I want to pound my Miata in the ass - Hardcore
 

Yes, indeed I do. I think this track car has pounded me in the ass long enough, without even a reacharound. So I think it may be time to be the pounder for awhile.

Here is the thing... I am trying to raise the height of the Ford Explorer 8.8 differential in the car. I have a plan, a spare subframe to modify, and the toolz and skillz to make it work.

But there is a problem... there is always a problem... and my ass is hurting again.

The 8.8 diff housing is a LOT larger than a Miata diff housing. If I had to guess, like 3-4 times the volume, and maybe twice the height. Because of its height, or to be more specific, the distance from the axle centerline to the top of the case, I am having clearance problems with the gas tank. I cannot raise the diff as high as I would like to in the chassis because the bottom of the gas tank is in the way.

I've been crawling around under the car a lot this weekend ass-essing the situation. I've raised the diff into place between the subframe members, I've measured everything twice, carefully, so I know what the issue is. Not enough vertical space. I cannot tilt the front of the diff downward to get more clearance since that will make the pinion angle too severe, and it would partially defeat the purpose of the whole exercise anyway.

So, like I said, I want to pound my Miata in the ass. Not in the harsh, unlubricated, don't-care-about-you way it has been pounding me in the ass. I mean gently, lovingly, carefully pounding it. Specifically I want to take a hammer to the gas tank, the rear, high portion of the tank, and pound it upwards about an inch. I'd like to do this to make a 'tunnel' about 4-5 inches wide that runs from front to rear, shaped like an ass-crack

So, I need some input from the hardcore Miata fabricators on whether this is WIN or FAIL. Though I will do it gently, and empty its ass before I start, I am not sure how much I can pound on it before its unpleasant contents will come gushing forth. Anyone ever done this?

My other option, and it is somewhat appealing, is to ditch the stock tank and go with a fuel cell. I am swapping subframes anyway so it should be pretty easy to remove the stock tank. I could also go with a smaller diff, but this one has nice internals and I have about $700 in it.I could probably recover a lot of that if I sold it and went to a Getrag which I believe is smaller.

OK you hardcore ass-pounders, tell me how it is.

Can't even give you my opinion cause I'm so horny from all the ass references:giggle:

*edit: fuel cell all the way. or install a smaller tank from some other car (don't know if there are much smaller tanks out there. or try to make your own. but to "pound" the stock tank sounds like a bad Idea and very dangerous.

I would say it's a very bad idea. Go with the fuel cell (you can get custom ones made), or the smaller diff. Just pounding away, even gently, is bound to create a crease and it will leak all over the place. Leaking fuel=very bad things.

And, I think you have some issues that have nothing to do with gas tanks.

Why can't you fit the 8.8? Mine fits fine with axle output exactly in the stock locations and clears the gas tank fine. Something's not right.

Many pics in my build thread. It's on jackstands so I can take more if needed.

You might try looking into the 90-93 tank it's about 2 gallons smaller. I'm not sure in what dimensions it's smaller since I don't have a 94+ tank to compare mine too.
It's a shot in the dark but it might keep you from whanging on your tank or replacing it with a fuel cell.

I would just go with the Fuel Cell option.

If you able, to pound in the extra metal and/or the stock tank. Your opening you self up to dangers situation. If anything to were to break free, like a driveshaft, or you run over something. The extra air/metal gap that is currently there, won't be there to absorb the damage. Allowing much easier for something to break the stock tank.

Now from the sounds of it, you willing to cut and demo anything. Wonder if you able to cut out some of the truck metal and remove the stock tank, if you could put a rectangular fuel cell in roughly the stock tank location but a few inches higher.

I'll check it out. I was thinking the same thing, that is I am not sure why I am having such a clearance problem. I have been looking at the FM conversion pictures, and some other conversions for that reason. I'll go check out your pictures. Which diff housing are you using, the T-Bird/Cobra, or something else?

Well, asking about hammering on the gas tank was the whole basis for the ass-pounding joke, so I had to ask :giggle: But yes believe me I know it is risky which is why I am asking. If someone else has done it successfully then I might try it. If not, then I will look for other options.

EDIT:
OK Stein I looked at your pictures. I remember your mount system now, the tray setup like the Boss Frog kit. You have the Thunderbird diff housing. You know what though, I believe the Explorer and T-bird diff covers are interchangeable, ergo your diff is about as tall as mine. How close is the top of the cover flange to the underside of the trunk deck? Where the housing slopes down to the pinion bearing carrier area, is there much clearance between that and the underside of the tank? No need for exact measurement, do not want to put you to any trouble, so approximate is OK.

