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post #1 of 33 (permalink) Old 08-11-2010, 08:16 PM Thread Starter
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7.5 IRS Limited Slip

I apologize if this is basic knowledge, I'm very new to Thunderbirds, so please forgive me. I've been hunting around for some information on a 7.5" IRS differential. I'm very interested in acquiring a limited slip version of this and I can't seem to find hard facts about what model of t-bird, cougar or mark viii (maybe there's even more!?) would have a 7.5" IRS limited slip differential (traction lock). As far as I can find, it would be a 3.8 LX??? Are there more?

With a little searching around here I've found that solid axle diff's cannot work in an IRS setup because there is no groove in the side gears for the half-shaft c-clip to latch onto. Correct?

My end goal is to either buy or piece together a full pumpkin/carrier assembly with the following specs: 7.5", IRS, limited slip, 3.08:1 gears. If anybody knows of somebody that's selling a differential like this/close to this or, better yet, a full carrier with all this installed I'd really appreciate it! (new member, can't post in the marketplace without a certain # of posts ).

Thanks for any help you can give me!

Last edited by foxk56; 08-11-2010 at 09:28 PM.
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post #2 of 33 (permalink) Old 08-11-2010, 09:42 PM
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Really, really silly question.

Why not the 8.8"?

I ask because my local U-Pull-Its have two 3.08 TLs and four 3.08 opens just awaiting someone to pull them and lay down $45 (yes, $45 for the open or the TL .. )

The 7.5 is the same price, but at least in 1991 (the year I have the factory service manual for), they don't list a TL 7.5. That is, TL was only in 8.8, 7.5 was always open.

If it must be a 7.5, then the 7.5 for most years was a normally aspirated (not-supercharged) 3.8 . The supercharged 3.8s got either, and the V8s always got 8.8s.

Open and TL was another matter ...

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post #3 of 33 (permalink) Old 08-12-2010, 07:43 AM Thread Starter
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Really, really silly question.

Why not the 8.8"?

I ask because my local U-Pull-Its have two 3.08 TLs and four 3.08 opens just awaiting someone to pull them and lay down $45 (yes, $45 for the open or the TL .. )

The 7.5 is the same price, but at least in 1991 (the year I have the factory service manual for), they don't list a TL 7.5. That is, TL was only in 8.8, 7.5 was always open.

If it must be a 7.5, then the 7.5 for most years was a normally aspirated (not-supercharged) 3.8 . The supercharged 3.8s got either, and the V8s always got 8.8s.

Open and TL was another matter ...

RwP
Why not an 8.8? Because it's way overkill for the type of vehicle I'm planning to build. Something like this



I've done a lot of research and it's told me that an ideal differential for this type of machine is a 7.5" ford since it is compact and comes in many different gear ratios. I know that traction lock/limited slip comes in 7.5's, be it solid axle or IRS.

I found a manufacturers specifications PDF from the sccoa.com website that shows that some thunderbirds DID come with traction lock in a 7.5 size: http://www.sccoa.com/articles/thunderbird_1991.pdf and http://www.sccoa.com/articles/scspecs.php .

I'm still slightly confused about specifically which thundebirds, cougars or mark 8's I need to be looking for though. If somebody could help me out I would appreciate it. Maybe one of the easier routes to go would be to fit a 7.5 solid axle traction lock diff inside a 7.5 irs carrier since the solid axle 7.5 TL's seem to be more plentiful (Rangers, Mustangs, etc???).
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post #4 of 33 (permalink) Old 08-12-2010, 07:59 AM
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We have a 7.5 TL at the shop. My boss had originally given it to me for my Bird cause we thought it was an 8.8. But after we inspected it closer, it proved to not by the 8.8 we had hoped. lol

I'm not sure what ring gear is on it, but it is different and we don't have the matching pinion. So you'd need a set of gears for it.

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post #5 of 33 (permalink) Old 08-12-2010, 08:19 AM
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7.5

Hello

I have both, But not together.

