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Discussion Starter #1
So, after 2 years I finally found the balls to start upgrading my 89 LX hatch. Going to use this on twisties, and I live in California, so engine upgrades are basically out the window.

Engine upgrade-wise, most I plan to do is add a supercharger or hopefully a CARB-legal twin turbo kit meant for hp increases on an almost stock engine.

Anyways, I want rear disc brakes for better handling, but looking at both a junkyard kit and a new conversion kit would already be about 1/2 of a good condition cobra IRS (excluding shipping costs if necessary). Should I sit tight and deal with having drums until I can get a proper IRS, or should I just go ahead and get a rear disc brake kit?
 

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Having run a 99 and 03 Cobra IRS there are far better options out there. I would say your best bet would be to upgrade ALL the brakes first. This would mean going with something like an M-2300-K kit which you could put together yourself pretty cheaply these days from junkyard parts.

Once you've done that I'd steer you towards at Maximum Motorsports Torque Arm and Panhard Bar setup. This will require all new springs, which means you'd probably want to convert to coil overs at the same time.

But somewhere between the brakes and the suspension mods my primary recommendation would be to upgrade the driver first. I generally recommend about 5-6 High Performance Driving Education events before you dive into the suspension. This is because you need to learn how to read what the car is telling you before you go changing parts around. What one car needs, another might not. Additionally what you learn at an HPDE will transfer with you to ANY car you drive now, or in the future.

Believe me when I tell you that doing HPDE BEFORE you start wrenching on the suspension will save you thousands of dollars in the long-run. Been there, done that, got a garage full of parts that I wasted money on to show for it.

I've been instructing HPDE and EVOC driving for over 25 years now with a list of tracks that spans the East Coast with one or two in places like California and Japan. I raced Mustangs in SCCA for over a decade (and hope to get back into it someday soon), and even I am still a student when it comes to driving.

But what I've learned is this: Only when you can't make the car go any faster with your driving do you mod the car.
 

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Discussion Starter #3
Having run a 99 and 03 Cobra IRS there are far better options out there. I would say your best bet would be to upgrade ALL the brakes first. This would mean going with something like an M-2300-K kit which you could put together yourself pretty cheaply these days from junkyard parts.

Once you've done that I'd steer you towards at Maximum Motorsports Torque Arm and Panhard Bar setup. This will require all new springs, which means you'd probably want to convert to coil overs at the same time.

But somewhere between the brakes and the suspension mods my primary recommendation would be to upgrade the driver first. I generally recommend about 5-6 High Performance Driving Education events before you dive into the suspension. This is because you need to learn how to read what the car is telling you before you go changing parts around. What one car needs, another might not. Additionally what you learn at an HPDE will transfer with you to ANY car you drive now, or in the future.

Believe me when I tell you that doing HPDE BEFORE you start wrenching on the suspension will save you thousands of dollars in the long-run. Been there, done that, got a garage full of parts that I wasted money on to show for it.

I've been instructing HPDE and EVOC driving for over 25 years now with a list of tracks that spans the East Coast with one or two in places like California and Japan. I raced Mustangs in SCCA for over a decade (and hope to get back into it someday soon), and even I am still a student when it comes to driving.

But what I've learned is this: Only when you can't make the car go any faster with your driving do you mod the car.
Alright, thanks for the info!
 

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I second what Mr. Ihara said. I had an 01 Cobra and aside from the superior ride the IRS provided, from a handling perspective it had lots of issues in stock trim. Ultimately, I installed the whole MM catalogue on my 86.

I would include autocrossing as an adjunct or alternative to HPDEs, as they are much cheaper and usually the venues are closer and events more often. You get much of the same feedback from the car but at much lower speeds. And set-up is similar. And seat time is more critical than random car mods. In general, the car in stock trim is more capable than a novice. It can be humbling.
 

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@exno254

Late to the post here and I understand that but I curious about what you're after? You bring up changing your brakes and or suspension but say nothing about why or what you're after. What are you trying to accomplish?

