MRM331

I think 3.27 is generally favored because factory 7.5's with 3.08's and Traction Loc are pretty rare if they ever existed at all. The rpm's at each mph with a Mustang tire diameter and a 3.27 were almost identical to the rpm's at each mph with a 23" Miata tire and 3.08.

-Jason

Rabitz

Awesome guys...just awesome.

I have an opportunity to get a cougar with a 3.27 rear, but it is open. Is it expensive to convert to a locker?

MRM331

Depends on what type of limited slip you want. Unfortunatly the Ford Traction Loc unit is no longer available from Ford although Yukon Gear makes one for a bit more than its worth. There are a few options in the $400 range but they are all more intended for drag racing than street/road racing. I'm looking at an Eaton True-track for mine which runs around $560.

Honestly I see no problem, nor have I had one with the Traction Loc on the street. If I were not trying to set the car up for 300-mile endurance sessions I'd just leave it as is (although I'd still swap out the ring gear and pinion). If you're interested I could sell you my 7.5 traction loc diff carrier after I pull the rear for its rebuild this winter. I'll even throw in the bottle of extremely stinky Ford friction modifier I was going to use if I kept it. It should be good to go as is although it could be rebuilt if desired (clutch pack rebuild kits are about $160 from Yukon).

-Jason

Rabitz

oh snap, PM sent.

V8droptop

That's not a bad deal on the 7.5. I kinda hadn't looked at that option before, but it has some nice features to it for sure.

I wouldn't discount the aluminum 8.8 as an alternative, though. Through your research you may know the axles are going to be a big ticket item on the diff. With the 8.8 some of us have found out how to get away with Factory Five racing axles for about $150/axle, which really sweetened the deal for me. So, I have a Mark VIII diff I got for $120 with axles, diff, and driveshaft, with 3.08s and trac-loc. I then took it, got a Ford Racing Torsen for around $300 on the mustang forums. Now I'm putting in 3.73s and will have a bullet-proof rear. It is a bit of coin, but I'm under $1000 installed with the V8r kit, and take out the torsen and you'd be more near $500. The caveat, you have to use 5-lug hubs for the axles I mentioned. You could either redrill t-bird hubs to 5x114.5, or look into trying to drill t-bird hubs to 4x100 which I've not heard of yet.

The only big benefit to the 8.8 for you would likely be parts availability, and that may be a non-issue, but if you have some near-by I'd check it out. There were about half dozen part outs on craigslist near me, where I picked up my parts.

P.S. there are a lot of mazda-speeders in your area, Eugene and Portland, I'll see if I can get their contact info.

Rabitz

So much good information here.

If I find a locked diff in the pick N pull, I wouldn't need to get a torrent or anything like that would I? The 8" sounds great, bit all the extra stuff I have to do with axles and hubs seems cost prohibitive for me...

Is the issue with the Mk VIII aluminum 8" that it only comes open diff?

charchri4

Yes the aluminum diffs were only open. They are also a bit tricky to work on as you need a case spreader to set them up.

Sunshine Guy

It is interesting that Jason created a chart to help him better visualize gearing choices. I recently did the same thing. I would highly recommend this process for anyone that really wants to think through their transmission and differential gearing options, and how each possible combo will affect drivability. Actually looking at the mph versus rpm numbers in each gear for different combos, visually, side by side, will allow you to see things you might never realize if each possible combo is just viewed individually. My chart is attached as an example.

I looked at six possible combinations of transmission and differential gearing. I am planning a very streetable engine with good low speed driveability down to 1500 rpm, a broad torque range and a red line of 5500 rpm.

My car currently has a standard, Mustang 5.0L T-5 with a 3.27 posi rear. It is "Combo A" on the chart. This is generally considered to be the "standard" Monster Miata Ford set-up. Honestly, I don't like it. Low gear is like granny low in a 1 ton pick-up. The car will only barely exceed 35 mph in 1st gear at the 5500 rpm red line, and that is with 24" tires that are taller than most on these conversions. Everyone talks about how available tires limit traction on these cars. So why start with granny low gearing that is just going to exacerbate wheelspin?

I could write many, many paragraphs on all the things I see by looking at this chart, but the gearing needs to be chosen based on each individual's engine characteristics, so I won't bore you with all my specific details. The bottom line is, money no object, for a mild motor combo the T-5Z transmission with the Astro Transmission's optional .73 fifth gear set and a 3.08 in the rear would be a great choice. It is interesting that Jason, looking at his chart, has come to a similar conclusion on the 3.08 or taller rear ratio.

The slightly taller first gear set in the T-5Z tranny coupled with the slightly taller rear gears nets out a jump from 35.8 mph at 5500 rpm in 1st gear to 43.1 mph in first gear. I guarantee you that would be far more usable and practical. Cruise rpm at 70 mph on my current combo is 2179 rpm (3.35 low/.68 overdrive tranny with 3.27 gears), while the 70 mph cruise on the 2.95 low/optional .73 overdrive T-5Z with 3.08 gears would be 2204 rpm...surprisingly close! The .73 overdrive is a great compromise between a pure track ratio and an economy-only, super tall gear.

