68 Firebird LS6/T56 GTO irs pro-touring

[dmac1974]

Ok so here goes. I have a 1968 Firebird and I'm doing a LS conversion pro-touring build. I have an Art Morrison subframe with the C6 front suspension, and I decided to try something different and fab in an irs from an 05 GTO. I was able to pick one up for $500 and I just thought I'd give it a shot. I had found a few threads about trying to use the gto rear suspension in a 1st gen, but they always seemed to just end. So after a lot of trial and error and plenty of head scratching I've finally got it stuffed in there.

Here is what it looked like when I first brought her home:

[dmac1974]

Working on it in my spare time for about 2 years now, here's what has been done so far:

The entire car was stripped down to the bare shell. I built a rotisserie and went to work with the welder, plasma cutter, and the angle grinder. I've replaced most of the floor, the dash, rear quarter skins, and the rear passenger upper control mount. I designed and built some adjustable mounts for the front two mounts of the gto rear subframe. Fabricated mounts for the rear coilovers and welded in a crossmember to tie the rear frame rails together and to hang the rear mount. Media blasting was done one the entire underneath of the car, the trunk, rear panel, and firewall. I recently coated the underneath, trunk, and interior floors with Rust Bullet, and for the first time in two years the car is off of the rotisserie. I still have a laundry list of parts and a ton of work ahead, but I'm looking forward to it. I have plenty of pics to post as soon as get time.

Here's how she sits now:

[Steve N 69 69 69]

Looks like a fun project. I'm looking forward to seeing the pics and the details on how the IRS works out.

[hotrodneb]

YA....nice work....It's too late for me but get up some pics of that IRS...

I am doing a '68 too....check my thread

[dmac1974]

Here is a shot of the crossmember I added. You can also see the "windows" I cut into the floor because I wasn't exactly sure where I need more clearance.

[dmac1974]

Upper coilover mounts and rear mount started.

[dmac1974]

Adjustable lower mounts fabricated. (I'll make them prettier later)

[dmac1974]

Rear mount finished and windows closed up

[dmac1974]

Shot of the front adjustable mounts. Turn the bolt on the end...moves vertical bolt forward and back

[dmac1974]

Coilover mocked up

[dmac1974]

Getting ready to tack in the last mounting bracket

[dmac1974]

All cleaned up and covered with Rust Bullet.

[dmac1974]

Here it is all bolted in place.

[dmac1974]

My rear tires haven't come in yet so I had to put the old rims and tires on the rear for now. I should have plenty of adjustabilty to set the ride height by either change holes in my lower mounts and then fine tuning on the coil overs themselves.

[Buryingthesun]

Maybe im looking at this wrong, but where the hell are the control arms????

[dmac1974]

Got the floors done too.

[dmac1974]

I'll need to lift the car up a bit to get a better shot of it in the car, but here is a shot of the gto rear where you can see the control arms better. You can also see the 3 main mounts on the subframe.

[dmac1974]

YA....nice work....It's too late for me but get up some pics of that IRS...

I am doing a '68 too....check my thread

Thanks...I had just went through your thread the other day :). Love that green and the mods to the front and rear bumbers.

[Buryingthesun]

I'll need to lift the car up a bit to get a better shot of it in the car, but here is a shot of the gto rear where you can see the control arms better. You can also see the 3 main mounts on the subframe.

I see them now im on my computer ;)

These damn phones are so pesky sometimes

[CamaroJesse]

Subscribed to this one. Lookin good dude. What kind of wheels are on the front? They look nice.

[mikes2nd]

Subscribed to this one. Lookin good dude. What kind of wheels are on the front? They look nice.

Yeah looks great. How much HP? I know some are concerned with the strength of the unit.

[dmac1974]

Subscribed to this one. Lookin good dude. What kind of wheels are on the front? They look nice.

Thanks, They are STR 609 wheels 18 x 8.5 30mm offset front 18 x 9.5 rear 35mm offset. Got a good deal on them and they were the right combination of the sizes and offsets I was looking for, and I liked how they looked too.

[dmac1974]

Yeah looks great. How much HP? I know some are concerned with the strength of the unit.[/QUOTE]

I have an LS6 block with a set of 799 heads. Just going to go with a mild cam and shooting for around 450hp at the crank or a little shy of 400hp at the wheels. I figure if I start having issues there are quite a few upgrades available to make it stronger. Maybe even an 8.8 or 9in upgrade down the road.