I agree with the suggested training but that aside there are advantages and disadvantages to IRS and a straight axle. The biggest disadvantage of IRS is it's more complicated. Adding IRS does not solve properly tuning your chassis for IRS. IRS isn't doing you any good and possibly extensive harm if not properly tuned to the car. And this doesn't matter how skilled of a driver you are. A major problem I see with people who think they can "buy" solutions with IRS on rear wheel drive cars is they are clueless about how to set up for rear wheel torque canceling by transferring it to the chassis. I can't count how many cars I've seen destroyed and worse because of this. Much easier to do with a straight axle which is why a straight axle is generally preferred over IRS on a strip. It's much easier to tune.

Bear in mind also that a lot of professional rally drivers even prefer straight axles over IRS. Easily verifiable. It has to do with the cars weight bias and style of driving. Too much about that to explain here. The Mustang is also not a preferred Rally car because of it's front weight. Rally drivers don't drift, they slide. They are losing traction because of geometry, not weight. Big, big difference when it comes to available power at the rear wheels. Drifters sacrifice their traction for geometry and have to regain traction when drifting. That's lost time.
 

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@River The problem with the SN95 IRS is that it was a compromise design. It had to fit into the rear of the car in the same amount of time as a stick axle and ride along the same assembly machines. It had to bolt to existing locations, and had to fit in the same space. The engineers did a pretty good job with packaging and were creative in their use of existing attachment points; however, these constraints also brought with them an entirely different set of problems.

While the IRS can offer some handling improvements with enough work, it adds a TON of weight and complexity to the car. If you get a '99 - '01 IRS instead of an '03-'04 then you have even more work to do. Then you have differential overheating problems under hard, sustained use, with any year from '99-'04.

After improving the loose nut behind the wheel first ;), there's no handling advantage to the SN95 IRS over a torque arm and panhard bar or watts link setup.
 

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@River The problem with the SN95 IRS is that it was a compromise design. It had to fit into the rear of the car in the same amount of time as a stick axle and ride along the same assembly machines. It had to bolt to existing locations, and had to fit in the same space. The engineers did a pretty good job with packaging and were creative in their use of existing attachment points; however, these constraints also brought with them an entirely different set of problems.

While the IRS can offer some handling improvements with enough work, it adds a TON of weight and complexity to the car. If you get a '99 - '01 IRS instead of an '03-'04 then you have even more work to do. Then you have differential overheating problems under hard, sustained use, with any year from '99-'04.

After improving the loose nut behind the wheel first ;), there's no handling advantage to the SN95 IRS over a torque arm and panhard bar or watts link setup.
Christopher,

I enjoy reading your comments. You have an obvious understanding of why you think the way you do. I also know a lot about Ford's but I have to admit I know little about Mustangs from the Mustang II on. Joining this site was due to advice I was given about some participants here and what they have done with the Fox Body platform other than the Mustang. Although I'm retired now most of my work was in Europe where the Fox Body didn't mean the same thing. After the Cortina, "Fox Body" was lost in the mix unlike here.

That said, IRS means something very different to me than it does to most and this may be opening a can of worms. I accept what IRS has become to mean but in reality there are very few cars made with an IRS suspension. Ford doesn't offer any IRS suspensions off a showroom floor. Among other things, inherent to the term "IRS" are some specific characteristics between sprung and un-sprung weight that just isn't met by what's being referred to as IRS. For true IRS the differential is completely un-sprung weight. It's part of the chassis. This is why almost all true IRS cars are performance mid or rear engine cars. Except for some rare breaking circumstances most people will never be driving at the extremes necessary to take advantage of what IRS can offer. Electronic traction control now offers more than enough of what most drivers can appreciate. And that includes me. Tuning is far from driving.

I completely agree with you on the traction bars and panhard or other options.

I don't mean to open a can of worms with this.

