IRS on a 1000HP car?

[SlowSVT]

Jefe, I'll call you when it's all bolted back together. The heads had a thin deck and were contorting under boost. The new heads just showed up. I need to have the turbo headers rebuilt because of the higher port on the head and the new header bolt pattern. It'll probably wont be back on the road until sometime this summer. I'm thinking about selling the AEM and doing a Haltech while it's down so it could be even longer. I've got lots of time to mull this IRS vs SRA thing over.

Another IRS owner mentioned that snapped axles can a lot more damage when they flail around. If lower HP cars are snapping axles I'm going to do it for sure, and probably on a routine basis. I drove Mike's Cobra back from Tombstone when it had the IRS and it wasn't anything to write home about. Granted it probably had all the stock rubber junk in it. Jefe, if you get the FTBP done if yours you'll have to take me for a spin up the mountain.

The IRS will shines in a bumpy corner and won't "skate" the rear end to the outside on a hard corner like an SRA will. It removes 65 lbs off the unsprung weight at each rear wheel which is huge, makes the suspension more compliant and can be de-cambered improving the cornering grip even more. For regular driving you probably won't notice the difference between the two.

The stock axles actually hold up better than you think after you fix the suspension and diff. As long as you’re not dumping the clutch a 4K with slicks they should hold up fine. A turbo should be easier on the axles over a PD blower with an explosive bottom end because the boost really comes on after the car has overcome inertia.

Very nice Fox body Mustang. I can just imagine the hardware you have taken down with this. Its appearance is just enough to rope-in some unsuspecting supercar owner looking for an easy kill :mj: until your motor gets on the "step" and doles out a healthy dose of humiliation from a 20 year old Mustang :banana:

 

[turbofiveoh]

I'd love to do an IRS because I don't really care about the 1/4 mile any more. The car took a long time to build and over the years straight line racing became less important. Going to the drag strip once in a while is fun, but it's not why I built the car. I just really enjoy driving it. I was hoping to hit 700-750 rwhp and I ended up north of 900.

My biggest concern with the IRS is breaking axles if the car plants the power. The car isn't lazy and as you can see from the dyno sheet it'll jump from 500rwtq to 900rwtq in ~1000rpm.

With the traction control and boost control on the power comes in very smooth and never shocks the tires. In fact, 80% of the time I drive the car on low boost which is good for ~600-650 to the ground. So, if I keep the traction control and boost control on, not try to stand the car on it's bumper, which I don't plan to do anyway, the IRS should liv....is that about right?

I would really like to have a well rounded car. It doesn't need to dominate at the drag strip or the road course, just perform well in either. I've got big 6 piston front and 4 piston rear brakes on it so it's by no means a 1/4 mile car.

With that being said, what do you guys think?

SlowSVT, it's pretty fun driving around but the blow off valve is so loud that anybody near the car knows it's not factory. In fact, Jefe can chime in on what the car sounds like just driving around. Between the whine from the turbo and the bov, most people want no part of it.

 

[Jibber]

Man. Those pictures are great. Thanks for sharing. Very nice car.

 

[Jefe]

The car isn't lazy and as you can see from the dyno sheet it'll jump from 500rwtq to 900rwtq in ~1000rpm.

:lol: Crazy

Good points on how you will be driving it and the way the boost is setup by gear/speed. That's really going to help keep parts together since there wont be that initial shock to the components. That 9 second Cobra video I posted made numerous passes on an IRS

In fact, Jefe can chime in on what the car sounds like just driving around. Between the whine from the turbo and the bov, most people want no part of it.

Yep it sounds like its in boost when its not in boost.

 

[SlowSVT]

I'd love to do an IRS because I don't really care about the 1/4 mile any more. The car took a long time to build and over the years straight line racing became less important. Going to the drag strip once in a while is fun, but it's not why I built the car. I just really enjoy driving it. I was hoping to hit 700-750 rwhp and I ended up north of 900.

My biggest concern with the IRS is breaking axles if the car plants the power. The car isn't lazy and as you can see from the dyno sheet it'll jump from 500rwtq to 900rwtq in ~1000rpm.

