89 Iroc Z

-----

The aftermath, I have created a full how-to at my website:
http://lukeskaff.com/projects/car/th...brake-upgrade/

-----

I decided to start a new thread about the LS1 brakes instead of adding this onto my C4 brake swap thread as originally planed.
For all of you who want to know about the C4 brake swap go here: https://www.thirdgen.org/techbb2/sho...hreadid=142115
Please read through the C4 swap post before asking questions it is very similar to the LS1 swap.

I got my LS1 front brakes in the mail and all I can say is they are HUGE! I hope they fit under my 16-inch wheels, if not I will trim down the cooling fins. The LS1 brakes have more pad area, bigger pistons, and a thicker rotor then 1LE or C4 brakes.

The mounting holes one the LS1 calipers are about 1/16 inch farther apart then the C4 brakes and use smaller bolts. So a new spindle mounting bracket design has been created (cannot use the C4 bracket). This is the main reason I decided to start a new thread. This first post has been updated a great deal since this thread was started and now has a lot of the needed information for the swap, check out my website for a full how-to.

The Baer (PBR) "Grand Touring [GT]" and GT+ brakes use the same caliper design as the LS1 brakes. In addition, the LS1 brakes are the same thickness rotor (1.25 inch) as the Baer GT brakes. As many of you know, Baer (PBR) uses GM (PBR) designs. The baer calipers are designed to be sturdier and flex less under load but good factory calipers like the C5 calipers are comparable.


Brake Specs:

STOCK BRAKES:
Rotor Size: 10.5 inches
Rotor Thickness: about 1 inch
-Caliper specs
Piston Size: 2 3/8 inch, 2.375 inch
Total Pistons surface area: 8.86
Material type: Cast Iron

C4 NON-HD 12-INCH BRAKES:
Rotor size: 12 inches
Rotor Thickness: 0.81 inches
-Caliper specs
Piston size: 1.5 inch outer diameter
Total pistons surface area: 7.068 square inches
Material type: Cast Aluminum

C4 HD 13-INCH BRAKES:
Rotor Size: 13 inches
Rotor Thickness: 1.1 inches
-Caliper specs
Piston size: 1.5 inch outer diameter
Total pistons surface area: 7.068 square inches
Material type: Cast Aluminum

1LE BRAKES:
Rotor Size: 12 inches
Rotor Thickness: 1.05 inches
-Caliper specs
Piston size: 1.5 inch outer diameter
Total pistons surface area: 7.068 square inches
Material type: Cast Aluminum

98+ CAMARO BRAKES “LS1”:
Rotor Size: 12 inches
Rotor Thickness: 1.25 inches
-Caliper specs
Piston size: 1.75 inch outer diameter
Total pistons surface area: 9.62 square inches
Material type: Cast Aluminum via gravity cast

C5 BRAKES:
Rotor size: 12.75 inches
Rotor thickness: 1.25 inches
-Caliper specs
Piston size: Somewhere close to 1.75 inch outer diameter
Total pistons surface area: around 9.62 square inches
Material type: Cast Aluminum via pressure cast (stronger)


Q: Why is this brake setup called "LS1" brakes?
A: It is a lot easier to type LS1 instead of "98+ Camaro/Firebird". In 1998 GM introduced the LS1 engine into the F-bodies that same year they also put bigger brakes and a few other goodies. I am just using LS1 as an easy way to identify the 12-inch, dual piston caliper brake setup on these cars. It is important to note that the "LS1" brakes also came on the V6 98+ Camaro / Firebirds also.

Q: Will the LS1 setup clear my stock 15 or 16 inch wheels?
A: This setup will not clear any 15 inch rims that came stock on thirdgens. Here are the rims that will work:
- Early model & Late model IROC 16 inch rims
- Some GTA and Firebird rims work some don’t you will need to read this post.
Some firebird/GTA rims need shims to space out the wheel, this can be done with GM part number 14091902 (.030 inches thick).