EDIT: Oops, it was late. I wrote sprung and un-sprung backwards. The differential in IRS is sprung weight.
 

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@River The Independent Rear Suspension for the Mustang sits entirely in a subframe that connects to the body with four bolts, surrounded by four fairly soft rubber bushings. The differential is mounted to the subframe by two tabs on the front pinion housing and is also bushed to help eliminate NVH from the gears in the diff. Unequal length double A-arms are also attached to the IRS subframe through bushings. 1999-2001 IRS use 28-spline half-shafts, and 2003-2004 half-shafts are substantially beefier and use 31-spline axles.

The IRS was prone to severe wheel hop during wheel spin that could be severe enough to snap half shafts, especially when used with drag radials. This was less of a problem in 03/04 models because of a change in bushing durometer. You can see in this drawing just how many bushings are in this entire design. Their relatively soft compliance rate means that not only do the control arms move around some, but the entire subframe as a whole moves around as well.

My IRS equipped 1999 GT made about 520 RWHP and exiting the high-speed, right-hand, turn 10 at my local track onto the 3000-foot front straight required that I wait until track-out before I could go Wide Open Throttle. If I went WOT shortly after apex the car would call what we termed "the waggle" where the rear of the car's rear would begin to oscillate back and forth. When this would happen you'd have to let off the throttle completely to let the rear end begin to track straight again. Staying in the throttle would result in successively greater oscillations as the rear-steer would induce more rear-steer.

Now, I was running all stock bushings which was a major contributor to the problem; however, as the SN-95 Mustang's IRS wasn't a widely produced piece parts to fix its woes are a bit more expensive. Once you fix all the busing issues it's not bad for what it is, but it is heavy. Only the lower control arms are aluminum, the rest is all tubular steel. Here's a picture to give you some idea of what it looks like:

1060360
 

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FWIW, my 01 Cobra IRS had 31 spline axles. But the same lousy handling, axle tramp, etc. occurred during my auto-cross experience. Now, my s550 also has the IRS center section mounted to the subframe, with aluminum control arms. The whole design is light years better than the sn95 version. Can it be made better? Sure but most of those mods focus on reduce axle tramp at the expense of NVH. I've done nothing to the s550 suspension except replace the shocks/struts/springs with KWv3s. My experience both auto-cross and OT with the s550 is far superior to the 01 Cobra.
 

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@exno254

Late to the post here and I understand that but I curious about what you're after? You bring up changing your brakes and or suspension but say nothing about why or what you're after. What are you trying to accomplish?

I agree with the suggested training but that aside there are advantages and disadvantages to IRS and a straight axle. The biggest disadvantage of IRS is it's more complicated. Adding IRS does not solve properly tuning your chassis for IRS. IRS isn't doing you any good and possibly extensive harm if not properly tuned to the car. And this doesn't matter how skilled of a driver you are. A major problem I see with people who think they can "buy" solutions with IRS on rear wheel drive cars is they are clueless about how to set up for rear wheel torque canceling by transferring it to the chassis. I can't count how many cars I've seen destroyed and worse because of this. Much easier to do with a straight axle which is why a straight axle is generally preferred over IRS on a strip. It's much easier to tune.

Bear in mind also that a lot of professional rally drivers even prefer straight axles over IRS. Easily verifiable. It has to do with the cars weight bias and style of driving. Too much about that to explain here. The Mustang is also not a preferred Rally car because of it's front weight. Rally drivers don't drift, they slide. They are losing traction because of geometry, not weight. Big, big difference when it comes to available power at the rear wheels. Drifters sacrifice their traction for geometry and have to regain traction when drifting. That's lost time.
S'all good, I'd rather hear how my ideas/plan are dumb as hell before they actually come to fruition. I want my LX to continue being a daily driver, hence why I can't get too much into engine mods because of Cali-smog. I can hot smog, but it feels much more satisfying staying legal especially in Cali. I'm not looking to go professional, just having simple fun on mountain roads with my fox if possible, maybe hit the track if my wallet and schedule allows (nearest one is 2 hours away). Guess I got that problem where I keep looking into random ass mods and thinking it'll make me faster.

