rear end ratio for -39 Century

[Guest 39convertible]

Hi

i´m in the middle of restoring a -39 century but i have never drive one.

this week me , my uncle and my friend visit a guy with a -39 century coupé.

And he tell us to take the car for a drive .

we discover that the engine rpm is way too high in normal trafic rythm,

we want to drive it in 60mph with lets say about 2000-2500 rpm.

i found a company who can do a new pinion and ring gear for us but we may need to hear from other guys too before whe can start.

please tell us about your experience, maybe the 1:st gear is too high for that ratio ??

all inputs is welcome.

Janne

[Grandpa]

Janne,

I think that the standard rear axle ratio for your car is 3.9. The optional rear axle ratio is 3.6. Many Buick guys change the rear end gear set to a 1955 Century, which is about 3.41. I think that the Buick rear axle gears are interchangable from 1938 through 1955. The 1955 3.41 gear set would be a good choice for you.

A new 3.6 gear set can be made for about $2100 USD. The first gear ratio has no effect on the engine rpm in high gear.

Grandpa

[Guest 39convertible]

ok Grandpa

thanks for the input

lets say our wheels actual outer diameter is about 30 inch.

and we whant our engines rpm be 2000 in 60 mph.

the 320 engine gives maximum torque with about 1800-2200 rpm.

so that would be a very good rpm if you should overtaking a vehicle on the highway.

that means we have to choose a rear axle ratio at 3.0:1

in 2000rpm that will give us :

1:st gear: 25mph.

2:nd gear: 39mph.

3:rd gear: 60mph.

and the engines rpm in 80mph would be 2660rpm.

with 3.9:1 and 2000rpm the results are:

1:st gear: 19mph.

2:nd gear: 30mph.

3:rd gear: 46mph.

and the engines rpm in 80mph is 3490rpm.

i think that the slightly higher mph on the first and second gear is acceptable and not noticeable.

about the 55 gears, i dont think they are interchangeable with 36-39 they have a different spline on the piniongear.

but that may be easy to come trough just weld the -55 piece of the -55 axle on the old axle as well.

Janne

[Grandpa]

Janne,

Your math is fine, but using a 3.0 final drive ratio would make starting from a stop an exercise in clutch slipping. Using a final drive ratio between 3.4 to 3.6 would be best. In any case, I wouldn't drive your Century above 65 mph with the original drum brakes. Also, the 320 cubic inch engine's cooling system may not be happy driving at speeds above 65 mph on a hot day.

Grandpa

[Guest 39convertible]

i talked to the owner of the coupé and he told me that he have tryed several solutions to solve the cooling problems.

he have electric fan installed , larger 7 blade fan , extra electrical water pump , and always drive the car with a small opening on the hood and still he have the trouble !!

only if he runs the car between normal and high speed he is troublefree.

i´ve talked to several -39 owners and they all talk about cooling problem so of course it IS a problem.

we are going to try to modify our cooler to a bigger size

and install a BIG electrical fan in front of it.

about starting , i dont think the slightly higher ratio is a problem.

When i drove the car i try to start the car from stop with the engine idles and i could drop the clutch pedal without having the engine stopped.

it was an impressing feeling.

Janne

[Grandpa]

Janne,

Re. 320 cubic inch engine overheating: One of the many causes of overheating is anti-freeze. The prewar Buicks were not designed for ethylene glycol antifreeze. A 50/50

mixture of ethylene glycol and water has a specific heat of about 0.85 at 200F. This means that it conducts heat only 85% as good as pure water. Thus, to use a 50/50 mix of ethylene glycol and water, the car's cooling has to be increased in capacity by about 20%. However, using 100% water is not an option in Sweden.

Another overheating fix is to install a smaller diameter water pump pulley. Also, installing a fan shroud would help. There is a BCA member that makes fan shrouds for prewar Buicks.