I would also agree that maybe you have a housing with a weird casting. In my little bit of reading I have done for 8.8 rear end swaps into the Miata, I have never heard of anyone having to beat stuff to shape to fit it in. If not that, I'm also voting fuel cell, I wouldn't beat the stock tank.

I think it may be that the Explorer housing is significantly taller than the t-bird housing. For one thing there is a decent gap between the ring gear and the top-inside surface of the housing. For another, there are a lot of cooling fins and protrusions on the top of the housing. So the distance from the center of the axle to the top of the housing could be significantly taller on the Explorer housing.

T-Bird diff housing (In a FFR Cobra frame)
http://parts.factoryfive.com/newcata...ctures/irs.jpg

Steins t-bird housing (can't see top but notice no fins)
http://i51.photobucket.com/albums/f3...7/HPIM0847.jpg

Mine
http://lh4.ggpht.com/_jh2HiMm4nKQ/TE...0/P7180136.JPG

Notice all the fins and protrusions and what not
http://lh6.ggpht.com/_jh2HiMm4nKQ/TE...0/P7180137.JPG

I nearly grabbed the grinder this afternoon and cut all of that crap off of the case. But I decided not to be hasty.

Don't mind all of the grime. That case was really clean until the CV boots exploded and coated everything with grease :facepalm:

Actually if those are all solid I don't see why grinding them off would be too bad of an idea.

Yeah I am 99% sure they are solid. I am thinking about it...

Here is another picture of a T-Bird/Cobra aluminum IRS. No fins, or at least not huge ones.
http://thefordsource.com/store/motor...M-4010-A88.jpg

Here is another of a T-Bird housing, looks like a cast iron case maybe. Bot notice it is completely smooth on the front. Ignore the red circles.
http://i53.photobucket.com/albums/g41/addaea/4361_1.jpg

Here, finally found a good shot of a T-Bird aluminum IRS. Much cleaner on the front top of the case where it slopes down to the pinion carrier area.
http://members.cox.net/rjsliwa/feb2005/9.jpg

Doesn't look like those other housings have that sensor either,like yours does.

I say try the grinder method. Cheapest option. Should anything go wrong or messed up, swap the housing. Either option would still be better than pounding on the tank or getting a cell.

Yes I do not need that temp sensor or whatever it is. There is another one on the bottom anyway.

I may just end up grinding it. My only concern is whether or not the fins are adding significant strength/stiffness to the housing. They are well away from any of the high stress areas so the forces should be well distributed at that point. Plus I can leave a bit there, and even more on the side fins.

I also have another wrinkle going on with the driveshafts that could impact what I do with this housing. I should know more tomorrow. Hopefully I do not get shafted... in the ass... :giggle:

I'd say get the grinder out and smooth those bitches to a reasonable level. The Miata is going to weigh half of anything else this diff ever came in, I really doubt that even if they were some type of partial load bearing element that it would cause any major issues.. I'm anxious to hear what the Driveshaft Shop has to say about those axles as well. GL with both ventures!

Thanks. I hope I can work something out with the Driveshaft shop as well. If I cannot, then the smartest path may be to sell this entire diff assembly and use the funds to get a 28-spline truetrac, a different R&P, and another housing. Then I can get the normal driveshaft setup from the Driveshaft shop.

Or, I take the cheap way out. Get some more Miata outer CVs, rebuild the shafts I already have, grind down the fins on this housing, and make it work.

For real. I am off wed and thurs, i would be glad to come help you, and if not, drink beer withyou

Would it be possible to modify a set of the factory cobra axles (or insert other 31-spline variant) to be useable in our conversions? Or is that more trouble than it's worth? Just popped in my head, so I thought I'd throw it out there.

One of those pics you posted (the smooth one in the middle of post 11) was the weaker 7.?" diff that they use in the non super coupe thunderbirds.

Swap to the 7.5 and be done.... you'll walk home with more cash in your pocket and have a diff that fits better.

For the Cobra, do you know offhand what the spline count is on the end of the halfshaft that mates with the inboard CV? Not the spline that goes into the diff, the one that goes into the inside component of the CV a.k.a the tripod. It is not something you can check unless the boot is off of the CV.

Sorry, I was away for a day. I have the T-bird and the FFR halfhafts in my car. Not sure if the NB subframe is different or not but it fit easily. I Sawsalled the old diff mounts off. At least a couple of fingers between the diff and the tank. I can't access my photobucket at work but there are assembled pics there if you follow the link from my build thread.

Thanks Stein, no worries. The two finger measurement is all I need to know, and somehow that method of measurement fits the theme of this thread.