I have a stock 7.5 with an open 3.27 ratio
My 7.5 TL came from a mini van with 4.10's

I used the 4.10's not the TL

Will sell you both $100

I am in western NY, Between Buffalo & Niagara Falls


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post #6 of 33 (permalink) Old 08-12-2010, 08:41 AM Thread Starter
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post #7 of 33 (permalink) Old 08-12-2010, 10:26 AM
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Quote:
Originally Posted by foxk56 View Post
Why not an 8.8? Because it's way overkill for the type of vehicle I'm planning to build. Something like this



I've done a lot of research and it's told me that an ideal differential for this type of machine is a 7.5" ford since it is compact and comes in many different gear ratios. I know that traction lock/limited slip comes in 7.5's, be it solid axle or IRS.

I found a manufacturers specifications PDF from the sccoa.com website that shows that some thunderbirds DID come with traction lock in a 7.5 size: http://www.sccoa.com/articles/thunderbird_1991.pdf and http://www.sccoa.com/articles/scspecs.php .

I'm still slightly confused about specifically which thunderbirds, cougars or mark 8's I need to be looking for though. If somebody could help me out I would appreciate it. Maybe one of the easier routes to go would be to fit a 7.5 solid axle traction lock diff inside a 7.5 IRS carrier since the solid axle 7.5 TL's seem to be more plentiful (Rangers, Mustangs, etc???).
I think you'll find that a traction lock for an 8.8 is a dime a dozen where as the 7.5 is much harder to find. Additionally, I think the gear selection for the 8.8 is also considerably better than for a 7.5. I don’t know if exterior dimensionally if they are the same though.

And what is wrong with overkill when it comes to an IRS differential? I can't imagine a 7.5 is that much heavier than an 8.8 (especially when the 8.8 is available in aluminum), nor can I believe the additional rotating weight of the 8.8 ring and pinion would be noticeable.

No Mark VIII ever came with a 7.5 nor did any of them come traction lock differentials, but all the Mark VIII housings are all aluminum.

Just my .02, but I really think you should consider the 8.8.

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post #8 of 33 (permalink) Old 08-12-2010, 10:58 AM
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On a solid axle TL, you can have someone machine the side gears to be more IRS friendly. But if your building something like that, you should be able to do it yourself. It is a bevel so that the axle with retainer will be able to be pushed in and fully engaged. There is an engineering like diagram floating around here someplace.

I have a 7.5 TL out of a Ranger I was going to put in my 89 Cougar, along with gears, before I basically just gave up on the whole car and let it sit.

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post #9 of 33 (permalink) Old 08-12-2010, 11:00 AM Thread Starter
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I think you'll find that a traction lock for an 8.8 is a dime a dozen where as the 7.5 is much harder to find. Additionally, I think the gear selection for the 8.8 is also considerably better than for a 7.5. I don’t know if exterior dimensionally if they are the same though.

And what is wrong with overkill when it comes to an IRS differential? I can't imagine a 7.5 is that much heavier than an 8.8 (especially when the 8.8 is available in aluminum), nor can I believe the additional rotating weight of the 8.8 ring and pinion would be noticeable.

No Mark VIII ever came with a 7.5 nor did any of them come traction lock differentials, but all the Mark VIII housings are all aluminum.

Just my .02, but I really think you should consider the 8.8.
Yeah, I bet you're right with 8.8's being easier to find. But with all things considered, the 7.5 would be a better choice (plus, read above, 2 people already offered to sell me theirs, so it's not impossible!). This project is for a bike-engined car. I've got lots of time to find the right carrier assembly, so it doesn't hurt to shoot for the stars. Here are some considerations with my project:
- Minimize weight; sure the 8.8 or even the aluminum 8.8 for that matter might be only marginally heavier, but why not try for lighter
- Minimize size; I havn't ever seen the two side by side, but I'd have to assume the 7.5 is probably smaller. Leads to better packaging and lower sizing requirements.
- HP and Torque; I definitely won't be producing anywhere close to V6 or V8 power in this car. A typical 1 liter motorcycle engine produces roughly 75 lb-ft of torque. I wouldn't be surprised if even the 7.5" could handle upwards of 300 lb-ft. Plus, the additional 1.3" diameter on the 8.8" setup is physically tougher to start spinning because of it's greater inertia, which saps horsepower
- Torque biasing; I'd have to assume that the 7.5 has a less aggressive 'limited slip' function, as in it more closely resembles an open differential versus the 8.8, not as "locked". This car will be anywhere from 1/4 to 1/3 the weight of a conventional car, so that is less weight to force the traction lock to slip as you're going around a corner