The problem with that combo in my case is it would cost me a boatload of money to make the changes. I already have a newly rebuilt, standard Mustang T-5 and a new, Martin built 3.27 rear end. So I'd be out somewhere around another $2000 for the T5-Z with the optional .73 fifth gear set plus maybe another $600 for the differential gear change.

So what looks like a practical solution to me is staying with the transmission I have and swapping out to a 2.73 rear gear. I know that statement is drawing gasps of disbelief and/or laughter, but look at the numbers...they don't lie!

First gear with the standard Mustang T-5 and a 2.73 rear is virtually identical in overall gearing to the T-5Z and 3.08, and nobody looks at you like you have three heads if you suggest you are going to use a 3.08. The car with a 2.73 rear gear will top out at 119.9 mph in fourth (direct) gear at 5500 rpm redline. That is NOT ridiculously tall gearing for a 325 HP, 2500 lb. aerodynamic car.

Because I am going to build a very flexible, well-mannered motor designed to pull cleanly as low as 1500 rpm, I can cruise that 2.73 / .68 overdrive combo all the way down to about 59 mph, and have a nice, relaxed 1820 rpm at 70 mph. So how then, as conventional wisdom would have it, is a 2.73 ridiculously over-geared? I would never have visualized this without creating the chart. That's why I think it's a worthwhile exercise, especially for anyone still in the planning stages of their car.

Meierznutz

Ford made ID'ing gear ratios on their IRS diffs pretty easy.... 3_73 is an open and 3L73 is a Trac-Loc in 3:73 ratio... I ran a Cobra 8.8 IRS and the only difficulty I had using it was trying to get the exhaust past the diff/axles. If I had the time to leave the car in Mass for the plumbing, I am sure we could have made it better. Instead we flattened out the pipe a bit and tucked it under the center of the rear end. Thinking back I could have done 2 1/2" duals into one 3" pipe past the diff on the passenger side and into the muffler.
For what it's worth, 3.73's with 205/50/15 tires gave me just under 2000rpm @80MPH in 6th gear.

tbone heller

Maybe it would work in a V-8 Miata, but the T-5 & 2.73 rear ratio was a horrible combo in my 1988 Mustang GT. Driveability as well as fuel mileage increased when Ford warranty paid me to replace it with a 3.55. LOL!

The issue that I see with that combo would be that the low RPM's at cruise speeds would not match up with the camshaft's "sweet spot" & cause a sluggish, crappy, no fun car.

Sunshine Guy

Tbone's point about matching cruise rpm to the engine's power band is an important one. A lot of Ford 5.0L engines end up with a cam in them somewhere near the 224 degrees @ .050 range, often with a lobe separation angle of 112 degrees. With that valve timing in a small displacement engine, you are going to want to be above about 2200 rpm to get any grunt out it, and 2500 is better.

I'm looking to do some decent heads with good low and mid-lift flow paired with a bit shorter cam timing of 217 degrees and a 113 degree LSA, so it should pull somewhat better at lower rpm. That's why I think a 70 mph, relaxed cruise at 1890 rpm should be quite acceptable. But I don't think it would be "snappy" in 5th gear, not by any stretch of the imagination.

However, by employing the 2.73 rear ratio, when you feel like surging along in highway traffic and riding the torque wave you simply downshift into 4th gear. This puts you at 2500 - 3000 rpm between 65 and 78 mph...perfect! So with the 2.73 I see 4th gear as great for "performance" highway cruising and 5th as the relaxed overdrive, similar to 6th versus 5th on a T-56.

If you instead used the typical 3.27 rear gears, in 5th you're between roughly 2000 - 2400 rpm in that same 65 - 78 mph range, fine for cruising but still low for "on the cam, snappy" performance, as tbone correctly points out. Yet if you downshift to 4th, you're in the 3000 - 3600 range, which for most folks would soon seem pretty buzzy just for surging along in highway traffic. Sure, you can do it, but I'd submit the 2.73 combo works better in both gears, giving you both a mile-eating overdrive and peppy direct gear.

Where the 2.73 combo looks to me to be hands down superior is in making first gear of the standard Mustang T-5 useable in the lightweight, traction-limited Miata conversion. Unwanted wheelspin in first would be reduced, which should help actual acceleration. In stop and go traffic, I currently am shifting constantly between 1st and 2nd, because 1st is almost unusably low. I know first gear with a 2.73 it isn't too tall to be drivable, because I can currently take off in second gear without undue drama or clutch slippage with a Ford e cam in the engine, which has longer valve timing than its planned replacement. The 2.73 gearing, in first gear, would only be about 60% of the way between the overall gearing difference between my current 1st and 2nd ratios currently turning the 3.27 rear gears.