I talked with the Driveshaft Shop this morning and they are looking into a driveshaft option based on the measurements I provided. I'll let you guys know what I find out.

I got my driveshaft from Southwest Speed. All new, $169 complete with front yoke (for my T5) and u-joints. IIRC $181 shipped. They sell them from stock in 1/2" increments. Shipped the next day. Might be an option to explore.

Sorry, should have said halfshafts. Driveshaft is fine AFAIK.

More information on Driveshaft Shop halfshaft options here. Linking instead of repeating. Bandit you probably saw this already.
http://forum.miata.net/vb/showthread...=366129&page=3

Update: Currently modifying a subframe I am going to swap in. I am cutting the excess fins off of the Explorer housing and using a template I made of the floor/gas-tank to check for clearance. It looks like I will be able to raise the diff about 2.5"-3" above where it was before which should give me the halfshaft angle reduction that I need.

Hell if I get this done quickly enough I will be able to track test it next weekend.

Updates
Fins removed from top of differential
http://lh6.ggpht.com/_jh2HiMm4nKQ/TH...0/P8210143.JPG

Modified subframe
http://lh6.ggpht.com/_jh2HiMm4nKQ/TH...0/P8210144.JPG
http://lh4.ggpht.com/_jh2HiMm4nKQ/TH...0/P8210145.JPG

Welds are not pretty but they are very beefy. Note the reinforcement plates. Also note the front diff mount, and the through hole mounts added for the diff.

New mounting location is about 2" higher in the car so it should take care of the problem. All I have to do is wait for the paint to dry, remove the old subframe, swap over the control arms, install this subframe, install the diff, and reinstall the rebuilt halfshafts.

So glad you have a decent rear end mount and not an obnoxious one...... Looks good man!

All that in 100 degree temps...what a bad ass. That's what I call motivation. I, on the other hand, spent the day installing part of a drip irrigation system at the house. I sure would like at least a little rain sometime before the year is up. I am finally picking up the Miata tomorrow so some work may actually progress on it for a change.

My excursions outside today were as brief as possible. It is freaking hot people. August in South Texas FTL. 100F+ temps with 107F+ heat factor. Hell it is well after 10 pm here now and the heat index is 97F. The only nice thing about it, and I do mean ONLY, is that black spray paint sitting in the hot sun dries very, very quickly. It is like I baked it on.

If I were installing drip irrigation today I would have had it irrigating me during the installation process.

Just got back in from dropping the subframe. Though I expected it would not be too bad, it was even easier than I thought. Going from sitting inside getting motivated to do it, to the subframe out of the car and sitting behind it in the driveway, took less than 30 minutes, single-handed. Pneumatic tools FTW.

Tomorrow I use rlogan's pressure washer to remove exploded cv grease and 16 years of road grime from the old subframe. Then the suspension gets swapped over to the new subframe. Then it all goes in the car.

It's in. No ass pounding required though the clearance between the top of the diff and the gas tank is close. I added a heavy foam spacer in between the two just to be safe. It could not be any higher in the car than where I have it now without removing the gas tank. The pinion angle also looks pretty good as well.

http://lh4.ggpht.com/_jh2HiMm4nKQ/TH...4/P8220150.JPG

http://lh5.ggpht.com/_jh2HiMm4nKQ/TH...2/P8220154.JPG

http://lh5.ggpht.com/_jh2HiMm4nKQ/TH...0/P8220158.JPG

UPDATE:
The suspension and halfshafts are back in the car and everything is looking good. The halfshaft angle is much, much better. At full suspension travel they are actually sloping downward towards the outboard end; at the midpoint of the suspension travel they should be very close to level.

I think I have this one solved though I will know for sure when I get it to the track. Hell if I ever do another rear differential conversion I'll just modify the stock subframe and save money on the conversion kit. This one was not too bad, but with a Thunderbird style housing it would be even easier. It makes me want to drop an 8.8 Ford truetrac into the turbo car as a preemptive measure :)

HOLY WOW!!!!!!!!!!! What is the measure difference? Looks like 3+ inches, and now it fits as well as the TII mutt I have in the back of my shitbox!

Looking good, thanks for the help yesterday...btw, I went to swap the input shaft oil seal and I need a gasket for that cover. Oh well, I need a few things so I'll order them all at once.

It is at least 2" higher, maybe 2.5". Before the bottom of the diff was lower than the bottom of the subframe. Now it is the same or even a little higher. It absolutely cannot go any higher with the stock tank :) I did not get to put it on the ground over the weekend but everything is bolted in. I just need to find some pinion nuts and then torque it all down.

DOH! I do not remember a gasket there. Oh well.