Grandpa

[critterpainter]

Check to see if the air deflecter/shroud is in place under the radiator. it fits below the grille and helps direct the air through the grille to the rad. It does help.

bill

[Guest 39convertible]

i´ve heard from several people that the early 50:s buicks have a ratio such low as 2.90:1 or something like that when the car is equipped with automatic trans.

i have 2 rear axles from a -47 and they look very similar in the banjo ,but the pinion gear have more splines and thats no problem for me.

unfortuneatley the ratio on them is 4.45 and 3.90 so they are useless for me.

janne

[Leif Holmberg]

What i can see in my parts book,1959 has a ratio on 2.78:1

Some of 1954-1955 has a ratio on 3.36:1.Some of 1940-1953 has a ratio on 3.6:1 and 3.9:1 It seems to be a lot of ratios between 4.54:1---to 3.36:1 from 1940 to 1955.

[Guest 39convertible]

do you know if there are other ratios than that on the -50:s

Janne

[Leif Holmberg]

Here are all the different ratios from 1940-1959.

Have you contacted Jan Guffin Stockholm as I wrote in an E-mail to you. (on Your Hot Mail address).I think he knows a lot about this. Leif.

[Guest 39convertible]

wow

thanks leif

i think 3.07 is just what i whant

do you think the gears is the sam as the late 40:s ?

Janne

[Leif Holmberg]

I really don`t know what you can use,but I think Grandpa is right,becuse all the input I have read about gears on late 30 to 1950:s Buick, is the same as Grandpa has written.What I know is that 1955 and 1956 differential are different. Ratio 3.36:1 can be found on 1954-1955 Buick:s ser 50Super-60Century-70Roadmaster.Still asking you, have you had contact with Jan Guffin,he knows a lot about your questions.

Leif

[Guest 39convertible]

yes i´ve talk to him.

[Guest imported_JPIndusi]

I am fairly sure that the rear end housing for all Buicks is the same from 1940 through 1955. This means that a complete pumpkin from a car from any of these years can be "bolted in" to any rear end housing. The change came about in 1956. I imagine for 1939 you may have to change the entire rear end asembly, selecting the correct length on the torque tube and the drive shaft. The splines on the drive shaft are another matching question. From my experience with a 1948 Special with the 248 engine going from 4.45 to 3.36, your Century should do fine with a 3.36, 3.41 or a 3.6.

Joe BCA 33493

[Guest 39convertible]

hi JPindusi

i understand that the pinion gear have moore splines.

i can take the end of the drive shaft from a newer style and weld it on mine, dont you think ?

Janne

[Guest imported_JPIndusi]

Yes, in theory this will probably work. However, this could be tricky as the length of the drive shaft has to be correct and the whole thing needs to be straight and balanced.

Good luck.

Joe, BCA 33493

[Guest 39convertible]

Thanks Joe

do you or anyone else know the difference between the -55 and the -56 and newer style gears ?(if there are any)

Janne

[Guest 39convertible]

anyone having a 3.07:1 gear set for me ???

Janne

[Guest 39convertible]

my email is :

portman@telia.com

[Only Buicks]

Janne,

I sent you a private email about a new gear set I have in 3.6 ratio.

Let me knowif you have any interest.

[Guest JUNKFLYBOY]

I HAVE A INTEREST IN YOUR RING AND PINION

[Guest JUNKFLYBOY]

HAS ANY ONE PUT A 59 BUICK 3.07 COMPLETE ASSY IN A 40 CENTURY OLDCARS@TAMPABAY.RR.COM ANY DIRECTION FOR HELP WOULD BE APPRECIATED. I AM IN FLAT FLA SO NO PULLING PROB

[Guest 39convertible]

do you have a 1940 ?

if so i dont think it´ll fit without modification.

the ring gear will fit but the pinion gear will not.

cant tell you the exact differennce.

you can have mine if you want but i live in sweden so you have to pay for the shipping too but i beleive the shipping cost would stay for about $30.00-40.00 (my guess)

Janne