With the traction control and boost control on the power comes in very smooth and never shocks the tires. In fact, 80% of the time I drive the car on low boost which is good for ~600-650 to the ground. So, if I keep the traction control and boost control on, not try to stand the car on it's bumper, which I don't plan to do anyway, the IRS should liv....is that about right?

I would really like to have a well rounded car. It doesn't need to dominate at the drag strip or the road course, just perform well in either. I've got big 6 piston front and 4 piston rear brakes on it so it's by no means a 1/4 mile car.

With that being said, what do you guys think?

SlowSVT, it's pretty fun driving around but the blow off valve is so loud that anybody near the car knows it's not factory. In fact, Jefe can chime in on what the car sounds like just driving around. Between the whine from the turbo and the bov, most people want no part of it.

I hear you on wanting a well-rounded car which are way more fun. To get precise handling you will need to reinforce the flexi Fox chassis and that is more than just adding a roll bar or sub-frames. I've compared the chassis structure of the Fox and Fox-4 chassis and there is quite a bit of difference which is the reason why the fox is so much lighter than the 4. Torsional loads to the chassis is your biggest enemy :cuss:

Based on what you’re saying perhaps an IRS may be a viable option. The sub-frame will tie-in and stiffen the back half as long as you beef-up the torque boxes. The IRS adds about 125 lbs but that will help balance the car better and plant the tires. As you stated you have it scaled back to 600 which should take the sting out somewhat (an overabundance of power is not always a good thing). Your car when the dial is turned up must be nothing more the a mechanical paranoid schizophrenic on methamphetamines :uh oh:

 

[turbofiveoh]

The IRS is becoming a more of a reasonable option. The torque boxes have been beefed up using a battle box kit with additional welding. When the car goes back under the knife the chassis will be stiffened up even more. If you have any advice on that it sounds like you've done your homework and I be interested to hear your ideas.

The car at 600 is more than fast enough to outrun anything on the street. The car is far more drivable at 600 and frankly more fun than trying to aim a 900hp missile.

I found a built IRS for 1200 but I'll need to put the FTBR bushings in it. When it's all said and done I'll have more in that setup than doing the torque arm and panhard bar. Is the difference really that big in handling and performance?

 

[mach1033]

Would anyone on here know the fastest record holder for the 1/4 mile on an IRS? Whether it be import or domestic I'm just curious. So far I came up with an RX-7 running 8's.

 

[turbofiveoh]

Would anyone on here know the fastest record holder for the 1/4 mile on an IRS? Whether it be import or domestic I'm just curious. So far I came up with an RX-7 running 8's.

Some supras and vipers have been in the 7s on an irs.

 

[SlowSVT]

The IRS is becoming a more of a reasonable option. The torque boxes have been beefed up using a battle box kit with additional welding. When the car goes back under the knife the chassis will be stiffened up even more. If you have any advice on that it sounds like you've done your homework and I be interested to hear your ideas.

The car at 600 is more than fast enough to outrun anything on the street. The car is far more drivable at 600 and frankly more fun than trying to aim a 900hp missile.

I found a built IRS for 1200 but I'll need to put the FTBR bushings in it. When it's all said and done I'll have more in that setup than doing the torque arm and panhard bar. Is the difference really that big in handling and performance?

Ford had the 2.3 liter engine very much in mind when they designed the Fox chassis and to achieve that metal was used very sparingly. If you remove the passenger interior side panels you will see how much sheet metal was removed from the stamping (that means the quarter panel is assuming more of the structural load). On the Fox-4 that same panel is more or less solid. That is just one example but I think you get the picture.

Not sure what brand of sub-frames you are running but the best ones are made by Hans and Global West.

Better yet, have them custom made out of thinner wall 4130 Chromemolly steel (maybe .060 - .093”) and fashion them after the Hans with the trellis framework or augment what you have. With some .050" thk 4130 sheet you can tie them into torque boxes and into the font frame rails. That would go a long way to stiffen the chassis and reduce metal fatigue. Avoid coilovers unless you beef up the shock towers as they are not designed to carry the weight of the car. I’m not a big fan of roll bars because they can actually increase the likelihood of injury should your body get slammed into the hard metal tubing (the DOT would never certify a car with a roll bar for this reason). Pretty much makes it a race car where a 5 point harness and helmet is recommended.

