I'm a bit surprised that the Miata is so wide?
As somewhat of a reference, my MkII/III with the X-pack arches has a front track of 1495 mm and rear track is 1555 mm.
The MkII/III is supposed to be around 2" wider than the MkI but the front is the same. Not sure how much it differs in the width of X-pack vs Cologne arches but I would suspect that rear Cologne arches are respectively wider than X-pack arches.
So that's some kind of reference to what track widths possibly can fit...
Gustaf
I looked at the miata IRS cradle(because yes, I have a Miata IRS cradle with a Ford 8.8" Trac-Loc IRS mounted to it sitting on the shelf as well....I have all kinds of stuff sitting on the shelf) to transplant into the '66 mustang...but it really wasn't a viable solution for me since the frame rails were sitting where the shocks/struts needed to be(guess I will keep it for when I get back to the Miata). Oddly the rear track between the '66 mustang and Miata were close enough that wheel backspacing would account for any difference. I am surprised the Capri is narrower than the '66 Mustang though....its a pretty narrow car. In the end I decided having a live-axle wasn't necessarily a detriment to handling for me.
In reply to therealpinto :
Track widths can be a bit misleading, as they are usually measured from the center of the tire, as opposed to the hub face. Internet says Mk1 is 1346mm up front and 1321mm out back. You can back out the wheel offsets to get hub measurements, which is what I've done, but that's based on what the internet thinks the measurements are on a nearly 50 year old car...
I'll only know for sure when I get some measurements on the car. At this point I'm just juggling ideas in my head and making notes because the real sticking point will be finding wheels and tires that fit, look right, and don't bankrupt me.
In reply to Wicked93gs :
I'm not at all opposed to a straight axle in the rear, and that may be the most practical solution in the end (pun intended).
The front is where I think things can really be improved.
I'm running a Fox body Ford 8.8 rear end in my Corolla project and the front suspension is Miata parts. I measured the rear end width of my street Miata and the Ford rear with the same wheels and they're the same width. I expected the Miata to be narrower, but it's not. That made it a lot easier.
Rodan said:In reply to therealpinto :
Track widths can be a bit misleading, as they are usually measured from the center of the tire, as opposed to the hub face. Internet says Mk1 is 1346mm up front and 1321mm out back. You can back out the wheel offsets to get hub measurements, which is what I've done, but that's based on what the internet thinks the measurements are on a nearly 50 year old car...
Yes, but at least in the front you'd wanna be pretty close to the Miata ET/offset not to upset the nice geometry too much.
I find numbers saying 1430 mm rear track for the NA Miata and 1410 mm front. I do think that the front end could surely work on a Capri with some wider arches and the rear too. But as you say, measuring in real life beats keyboard research!
I also agree that the front end is what really needs improvement...
Gustaf
Having blowed up a Capri 4 speed due to axle wrap leading to axle hop under power (I may have been quite stupid at the time) I would suggest that the Capri rear suspension (outside of cars like the RS 3000 and whatever special stuff FAVO did there) is not all that great. The front is bad! The rear is fairly poor. Get the wrap under control, maybe add a touch more rate, and a mild anti-roll bar and the stock stick axle should work better. There are a few folks on this board that have some experience in getting a rear-leaf sprung vehicle (DusterBD springs to mind) to handle better. Weren't you talking about going to a 3-link, possibly torque-arm style, with coil-overs?
Also yes the rear axle on these are quite narrow. The factory Atlas rear axle most American market cars got is said to be quite strong but LSD's are not cheap (assuming they're even available anymore) and rear discs are probably custom projects. A custom-width Ford 8" or 8.8" seem like good solutions based on planned power output.
In reply to pres589 (djronnebaum) :
I had a similar moment of stupidity with my original '74 back in the day that resulted in the 2 piece driveshaft becoming 2 separate pieces on the 1-2 shift. Yes, axle hop was a big issue.
If the rear remains a straight axle it won't have leaf springs.
There's very good aftermarket support for the 8.8, and they're pretty easy to find, so that's the likely direction if it comes to that.
