28 Spline or 31 Spline

[Shelbyco427]

I am totally new to this whole process so I apologize up front on any idiotic questions that might come up. I'll start with the 1st one regarding a donor rear end that I picked up. I picked up a donor 8.8 axle out of a Mustang with the tag that reads S223B 2L73 8 8 3L22. Out of this I assume a 2.73 Limited Slip which at 1st internal glance I agree. I am curious on the date code if anyone knows what YEAR it is as I believe the month was December and 22nd day... so a Christmas baby? I also see that it is a 28 spline when I pulled the axles.

Here is my big question. If I am simply going to keep the 2.73 gearing and build a 351W, possibly bored out for 427 but going to use as a pleasure car and not a track car, will I be concerned if I kept the 28 spline setup? I am going to need to upgrade to get the Fox Body length axles for my build, but I really don't want to tear apart all the internals as they are in great shape.

Am I asking for potential problems if I don't plan on hammering the pedal in regards to destroying the 28 spline axles?

[svassh]

You'll be just fine with a 28 spline. Not likely to break one unless you were doing clutch dumps with slicks on the car.

[Jeff Kleiner]

As he said it'd take a REALLY sticky tire and a hard shock to hurt it.

With that said, you might want to do the math and put some heavy thought into the 2.73s. Even if you go to the TKX with the lower 3.27 first gear that's still a pretty tall first overall of 8.92:1. Additionally, if you use the TKX with a 3.27 first the fifth gear ratio is .72 resulting in a 1.96 overall final. You won't be able to effectively use fifth gear until >80MPH. I think even with a torquey 427 a 3.27 or even 3.08 would be a better choice. If you go to the TKX with 2.87 first and .68 fifth I suggest coupling it with a 3.55 rear (10.18 overall in first and 2.41 overall in fifth).

Jeff

[Shelbyco427]

Now you have me jumping into the deep end already. However, I'm game for some additional education. I blindly did a comparison of different gearing, but I have no idea of what I am looking at outside of the obvious. I believe I have the same data as you in regards to the trans gear ratios, but I only equated that to what theoretical speed that would be at 5500 rpm shifting (i.e. 1st gear to 2nd gear on a 2.73 with a trans gear of 3.27 would be 36.15 mph). However I could be completely ignorant on this and ABSOLUTELY am when it comes to understanding your 8.92:1 and 1.96 overall final.

Do you have any resources you can point me to to help further my education while I wait on my kit to show Mid May?

Thanks for reaching out and offering help. I am quite mechanical, but engines passed me by in my late teens and now I'm playing catch up.

[Jim1855]

I ran 55k miles with a 28 spline behind my 427 w/o any problems. I do have a friend that ran 28s with his 462 SBF. He twisted an axle and broke it with drag slicks on a hard launch. No damage to the splines.

I ran 3.27s with a TKO600 and the .64 OD. First gear was 2.87, overall a great combination. Other than switching to 3.31s and the 2015 Mustang IRS this build will use the same pieces.

Just a thought, explore your options on building (bore & stroke) a standard Windsor block to 427. Not all blocks will safely go there.

So now that you have more information than you asked for, I wish you well. Stop back with more questions, this site is a great resource.

Jim

[Jeff Kleiner]

....However I could be completely ignorant on this and ABSOLUTELY am when it comes to understanding your 8.92:1 and 1.96 overall final.

Do you have any resources you can point me to to help further my education while I wait on my kit to show Mid May?

It's just simple math. To calculate the overall reduction in a given gear multiply the transmission gear ratio by the rear end ratio, hence my example of a 3.27:1 first gear coupled with a 2.73:1 rear is 3.27x2.73=8.92 and so on. Personally, for our cars I like to see a first gear overall that is >10:1 and <11:1 and for fifth I like to see an overall somewhere in the neighborhood of 2.2:1 to 2.4:1.

Jeff

[BUDFIVE]

Shelbyco427-I think most of us engineers have a spreadsheet. this picture is a spreadsheet in Google Sheets, much like Microsoft Excel. You enter the blue fields and green fields with bold numbers, and the spreadsheet fills in the gray cells which are the RPMs at each speed in each gear. This data is for a close ratio TKX (18084), 3.55 differential gears, and 315/35-17 tires. I don’t know how to put a spreadsheet into the forum post. If you would like the spreadsheet, PM your email to me and I’ll send it, either as a excel file or sheets file.

IMG_0972.png

[AndrewIdaho]

It's just simple math. To calculate the overall reduction in a given gear multiply the transmission gear ratio by the rear end ratio, hence my example of a 3.27:1 first gear coupled with a 2.73:1 rear is 3.27x2.73=8.92 and so on. Personally, for our cars I like to see a first gear overall that is >10:1 and <11:1 and for fifth I like to see an overall somewhere in the neighborhood of 2.2:1 to 2.4:1.

Jeff

Tremec has a very nice calculator that is fun to play with to see the impacts of gearing and RPM for each gear ratio: https://www.tremec.com/calculadora.php

Best Regards, Andrew

[DanQ]

I am running a Ford 8.8 rear axle with 3.00 gears in my '33 hot rod, and drive mine just like you described you will, a mellow cruise, going out for an ice cream cone, to a car show, etc. For that application, I would not worry about the 28 spline axles or the 2.73 gears, in fact you will love the lower engine RPM while cruising down the highway, appreciate the lower noise level, and the increased fuel economy. With that light of car driven mildly, that axle should suit you just fine. Besides, that is something easily changed down the road should you not like it.

[Dgc333]

I recommend that you build your car and drive it for awhile before deciding to change out the 2.73 gears.

I picked up a used axle with 2.73 gears and fully expected to have to rebuild it which I would have done with 3.55 gears. But once I opened up the rear the gears were in great shape so I just replaced the axle bearings, seals and installed the 5 lug axles.

My Speedstar is not quite finished but I do have about 30 miles on it. Even with 305/35-20 tires on the back the 5.7 Hemi will effortlessly break the tires loose in 1st and 2nd, it will also break the tires loose in 3rd if I try. At this point I can't see any reason I would want to change from the 2.73s.

[Railroad]

I would have no problem looking for a set of gears someone has taken out of their ride.
I have used numerous sets from junk yards and enjoyed them.
My point is to find some Mustang clubs and gears they have available.
If your location was posted, you might get some offers.
Good luck,

[Railroad]

I would have no problem looking for a set of gears someone has taken out of their ride.
I have used numerous sets from junk yards and enjoyed them.
My point is to find some Mustang clubs and gears they have available.
If your location was posted, you might get some offers.
Good luck,