If you do any welding be real mindful of corrosion and make sure you can get to any area where the paint has been burned off. Use “weldable” primer on exposed metal prior to welding if you can’t get to it after it’s been welded to the chassis (the stuff holds up very well to the heat).

If you go with the IRS consider a torque biasing posi carrier like a Torsen T2R over the Eaton Trac-Loc which acts more like a light switch transferring the power from one wheel to the other. My Terminator would “skip” sideways a little in a lazy corner at part throttle from the Trac-Lok intermittently engaging. Keep in mind with a Torsen your days of 4000 rpm clutch dumps and slicks are over. I rarely shock my driveline so I’m not losing sleep over it. FTBR also offers upgrade toe link bars which will make it hold the toe settings better under braking and acceleration.

Here is some eye candy. Note how the rear cover is plumbed for an oil cooler. Just imagine the cornering speeds this one will allow with the wheels de-cambered 2°

 

[Posi]

You know what though come to think of it the IRS hooked from a roll no matter what the speed a lot better than the SRA on the street. From memory at least.

Anybody else notice this?

 

[novi2000]

You know what though come to think of it the IRS hooked from a roll no matter what the speed a lot better than the SRA on the street. From memory at least.

Anybody else notice this?

I have noticed this as well, the IRS does seem to do realy well on the street even on 800+ Hp cars.

 

[Jefe]

Interesting point. I never had a SA in a car with enough power to notice that. Turbo you might check with Mike on that. He was running 700rwhp+ when he swapped in the solid. That thing would do a burnout from a 4th gear roll no problem

 

[turbofiveoh]

Interesting point. I never had a SA in a car with enough power to notice that. Turbo you might check with Mike on that. He was running 700rwhp+ when he swapped in the solid. That thing would do a burnout from a 4th gear roll no problem

Who is Mike and how do I get in touch with him?

 

[turbofiveoh]

Ford had the 2.3 liter engine very much in mind when they designed the Fox chassis and to achieve that metal was used very sparingly. If you remove the passenger interior side panels you will see how much sheet metal was removed from the stamping (that means the quarter panel is assuming more of the structural load). On the Fox-4 that same panel is more or less solid. That is just one example but I think you get the picture.

Not sure what brand of sub-frames you are running but the best ones are made by Hans and Global West.

Better yet, have them custom made out of thinner wall 4130 Chromemolly steel (maybe .060 - .093”) and fashion them after the Hans with the trellis framework or augment what you have. With some .050" thk 4130 sheet you can tie them into torque boxes and into the font frame rails. That would go a long way to stiffen the chassis and reduce metal fatigue. Avoid coilovers unless you beef up the shock towers as they are not designed to carry the weight of the car. I’m not a big fan of roll bars because they can actually increase the likelihood of injury should your body get slammed into the hard metal tubing (the DOT would never certify a car with a roll bar for this reason). Pretty much makes it a race car where a 5 point harness and helmet is recommended.

If you do any welding be real mindful of corrosion and make sure you can get to any area where the paint has been burned off. Use “weldable” primer on exposed metal prior to welding if you can’t get to it after it’s been welded to the chassis (the stuff holds up very well to the heat).

If you go with the IRS consider a torque biasing posi carrier like a Torsen T2R over the Eaton Trac-Loc which acts more like a light switch transferring the power from one wheel to the other. My Terminator would “skip” sideways a little in a lazy corner at part throttle from the Trac-Lok intermittently engaging. Keep in mind with a Torsen your days of 4000 rpm clutch dumps and slicks are over. I rarely shock my driveline so I’m not losing sleep over it. FTBR also offers upgrade toe link bars which will make it hold the toe settings better under braking and acceleration.

Here is some eye candy. Note how the rear cover is plumbed for an oil cooler. Just imagine the cornering speeds this one will allow with the wheels de-cambered 2°

Great information!

