Rabitz

Hi am am going to look at a 94 mustang gt convertible this weekend. it will be the donor car for my v8 build. has a nice built 347 (the reason I am going). unfortunately it is an automatic ( kind of a plus, because he is selling it cheap because of this...). I have been doing a lot of searching, and only see the cougar/tbird rear ends being okay to use.

will it be possible to use the rear diff from the mustang? or are the axles all wrong?

thanks for any help.
finally have the funds together, but am having issues with flakes on CL+donor cars

Jason

tbone heller

The 1994 Mustang uses a solid rear axle where as the T-bird/Cougar is an independent suspension, like what is in the Miata from the factory.

Rabitz

i understand that it is a solid axle. I was wondering if anyone has swapped one in/ shortened it/ flared miata fenders. my question was kind of not really there- I posted pretty late

Gator Bait

There's one with a solid axle 4 linked in the classifieds. It can be done but more work.

Sunshine Guy

I would suggest a straight rear axle could be of value if you want a car mostly for drag racing. Properly set-up, it will do the best job of launching the car very hard with minimal wheel hop and especially maximum weight transfer.

If you want a car that retains the famous Miata nimbleness in cornering with better ride quality as a bonus, an independent rear suspension is a better choice.

mrmustang

If you want to use a Mustang as a donor and use a straight axle, perhaps a FFR Cobra replica is more suited to your idea(s).

Rabitz

nope. have a miata that im gonna use. Im pretty clear on what i want, but the shimmering of the mirage as I get closer to it is oh so frustrating. I appreciate the idea of a cobra replica, but in all honesty, I would be posting on a different forum if that was my interest.

I am buying the mustang for the engine that is in it. that is all. Just thought I would see what else can be done with spare parts before I part it out.

Sunshine Guy

Since you are choosing a Ford build, it would really make sense to take a look at Martin's kit (Monster Miata). It was created to do exactly what you want to do, i.e., put a Mustang drivetrain, electrics, and supporting systems into a Miata. It's a system proven over two decades and many dozens of cars.

If your fabrication skills are especially good and you feel you don't need every piece in the kit, if you ask nicely Martin will usually sell you only the pieces you do need.

He is very supportive of his customers, and will sell you the "how to do it manual" separately if you don't buy a kit. Both are priceless when you get stuck. I recommend starting by purchasing the manual. He will credit the price of the manual against a kit if you decide to purchase one later. If you don't, IMHO the manual will still be the best money you spend on the conversion.

Looking forward to following your build progress!

Rabitz

Yes. I agree. His build manual is a great place to start. Martin is a wonderful guy. Very helpful. I was just thinking we could discuss more than the one way street everyone is driving down. Maybe I'm in the wrong section or something.

charchri4

There is a build on here where a guy narrowed a solid axle for the car. I think it was a 9 inch but don't recall just do some searching and you will find it. It would certainly not be easier or cheaper than using an ISD or there would be lots of them running around. Great to see you think out of the box though and I look forward to seeing your build!

Rabitz

just trying to reach some of the higher hanging fruit!

im having some serious issues with craigst list people here in Portland. im afraid the 347 stroker may have fallen through. if so, im gonna go to my pick n pull, and pull a bunch of low hanging fruit this weekend! i really am busting at the seems here to get this going. I was hoping to start my build thread this weekend...but it is not looking so good. ill keep you informed.

in the meantime...more research

Sunshine Guy

Don't be discouraged. You will find people here supportive of whatever you want to create. There are people with good experience, fabricating skills, and innovative ideas.

However, the great majority of people do choose an IRS rear end. Although it is easier to do IRS because 1) proven parts already exist, and 2) staying within the same design parameters makes for less time, effort, and probably cost, there are certainly people here and elsewhere who COULD have done a straight axle if they wanted one.

To the point of my previous post, the reason they didn't go that route is the high value placed on preserving the nimble handling of the original car. A straight axle will be harder to design and fabricate, and it will depreciate the car's handling. However, if you WANT a straight-line bomber, there is no reason you shouldn't build what tickles your fancy. We will offer whatever support we can along the way.

charchri4

+1 to what Sunshine said. Not trying to discourage you at all from building it the way you want to and I'm glad you posted the question. What mmustang was referring to was not building a cobra but using the factory five or FFR Cobra rear end set up as an alternative to a tbird one.

As for solid vs straight like anything else there is advantages to both but as Sunshine pointed out most folks feel the IRS is the way to go in a Miata. For what it's worth it would appear the factorys do too. After a long successful history with a solid axle both the Camaro and the Mustang are now IRS. And it only took them 25 years to catch on our cars are better!

Rabitz

well...got home an hour ago. NOT with the Mustang. drove 4 hours, and the guy wouldn't sell the car at our agreed upon price...so, I did not buy the car. never had that one happen to me before, so iwas kind of surprised, and unsure how to handle the situation. I am a pretty even tempered guy, but this one really fried my butter.

I was hoping to post some pics and start a new build write up.

I blew the motor in my '94R, so I am thinking of just pulling it in to the shop, and getting it stripped down, and start on that to alleviate my befuddled mind. That way I can (in my mind) start on this endeavor.

My wife was more upset for me then I was For myself, I think.

well... thought Id update here, since I was supposed to be getting this "great deal" lol.

any one here in Portland area? the guy had me drive down past Eugene...great engine, but it was too sheisty of a deal.

thanks for all the advice and suggestions. always good to hear options.