In reply to Rodan :
Don't dismiss an 8". They are cheap, more than strong enough for any 3.7 you are likely to build, and easy to find and come in several widths. They are also a lot lighter than a 8.8"(that explorer rear end is HEAVY) or 9". In addition to that...in a vintage Mustang they are 5-lug if you need that pattern...or you can grab a Maverick unit if you need 4-lug...either the 65-66 Mustang rear, or the Maverick rear are likely narrow enough to fit(hard to say with the way track widths are though)
1430mm for the Miata rear track sounds right...since the '66 Mustang is 1422 and when I measured them by hand they were nearly identical from what my tape measure said.
On another note....you likely wouldn't even need rear disc brakes in a small car like the Capri, drums on a 8/8.8/9 will give you more stopping power than you can put to use anyway. For me rear disc conversions are because I am lazy when changing brakes and don't want to take the extra time to fool with springs and star adjusters.
In reply to Wicked93gs :
8" axles are extinct in junkyards here, and anything I've found for sale is at least as expensive as, if not significantly more than the equivalent 8.8" part. I'm willing to put up with a little extra weight for parts that are easy to get, and don't break the bank. Ideally, I can make a Miata subframe work... measurements look workable, gotta get under the car and see what kind of fabrication will be involved for the structure.
This car will be driven hard on the track, so it will definitely be getting discs in the rear. Even if I were going to use it to drive Miss Daisy, it would get discs. After doing the rear drums on our Bronco last year I have no interest in ever doing a drum brake job again. Ever.
Going down the research rabbit hole, I discovered you can actually still buy a Cosworth GAA, complete...
Definitely drool worthy... 
Unfortunately, also waaaay out of my tax bracket at ~$71,000... 
I did, however, find some reference books that I'm working on acquiring.
In reply to Rodan :
There is always the 7.25" rear as well....people here always upgrade to an 8"...but its been proven to do 12s in drag racing applications...so its tough enough. For that matter the 7.5" would be as well....but yeah, the 8.8"(either the solid axle or the IRS) has a lot of advantages...cheap gears, options like air-lockers, etc etc etc.
The "easy button" one for a Capri is probably a 4 lug Fox body, although I'm not sure they were available with disc brakes. They're the narrowest if you were trying to do it without actually cutting and welding a housing. If you want to narrow a rear, then any disc brake 7.5 or 8.8 will be good. I assume the 7.5 is lighter, but have no idea what actual weights are and would be curious to know since my Challenge car is light and using the 8.8.
My 1st choice right now is NC Miata, pending real measurements. From my estimates so far, it looks like it will work.
NC is the widest, but has 5 lugs, good brakes, and much stronger wheel bearings than the NA/NB. Easy swap to RX8 hubs, which are even stronger. Good aftermarket support for brakes and shocks/springs. 5 lugs will open up more options for wheels, which need to be 18" to get even close to the necessary tire size with any kind of availability. NC is also close to the same weight as I expect the Capri to come out. NC diffs were available with 3.73, 3.91 and 4.10 gearing... I need to run some gearing calcs to see how that works with the Mustang trans, but the Capri should have an inch more tire height than an NC to give it more legs.
The question is how much fabrication will be required? I'm kind of willing to scratch build a frame and drop the body over it, if necessary, but hopefully it won't come to that. I figure if NOHOME can get a whole Miata under a P1800, I should be able to get a couple of subframes under a box-flared Capri... 
In reply to Rodan :
Well, I had originally intended to stick NA/NB subframes under my '72 Corolla, but it quickly became apparent that the frame rails on the donor car and the Corolla are in very different spots. I have never looked at an NC, but the NB front frame horns are much higher to clear the subframe and the Corolla was very low, to the point that I went to a fabricated frame. The rear suffered the same problem but to a lesser degree. Measure the height of the Capri chassis at various points and see how high the mounting points of the Miata subframes are. I ended up with Miata front supension pickup points inside the frame horns, so off came the whole front chassis structure. The rear didn't have enough fore-aft length in the frame rails to accept the subframe so it would have required complete removal of the frame rails. At both ends the frame rail width was right at the A-arm mounting point widths. Basically everything interfered with everything. Start measuring !
From putting the Sierra IRS under my MkII/III, I would expect fitting a Miata subframe would mean basically new frame rails from the rear seat back but that's not really to big of a job IMO.
