Transmission / Overdrive: Anyone willing to give me a quick primer on Differential gearing, common swaps for 289 pwr'd Hawk?

[cavanbound]

I'm capable enough under the hood (engine-side), but have never changed a differential (back-end) on anything I've owned. I've seen some references (specifically for the 289-powered Hawk) to swapping the 3:31 differential to a 3.07. I see a number of posts (including June 2018 thread on "Changing '65 Borg-Warner to GM powerglide") but I think I need a quick explanation - and if possible, with specifics of putting the stock 289's power to the road. Plenty of info out there on differentials -and I know the basics, but would like to know what's common for my 1960 Hawk. My engine is stock, but for an upgrade in carburettor (2bbl current, 4bbl planned, everything in-hand except the time). The car has overdrive, and it works well.

I'm just trying to figure out if I'd benefit from anything here. I also have a '62 GT without overdrive - and I also interested in finding ways to smooth out/lower-revs at highway speed.

Thanks for any insights.

[PackardV8]

As much as we'd like to believe otherwise, the 289" is a small engine. It has good torque and if quickest acceleration is not a must, a 289" can pull an overall 2.6 to 2.8 in overdrive on relatively flat ground. This is why most V8-overdrives came with 3.73 to 3.90 rear axle ratios. Go much below that, say 3.54 or 3.31 and the ground had better be really flat.

Your '62 GT without overdrive, with a caveat for those few with the close ratio 4-speed, can use a 3.31 or 3.07, if quiet highway cruising is the prime directive.

jack vines

[jrlemke]

My '61 Hawk is stock 4bbl, 3spd/o.d. and has stock 3.31 T/T rear end. No problem in normal driving. Just don't try climbing very steep hills with o/d engaged. Happy with it as is.- Jim

[Mike Van Veghten]

The higher the ratio (smaller number) the more sluggish the car will be pulling away from a stop. The highway gain will be little, plus the passing of another vehicle will be more sluggish with the higher ratio.

You'd be well off leaving the 3.31. If you drive mostly around town with a little freeway, even going to a 3.55 would make the car more driveable/fun.

Mike

[StudeRich]

I really doubt anyone in their right mind would even consider changing a Flight-O-Matic 3 Speed to a 2 Speed Powerglide.

To get Overdrive and Automatic, maybe a GM THM 700R4 or 2004R with an adapter to the Stude. V8.

If you have a '60 Hawk with 3 Speed Manual Overdrive and you think the RPM's are too High, you should look at the Tag on the Left side of the Diff. rear cover to learn the Ratio.

It should be 3.73 and drive just fine, but if someone has put a Performance Ratio of; 4.09, 4.27 or 4.54 into it THEN you need to think about a swap to a better "Highway Gear."

[TWChamp]

I agree with Jack and Rich. My 1950 Champions and Commanders with overdrives have 4.54, and I wish they came with a 4.11.

[bensherb]

My '62 GT came with a 3 speed overdrive and a 3.54/1 rear axle ratio. I couldn't imagine a need for a lower axle, it worked great until the trans self destructed and I swapped it for a 4L60 AOD, which also works great with the 3.54 axle.

[Jeffry Cassel]

My 64 Hawk has 3.31 rear end. My 61 has a 3,07. There isn't that much difference. Very hard to imagine anyone switching to a Powerglide!

[Caso wannabe]

Talk about being sluggish, a powerglide WILL KILL any off the line acceleration you have. 1st gear is approx 1.82....

[wittsend]

1. It is important that the existing rear ratio is known before any advice becomes applicable.

2. The mentioned link regarded a '65 Borg Warner to Powerglide transmission: It would highly be likely that the car had a McKinnon (GM Chevrolet) engine which a Powerglide would directly bolt to. For a Studebaker powered car it would be a very expensive alteration (adapter) with a loss, not a gain. The Powerglide is a low cost, simple, 2 speed transmission. In some instances it can be modified for high power applications where the two speeds are applicable. For a street car it is going backwards. This is supported by virtually everyone who replied in the link saying it was not a good idea.

So, knowing the existing rear ratio is is paramount. Find that out and report back. You also need to state what transmission you currently have. Remember that just changing the rear ratio will have trade-offs. Less RPM's on the highway means a more sluggish start.

