Rear axle delimma

[Texas Driver]

Hey all. I have been reading a lot of build threads and other discussion. Love a community like this that come together for the same love.
I am trying to figure out if I want to spend the extra $ on getting the IRS or solid axle. I have been a road racer for about 15+ years so that side of me wants to go with IRS. The logical side of me that says this will be mainly street driven and tracked from time to time says to save the $ and go solid axle. Any input would be great to others experiences one way or the other.
Second question that I can think of today, order the turn key engine and trans at the time of kit order or wait till I have a little extra cash flow to do so? Has it seemed to slow down the build if waiting or find yourself wanting to work on certain parts of the car and can't because you didn't have the motor yet?

Thanks in advance for any input.

[phileas_fogg]

Go with the IRS. You'll enjoy driving the car more.

Wait with the engine. Depending on how fast you work, you may not need the engine for a year or more.

John

[michael everson]

If you catch a sale that includes options, then its a no brainer. Either way look at all the costs and then decide. I personally would go IRS.
mike

[chuckster]

Go with the IRS. You'll enjoy driving the car more.

Wait with the engine. Depending on how fast you work, you may not need the engine for a year or more.

John

What he said, I have the IRS in my car, and the ride is downright plush.

[cgundermann]

My own car is a 3-link solid axle and after 4 upper back surgeries, it's not comfortable on rough roads. Having driven my Pal's 20th Anniversary IRS equipped MK4, I can tell you it's definitely worth the upgrade. A consideration in timing your motor purchase, is its warranty period. So if your build slows (like mine did) and you purchase your motor early, it might be out of warranty during your first start or when road worthy.

Chris

[FlyingCobra]

I can't comment from a driving perspective, but one point is that the IRS gives you the option of a T-56 transmission with a 302/351. I now have a T-56 that I want to put in the car, but because I chose the 3-link suspension that won't fit.

However, the 3-link is noticeably cheaper. So there's that aspect and I chose the 3-link as a balance between price and performance. We'll see at the end of the build what I think of that design decision.

On the engine, I'd definitely wait. Sitting isn't good for engines. Besides, you might come across a deal you can't pass up or change your mind on what you want to do in the meantime within the family of engines you chose.

[Texas Driver]

Thanks everyone on the advice. I think my mind is made up now. IRS it is and great point of coming across a deal on a motor and or trans for the car would be great.
Hope to start a build thread early next year.

[Avalanche325]

In case you go searching for past threads...

There is an old version of IRS that had issues and was not as good for performance. I have not seen anything but praise for the new IRS.

If I were building today, I would go IRS.

[CraigS]

I know, don't want to think about it, but having an IRS car if you ever want to sell it will be a huge plus. Now here is another bass ackwards way to justify it or the engine. The salvage rear suspensions are significantly less expensive if you go w/ the 315 ratio. So go w/ a 351 engine to help w/ a little extra grunt to overcome that gear. Another tip, be sure to get a salvage setup that includes the rear brakes. So now you have free rear brakes. Nice huh? And the brake size is just fine for us and all the aftermarket people make pads since it's from a Mustang that the sell zkillions of per year. You will notice that some of the rears are aluminum pumpkin and some are iron. Ford uses iron in the higher perf and/or standard trans Mustangs. But I have been running my Al 315 diff for two seasons of autocross (18 or so per year) on Hoosier 335x17 autocross tires. It is plenty strong enough for our use. I know you won't track teh car much but here is another IRS advantage. I had solid axle for 9 yrs and never 100% got rid of axle end play (and the axle flex) so I always had a lower brake pedal for the last half of an autocross run because of pad knock back. My first Ax w/ the IRS and the brakes were the same at the end as the beginning. And it is still that way. I guess you can tell, I think the 2015 IRS is fantastic.

[Murd]

Can’t comment on irs but it’s nice to have all the parts and components you need on hand. I picked up my kit end of February and dropped engine in probably mid June? First start in July 2017. Still took me till May 2018 to get it on the road without paint. If you are planning a multi year slow build maybe hold off on the engine, if you want it on the road in a year or so I’d say get everything quick as you can.
Have fun!

[GoDadGo]

My pal Donnie has the IRS (MK-4) while my buddy Jerry (Type-65 Coupe) has the 3-Link as does my car (MK-4).
The IRS rides considerably better than the 3-Link; however, the 3-Link plants the crap out of the tires way, way better.
If you intend to road race and/or autocross the car, get the IRS, but if you like to traverse the 1,320 then the 3-link is the better option.

PS: If I ever figure out how to turn left and right I guess I'll need to upgrade or build a new car.

[Jimtmich]

Getting a salvage 2015 IRS is a great way to go. I believe the V6 cars have solid disks, the GT have vented disks. There is a "Track Pack" mustang option with very large Brembo brakes that may push you into larger wheels that you want. the Torsen differential was only available in 3.73 ratio from the factory. I read that the 3.15 ratio diff cannot be changed, but I haven't verified that. The Aluminum vs cast iron differential housing as mentioned before is a automatic vs stick shift from the factory. For these lighter cars, probably not an issue.

[seagull81]

Welcome to the Cobra family. You have a PM.