Jason

charchri4

Oh that's the pits! So what's the plan to find a motor then? What this Mustang the reason you were going Ford or do you just want to go Ford? Always interesting to hear what guys like and why they build what they build...

Rabitz

I'm open to do whatever. Ford is a lot more cost effecrive, that is why I was going that route. Now I'm gonna be open to deals I find from friends/net. My plan is to build a rough rally style car. Built for the mountain/rough roads to get crazy.

Rabitz

okay, got the donor car. unfortunately, it is an automatic, so now I am gonna start searching for the trans and rear end.

I have been reading some posts about the torsen in the miatas and whether they can handle a V8 or not. i was running a boosted stock engine and makeng roughly 260hp with it using my Torsen with no problem. The engine I have aquired that is going in, has even more than that (around 380), so I am definitely going to sell my torsen and use that money to buy something else. the RX7 rear is a direct drop in, and I heard it can handle the higer torque output of the V8s, but I htought I would get some input from everyone here on what they are using.

charchri4

You going to run the auto then or hunt for a T5?

The vast majority of swaps fall into 3 camps for diffs. The Ford 7.5 or 8.8 from a Tbird or the Getrag from a Cadillac STS and CTS. The have different levels of complexity to install and different price tags to put them in.

It's been a couple of years since I did the research on this but I spelled out how I would do it in this thread yesterday. https://www.v8miata.net/v8-miata-bui...ge3/#post11317

Congrats on the donor score!

cruizin54

One other thing to consider if you decide to put in a solid axle - you will destroy the resale value. The beautiful part of doing a proper conversion is that you keep the handling and improve the power. It will still drive like a Miata but will be a LOT more fun to drive. Sell the axle and use the money to get a 7.5" traction lock from a Tbird / Cougar.

Rabitz

I will definitely NOT be running a solid axle. Gonna pull the miata in the shop, strip her down and sell the engine drive train and supercharger and rear diff and axles. Pretty much everything I can part out. Use that to buy stuff for the conversion. Gonna go with a red rear, or ford 7.5 maybe the 8 inch, but not sure if I need to go that big. Also gonna be deleting the automatic and look for a t5. Those are the plans at least...

Rabitz

Rx7 rear, not red

MRM331

I think one issue you may run Ito with the rx-7 rear may be how to support its front end. I'm not sure but I thought the rx-7 had a PPF like the Miata...I could be wrong though.

If you do go the ford 7.5 route you should be able to find one with a 3.27 and factory tractionloc in yards in your area. It seems like thunderbirds sold in the colder areas of the country came with them often. You'd be looking for one with an "E" stamped on the doorjam label under "AX".
One note of caution with the tractionloc and anything more than street use...while it never occurs on them street, track use seems to over heat mine fairly bad and always lead to it boiling over out the breather. From what I have read this does not seem to be an issue with 8.8 tractionlocs possibly due to the larger volume of coolant they have over the 7.5. I'm pulling my current 7.5 over the winter and rebuilding it with an eaton truetrack and 3.08 gears for this reason.

-Jason

Rabitz

There are a couple mk VIIIs around the corner from me. I understand they have the aluminum 8". I'll go check tomorrow. That was the most understanding I have gleamed from the different rear differnetials. I really appreciate that input.

On a different note, why 3.08 gear? Seems low.

MRM331

If I didn't have so much invested in the 7.5 I'd probably go with an iron 8.8. I've read that the aluminum 8.8 cases have a tendency to flex in IRS Mustang applications leading to pinion/ring gear misalignment during hard acceleration. I'm not sure this would even be an issue in a Miata but I'm happy that I'll never have to visit it with a chunk of iron back there.

I made up a spreadsheet that lets me compare different drive line gear combinations and tire sizes relative to each other. It looks like going from a 3.27 to a 3.08 cancels out the smaller tire size of the Miata and brings the rpm's-to-speed to where they would be in a stock Mustang or T-bird. A 2.924 ratio would be perfect but a 3.08 is the closest you can actually buy. I'm planning on putting a thread together about it over the winter break.

-Jason

charchri4

On the 3.08 he is after, as I recall from other threads Jason is looking for better track gearing so with a 3.08 he will spend a bit more time in 5th and a bit less at the top of 4th. For most guys the 3.08 would be a bit on the low side and 3.27 is generally considered ideal for the T5 V8 Miata.

This might help in your hunt if you don't have it already.


Thunderbirds and Cougars that show an "E" axle code on their door pillar tag usually have a 7.5 inch limited slip differential. Once you see that look under the car and look at the differential itself. If it reads "3L27" it's a Limited Slip with 3.27 gears. If it reads "3.27" it's an open 3.27. These are the axle codes on the door tag and axle tag:

Code Ratio
1.. 3.58C
2.. 3.55C
4.. 3.42C
5.. 3.27C
6.. 3.73C
7.. 3.07C
8.. 2.73C
9.. 3.31C
53.. 3.58C
A.. 3.63C
B.. 2.47C
D.. 3.42L
E.. 3.27L
F.. 3.45C
J.. 3.85C
K.. 3.55L
M.. 2.73L
R.. 3.45L
W.. 3.73L
Y.. 3.08C
Z.. 3.08L

C meaning open and L meaning Locker. If you find a K or W pick it up for me and consider it sold...