As for the IRS vs Cobra brake setup, it's more along the lines of me trying to plan out a budget since a cobra disk brake setup could cost much cheaper than around $7-800, versus just holding on a bit longer to buy an IRS since IRS' make the ride quality and handling much more better as far as I've heard. No I don't have any knowledge in regards to geometry, I'm a simple guy, so I'm still thankful for the info yall have given thus far. I just want to get rid of the drum brakes because it was a massive PITA when I first did them myself.
 

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Christopher,

I enjoy reading your comments. You have an obvious understanding of why you think the way you do. I also know a lot about Ford's but I have to admit I know little about Mustangs from the Mustang II on. Joining this site was due to advice I was given about some participants here and what they have done with the Fox Body platform other than the Mustang. Although I'm retired now most of my work was in Europe where the Fox Body didn't mean the same thing. After the Cortina, "Fox Body" was lost in the mix unlike here.

That said, IRS means something very different to me than it does to most and this may be opening a can of worms. I accept what IRS has become to mean but in reality there are very few cars made with an IRS suspension. Ford doesn't offer any IRS suspensions off a showroom floor. Among other things, inherent to the term "IRS" are some specific characteristics between sprung and un-sprung weight that just isn't met by what's being referred to as IRS. For true IRS the differential is completely un-sprung weight. It's part of the chassis. This is why almost all true IRS cars are performance mid or rear engine cars. Except for some rare breaking circumstances most people will never be driving at the extremes necessary to take advantage of what IRS can offer. Electronic traction control now offers more than enough of what most drivers can appreciate. And that includes me. Tuning is far from driving.

I completely agree with you on the traction bars and panhard or other options.

I don't mean to open a can of worms with this.

EDIT: Oops, it was late. I wrote sprung and un-sprung backwards. The differential in IRS is sprung weight.
@River The Independent Rear Suspension for the Mustang sits entirely in a subframe that connects to the body with four bolts, surrounded by four fairly soft rubber bushings. The differential is mounted to the subframe by two tabs on the front pinion housing and is also bushed to help eliminate NVH from the gears in the diff. Unequal length double A-arms are also attached to the IRS subframe through bushings. 1999-2001 IRS use 28-spline half-shafts, and 2003-2004 half-shafts are substantially beefier and use 31-spline axles.

The IRS was prone to severe wheel hop during wheel spin that could be severe enough to snap half shafts, especially when used with drag radials. This was less of a problem in 03/04 models because of a change in bushing durometer. You can see in this drawing just how many bushings are in this entire design. Their relatively soft compliance rate means that not only do the control arms move around some, but the entire subframe as a whole moves around as well.

My IRS equipped 1999 GT made about 520 RWHP and exiting the high-speed, right-hand, turn 10 at my local track onto the 3000-foot front straight required that I wait until track-out before I could go Wide Open Throttle. If I went WOT shortly after apex the car would call what we termed "the waggle" where the rear of the car's rear would begin to oscillate back and forth. When this would happen you'd have to let off the throttle completely to let the rear end begin to track straight again. Staying in the throttle would result in successively greater oscillations as the rear-steer would induce more rear-steer.

Now, I was running all stock bushings which was a major contributor to the problem; however, as the SN-95 Mustang's IRS wasn't a widely produced piece parts to fix its woes are a bit more expensive. Once you fix all the busing issues it's not bad for what it is, but it is heavy. Only the lower control arms are aluminum, the rest is all tubular steel. Here's a picture to give you some idea of what it looks like:

View attachment 1060360
@Christopher Ihara @qtrracer acer

First: Please notice a correction I made from my post last night. It was late and I don't know why but wrote sprung and un-sprung weight backwards. The differential in true IRS is "sprung" weight. What that amounts to is the torque forces being transferred to the wheels is tunable to the chassis (suspension) which offers much more control. It's not floating from the chassis.