Lastly know that the transmission ratios, the rear ratios and and the tire size all factor together. All three need to be taken into consideration.

[bensherb]

I've got a powerglide in my '46 Chevy, with a 283 and a 3.0/1 axle, all from a '62 Biscayne, and it works fine, at least as quick as most of the average new pigs on the road today. Or is it just the average driver today is too busy playing around with their phone and other misc crap to actually hit the gas when pulling away from a stoplight? Either way it's a fine driver.

[whitehawk759]

I am not sure where folks got the idea that you wanted to put a Powerglide in your car but I think your just asking about changing from a 3:31 to a 3:07. I have a 61 Hawk that had been changed to a 3:07, sluggish on take off. Changed back to the original 3:31, world of difference. I would leave well enough alone, unless yours is not a T/T , then I would change it. Although most of the Hawks had T/T stock.

[wittsend]

I am not sure where folks got the idea that you wanted to put a Powerglide in your car ...

From the original post: "I've seen some references (specifically for the 289-powered Hawk) to swapping the 3:31 differential to a 3.07. I see a number of posts (including June 2018 thread on "Changing '65 Borg-Warner to GM powerglide") "

I get your point but there was a bit of vagueness with statements like, "... with specifics of putting the stock 289's power to the road." And given the proceeding link to the Powerglide related post I'm sure many like myself felt it was at least a secondary point of consideration.

I'm even more confused because the main (first) car in the post regards on having an overdrive but there is discussion about swapping from 3.31 to 3.07 gears. Then there is wanting to know about a how to drop the RPM's in a non-overdrive car that would seem self explanatory given the experience from the first mentioned car which is stated to have overdrive (" The car has overdrive, and it works well.").

[cavanbound]

Sorry for the vagueness, guys. Maybe my inexperience showing through. Here's what I've got:

- 1960 Hawk: I'll go look for transmission tags, and report back. It's overdrive-equipped, and I'm not unhappy with the engine performance or how that translates through the tranny to the road. My inquiry was primarily because I've seen so many references to swapping out differentials, I wanted to understand what could be achieved. The car, in overdrive, is great to cruise in at 65-70 mph.

- 1962 GT Hawk: No overdrive, 3-spd automatic. Engine is to "R2" build (based on receipts/specs from a late 90s rebuild). This car has power, but feels wound up at 65-70 mph. A non-functioning Tach that I still need to rectify means that I can't give good specifics. However, gut feel is that I certainly wish I had overdrive. Additionally, because I live in the country, "town" is 65-70 mph for 10 minutes, so its relevant.

I really appreciate what's been offered here - thank you.

[wittsend]

Thank you for the clarification. The situation is now much better understood.

The term, "what you get in one situation, you give up in the other," is never more applicable with the gearing ratios of cars. The goal should be towards the type of driving you predominantly do. Those with the experience of overdrive cars have mentioned what they feel is the better ratio. If you go to a lower rear ratio (oddly identified by a higher number) you will get better acceleration at the cost of higher RPM's at speed and likely less gas mileage. If you go to a higher gear ratio (defined by a lower number) then the acceleration will be slower but less RPM's at speed and possibly better mileage.

It sounds like you are content with the 1960 Hawk but would like to have a lower RPM at speed in the 1962 GT Hawk. Thus if you are looking at swapping the rear ratio you will be giving up one aspect for the other. Another option is to change the transmission. Something like the Automatic 700R4 has a very low first gear but an effective overdrive. The T-5, 5-speed Manual does similar. The "spread" of the ratios offers better acceleration and lower RPM's for any given rear ratio.

All that said this can be taken too far. I had a Datsun 510 with 3:90 gears and 215-50-13" tires that was shifting into third gear halfway though an intersection (rear gears too low). Inversely my Studebaker has a 700R4 overdrive automatic, 3:07 gears and 225-60-16" tires and is at 1,600 RPM going 65 MPH on the highway (rear gears too high). A 350 Chevy's torque somewhat saves the problem from being too great.

As I stated earlier knowing your rear ratios would be helpful. Also, realize that changing tire side (diameter) has the same effect as changing the rear ratio but in most cases it is less that the standard jump in commonly available gear ratios. Anyway, I hope this is helpful and this time I'm, not the one causing confusion. Basically you need to define a goal yet understand the trade-off before venturing into changing the rear gearing ratio.