Q: Will my stock spare clear the LS1 brake setup?
A: No it will rub.

Q: What proportioning valve and master cylinder combo will I need to use the LS1 brakes?
A: I have not determined this yet. If anyone could find out what Bear Brakes uses on there thirdgen GT kit’s that would be greatly appreciated.

Q: What brake hoses should I use?
A: The stock hoses will work.

Q: What year Camaro / Firebird can I get these brakes off of?
A: All 98+ Camaro / Firebirds with the LS1 engine came with the 12 inch, dual piston caliper brake setup.

Q: What is a hub?
A: This is fully explained in the C4 brake upgrade thread.

Q: What outer diameter should I take hub down to?
A: You should take the LS1 rotor with you to the machine shop and have them machine down the hub until it fits in the hat of the LS1 rotor. My hubs have an outer diameter of 5.85 inches.

Q: What is the difference between the LS1 and C5 brakes?
A: They are based off the same design but there are more differences then I originally though. The C5 caliper has stiffing / cooling ribs that extend the full length of the caliper. Where the LS1 caliper only has them on the back (as you can see in the picture below). The C5 caliper is produced by a method called pressure casting and the LS1 caliper is produced by gravity casting. This means the C5 caliper is stronger and more durable.

Q: Are the C5 and LS1 caliper interchangeable?
A: Yes, You can go to a parts store and purchase a C5 caliper and use it on a LS1 abutment bracket (mounting bracket, some times called a caliper carrier). The C5 and LS1 calipers abutment brackets are different though so only the calipers are interchangeable.

Q: What about the spindle bracket?
A: Here is my finalized bracket:
Bracket design page on my website

Q: Where can I find out about putting C5 brakes on my car?
A: http://82lt1.cz28.com/brakes/brakes.html
Vexter was kind enought to make up a drawing of the C5 bracket which can be downloaded here.

Q: Brake Does and Don’ts
A:
- Do NOT make your spindle bracket out of aluminum!
- Always round off edges on your spindle brackets to reduce stress points.
-When replacing rotors or pads always perform proper braking in procedures before putting your brakes through its paces. I know it's tempting to slam on the brakes to test out your new setup but it's a lot better to many gradual and normal stops first to start a wear pattern going first.


Useful Part numbers:
- Front Left LS1 caliper #18042491
- Front Right LS1 Caliper #18042492
- Caliper abutment bracket (caliper carrier) 18026163

Last updated: 06-18-2009

Fun legal disclaimer: Thirdgen.org nor I can be held responsible for any damage done to you or anyone else from the use of this information. The very nature of modifying brakes from factory setup can be very risky if not properly done. Always use common sense when modifying brakes if you are unsure about something have someone else do it for you who is knowledgeable or ask here!


Here is a picture comparing the C4 12 inch NON-HD caliper (left, the C4 HD caliper is bigger to accommodate a wider rotor) to the LS1 caliper (right)

89 Iroc Z

The C4 caliper is the same as the 1LE caliper except it has a smaller rotor opening

Another picture comparing the C4 caliper (left) to the LS1 caliper (right)

89 Iroc Z

The LS1 rotor has a different offset then the C4 rotor but this should help move the caliper out more so that I can squeeze in a thicker mounting bracket.


Another picture comparing the C4 (12-inch, 0.81 inch thick) NON-HD rotor (left) to the LS1 rotor (right)

355SS

iTrader: (1)

C5 uses a 12.75" x 1.25" thick rotors. The only difference in the LS1 and C5 calipers is the production method. C5 calipers are pressure cast making calipers slightly denser and more rigid. The ZO6 calipers and standard C5 calipers are the same except for the color. The part of the caliper that you are referring to as the "heat sink" is actually stiffening ribs, so avoid grinding if at all possible.

89 Iroc Z

Thanks for the info, post updated

machinist

I totally agree about the ribs. If you look at the caliper you can see how they are designed to strengthen the caliper, and also act as heat sinks. If you grind them off you will be weakening the caliper substantially.

