IRS vs Live Axle

amy98gt

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I talked to my buddy the other day who owns a sick performance shop in Sumter, SC. Hes finishing up the motor in my car and offered me a good deal. He wants to do a IRS swap out of his 04 cobra to a Live Axle, out of my 98 gt.
The only problem is I want a more of a course setup than anything else. All my research has been adapting the live axle. so I'm very limited on my knowledge of the IRS and its benifits over the live axle. Is this swap even worth it? Or am I better off with what I got?
Please help! I need to make a decision this week.
 

Saint

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You want a road course set up?

An IRS would give you the most benefit for a road course set up point blank.
 

mikey94gt

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So, u wanna go IRS, eh? The IRS in the mustang is a decent piece, definitely better than the factory suspension you now have. No more binding in corners and causing snap oversteer, and it will be more predictable. It does add weight to the car, right where mustangs need it, and will even out your f/r weight biasThat said, there are some things the IRS will need, like an anti hop and grip kit from MM. Since you're not gonna drag race it, I wouldn't really worry about the axles and bracing the center diff. The biggest killer of those is axle hop, which the IRS is prone to in factory trim
 

justinschmidt1

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If you want to turn, go irs, if you want to drag race, stay live axle

Simple as that.
 

twovalveterror

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If you want to turn, go irs, if you want to drag race, stay live axle

Simple as that.

if only.

what mikey said was spot on. massively better than a stock suspension, but you'd definitely want the bushing kit. but you say you've done some work on the live axle set up? depending on what you've done, it may in fact be better than a stock IRS. you can get a live axle to handle just as well as an IRS on roads as smooth as glass. an IRS's main benefit is stability and predictability. but it does have it's drawbacks.

it takes a little bit more work to make it work on the drag strip, and will never be as strong as a live axle (from my limited experience that is) and it even has disadvantages to a live axle on the road course. the main one being heat production. I've been told by self proclaimed "experts" that a stock IRS will be fine for occasional track use. but some of the more reputable shops say that if you race frequently enough, you might want a diff cooler. the IRS does not have axle tubes that act as heat sinks, and depending if you choose to route your exhaust under the diff or not, even that can factor into diff temps. the IRS will get considerably warmer than a live axle.

I'm halfway through an IRS swap myself. but I'm waiting on a couple more parts to get here, so I won't be able to give you first hand impressions in your time frame.
 

justinschmidt1

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I hear ya.....you can definitely make a live axle handle great if you put the work in to it, im a firm believer in that.

I was just saying out of the box it seems like the irs is a better options for the occasional autox or spirited driver as far as stability in the twisties.
 

castine917

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I read earlier today that after MaximumMotorsports built their solid axle black car with their parts they converted to irs and built that. It ran 2 seconds quicker and new lap records at the course they tested at.
 

twovalveterror

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I hear ya.....you can definitely make a live axle handle great if you put the work in to it, im a firm believer in that.

I was just saying out of the box it seems like the irs is a better options for the occasional autox or spirited driver as far as stability in the twisties.

for sure, no disputing that.

I read earlier today that after MaximumMotorsports built their solid axle black car with their parts they converted to irs and built that. It ran 2 seconds quicker and new lap records at the course they tested at.

yep! reading the drivers impressions on that car made me finally quit bouncing back and forwards between live and irs. that and the deal I got on my IRS!
 

mikey94gt

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Exactly, fellas. I have solid axle in my car with the steeds 5 link, drives just as good as an irs car, until the road gets bumpy anyways. I forgot to mention the diff cooler deal, all the vette guys run one if they play on the track more than once or twice. If you don't do the diff cooler, just run a good synthetic and change the oil before and after each track event. I do my engine oil and trans that way. The main reason I run solid axle is reliability, because I do a ton of track days, road trips and occasional drag strip assaults.
 
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amy98gt

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So, u wanna go IRS, eh? The IRS in the mustang is a decent piece, definitely better than the factory suspension you now have.

Ok let's up the ante. Both IRS & Live Axle are fully built. Is the IRS still the leader?

It's either I pay for the parts and build up mine, or I do the swap and use his fully built IRS. Either way I'm paying the same amount.
 

justinschmidt1

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Ok let's up the ante. Both IRS & Live Axle are fully built. Is the IRS still the leader?

It's either I pay for the parts and build up mine, or I do the swap and use his fully built IRS. Either way I'm paying the same amount.

No, thats the thing...

when you build the live axle I believe it out performs the built IRS.
 

twovalveterror

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what is your definition of fully built?

if the IRS has full bushings, diff brace and cooler, and beefed up internals ( which you might not need, depending on power level) then it is the winner for road racing.

but if the SRA has corrected geometry (ta/phb/rca) with internals. you have a pretty mean set up already.
 

mikey94gt

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It depends on what you use to build each one, especially for road racing. If you keep the triangulated upper control arms, the rear suspension starts to bind when driving the car hard, and that's when the rear end breaks traction. I think the factory style triangulated 4 link is limited, because the instant center is off, and it allows the axle to move around under the car. If you take a look under any of the seriously fast mustangs on the road course, none of them will have the stock style suspension, and if they do, they usually move the upper control arms somehow. hence the question, what do you plan to build the axle suspension with?

Now, for instance, if you were just going to do basic upgrades such as control arms and roll bars, I think the IRS would be a better setup for the money. If you wanted to go all out and ditch the triangulated 4 link, swap to a torque arm or other complete package of that type, then the IRS/solid axle question may be a toss up.

One last thing to remember, you can adjust toe in and faster, etc on the IRS, where the axle is pretty set to 0*. Something to consider when wheighing options
 

Slykin

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I believed IRS was definitely the way to go... until I test drove a 2011 5.0 yesterday. Of course, the whole chassis is set up better than our cars, but the rear end on that car was PLANTED. I was definitely impressed, and I think for most guys a live axle is the better way to go. I'm sure the IRS is better on rough roads, but on smooth roads... Live axle FTW. Plus on a live axle, you don't lose/gain hardly any camber through the corners, you basically get 100% contact patch if I understand correctly.
 
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amy98gt

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I know he already has the diff brace, bushings, and internals. I have uca, lca, 31 diff, 31 axles. he puts 802 to the feet while mine 610
 

framda

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One of the major drawbacks to a solid rear axle is that a bump on one side is transmitted to the other side. If it was better for handling, the manufacturers would still be using the cheaper solid front axle.
 

twovalveterror

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I know he already has the diff brace, bushings, and internals. I have uca, lca, 31 diff, 31 axles. he puts 802 to the feet while mine 610

you have aftermarket UCAs? swapping to the IRS from that will make it a completely different car. have you not had any problems with snap oversteer? I'd be amazed if you told me you had any mid-corner confidence with your current set up. both of those power levels seem pretty sketchy for an IRS though.

but billetflow's website has people saying they're running 10's with the IRS so it may be able to handle it. how long has your buddy been running it with 800 hp?
 
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amy98gt

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I havent installed the rear parts yet. I got the mm rear grip package and was thinking about adding a torqe arm bar.
His cobra hasnt been strong long actually. We did a 732 03 cobra and it has 3k miles on it since.
 

Gallows

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In regards to the heat buildup on a road course what would be different on a 200-300 mile drive with deeper gears? Would you not get a lot more heat buildup doing that?
 

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