be driven on the open road. IMO, that's what they were designed for. I would have a 455/400 all day long in that baby, no matter what the mileage. In fact, I've always found that the big motors get better mileage on the highway because they are hardly working. Plus, when you want to pass, the reserve power is always there. As you probably know, it's just plain fun to put the pedal down, no matter how fast you're going.

As the marketing song went back in the day " There's a special feel in an Oldsmobile, it's more than a luxury ride, it's a feeling of pride that you have inside ". Why waste it on downsizing a great motor in a truly luxury car?

 

I was really surprised by the mileage I got with my '69 Ninety Eight LS. With a tired 455 and my heavy foot, I still got 16 MPG.

 

I'm not. That motor was designed for that car. Even with a heavy foot you're not punishing the engine. Also, something else to consider. If you keep your maintenance up on the engine, fluids, timing, carb etc there is no good reason in the world why you can't get decent mileage. (Most of the newbies to the vintage scene don't seem to know that 16 mpg on a big car like that is not unheard of. On the highway, you could get even better - probably around 22)

 

Ive owned quite a few land Yahts in my life. 1 73 Olds 88 4 door, one 74 Olds 88 Convertible, and a 75 Cadillac Coupe Deville. The Olds 88's both had 350s the 4 door had a 2 barrel which around town got better mileage then the 74 4 barrel car but the 2 barrel guzzled on the highway over 55mph. I swapped a 70 455, with a Edelbrock Performer cam and intake, 2.5 inch true duals, re-curved HEI distributor, and a re-jetted Q jet in the 74. It got better around town mileage and better highway mileage because my foot wasn't in it all the time begging it to move. Believe it or not the 500 in the Caddy got the most mileage out of them all. So This leads me to believe 3.08 gears and an engine with gobs of low and mid range torque will get you better mileage then a tiny motor busting its *** to get you moving. Im looking into a 200r4, if you swap a few parts out to the Grand National parts and it will hold up fine, and is the same length as a short tail 400, not sure about the long tails our cars use.

 

I agree with everybody, get an overdrive tranny and be done with it.

It will have to have a vacuum lockup, with is an easy mod to do, but the tranny will have to be built up a bit for a 455 (talking the 200r4 here)

I swapped 3.42 into my Old Delta because I wanted it to be quicker than the 2.73's is came with and it was quicker but with the 200r4 trans I dropped from 18 city/highway to 15 city/highway

 

I bought my '73 Delta with a 260 in it, and have been playing with the suspension while I've been doing preliminary work on a 350 for it.

Estimates so far have the 260 getting 12 to 15 mpg through a 3.08 rear in fairly steady 40-50mph driving.

Don't do it.

- eric

 

X4 on the transmission swap, you will have to save a lot of fuel to get your money back on the initial cost though. If you are only putting on a few miles each year you might never get your investment returned.
To get the best from a suitably built 200r4 don't forget you will need a suitable rear end ratio. IMO the engine should be at peak torque for your desired cruising speed in O/D.
Roger.