The IRS in the picture is sweet, I need one of those! Is that yours? Ideally I can find one for sale that doesn't require an SRA in return, take my time building it up and then doing the swap over a weekend.

I've been looking into differentials and if what I'm seeing is correct Eaton references the same part number for the trutrac diff for the 8.8 and the 03/04 IRS. I've got one in my 8.8 that's practically new, that would really soften the blow in building the IRS.

I've been keeping an eye on the local classifieds for loose IRS. Most of them are pretty far away and shipping can get pretty crazy. If anybody has any leads on a reasonably priced IRS in good condition please keep me in mind.

 

[REX-RACER]

Sweet car! Subbing!

 

[stangposse]

I agree with a solid axle being a better choice for high powered/hard launches. Although some have made do with the IRS, I like the odds of the SRA.

However....those who say that there is no difference between the feel of an IRS and a SRA on the street make me laugh. I've driven an '05 GT, '11 GT and a '12 GT500 over the same expansion joints and thru similar twisty roads and the behavior of my IRS is CLEARLY better than those other three....and my car is much lower. Hitting an expansion joint during a turn and accelerating out of any twisty, with wheelspin, in my Cobra is a much less dramatic event than the SRA cars.

 

[c6zhombre]

isn't the irs far heavier than the solid setup? it probably keeps more weight right over the rear on a roll and increases hookup from the punch. the solid is superior launching from a dig, but it needs the brace of the tires biting to really show off.

 

[Jefe]

Who is Mike and how do I get in touch with him?

You know...the guy that gets paid to shoot illegals

 

[brian97cobra]

If drag racing is your primary concern and you don't care about ride comfort or handling, then you may be happy with an SRA.

BUT........................... most of the people that I know drive their car on the street 99% of the time and that should be taken into consideration. :read:

i ditched my IRS for a built SRA and came out with money in my pocket. i also have full race suspension and it still rides great to me. my only complaint is that if you hit a bump u will def feel it. it does not cushion against bumps. but besides that i feel the road a lot better, my steering and braking response is better and i dont have to worry about breaking another cover.

if something does break it is cheaper and faster to fix a SRA vs IRS.


you have good products for the IRS but lets face it 1000 rwhp.... needs a SRA


EDIT: i have a rear torque arm with panhard bar from maximum motosports, tokico d-spec shocks and h & r drag launch springs in the back, 2000 gt sra with 31 spline ford racing axles, 3" long race studs, 31 spline trac lock with carbon fiber clutches, 3.73 rear gear.

if anyone was wondering

 

[ac427cobra]

you have good products for the IRS but lets face it 1000 rwhp.... needs a SRA

It all depends on the intended use of the vehicle. In case you didn't read the OP he stated: (and I've highlighted in RED the pertinent information)

I've been strongly considering doing an 03/04 IRS swap into a turbo Fox body that makes 903/955 RWHP/TQ in lieu of the conventional built SRA with TA and PHB. I've been asking around and have gotten mixed feedback. So I thought I would post up here and see what people think.

If I were to use a full tilt boogie or maximum motorsports delrin kit with stock 03/04 axles or DSS level 3+ axles would the IRS live with the occasional launch with sticky tires?

Thanks for the feedback.

I would say absolutely the IRS is suitable for his application with IRS upgrades such as removing ALL OEM rubber or aftermarket poly and upgrading his rear cover.

Now, if he said he goes to the drag strip four, five or ten times a year and likes to launch at 5k, I will, and have ALWAYS recommend to these people to convert to an SRA. If going in straight lines all of the time with constant launches, massive power and a high concern for drag racing ET's is your primary focus, you are going to be better served by an SRA, no question! But................... most of these guys drive their car on the street 99% of the time and I like to make them aware of the sacrifice they are making in ride and handling by swapping to an SRA.

Besides that, when it comes time to sell the car and the new buyer finds there's an SRA underneath it they are going to know someone pounded the crap out of it at the drag strip because that's the only reason the axle would have been changed.

Last I checked Terminator owners weren't really happy about the resale value of their cars. Converting to an SRA only makes the situation worse.

FWIW

:thumbsup::coolman::beer: