I was posting in someone else's thread but my question ended up not being a quick one so as not to hijack the other thread and to get input from the target audience, I am opening a new thread..

All input/recommendations appreciated....

The car will be making over 400whp with a centrifugal blower and eventually 500-550. it Will be able to use the power and put it to the ground relatively well, and I do not plan to drive it easy. I want to be able to drive it to my fullest capability, which means right foot on the floor, never lift, kick the clutch and ****** a gear.

Can the 7.5 with upgrades in these light cars withstand 400whp for now and this driving style? If there are upgrades that can beef up the 7.5 to handle it, I'm interested to know what they are and where to get them. If I need a different rear, I'm interested in feedback on what other units may hold the power and driving style.

I'm very familiar with the 5 liter engines/drive lines and the 8.8 solid axles having raced them for over 20 years....but have never owned a 7.5 IRS and know very little about the IRS so your personal experiences and recommendations are appreciated..

 

The 7.5 will be fine. That is what I am using. The vast parts selection is the only thing that makes the 8.8 appealing to me. It also has a larger fluid capacity which could be beneficial on a road raced car.

The lack of traction will be the weak link followed by the axles. Remember that everything has a weak point.

What special voodoo are you going to use to get to that 500-550whp? Or even to that 400whp?

 

Thanks TBONE for the reply and to PAPABEAR for the response in the other thread. These are the kinds of responses/experiences I'm looking for....

I'm adding a centrifugal blower to get the 400whp number initially....I'll need to add~120 wheel hp to get there so it will take roughly 9-10 lbs of boost I'm estimating.

Later, I'll bump up to 15lbs of boost to get in the 500-550 who range. I have a few cars that make this or a lot more but in a mustang body.

I plan to use a 225-50-15 drag radial on a kosei K1 wheel in 15" when I'm "playing" since it will match my kosei K1 16s. I'll run the 16s front and rear when I'm not "playing". I expect I will take it to the drag strip occasionally and will get it into the low 1.60 sixty foot range.

The goal is to be able to use the car the same way I do now for street use and corner carving but also be able to take it to the drag strip occasionally and run well into the 6 second 1/8 mile territory on drag radials.

 

Well running the centrifugal blower will help. Since it's boost will come on progressively vs a roots or nitrous car. Your shock load will not be as hard on the drivetrain at launch. In my estimation your weak links are going to be U joints and your transmission unless your running a tko-600 your going to destroy even a upgraded t-5 in your no lift grab a gear driving style. Running a 225 drag radial I still believe you won't create the kind of traction needed to snap a half-shaft. I have seen some pretty out there engines on RX-7 swaps and the turbo rear ends they are using are living and I doubt they are stronger than a 7.5 that lived in a v-8 3800 pound Thunderbird/Cougar.

 

That's pretty much what I was thinking but theory and actual experience tend to conflict one another sometimes.....so definitely interested in what others have seen and experienced.

The reasoning you provide is why I chose the centrifugal blower with more of a linear power curve. Plus I've been down this path before with One of my mustangs... It made 540 rwhp and ran 6.70s at 105 with a 1.57 sixty foot. It's a 5 speed car on a 255-50-16 at 3200lbs.

Looking to do something similar but in the much lighter miata. Just don't want to break the rear end first pass like a goofball so I'm planning to start off with lower boost and ~400whp. From what you guys have seen, it Sounds as if the 7.5 is much stronger than I was thinking even at higher HP levels....especially when factoring the linear power of the centrifugal and the light weight of the car. Time will tell for sure....

I'm expecting to have the SC back on and the tune wrapped up in the next 2-3 weeks. I also plan to be at the Mitty with the car for my first time in April..

Does anyone make upgraded U joints for the 7.5 rear?

