Howdy. I've been reading this forum and getting lots of useful info so I decided to join. I'm the proud new owner of a 72 Cutlass Supreme with a 350/4 barrel. The engine was rebuilt by previous owner but I have no specs on what was done. I know they put in a cam and pistons, but don't know if it was bored over. Has headers, original rear end. Trans was rebuilt, but again, no specifics. Shifts really well, may have a shift kit in it.

This car runs well but I don't think it's making as much power as it should. I'd like to change out the vacuum advance can. What's the best can or adjustable can?

 

Welcome to the site, congrats on your new toy. BTW we do like pictures. As far as the vacuum advance here is a good read.
http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf

 

Welcome.
I don't know what your experience is with 72 Oldsmobiles is but I was driving them when they were new. The CS wasn't a powerhouse with the stock 350 4v engine. That L34 only puts out 180 BHP. If you think the car doesn't have the guts it should, I'd strongly suggest you check and see what the rear axle gearing is before you do anything else. If it has the (stock) 2.56:1 ratio, that would explain its lackluster performance.

Back in 72, it was the beginning of the oil embargo and most of the production cars were geared for better highway mileage. If the engine fires well and runs well? It's not broken. It's likely tuned properly too or you'd notice performance issues.

Check that gear ratio - I'd bet that's your solution to the lack of performance. Don't compare these cars to modern cars that can get 200+ HP out of a V6 or 180 out of a 4 banger. The newer cars power to weight ratios are much better.

 

No doubt you're right, Alan R. I know the rear end is stock and I plan to change that. Which, leads me to another question. Is a 3.23 rear end with posi going to let me chirp the tires, or do I need to go with a 3.5 or higher?

 

With the 8.5" differential that is original to your car your gear ratio options are 2.73 3.08 3.23 3.42 or 3.73. 3.23 gears are a good compromise choice for performance and drivability. Note 3.55s don't exist for the 8.5" differential.

 

Thanks, oldsmobiledave. I've hear lots of good things about the 3.23. But I wonder, if I put that in along with posi, will it do a burnout? Not something I'd do very often, but I think anything called a "muscle car" should be able to spin those tires on occasion.

 

If you're planning to use it for cruising and/or highway you might consider a 3.08:1 change. IMO it's among the most versatile that fits that profile, and yes it will chirp your tires. You'll definitely notice a difference in launch with any of the 3 series gears.

Going to a posi is an upgrade that's your call.

 

You can't decide what gear to run without knowing rear tire height. Are you planning to run stock height tires?, short tires?, tall 28" tall tires?


Pick your tire height then your gear ratio.

 

So my tires are 25" tall. P245/60R14

 

Shorter than stock. Very short in fact. If you are staying with that size tire then go with 3.08 gears or 3.23 maximum.

 

Is it worth getting 15" rims? Would that make a significant difference in lowering rpm's at highway speeds running the 3.23 gear?

 

Thanks! How do I have my picture appear alongside my name when I post?

 

You should familiarize yourself with this rpm calculator so you can make your own calculations. Also the tire height calculator.

http://www.csgnetwork.com/multirpmcalc.html


https://tiresize.com/height-calculator/

 

Thank you, Oldsmobiledave! These will be super useful.

 

IMO, yes. Especially considering how hard it is to get 14" rubber nowadays. With 15" rims there's better selection, and you can use the calculator Dave provided to equate the aspect ratio you might like.

 

2.56 non posi, lay a little rubber and still cruise at 60 mph without breaking the bank. This with a 455. For everyday use, leave 2.56 . JMO

 

Cruisin speed...rpm

 

Just below 50 mph about 1500 rpm...at 60 mph about 1900 rpm...this is with 455/400 setup..

And welcome to co!

 

Good point about getting tires for 14" rims. Guy on craigslist has a set of 15" Cragar SS with good tires for $875. I might have to go for that.

 

Lots of torque with that big 455. Can't do that with my 350.

 

Since you posted that picture of your tach I'll ask you, what is redline on a 350?

 

Gee Ted, that pic sure looks familiar! Yeah, a 455 is a torque monster but a 350 with a 2.56 isn't getting the same results.

I'm leaning more and more towards a 200 4R for my 350 with the 3.42 gears BTW. Based on calculations of tire size and gearing, I'd be pushing close to 2700 rpm at 60mph, and 3200 @ 70. It won't hurt the engine or trans, but it will eat fuel. By comparison, with a 200 4R I'd only be pushing 1750 rpm at 60 and just over 2000 at 70 mph. I like that, especially since the gearing for 1,2 and 3 are almost identical to the TH350. Final drive on the 200 4R is .67:1

By comparison the OP would be 1950 @ 60 mph with 2.56 gears, and 2280 @ 70 mph.

 

2.56 gears with a 455 are lazy but drivable. 2.56 with a 350 is boring and lethargic.

Put 3.08 or 3.23 gears in it depending on your tire height and drive that car.

 

I had 3.23 posi in my 455 cutlass and from a stop, floor it and the circus was in town any time you wanted! Had 326 hp and 450 ft lbs torque though. Going with 3.42 next time though.

 

I guess it all depends on what type of car you want and how fat your fuel budget is going to be......

 

Exactly right. And I do drive my car everywhere. I'm 55 yrs old...I'm not saving it for "the future."

 

But you did not mention tire height. Comparing gear ratio without knowing tire height is an incomplete discussion. A car with 245 60 14s and 3.23 gears will drive very differently from one with 275 60 15s and the same gear ratio.

 

I'll post it wnen I get home.
P225/70R14. 98S M+S
3:23 posi
455 motor with 326 HP and 450 ft lbs torque
TH 400 trans