The only caveat I'd offer is that the taller first gear would be less desirable, and at some point unusable, as you advance to more radical cam timing.

Perhaps down the road here I'll be the first idiot to waste $1000 swapping out a nearly-new 3.27 set-up for a 2.73, hating it, and swapping back to the 3.27, LOL. It wouldn't be the first time I ventured down the primrose path.

Anyway, the real take-away from my original post was that it's a worthwhile task in the planning stages of your car to create a comparison chart of different transmission and differential ratios to help with visualizing how the overall gearing will impact your driving at all speeds and common driving situations. Hopefully your interpretation of the data for your combo will be less misguided than mine may well be.

charchri4

Sunshine +1 on the 2.73s being perfect on the road. It's no big deal to down shift when needed and 4th puts you in a great power band.

The comment that puts me into ponder mode here is the nearly unusably too low 3.35 1st gear with the 3.27s. At 5500 you are at 35.8 mph with it. The sort of normal for the GM guys is a 2.66 1st gear and 3.73 diff which puts them at 39.5 mph at 5500. The typical LS probably has a few more ftlbs to the ground as they pass 3000 rpm so my thought is maybe they are about an even match?

I've really never heard any complaints of 1st being too low for the dark side boys so I'm wondering about your 1st being too low for comfort. Maybe they are more different than I think. Thoughts?

PS if you want to play with the spread sheet the T56 gears are 2.66 1.78 1.30 1.00 0.74 0.50 (all F bodies and base C5 vettes) or 2.97 2.07 1.43 1.00 0.84 0.57 (all GTOs and C5 Z06 vettes)

Sunshine Guy

The most important point I'd make about the table I posted is it is intended to be an example of a method for creating a very visual gearing selection tool that replaces assumptions with facts. It makes character differences in road performance very easily recognized and understood.

The six combos I chose to calculate are not those I might have chosen as potentially "best" in general, but rather to help me recognize the most practical and cost effective way to get performance characteristics more to my liking considering what I already have. People planning from scratch or having motors with different power bands should definitely consider other combos.

I already have a like-new 3.35 low T-5 so I looked at three combos using it; the combo I currently have, changing to 2.73 rear gears, and doing the 2.73 gears with the optional .83 overdrive gear set installed in my tranny. I looked at the TKO because I already own one (it would be perfect for a motor with a hotter cam and a 3.08 or 3.27 rear gears, but the ratio s-p-r-e-a-d is a bit narrow to take advantage of my planned engine's extra wide powerband. Finally I did a couple combos with the T-5Z just for comparison purposes.

Yep, 4th in this T-5/2.73 combo would be like running down the highway in your .80 5th gear with a 3.43 rear gear. Very useable.

Where the GM guys are seeing 39.5 mph at 5500 rpm; my proposed 3.35 low T-5 with 2.73 combo would run 42.8 mph at 5500. Not that radical of a change?

FWIW, a 3.08 gear (instead of the 2.73) with the 3.35 low tranny would produce 38.1 mph in 1st...which is what led me believe a 3.08 won't be enough change from my current 35.8 mph to really make me happy. I do think it wise to finalize my engine upgrades and drive it for a bit to really get a feel for its powerband before making a rear gears change. Perhaps when I get the engine running smoother/better, I'll change my mind and consider the 3.35 first gear acceptable with 3.08 gears in the rear.

Lest it sound like I'm trying to sell this particular combo, I'll repeat that I like the T-5Z with optional .73 overdrive and 3.08 gears slightly better than the 3.35 low/2.73 rear combo, mostly because of the former's narrower intermediate splits. But that T-5Z combo is at least $2500 more expensive in my situation, with very similar results to what I can achieve for nearly $2000 less.

Not so much here, but on other forums I've read where several other V8 Miata owners complained about the "granny low" of the 3.35 tranny. It works well in the 600+ lb. heavier Fox body because it's needed to get the car moving with the small displacement V8, but to at least some owners of the far lighter Miata conversion, it seems superfluous.

I'm aware of the T-56 ratios because I intend to steal a cousin of one of these GM-originated boxes for my higher HP stroker build! Tremec now makes the aftermarket T-56 Magnum available in a couple of Ford versions. The close ratio version has excellent gear ratios for a stroker motor with a wide power band that starts a bit further up the rpm scale.

Final take-away: Comparing speed versus rpm for each gear on multiple combos, side by side, before selecting your components may well help you build the most satisfying combo the first time.

tbone heller

I think that your money would be better spent installing T-5Z spec gears into your current T-5 & leaving the rear gearing alone. That way you will have a good base for engine upgrades if desired in the future. Also. if you wanted to convert to a 3.08 gear, you could easily find a complete unit in the junk yard.