 

U joint size it going to depend on your rear end yoke. I think the standard is a 1310 Universal for a 7.5. From a quick search looks like a 1330 universal is going to be the only upgrade for that rear depending on if its a small or large bolt pattern. To get into a 1350 you would need to go up to a 8.8. But changing yoke size means your going to have to get a new driveshaft made. Probably cheapest insurance is to get universals that are non-serviceable if you do not already have them. The universals with the grease fitting are weaker. The hole for the fitting and internal grease channel to the cap makes the joint weaker. A non-serviceable is a solid piece of steel.

 

What rear did/are you using in the high HP Mustangs? The internals are the same for a 7.5 solid axle or IRS.

The V8 Tbirds used the 8.8 rear, as did the supercharged V6 from the factory.

High hp axles are available. Pick your hp, open your wallet. Per Martin's (and others) comments on axles, I wouldn't think you would want to use the Miata outer CV's.

Mike

 

Ah...driveshaft u-joints....

On my list was to research if the FMS aluminum DS for a mustang could be cut down and adapted to work with the 7.5 rear. We have a very good DS shop locally so I'll be sure to have them install the strongest u-joints possible when I get to the aluminum DS swap. Thanks for the info on the joints..

Based on feedback here from you guys, I'm going to try out the stock 7.5 and axles. If they break, then I'll replace them with something stronger. Currently researching available options for beefier axles.....

 

Using a straight axle 8.8 in the other cars. 31 spline axles, motorsport 31 spline carriers with extra clutch materials, different gear ratios in different cars...

Based on comment/feedback thus far, I'm leaning toward keeping what I have for now and seeing if it will last at the 400whp level, then save up for an aluminum 8.8 rear from a cobra and do upgraded axles for the 8.8 at the same time...before I up the power to the 500+ range.....

 

I would much rather have a cast iron center section from an 8.8 Cougar/T-Bird than an aluminum one from a Cobra or a Mark VIII. Not only will it hold its tolerances better, the extra weight will help in the traction department.

As far as drive shafts & u-joints, talk to the guy at Drive Shaft Services in Kennesaw. I have used him for 30+ years.

 

Thanks....I'll talk to them.

Do you happen to know how much lighter the aluminum version is...ballpark?

 

+1 there is lots of discussion on Mustang and Tbird sites about the differences and plenty of stories if ears being broken off aluminum housings. But that is on a 3600+ lb car and the Miata is a whole different deal.

I don't know the weight difference off the top of my head but I was hot for an aluminum housing too till I did the research on it and can tell you why I didn't go that way. The aluminum case is less stable, more expensive, takes a case spreader to work on AND the Cobras are 31 spline and the MarkIIIs never came with posi so there is no bolt in option.

PS if you do go to an 8.8 make your mount like Travis and Mikes don't use the Monster Miata one.

See
MiataV8 Conversion: Rear subframe

And if you are really board...
Jim and Jenna build diary, Miata LS1 Conversion: Important stuff for building a V8 Miata - what I would do different.

 

Thanks guys...this really helps me determine what path I will want to go in the future when it's time to upgrade the rear. I'm real familiar with the 5.0 driveline from crank pulley to pinion flange....lol; however, from pinion flange back....clueless........MUCH to learn....

 

Early cobras are 28 spline.. And the difference is about 14 lbs iirc. And you can easily make a case spreader with the angleiron and bolts and stuff

 

LOL well yeah you are right but lets see a show of posts on who would go to the trouble of making a case spreader and spending 300 to 400 more than iron for 14 stinking pounds that is 6 inches off the ground and on the right end of the car for it!

 

After looking for a year for an 8.8 with a trac loc my search was fruitless. I finally found a whole IRS assembly from a 99 cobra (99-01 was 28 spline 03-04 was 31) I paid $600 for the complete IRS assembly from rotor to rotor. Took the diff out and sold the rest for $400. Could have got more but it was to a buddy.

