Rear end gears for automatic w/SBF

[60's Kid]

I am looking for someone that has had some experience with running a sbf with an automatic and what gear ratio's might be the best. I am just making plans for a 33 (hope to order before the end of the year) and putting together some doner parts. I've got a 302,AODE tranny, and the 8.8 rear end that I need. I am open as far as tire size so I can vary that later if need be. If anyone with some experience that can give me a ball park idea that would be helpful.
thanks Rick

[progmgr1]

I don't remember where I found it (probably on the forum, long ago), but I have an Excel spreadsheet gear / speed calculator that has a lot of standard transmissions built in. However, the forum won't allow me to upload a copy - it's an old version of Excel and the forum thinks that it's an "invalid file". PM me your email address and I'll send you a copy that way.
Keith

[Svtfreak]

Not sure what ratio your first gear is. But my mark 4 (close to same weight) is just about pierce twitch its T-5 (2.95 1st) and 3.55 rear cogs and 315/35/17 tires.

[Railroad]

I think it will be a toss up between 3.55 and 3.73. If you do a lot of highway driving, might want to do OD, tire, rear gear calculation to see where your rpm will be, at your cruise speed.
I considered an auto in my on going build of a roadster. Tony at FFR was definite about the stall speed of the converter. I will not get into that since your question was about gears.

[NAZ]

You're asking about what are the best gear ratios for running an auto with a SBF 302 but in the next sentence you list the parts you have collected including the 302, AODE trans and 8.8 rear end. So one can surmise that the question of what's best in regards to gearing is academic at this point since you have selected the gearing. Yes tire size is a variable but not nearly as much as gearing if you plan to run fenders that limit your size choices. The good news is that if you have an 8.8 with anything less than 4.10:1 and all you plan to do is cruise the street you'll likely be happy with whatever combo you end up with. The 33HR is a light car so it doesn't need a lot of power to make it fun to drive.

Typically when you select gearing you start with the intended purpose of the vehicle and look at the powertrain as a system. The engine power band, the weight of the vehicle, and the intended use (along with many other variables) will dictate the ideal gear ratios. A 302 is a generic term for a SBF that gets thrown around but really doesn't tell us anything about the power band as it could be a mild stocker or an all-out modified screamer running 20 PSI of boost. The AODE trans has a 2.4:1 first gear and a .67:1 OD so those numbers can be used along with the rear gear ratio and tire size to get an idea of performance. The Ford 8.8 can come with a variety of gearing (3.08, 3.27, 3.31, 3.55, 3.73, & 4.10) so you'll need to determine what you have so you can bake that number into your performance calculation. And finally tire diameter will give you the RPM in OD at highway speed which will probably be the most important number you'd be looking at for a typical street driven car.

Lastly, as was mentioned above the torque converter stall speed is an important consideration, not only from a performance aspect but also to make the car more enjoyable to drive. Holding 800 PSI of brake pressure at a signal to keep from rolling into the intersection is not enjoyable. The best thing you can do is to contact a performance converter manufacturer and have them spec a convertor that will work well in your application. Expect to provide them with details of your intended use, transmission, car weight, and engine bore and stroke as well as peak HP & torque and RPM of the peaks. A custom converter may cost you $400 or more but is worth it.

[60's Kid]

Good info...thanks for the responses. The rear end has 2.73 gears and I plan on changing. I am planning on a mild engine build...350 to 400 hp.
The convertor I plan on looking at when engine is done. I was looking for info of builds that had the set ups. This is very helpful.

[60's Kid]

Good info...thanks for the responses. The rear end has 2.73 gears and I plan on changing. I am planning on a mild engine build...350 to 400 hp.
The convertor I plan on looking at when engine is done. I was looking for info of builds that had the set ups. This is very helpful.

[myjones]

You're asking about what are the best gear ratios for running an auto with a SBF 302 but in the next sentence you list the parts you have collected including the 302, AODE trans and 8.8 rear end. So one can surmise that the question of what's best in regards to gearing is academic at this point since you have selected the gearing. Yes tire size is a variable but not nearly as much as gearing if you plan to run fenders that limit your size choices. The good news is that if you have an 8.8 with anything less than 4.10:1 and all you plan to do is cruise the street you'll likely be happy with whatever combo you end up with. The 33HR is a light car so it doesn't need a lot of power to make it fun to drive.

Typically when you select gearing you start with the intended purpose of the vehicle and look at the powertrain as a system. The engine power band, the weight of the vehicle, and the intended use (along with many other variables) will dictate the ideal gear ratios. A 302 is a generic term for a SBF that gets thrown around but really doesn't tell us anything about the power band as it could be a mild stocker or an all-out modified screamer running 20 PSI of boost. The AODE trans has a 2.4:1 first gear and a .67:1 OD so those numbers can be used along with the rear gear ratio and tire size to get an idea of performance. The Ford 8.8 can come with a variety of gearing (3.08, 3.27, 3.31, 3.55, 3.73, & 4.10) so you'll need to determine what you have so you can bake that number into your performance calculation. And finally tire diameter will give you the RPM in OD at highway speed which will probably be the most important number you'd be looking at for a typical street driven car.

Lastly, as was mentioned above the torque converter stall speed is an important consideration, Holding 800 PSI of brake pressure at a signal to keep from rolling into the intersection is not enjoyable. The best thing you can do is to contact a performance converter manufacturer and have them spec a convertor that will work well in your application. Expect to provide them with details of your intended use, transmission, car weight, and engine bore and stroke as well as peak HP & torque and RPM of the peaks. A custom converter may cost you $400 or more but is worth it.

All of what Naz said is true to get the right TC for performance but it's driver comfort that also comes into play on the 33 with it's underpowered brakes.
FF has taken a power brake system and it's parts and drastically reduced the line pressure/clamping pressure by going back to a manual set-up with it.
SO
When you talk to a convertor company be very specific and say that it's not the stall speed that's the problem it's the (flash) speed.
With minimal brakes on the 33 it's not the lock-up or "stall speed" that's the problem, it's the flash speed or the RPM's where the car will start to creep
forward and overcome the braking power at idle. You won't be anywhere near even an 1800 stall at a stop sign but you will be into the flash range of
it so it will be moving the car.
The typical way to control this is to go to a higher than ideal stall speed for your combo so that the flash speed also rises to a point that won't make the
car creep forward under a reasonable amount of pressure on the brake pedal. Go read up on those two terms and see how they relate here. HTH

Dale

[NAZ]

If your rear end really has 2.73:1 gears (or was that a typo and you meant 3.73:1?) I don't think you're going to like that combo. With 26" diameter tires it will move down the highway at 75 MPH running ~1775 RPM which would be close to the sweet spot if you had a Scorpion engine like my F550 instead of a 302 SBF. My car idles at 1000 RPM and would barely crawl out of its own way at 1800 RPM down hill (4500 stall speed torque converter). Now if you had the same tires and a 3.55:1 rear end ratio your 1st gear total ratio would be 8.52:1 which even 300 HP would seem sporty in a light car and your 75 MPH cruise would have the engine running ~2310 RPM.

[TxMike64]

Just to confirm the OP's numbers: there were quite a few '94-'04 Mustangs with 2.73 ratio rear-ends - I know it seems like a typo, but it's true. A vast majority of GTs had 3.27 tho.

[60's Kid]

The rear end had tag on it indicating 2.73...and I have opened it up and by counting teeth on ring gear and pinion...it is a 2.73...not a typo.
That's why I'm asking questions. I figure it's too low.
Thanks for the TC info, Dale.