LT1RX7

I have decided that I want to turn the LS1/FD into a strip car. Not a full out drag car, something I can still drive on the street but a more dedicated "drag" car. I am trying to decide what way I want to go in regards to the rearend.

Should I:
1. Cobra IRS- Already have the diff and cradle for it and it is basically a bolt in at this point. Problem is I am still limited by the IRS and the Cobra rear although much stronger than stock with better gear choices is still a "weaker" rear diff.

2. Ford 9" Solid axle- This I would have to fab something up to get it to work, not even sure how a solid axle would work in the car the way it is set up right now. This is a step up from the IRS but not very adjustable and still not bullett proof.

3. Art Morrison 4-link or similar- This is the ultimate setup for what I am looking for. Infinate adjustability, strong as hell and comes as a "weld in" unit. The problem I have here is can it fit? Will I still have some ground clearance? Should I just tub the car out while I am at it(which makes my NHRA rules more complicated)?

I cant decide where I want to go with this. It is still going to be a street car too but it is never going to go on a road course and going fast around turns doesnt do it for me that much so the IRS isnt critical.

I am strongly leaning towards the 4-Link or the solid axle but I am worried about welding it into the car and how well it will fit, then if I want to put more tire under the car. Decisions decisions.

Any input? Any one done anything similar?

Please if you dont have anything constructive to say please keep it out of this thread. No FLAMES!!!

Mike

FarNorthMotoring

iTrader: (2)

I don't know how fast you want to go, but a good friend of mine, runs ET streets on stock rear with Kaaz and DSS stage 5 axles and was pulling 1.5-1.6 60 foot times last weekend on poorly prepped track. I think best was a 1.51.

kwiktsi

If it were me personally, I would go with the Cobra IRS just to keep the car decent handling also. if you absolutely do not care and want just a drag car out of it, the 4 link setup would be ideal. It depends on your goals. Just another gutted strip car, just hack it and go 4 link with a solid axle- "high tech and modern" (since you have the LS1 Also- this is what i would do), go with the IRS and a 6 speed .
Joe

LT1RX7

The problem with the stocker is the gear selection, not to mention the strength. I need a lower gear than the 3.9's.

The high tech and modern crap is the stuff I would prefer to keep out of this thread. You all know all the guys you look up to.....Abel etc dont use the IRS rear so why hate on me. I still love the 7 just have to overcome it short comings in places I want it to shine.

kwiktsi

I meant no offense with the high tech comment. I do not know your goals with the car- 9 second drag car- go 9"/4 link. low 11 sec street cruiser that is nice to drive daily if needed, LS1/6 speed with IRS. That is what I am getting at. I was assuming using an LS1 that you were looking to do a modern setup, so I was just saying what would accent that nicely. Hell, you could put a tunnel ram on a stock 305 with glass packs and a 10 bolt open rear for all I care, I was just trying to be helpful and don't know the "theme" you are trying to follow with the car.
Joe

John Magnuson

Heck... if it's going to just be a drag car throw a beefed up Ford 9" Rear End under there... tube the back half... run some huge slicks...

SPOautos

Why would the 3.9's be a problem?

93BlackFD

tire diameter affects final drive too...keep that in mind, if you want lower than 3.9 you can use that to your advantage

kwiktsi

Yeah, but smaller tires don't hook . That is why you always see back halved cars with huge tires running in the 5.00's for gear ratios.
Joe

SPOautos

I dont think Hinson had any problems with the 3.9's

jimlab

The differential isn't weak, it's the configuration that's weak.

Two things you have to remember are that the '03+ Cobra Mustangs weigh 800+ lbs. more than the FD, which puts a lot more load on the components when drag racing, and that Ford tried to give the car a "cushy" ride by suspending the rear subframe with an extra set of bushings. That's right, there are two "levels" of bushings; suspension component bushings and bushings which attach the rear subframe to the car.



Add to that the rubber bushings on the front and rear of the differential and it's no wonder that they have repeated complaints of drivetrain noise and lots of breakage claims from wheel hop, even on stock cars. My setup eliminates all that slop.

For occasional drag racing, you should be just fine. Keep in mind that these components are considerably stronger than any possible combination of FD parts, including the KAAZ differential and RP 300M axles.

For a full-on drag car and if you plan to run faster than mid- to low-10s, this is the only way to go. Besides the fact there are tracks that will boot IRS cars running faster than a certain E.T., it's just not worth it from a saftey standpoint. If you look at all the 9 and 8-second cars, most are running solid axles. Vinny Ten, Craig Paisley, Ray Locchead, etc. The noteable exceptions I can think of are the Bullish and MVP Motorsports Supras, but as soon as they have a near miss with a wall (or worse), I suspect they'll get rid of the IRS.