[40STY]

I have to say a HUGE 'Thank You', you have inspired me! Im also doing a 68 Bird, transferring as much from a G8 (VE commodore here in Australia) as possible. Ive still got the IRS in the donor car and have been toying with the idea of trying to get it in the Bird, but thought it would be too much effort. I'm sure it took a lot of thinking and trial & error to get it right, but in the end, the mounting looks reasonably simple. The G8 IRS looks like it would be even simpler, because its all one unit, with the springs included, plus theres only 4 mounting points on the outside, not in the centre of the car. Some more pics of how yours is mounted, especially at the front would be awesome.

Congrats mate, nice work (and great that someone has actually done this and posted it, like you I found heaps of threads/posts that never showed the finished product).

[dmac1974]

I have to say a HUGE 'Thank You', you have inspired me! Im also doing a 68 Bird, transferring as much from a G8 (VE commodore here in Australia) as possible. Ive still got the IRS in the donor car and have been toying with the idea of trying to get it in the Bird, but thought it would be too much effort. I'm sure it took a lot of thinking and trial & error to get it right, but in the end, the mounting looks reasonably simple. The G8 IRS looks like it would be even simpler, because its all one unit, with the springs included, plus theres only 4 mounting points on the outside, not in the centre of the car. Some more pics of how yours is mounted, especially at the front would be awesome.

Congrats mate, nice work (and great that someone has actually done this and posted it, like you I found heaps of threads/posts that never showed the finished product).

I tried crawling under there and getting some better shots, but I really need to get it up on a lift to get the whole picture.

[dmac1974]

Here's a shot of the front mounts from inside the car.

[dmac1974]

Finally got my front coilovers in the mail today. Got them and my sway bar installed.

[40STY]

Thanks for the pics. All 4 of the G8 mounting points are the same as the front mounts for the GTO. I might steal your sliding mount idea, but vertically to give height adjustability.

[dmac1974]

Made a little more progress. Got the shell into epoxy primer.

[40STY]

What length are the shocks you got? And what spring rate? Ive got the G8 IRS bolted up and just need to get some coilovers, but I can't decide on what spring rate to get.

[dmac1974]

What length are the shocks you got? And what spring rate? Ive got the G8 IRS bolted up and just need to get some coilovers, but I can't decide on what spring rate to get.

I used the Strange coil overs that came with the art morisson subframe. The are single adjustable and approximately 11.25" compressed and 15.375" extended. I used the vehicle weight database on RideTech's site to make an educated guess and went with the 10" 250/lb springs. I made the lower mount adjustable to give me some more ride height adjustment. You got any pics of that rear bolted in the car?

[40STY]

They shock and spring are exactly the same as what I was considering, cheers.

The front mounts were simple, just 2 90 degree plates bolted to the chassis rail. Similar plates would have done the job for the rear mounts, but since I had the entire G8 shell, I did it the hard way. I cut out the rear chassis section that the IRS originally mounted to and used that. Its got 2 threaded shafts that the rear mounts sit on, giving a better alignment and makes removing the IRS cradle easier.

After bolting the IRS in, although the 4 mounting points made it reasonably easy (especially the fact that no welding was required), I would probably suggest to anyone thinking of putting an IRS in to use the GTO rear, not the G8. The main reason being wheel offset. The G8 has a track width 30mm wider than the GTO, so sits an extra 15mm out each side. As you can see from the pics below, the standard 17x7 +43 offset rims fill out the guards and are only just inside. Thats, fine, but getting wider rims with a bigger offset to keep the wheels inside the guards isn't easy, especially when the PCD of the wheels is 5x120mm, not 5x4.75". So far the options I have found are either custom wheels ($2500+, no thanks) or getting japanese car blank wheels (for mitsubishi, subarus etc that have high offsets) re-drilled with the 5x120 PCD, or to try and get the replica corvette wheels from OE Wheels (18x8.5 +56) PCD changed from 5x120.65 to 5x120. The other problems with the G8 IRS is that the mounts for the top control arm sit up fairly high and hit the underside of the trunk, limiting how high the IRS can be mounted and also the fuel tank will need to be altered, as the back of the rear mounts looks like it will hit the corners of the tank.