Now, can of worms I guess? lol

Name dropping is something I try to avoid but I use it here for explanation. I came back to the states in '97 as a representative consultant for a German car manufacturer that had hired the engineering firm, Applied Technologies, to design the changes needed to get one of their cars compliant to DOT regulations. AT was owned by Jim Thornton and the Kamm brothers. Jim is credited by NHRA as the inventor of the torque cancelling chassis and the altered wheelbase "Funny Car," among many other things. He also ran Chrysler's performance department and was the principle driver for Chrysler's racing team, The Ram Chargers. He owned the NHRA in the early 60's. I learned a lot from the man over many lunches.

I bring this up to suggest you look in to his ideas about the torque cancelling chassis. The concept of it was all about using the chassis rather than the throttle to control the difficulties with transferring torque to the track surface with the suspension and the rotational dynamic changes inherent to wheelbase weight bias.

Thanks for the IRS pic. It clearly points out the problems with creating true IRS. To start with, ideally the spring and shocks will be on perpendicular to and on the axle. That's all about wheel rate and spring rate matching and harmonic interference. Because of all the mass (weight) you mention needed to fit the designs in with a FWD car it's almost impossible to do with a compact, low CoG design without the added mass. Early designs started with the De Dion Tube and\or integrating the control arms with the differential. Both concepts were tuning nightmares and didn't really solve the added mass problems with attempts to enhance tuning.

Just thought you might find looking in to all of this interesting.

@exno245,

I have to agree with Christopher after reading your last post. You will be much better served with improving your driving skills instead of your car. If your goal is a good time that's in the driving, not the car. What you do put in to your car would better serve you to focus on your transmission and clutch anyway. When I watch drivers starting in Rallying the biggest mistake they make is with trying to transfer there street shifting habits to the track. A driver should rarely touch the clutch pedal after the start of a race. They seldom will let up on the throttle. That's where the time and power is at during a race. Look at this to get an understanding of what I mean. Also, I don't mean to suggest you save up for something like this. You really don't have the car for it but like you said, you're looking to have a good time and there are methods to get you a lot of what this offers with good training. You'll have a blast too taking the training. Also, you're driving by yourself, correct? That makes a huge difference along with being on a closed track compared to an open circuit. Bear in mind this is for a manual transmission and clutch. Not an automatic with manual control abilities. Similar systems are also available with bang shift instead of paddle shift.

 

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S'all good, I'd rather hear how my ideas/plan are dumb as hell before they actually come to fruition.
I wouldn't say "dumb as hell". Rather, ill informed. But if you persist with the plan, then perhaps "dumb as hell" LOL.

As for the IRS vs Cobra brake setup, it's more along the lines of me trying to plan out a budget since a cobra disk brake setup could cost much cheaper than around $7-800, versus just holding on a bit longer to buy an IRS since IRS' make the ride quality and handling much more better as far as I've heard. No I don't have any knowledge in regards to geometry, I'm a simple guy, so I'm still thankful for the info yall have given thus far. I just want to get rid of the drum brakes because it was a massive PITA when I first did them myself.
Putting on Cobra rear discs is not all that pricy. What can get pricy are the axles and wheel/tires. And these days, finding a 03/04 Cobra IRS for under $1K may be very hard. Moreover, given how old these things are now, you still may need to rebuild/replace some of the pieces. Just say'n.
 

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I wouldn't say "dumb as hell". Rather, ill informed. But if you persist with the plan, then perhaps "dumb as hell" LOL.