 

I'm actually near the end of the process of upgrading my 7.5 for endurance-type track events. I had the "standard" 7.5 3.27 Thunderbird rear with the factory tractionloc limited slip. While fine on the street the much lower oil capacity of the 7.5 compared to the 8.8 always resulted in oil spilling out the breather after even only a couple of laps. I looked into a few different options such as adding a cooling system ($$$$$$$) or even upgrading to an 8.8. The cooling system seemed like an expensive bandaid and the 8.8 would have required a new rear subframe solution, new exhaust to clear it, etc. I finally settled on having my existing 7.5 completely rebuilt to my spec and keeping literally only the case and pinion flange. For around $1000 I was able to have the 3.27 ring and pinion replaced with a new 3.08 set, all new bearings installed and the tractionloc replaced with an Eaton Tru Track. The 3.08 is just enough of a difference to make up for the Miata's smaller overall tire diameter when compared to the Mustang the T5 originally came out of. The hope was to make first gear "as useful" as it would be in a Mustang. The main cause of my boiling it over was the friction caused by the traction loc's plates and clutches. The Tru Track uses only gears to vector torque and generates a fraction of the heat as a result. All of this gets me the differential I want for about the same cost as a new GM unit (I think) which again would have required additional modification to the car's current structure to work ($$$$$).
Unfortunately I finally got the built differential in the car in late November and was greeted by an insane amount of gear whine. After removing it again and dragging it down to the builder he's got it reset and ready to try for a second attempt. He's done a great job of standing by his work, it's just a pain pulling a diff from a V8 Miata when you have other things that require your attention (yes, that means everyone's harnesses I'm working on currently).
I'll be reinstalling it this weekend to see if the second time's a charm. If it's quieter this time around I do think going the custom built diff route is a good choice. Just from driving it around trying to diagnose the gear whine I can tell the gearing of the 3.08 is much more natural feeling and the Tru Track's superiority to the old tractionloc is very noticeable. I can't wait to really get into it in the spring and really enjoy the effects of this and the slew of other upgrades the car received while down for a year.

So...long winded response: consider custom building a 7.5.

-Jason

 

I'd like to use a 7.5 for size reasons (every little bit helps) but want to use 2.73 gears for my application. Do any of the IRS options have that gear ratio? I know I can get a R&P from a solid axle car and probably just do the build w/true trac from the get go, but wondered if there is one out there that would work.

 

My 1988 Mustang GT came with 2.73 gears. What a sluggish turd that was until I swapped in some 3.55s.

 

The 8.8 is clearly stronger. Yes, it's a little bigger and a little heavier. But so what? Unless you're trying to eek out over last little bit of performance, what's the difference? IMO, build for strength and durability over everything else. You know it's going to break at the worst moment. Like lap one on saturday morning.

Bite the bullet, spend the bux, build it right the first time, and never worry about it again.

 

In my case, I'm considering a manual transmission w/4.4 1st gear; 2.73 should be more than enough for that.

Back to the original ?; any 7.5 IRS with that gear? My preliminary web searches came up in vain, but thought someone here might know better.

 

The only ratios I've been able to find are 3.27 and 4.10 in stock irs 7.5's. I'm pretty sure this is why 3.27 became the "prefered" differential to pick from a yard. I made a spreadsheet a year ago that compaired rpms/speed based on final drive ratios and tire diameter. It showed that moving down to a 3.08 results in the same rpm/speed ratios with our 23" tire as a 3.27 with a standard Mustang tire. While I'm still working on getting it set up correctly the limited amount of driving I have done showed that first gear way a lot more usable.
All this comes with the caviat that my rear subframe, axles, exhaust, etc are already built around the 7.5. Having it built with a 3.08 and TruTrack cost the better part of $1000. Building from the start with the larger rear could have been cheaper; reworking the supporting components at this point would not be. If I were starting from scratch I'd look at both options and weigh the total costs of reaching my ultimate goals with ether before choosing one over the other.
-Jason

 

https://www.yukongear.com/partslist....26+Pinion+Sets

 

Motive gear offers most ratios as well in their "oem" line. Some are also offered in thier heavy duty line.

-Jason

 

Anyone know if a clutch pack for a solid axle diff will work in the IRS trac lok? I'd like to rebuild my 7.5 trac lok, and cannot find a clutch pack kit that says it is compatible with the IRS diff.

Thanks!