Lexus GS400/GS300 IRS

[Blackarrow222]

I saw one of these for sale locally and it got me wondering how appropriate it might be for a GM A-body. Does anyone know anything about the geometry of these and how sports oriented they are? It appears they attach to the frame with just four big bolts.

[matthimself456]

I don't have any pre-existing knowledge to share but you definitely caught my interest. I was just doing the math to see if there was any chance of tucking a S550 Mustang rear suspension under an A-body when you posted this. These will be significantly narrower and therefore easier to package under all kinds of old cars.

Digging into the internet some it seems like there are strong similarities between the Lexus IS, GS, and LS of different generations and most or all mount in subframes like this that are self contained other than an upper coilover mount. I haven't found much analysis of the geometry yet but looks like older ones like the GS300 pictures are double A-arm style and newer ones might be more complicated multi-link. Toyota is not known for under-engineering things so I'd expect any of them will have a better camber curve than the non-existent camber curve of a solid axle and much better roll center placement than the GM 4-link.

Information on differentials isn't super easy to find but it least it seems like ratios are available from high 2's to high 3's and even 4.10. The only info I've found on strength is that most (maybe all) ring gears are only about 8" diameter but breakage does not seem to be a major concern even with high power turbo builds. Limited slip options seem to be semi-available on the aftermarket and some models may have had factory torsen-style diffs. I don't know if they share any design roots with Toyota truck/SUV rear axles but if they do then differential ratios, limited slip, and lockers should be very available from the off road crowd.

Big-picture IRS seems unnecessary. But for an A-body not factory equipped with a 12-bolt or 8.5" 10-bolt the expense to get decent strength and performance adds up awfully quick. Especially when you get into full-floater kits and stuff like that. Something like this combined with some skilled fabrication work could be a very worthwhile alternative.

[Blackarrow222]

I've done a little research on Lexus forums as well and I can add that the GS400 and GS300 assemblies seem to be identical save for the ratios. GS400 is 3.266 and GS300 is 3.916. Unfortunately, I believe they're both open diffs from the factory so this will require more coinage to address. Still, a high performance Ford 9" with good brakes at the ends can easily push 5k so the Lexus IRS does present financial incentives along with its performance/ride quality characteristics.

[Motorcitydak]

Does the track width and wheel bolt pattern work for your project? As mentioned, any IRS will be an upgrade from all old solid axle setup. As you said, they all typically attach with four bolts. That said, any oem setup will have some built in compromises. Be it suspension bushings, roll center, camber curve……it’ll be there but still a better experience for you.

With the noted limitations of gearing and differentials, I’d consider the offerings from a Mustang, Camaro and challenger. Check the widths and bolt pattern with availability. I’m sure you will be tripping over those units and they will attach in similar fashion. I think you’ll have an easier time finding brake upgrades as well.

The cradles are usually tubular steel so the mounting locations can be adjusted to fit your setup as well. I’d rather alter the cradle than the frame and floor of the car unless absolutely necessary.

[Blackarrow222]

Seems like the track width is easy to fine tune with wheel offset, which would probably be fairly minor. Wheel bolt pattern is an inconvenience but I guess if you had to have your rears made with a different pattern than your fronts it wouldn't be the end of the world. Just goofy.

I just looked up a Camaro IRS assembly comparable to this Lexus unit and the Camaro setup is like 10x the price on Ebay. Yikes!

Somehow I feel like a Lexus setup in a Pontiac is less sinful than one from a Mustang or Challenger but that's just me being silly about mixing brands. Those are good recommendations so thank you.

Another consideration for me personally is the ability to fit 15" wheels over the spindles. This is why the C5 and up Corvette stuff isn't an option for me and I suspect the modern domestic stuff all has taller spindles than the Lexus pieces from years prior.

[matthimself456]

Does the track width and wheel bolt pattern work for your project? As mentioned, any IRS will be an upgrade from all old solid axle setup. As you said, they all typically attach with four bolts. That said, any oem setup will have some built in compromises. Be it suspension bushings, roll center, camber curve……it’ll be there but still a better experience for you.

With the noted limitations of gearing and differentials, I’d consider the offerings from a Mustang, Camaro and challenger. Check the widths and bolt pattern with availability. I’m sure you will be tripping over those units and they will attach in similar fashion. I think you’ll have an easier time finding brake upgrades as well.

The cradles are usually tubular steel so the mounting locations can be adjusted to fit your setup as well. I’d rather alter the cradle than the frame and floor of the car unless absolutely necessary.

Track width is the really tough thing. If old cars were 6" wider then the S550 Mustang would be a no-brainer. But as it is, even with impossibly high wheel offsets, the math doesn't quite work out. Camaro and Challenger have the same problem. But for whatever reason Japanese and European cars tend to be more narrow. Even the big Lexus LS is much more inline with the width of a 60s-70s Chevelle or Camaro.

Wheel bolt pattern is a factor, too, but one that has more available solutions.

[dhutton]

What about the 04-06 GTO? Wider than a first Gen but workable I think.

https://www.hawksmotorsports.com/04-...spension-used/

Don

[simon455]

I saw one of these for sale locally and it got me wondering how appropriate it might be for a GM A-body. Does anyone know anything about the geometry of these and how sports oriented they are? It appears they attach to the frame with just four big bolts.

Would you happen to know the width of it?