So for many reasons, I'm not considering an 8.8

Last edited by foxk56; 08-12-2010 at 11:19 AM.
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post #10 of 33 (permalink) Old 08-12-2010, 11:16 AM
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Can you please provide the information you're quoting? I'm curious to know the actual weights of an iron 7.5 T/L, iron 8.8 T/L, and an aluminum 8.8 T/L. (I think an aluminum 8.8 T/L is about 65 lbs).

I didn't know the 7.5 housing and everything was smaller. I just thought it was basically the ring and pinion. Does anyone have any comparison photo's?

Again, just curious since you’ve made your decision.

But a tip on used T/L's.... get them rebuilt. IMHO the Ford clutch packs seem to be shot after about 50k miles if they are behind anything with any power (which is usually where they were installed from the factory). Or at the very least do the torque break away test to ensure the clutches are still good. The manual has the test procedures.

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post #11 of 33 (permalink) Old 08-12-2010, 11:35 AM Thread Starter
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Can you please provide the information you're quoting? I'm curious to know the actual weights of an iron 7.5 T/L, iron 8.8 T/L, and an aluminum 8.8 T/L. (I think an aluminum 8.8 T/L is about 65 lbs).

I didn't know the 7.5 housing and everything was smaller. I just thought it was basically the ring and pinion. Does anyone have any comparison photo's?
Like I said I've never seen comparison photos, and I've also never researched comparison weights. I'm going off of educated assumptions. If 8.8's were installed in V8's and 7.5's in V6's (roughly) then I'm gunning for the "smaller" of the two because of my application.

Quote:
But a tip on used T/L's.... get them rebuilt. IMHO the Ford clutch packs seem to be shot after about 50k miles if they are behind anything with any power (which is usually where they were installed from the factory). Or at the very least do the torque break away test to ensure the clutches are still good. The manual has the test procedures.
Thank you very much for the tip, that is helpful
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post #12 of 33 (permalink) Old 08-12-2010, 11:44 AM
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I mis spoke on the differences between the side gears for solid axles versus IRS. The bevel is there so that you can REMOVE the axle. Without the bevel, the retainer can't be pulled through without somehow compressing it.

So you can insert the IRS axle with the retainer, and won't simply fall out in a normal situation, but if you needed to remove the axles, you would be fighting with it. Some here say they run without the retainer on the axle.

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post #13 of 33 (permalink) Old 08-12-2010, 12:24 PM Thread Starter
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I mis spoke on the differences between the side gears for solid axles versus IRS. The bevel is there so that you can REMOVE the axle. Without the bevel, the retainer can't be pulled through without somehow compressing it.

So you can insert the IRS axle with the retainer, and won't simply fall out in a normal situation, but if you needed to remove the axles, you would be fighting with it. Some here say they run without the retainer on the axle.
Yeah I've seen numerous threads on here saying people run without the retaining clips. I wonder if, alternatively, you could put a bevel on the retaining clips instead of the gears? Maybe if you do it enough to have the same effect though you risk the clips breaking apart and causing mayhem inside the carrier. It would certainly be easier than disassembling and cutting the bevel in the side gears though.

And yes I've found both a drawing and instructions on how to cut the side gears. Here: http://forums.tccoa.com/showthread.php?t=126533 and here: http://forums.tccoa.com/showthread.php?t=119374
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post #14 of 33 (permalink) Old 08-12-2010, 12:25 PM
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For weight, I don't have actual numbers, but having held the iron 8.8, the aluminum 8.8, and the iron 7.5 I can tell you the aluminum 8.8 is much lighter. As for external dimensions, the 8.8 is not that much bigger externally, and will mount in the exact same place. The axles are the same. Unless you already had a 7.5TL laying around, I can't see any disadvantage to going with the 8.8.

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post #15 of 33 (permalink) Old 08-12-2010, 12:40 PM Thread Starter
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For weight, I don't have actual numbers, but having held the iron 8.8, the aluminum 8.8, and the iron 7.5 I can tell you the aluminum 8.8 is much lighter. As for external dimensions, the 8.8 is not that much bigger externally, and will mount in the exact same place. The axles are the same. Unless you already had a 7.5TL laying around, I can't see any disadvantage to going with the 8.8.
Thanks for the information. I'll keep the 8.8 as a backup idea.
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post #16 of 33 (permalink) Old 08-12-2010, 06:28 PM
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For weight, I don't have actual numbers, but having held the iron 8.8, the aluminum 8.8, and the iron 7.5 I can tell you the aluminum 8.8 is much lighter. As for external dimensions, the 8.8 is not that much bigger externally, and will mount in the exact same place. The axles are the same. Unless you already had a 7.5TL laying around, I can't see any disadvantage to going with the 8.8.
I used to think they weren't that different, either. Until I saw two Mustangs up on racks side by side at work a few months ago. One had a 7.5 and the other an 8.8; I gotta tell ya, the 8.8 looks HUGE next to the 7.5. I was literally shocked.....

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post #17 of 33 (permalink) Old 08-12-2010, 10:23 PM
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The 8.8 is about as high as it is round on a MN12, the 7.5 isn't as high as it is wide (but it's the same width).

Use an aluminium 8.8 out of a Mk VIII and use a 8.8 gear set with TL out of a TBird/Cougar - best of both worlds (lightest of the three choices, etc.)

'Course, you could also drop the friction of the traction lock and run open ...

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post #18 of 33 (permalink) Old 08-15-2010, 06:34 PM Thread Starter
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Will this set of gears fit in a 7.5 IRS pumpkin from a t-bird or cougar? http://cgi.ebay.com/ebaymotors/ws/eB...T#ht_500wt_807
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post #19 of 33 (permalink) Old 08-15-2010, 07:01 PM
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Theoretically, yes, but if you are running a motorcycle engine, all of which like to rev to like 10K, you definitely don't want a 2.73 rear. Look for at least some 3.73s, or possibly even 4.10s

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post #20 of 33 (permalink) Old 08-15-2010, 07:21 PM Thread Starter
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Theoretically, yes, but if you are running a motorcycle engine, all of which like to rev to like 10K, you definitely don't want a 2.73 rear. Look for at least some 3.73s, or possibly even 4.10s
4.10's are way out of the realm of my project scope. At highway speed I would be turning 7k RPM. No thanks.

This will be a mostly road use car. I've done the calculations. Using an FZ1 motor (my preferred) and 2.73 gears my overall 6th gear ratio would be 4.8125 and a full FZ1 motorcycle's 6th gear overall ratio is 4.8477, so it's very comparative. At highway speed I will be close to 5k rpm's, right in the meat of the powerband, and I don't want to go much higher, for driving comfort. I would also probably be happy with 3.08's, but the ebay gears I just found are cheap. Believe me, I'm asking the right questions.

So any other answers to whether those ebay gears will fit the case or not?

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post #21 of 33 (permalink) Old 08-15-2010, 08:02 PM
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OK, I was thinking along the lines of a car transmission, but if that is how the gear ratios work out, then go for it.

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post #22 of 33 (permalink) Old 08-15-2010, 09:16 PM
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I bought cheap ebay gears once, got a beautiful pattern and they sounded like a dump truck going down the road. Good used 7.5 3.73 gears are plentiful. Most 4cyl rangers had them.
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post #23 of 33 (permalink) Old 08-15-2010, 09:32 PM Thread Starter
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Thanks for the input Alan. Hmm, hopefully that was just a lemon situation or the seller wasn't honest. Oh well, I'm not out much money if they don't work.

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post #24 of 33 (permalink) Old 08-15-2010, 10:35 PM
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I will try to get pictures of both 7.5 and 8.8 rear pumpkins under Mustangs so you all can see the differences....

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post #25 of 33 (permalink) Old 09-23-2010, 09:36 AM Thread Starter
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I just picked up a 7.5 IRS full pumpkin and 7.5 TL from tbirdtess (thanks!). On the subject of IRS axles, he mentioned that I might be able to avoid machining a bevel into the side gears in the 7.5 TL (which came out of a solid axle and thus do not have the beveled edge for the c-clips) by swapping the open differential side gears inside the pumpkin he sold me into the trac-lok unit he also sold me. Do any of you know if this is a viable option or not?
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post #26 of 33 (permalink) Old 09-23-2010, 09:48 AM
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Depending on the year TL, it might not matter. I know all the new TL's are usable for both solid or IRS rear ends, but I don't know when they went to that design.

I don't think the side gears are interchangable between TL and non-TL.

But hopefully others will chime in with specifics.

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post #27 of 33 (permalink) Old 09-23-2010, 10:02 AM
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The open diff side gears will not work with the trak-lok because they don't have the teeth machined into them for the clutches. You will need to machine the bevel on the side gears, unless they already have one. The simple solution is take the diff apart, and slide just a side gear on an axle and see if you can get it off. If it won't come off relatively easily, then you would be screwed if you put it in the car like that because you'll never get it out.

As for the new ones already having the bevel, I think that is only for the 8.8 diffs. Ford doesn't have any newer cars with a 7.5" diff and IRS, so they probably wouldn't waste time machining the 7.5 gears, but the 99+ cobras had the 8.8 with IRS, so it was cheaper to just make all the gears the same.

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post #28 of 33 (permalink) Old 09-25-2010, 06:59 PM Thread Starter
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Thanks for the info guys! I was just curious, and even if it did work I most likely for peace of mind would have just beveled the gears anyways so I wasn't worried about the fit or the teeth on the new side gears having a poorly matching wear pattern or something.

Another general question about the 7.5 TL's.... will all available R&P ratios fit on the same differential? I was talking about my project with my father and he mentioned that, for example, some chevy differentials will only accept a range of R&P ratios, and then if you want to go much higher or lower than the original you need to get another differential that is just slightly different that will accept another range of ratios. He got me curious if that was the case with Ford or not. Most likely I will stick with 2.73 or go just slightly higher to 3.08

Thanks again!

-Jason
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post #29 of 33 (permalink) Old 09-27-2010, 04:01 PM
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Nope, ford does not have that problem. If the gears say they are for a 7.5, then they will work in a 7.5.

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post #30 of 33 (permalink) Old 09-28-2010, 05:41 AM
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Quote:
Originally Posted by MadMikeyL View Post
Nope, ford does not have that problem. If the gears say they are for a 7.5, then they will work in a 7.5.
Agreed, but I think with some sets and higher gears, you have to use a smaller (or different) cross shaft.

I did a quick search for 7.5 R&P's and found this:
http://www.drivetrain.com/parts_cata...ion_gears.html

Note 3.73's and 4.10's
Quote:
*Requires cross shaft F7.5CS (will not fit Auburn carrrier)
And this:
http://www.precisiongear.com/ford7.htm

Note 4.56's
Quote:
• Requires a modified cross shaft for installation. Use part number F7/XSM for open and factory L/S differentials. Use F7/AKIT for Auburn differentials.
But I'm not that up on 7.5's.... (or 8.8's for that matter). Just something to keep in mind and/or research more.

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