[captainofiron]

wow thats pretty awesome

[Buryingthesun]

They shock and spring are exactly the same as what I was considering, cheers.

The front mounts were simple, just 2 90 degree plates bolted to the chassis rail. Similar plates would have done the job for the rear mounts, but since I had the entire G8 shell, I did it the hard way. I cut out the rear chassis section that the IRS originally mounted to and used that. Its got 2 threaded shafts that the rear mounts sit on, giving a better alignment and makes removing the IRS cradle easier.

After bolting the IRS in, although the 4 mounting points made it reasonably easy (especially the fact that no welding was required), I would probably suggest to anyone thinking of putting an IRS in to use the GTO rear, not the G8. The main reason being wheel offset. The G8 has a track width 30mm wider than the GTO, so sits an extra 15mm out each side. As you can see from the pics below, the standard 17x7 +43 offset rims fill out the guards and are only just inside. Thats, fine, but getting wider rims with a bigger offset to keep the wheels inside the guards isn't easy, especially when the PCD of the wheels is 5x120mm, not 5x4.75". So far the options I have found are either custom wheels ($2500+, no thanks) or getting japanese car blank wheels (for mitsubishi, subarus etc that have high offsets) re-drilled with the 5x120 PCD, or to try and get the replica corvette wheels from OE Wheels (18x8.5 +56) PCD changed from 5x120.65 to 5x120. The other problems with the G8 IRS is that the mounts for the top control arm sit up fairly high and hit the underside of the trunk, limiting how high the IRS can be mounted and also the fuel tank will need to be altered, as the back of the rear mounts looks like it will hit the corners of the tank.

Front mount
https://static1.pt-content.com/images/pt/2015/01/DSC_0445_zps9522c12e-1.jpg (http://s218.photobucket.com/user/stuchy81/media/Firebird%20build/DSC_0445_zps9522c12e.jpg.html)

Rear mount. The bracket in front is where Ill attach the coilover.
https://static1.pt-content.com/images/pt/2015/01/DSC_0444_zps34de4858-1.jpg (http://s218.photobucket.com/user/stuchy81/media/Firebird%20build/DSC_0444_zps34de4858.jpg.html)

Rear mount from underneath. You can also see the top control arm mount on the left, which is only about an inch away from the trunk floor. Mounting the IRS any closer to the body would require holes to be cut in the trunk. Even as it is, with the car dropped to the lowest ride height, there is still 11cm clearance from the lowest point of the IRS to the ground.
https://static1.pt-content.com/images/pt/2015/01/DSC_0450_zps66488563-1.jpg (http://s218.photobucket.com/user/stuchy81/media/Firebird%20build/DSC_0450_zps66488563.jpg.html)

Guard clearance
https://static1.pt-content.com/images/pt/2015/01/DSC_0438_zps7882da1e-1.jpg (http://s218.photobucket.com/user/stuchy81/media/Firebird%20build/DSC_0438_zps7882da1e.jpg.html)

Sitting at its lowest possible ride height. Normal ride will be a couple of inches higher than this.
https://static1.pt-content.com/images/pt/2015/01/DSC_0440_zps09587642-1.jpg (http://s218.photobucket.com/user/stuchy81/media/Firebird%20build/DSC_0440_zps09587642.jpg.html)

Do you have any pictures from the back of the car? And can you get pictures of wheel to shock clearance? Also the 250lb spring you chose seen REALLY soft, I'd have done at least 550lb-750lb but I'm also not concerned with a soft comfy ride. I haven't done the math on mine yet but for the front I'm going to in the 1,000lb-1,200lb area for my springs

[Justin@EntropyRad]

Nice use of the IRS

[40STY]

Do you have any pictures from the back of the car? And can you get pictures of wheel to shock clearance? Also the 250lb spring you chose seen REALLY soft, I'd have done at least 550lb-750lb but I'm also not concerned with a soft comfy ride. I haven't done the math on mine yet but for the front I'm going to in the 1,000lb-1,200lb area for my springs

I dont have any pictures from the back. Ill take some next time Im working on it. And I dont have the shocks in yet, but when I took the above photos, I measured from the tyre to the chassis rail and there was 8cm (just over 3") of space. The shock will mount nearly directly infront of the rear mount (pretty much the same as the stock G8, but with a skinnier coilover), so should leave plenty of wheel clearance.