Contrary to popular belief, a 4-link suspension can handle as well as or better than an IRS setup, if setup properly. That's the key... it takes a lot of suspension tweaking to get them dialed in. IRS is simpler, since there's no adjustability to fool with.

You'd have to seriously modify the car to go this route. A good chassis shop would be able to do it, but you're talking about some invasive surgery to the rear of the car. Ground clearance won't be a problem, since they'll cut your floor pan to integrate the rails, but you'll be limited to running slicks all the time if you use a narrowed rear end and tub the car. If you go "street width" with the axle housing and don't tub the back of the car, then the widest tire you'd be able to run would be a 10.5 or 11.0" slick, but there's nothing wrong with that. Mustangs run 7s on them all day long.

Personally, I'd go with the Cobra IRS until (if) something broke, then worry about it. It's the least invasive option and doesn't keep you from going back to stock, plus it's a considerable increase in strength over the stock setup, which many people are using to run 10s and even 9s. (Ari Yallon, for example).

jimlab

Hinson is also running a stock bottem end LS1 and getting a lot of his power from a bottle. A stroked or forced induction LS1 is going to have a lot more traction problems with 3.9s or 4.10s. This isn't about buzzing the engine, this is about effectively putting your power to the ground.

jimlab

This brings up an interesting point... If drag racing is your goal and not corners, then not only are you going to be quicker if you use an automatic transmission, but automatics are a lot easier on drivetrain components, even with a transbrake.

kwiktsi

Yeah, that's why I made the comment about not knowing what he wanted to do with the car- he said track and some street- to me that is a street car and would be an LS1 6 speed IRS car running low 11's to mid 10's and is awesome to drive daily too. If it is purely the ET, an A4 or even th 400 with a 9" would be the way to go. I was going to make the same comment about the cobra IRS and the weight difference between the stang and FD, but you beat me to it . I think the cobra IRS will hold up fine- especially if he uses an auto. I am all about drivers, not drag cars. I just got rid of a 396 stroker 94 Z that ran 10.3@136 on BFG drags- drove it to and from the track with the AC on the whole time and when I raced, it just sat in the pit lanes locked waiting for us to be called- no tinkering around and messing with it between runs- just drive there, do my thing and go home. That is what I consider a street car. If you back half it and go 9", 4 link, tubs, etc. it is just a drag car with plates on it .
Joe

kwiktsi

also Jim- am I understanding right that you are running the Cobra IRS also? How involved was the swap?
Joe

jimlab

I'm the one who made it possible. Check the pictures in the group buy thread...

https://www.rx7club.com/group-buy-center-69/03-cobra-8-8-irs-cradle-group-buy-313112/

turbojeff

iTrader: (30)

I was thinking the Cobra IRS would be the entire Independant Rear Suspension. It looks like when people refer to the Cobra IRS they are just speaking of the diff (axles and subframe changes too)...

Sounds like a good idea to me, I don't think putting the entire rear subframe would be a good idea, I think Ford made that rear suspension for the T-bird right? It doesn't seem any better than the Mazda rear suspension and is probably worse.

jimlab

Look at the picture on the previous page.

Alpha-7

iTrader: (1)

just curious lt1rx7 did you ever run your rx7 with the setup you have now???

Alpha-7

iTrader: (1)

1/4 wise...

turbojeff

iTrader: (30)

I did, I don't understand how that will show that it is better? Double bushings?

Edit...

And big ugly coil springs, mounted in a wierd place.

kwiktsi

I am going to go out on a limb here and say I *think* he meant to say "look at that pic in the thread I mentioned above". There is a pic of the setup- basically it is a modded FD subframe that has been modded to accept the cobra center section.
Joe

jimlab

Who said it was better? I was agreeing with you.

turbojeff

iTrader: (30)

Cool, I didn't know what you were saying.

So for the record, when your referring to "Cobra IRS" your really just talking about the diff and associated bracketry.

I think the Cobra IRS (suspension, not diff) is a POS but I didn't want another battle.

If I were building a V8 FD I'd probably stick with the stock diff until it gave me troubles only if I wasn't planning on building a drag car. Then I'd probably go right for the Cobra diff.

jimlab

In an FD? Sure.

If you had an engine that made over 300 lb-ft. of torque at idle, you'd probably want something a little taller than the stock 4.10 or automatic 3.90 gears in the back.

Even the 3.55 R&P in my case is a bit shorter than I wanted for 1st through 3rd, but I didn't want to give up any more gearing on the high end than I had to. In fact, with the 2.97:1 first gear of my M29 T56, the 3.55s are about the same as 4.10s behind a standard T56 (10.54:1 vs. 10.91:1).