Putting on Cobra rear discs is not all that pricy. What can get pricy are the axles and wheel/tires. And these days, finding a 03/04 Cobra IRS for under $1K may be very hard. Moreover, given how old these things are now, you still may need to rebuild/replace some of the pieces. Just say'n.
I can't speak to the area anyone is in but here in the Detroit area there are plenty of shops selling "take off" IRS systems at good prices. They are even available on occasion from cars straight from the show room floor. There are even shops that will give you a used system if you pay them to put it on. It didn't cost them anything.
 

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I can't speak to the area anyone is in but here in the Detroit area there are plenty of shops selling "take off" IRS systems at good prices. They are even available on occasion from cars straight from the show room floor. There are even shops that will give you a used system if you pay them to put it on. It didn't cost them anything.
Take off 99-04 Cobra IRS units?
 

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Okay that is what I thought, not applicable to this conversation.
Really? Why not? I've swapped numerous front and rear suspension. Granted, it's not a bolt on but where's the fun in that? Also as I said, the shop selling this would probably give it to someone if they where hired to put it on. And as others have said, this is a better set up than the SN95 IRS.
 

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No doubt it is way better, the difference is one can be installed into an SN in a weekend the other requires a skilled fabricator. Would be badass tho
 

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Really? Why not? I've swapped numerous front and rear suspension. Granted, it's not a bolt on but where's the fun in that? Also as I said, the shop selling this would probably give it to someone if they where hired to put it on. And as others have said, this is a better set up than the SN95 IRS.
The S550/S197 chassis is COMPLETELY different than the Fox3/SN95 chassis. The original S197 is derived from the Lincoln LS platform from the 2000's. If you compare a Fox body car to a new S550 car you will immediately see how the S550 Mustang DWARFS the Fox body.

Track width on a 1987-1993 Mustang is 57"
Track width on a 2005-2015 Mustang is 62.5"
Track width on a 2016-2020 Mustang is 64.9"

A Fox body (1979-1993) Mustang's entire rear axle with tires will literally fit INSIDE a 2016-2020 IRS. You'd have better luck putting the Mustang IRS into a Ford Fusion as it is almost a direct fit. (
) This guy did just that.
 

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The S550/S197 chassis is COMPLETELY different than the Fox3/SN95 chassis. The original S197 is derived from the Lincoln LS platform from the 2000's. If you compare a Fox body car to a new S550 car you will immediately see how the S550 Mustang DWARFS the Fox body.

Track width on a 1987-1993 Mustang is 57"
Track width on a 2005-2015 Mustang is 62.5"
Track width on a 2016-2020 Mustang is 64.9"

A Fox body (1979-1993) Mustang's entire rear axle with tires will literally fit INSIDE a 2016-2020 IRS. You'd have better luck putting the Mustang IRS into a Ford Fusion as it is almost a direct fit. (
) This guy did just that.
Boy, had a hard time getting to that video. I liked it though.

The 86 Porsche 944 had a wheelbase of 94.5", front track of 58.1" and rear track of 57.1".
The 69 Escort MK1 2 door saloon had the same wheelbase of 94.5", front track of 49" and rear track of 50".

There are a number of MK1 Escorts running today that have the 944 drivetrain and suspension on highly modified but original frames today. It's been almost 30 years but I'll see if I can find anything I can put on line. It was film not gifs and I have no idea of what box or where to look. lol

Here's a highly modified Escort MK1 Van with a 2JZ GTE VVTI and assembled Skyline IRS rear differential and suspension.

As I wrote, it wouldn't be anything like a bolt on swap but there's not much that can't be done this side of stupid or ridiculous. That's the domain for Roadkill. lol Seriously though, altering that subframe assembly, finding suitable CV axles along with finding or making suitable control arms. I have no idea on heights appropriate for the car but that's not to hard to solve as needed. It's about having the time and will to do it or the money to have it done.

I have a NOS 2013 Coyote leftover from Watson Racing and Hellion when they prototyped and built the twin turbo Mustang Turbo Jet's. Now that was a nightmare I wouldn't have gone near. Except for the basic GT body style the finished car didn't resemble the original car at all because of the power it could make.
 
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