I tried to retain most of the frame rails but it was tedious and only possible thanks to the semi trailing arm Sierra IRS that is far from ideal in any other way.
Gustaf
In reply to Rodan :
18s? good lord, those are going to look cartoonish on a MKI Capri...they look terrible on a 65-66 mustang and those are visually far larger. Everyone always cites tire size choice as a reason to go with 17"+ wheels, but the reality is that there are perfectly acceptable tire sizes in 15" and 16"...you don't need a "greater variety of tire choices" you just need a single tire size that works for your application....and there are plenty of choices on reasonably sized wheels to handle way more power than a NA(or even turbocharged) 3.7L will ever put down....especially one with IRS. Even if you assume road course or auto-X, there are still good sizes to be had without going to 18" wheels. The only advantage 17s and 18s have is that a replacement might be easier to get down at a used tire place in a pinch instead of ordering them online.
I knew someone was going to freak when I said "18s"... LOL
You know what would really look cartoonish? 9-10" wide tires under these flares:
The 3rd gen flares add over 7" on each side... original Group two cars ran 16" wheels that were 14" wide in back and 12" up front.
Here's a car on 16s:
And one on 18s:
Can you tell the difference? Sure. Does it look 'cartoonish'? I don't think so.
And the truth is that tire availability is absolutely an issue... if I want to actually drive it.
The rear tires are the problem. They really need to be at least 305 width to look remotely correct, with 315 or 335 being preferable. Those widths just aren't available in 16" tires. Hoosier makes the R7 up to 275 section width in 16"... I haven't found any street legal performance/track tire any wider in a 16". The only wider option in 16" is full on road race slicks.
If I jump up to 17s, I get to choose from R7s, R888Rs and maybe a couple others. Michelin made a PS2 in 335, but I don't expect those to be available forever.
18s is where it's at. Great selection of 275/35-18 for the front, and matching 315 or 335 in the rear depending on which tire I choose. Azenis 615/660, Rivals, Conti ECS, NT01, Toyo RR, R7s... much more availability.
Would I prefer to run 16s and be period correct? Sure. Is it practical? Nope.
With those sorts of flares I could see going to the tire/wheel packages you're talking about. With the original bodywork, which is pretty slab-sided around the wheels, a small change in wheels can look pretty different. I had my mk1 on 14" Fox Mustang rubber-faced wheels with 195-60-14's and it seemed like plenty of tire for the car but the body was stock. I didn't know you were hoping to go to something like the Group 4 flairs or wherever those are from. Seems like there are a ton of variations from one car/series to the next.
Yeah, the plan has always been for a Group 2 replica/tribute build.
With a stock body, or even the street flares commonly seen I absolutely agree that 15s would be the way to go, and there are plenty of tires in that size group. That's essentially what I'm running on our NA Miata. On my first Capri, back in the early 80s, I was running 205/50-13s. But the big race flares are a whole different animal. There were basically 3 versions of the flares for the factory racing Capris:
1st Gen:
2nd Gen:
3rd Gen:
The plan for this car, unless something changes drastically, is to run the 3rd gen bodywork.
I think you are spot on in the analysis with one important key factor - you plan on redoing the entire suspension.
To get a Capri to work (both looks-wise and performance-wise) with large diameter wheels the suspension needs lowering to a point where the stock (especially front) suspension just won't work right. The factory race cars had numerous tricks up their sleeves to handle this.
The Capri we built had a version of the 2nd generation arches. We only ran it on 15" wheels and mostly drag racing, the 26x10,5 ET Streets looked pretty nice but the 205/50-15's in front always felt small.
Gustaf
I'm not sure how much this will help, but here's some body data from the shop manual. The frame rails do appear fairly low in the back.
For the Capri that only exists in my head currently I would rather have the Basil Green Racing Perana's flares;
Pretty mild compared to the Group 2 cars but still a big change. Gets rid of the square wheel openings as well which I never liked.
Burton power might be able to help with spending money on a ford engine. I bought stuff from them back when I raced a old fiesta.
In reply to pres589 (djronnebaum) :
Always liked this one, will definitely be an influence on mine when I start putting it together.
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