250lb sounded soft to me as well, but thats what the ridetech calculator suggested. Its fairly cheap and quick to change to stiffer springs down the track if it does end up being too soft. Ive since has a look at the spring rates for the '05 GTO (Monaro), which will have a slightly heavier rear end than the bird and uses stock springs about 440-520lb. So maybe 350-400lb in the bird would be better than 250lb, but the 550-750lb your suggesting might just make it far too stiff. Time will tell I guess.

[rohrt]

Subscribed.

[Buryingthesun]

I dont have any pictures from the back. Ill take some next time Im working on it. And I dont have the shocks in yet, but when I took the above photos, I measured from the tyre to the chassis rail and there was 8cm (just over 3") of space. The shock will mount nearly directly infront of the rear mount (pretty much the same as the stock G8, but with a skinnier coilover), so should leave plenty of wheel clearance.

250lb sounded soft to me as well, but thats what the ridetech calculator suggested. Its fairly cheap and quick to change to stiffer springs down the track if it does end up being too soft. Ive since has a look at the spring rates for the '05 GTO (Monaro), which will have a slightly heavier rear end than the bird and uses stock springs about 440-520lb. So maybe 350-400lb in the bird would be better than 250lb, but the 550-750lb your suggesting might just make it far too stiff. Time will tell I guess.

Awesome, Anytime you want to take a horde of pictures you can direct message them to me.

Thanks again!

[dmac1974]

So I've been slacking on updates, made quite a bit of progress. Here are some pics.

[dmac1974]

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[dmac1974]

And a few more...114048114050114051

[dmac1974]

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[KB Camaro]

awesome build, Merry Christmas!

[rohrt]

Looking good!

[dmac1974]

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[dmac1974]

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[dmac1974]

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[dmac1974]

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[dmac1974]

Just decided to hang the fenders on the car to make more room in the garage. Anxiously waiting for warmer temps so I can finish up the body work and get this thing painted.

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[ryeguy2006a]

Awesome project and nice use of the GTO IRS. You made that look easy!

[dmac1974]

Awesome project and nice use of the GTO IRS. You made that look easy!

Thanks, I wish it was easy. Can't wait to get it on the road and see if it actually works.

[dmac1974]

I have been completely slacking on my updates again...Here are some more pics.

[dmac1974]

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[dmac1974]

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[1965gp]

Car looks great

[captainofiron]

Love the color

[wyattroa]

Great work, very inspiring!

[dmac1974]

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[dmac1974]

https://youtu.be/COog04GT3aU

[1sikride]

Greasy !

[gator68428]

Coilover mocked up

Wow, bold build!

I'm still trying to wrap my head around this one.

You're no longer using the spring in the stock location for the GTO. Now you're adding the weight of the car to all shock loads to the shock knuckle. Also because the base of the coilover mount is moved a few inches away from the knuckle than it was originally, there is now a twisting torque applied to the knuckle (in addition to continuous weight of the car).

Just to be safe do you think you should add a bump stop separate from the coil over? That's usually recommended anyways for coil over health in general. Let it hit your bump stop before the coil over bump stop. One for each side somehwere between a-arm and frame/car. In this case would give a little more insurance.

Are you using a MIG welder?

Kudos to you for going for it. I'm in full support and always a fan of experimentation.

[dmac1974]

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[813Demon340]

Looking good, love the color with the black wheels. What is the console out of? Looks good whatever its from.

[dmac1974]

The To Do list is rapidly shrinking. She runs and drives and most of the interior is done. Still need to get the back seats covered and installed, but I don't need back seats to drive it! Still working out some bugs but overall very pleased with how it runs and drives.

[dmac1974]

Looking good, love the color with the black wheels. What is the console out of? Looks good whatever its from.

Thanks, the piece with the cup holders and lid were purchased from TMI, but the rest I made myself.

[dhutton]

Thanks, the piece with the cup holders and lid were purchased from TMI, but the rest I made myself.

Looks good and very clever solution.

Don

[ragecage92si]

@dmac1974 I am a new member building a 68 firebird, your post caught my eye and I was wondering if you are still an active member?

[dhutton]

@dmac1974 I am a new member building a 68 firebird, your post caught my eye and I was wondering if you are still an active member?
He has not been here since 2017…