89 Iroc Z

If they needed to be ground down it would only be a very small amount. Hopefully they will fit. 98+ Camaro’s have 16’s and these brakes. I am going to do a test fit tomorrow then I’ll see if I need to even worry about it.

I agree with you guys the cooling ribs do add some stability. But I also feel taking a hair off them to add a small extra amount of clearance would not be dangerous.

machinist

Just out of curiousity I did some checking on brake parts from A-Zone for this. There wasn't any designation as to LS1 or a standard camaro that I could find. If there is, then please let me know.

Rotors $ #55034 $38.99
Aimco Extreme $93.99

Calipers #C446
Caliper core $38
Caliper with exhange $37.40.
Total for caliper without core $75.40

So if these are the correct parts, for rotors and calipers without caliper cores, both sides $228.78

With extreme rotors $338.78

This is for NEW parts.

The calipers probably don't include the caliper carriers, but I'll check on these and let everyone know if I find a price on them.

89 Iroc Z

Not bad price for the whole setup new. Alloy, A.K.A. machinist your right the calipers do not come with carries so that’s probably another $90 if you get them from the dealer. Don’t know any other place to get them new.

I got my whole setup for $200 from a guy on a fourth gen board which included 2 calipers, 2 caliper carriers, 2 stock lines, 1 caliper rebuild kit, 4 abutment bolts, 4 stock pads, 2 stock rotors, 4 guide pins and bolts. The setup had 10k miles on it and it is as good as new.

machinist

The price isn't bad that bad is it. Just need to see how much the caliper carriers are.

Other than the rotors being thicker, what benefits do you see with the LS1 setup over the C4 stuff? From your pictures the brake pistons seem to be approximately the same diameter, so probably about equal braking force to the C4 calipers. The rotors are thicker, but not larger diameter.

89 Iroc Z

The befits of the LS1 over the C4 and even 1LE setup is a thicker rotor, which will help a lot to keep the brakes cooler, when doing spirited driving . The LS1 brakes have bigger pistons (see specs above) and from what I have read are more efficient. Also I have read that they exert more braking force with the same amount of hydraulic line pressure. Therefore, it may use a different proportioning valve then the 1LE setup. I have to find out what Baer uses with there GT & GT+ setups

The C4 setup remains the ultimate swap for those who want need a budget brake upgrade and should perform with the same braking power as the 1LE setup.

The LS1 brakes are for those who are willing to spend a little more money then the C4 setup and have better brakes that should outperform the 1LE setup while still remaining lower cost then the 1LE setup.

355SS

iTrader: (1)

Most of the benifit is going to be from the LS1 brakes having a larger pad unless you are overheating the C4 brakes. People call them LS1 brakes because they were released on the f-bodies at the same time as the LS1. So actually they are 98 and newer brakes and they came on all f-bodies of these years.

lock

You saying the C5 and LS1 calipers are the same? I think in the C5 thread that the C5 calipers were around $80.

machinist

Luke,

What have you based this statement on?

89 Iroc Z

They are not exactly the same. As 355SS said “The only difference in the LS1 and C5 calipers is the production method”. Everything is interchangeable betweens the C5 and the LS1 Calipers; Brake Pad’s, Caliper carries.
Note: The C5 caliper does have a different caliper carrier, The mounting bolts are spaced differently.


I came to this conclusion after reading up on the late model PBR brake setup before I purchased a set of LS1 brakes. I used to have a bookmark to a site with tons of info, which I don’t have anymore because the computer that has the bookmark got all messed up I am going to reinstall windows. I will post the link once I get that computer up and running again.

lock

Any updates on installing the LS1 brakes 89 Iroc?

MartinCadek

thumbs up to all that participated in the C4/C5/LS1 threads , but this is a lot of info to read and make sense of for a novice like me seeking better brakes.

I think it would be awesome if you guys could write up some tech articles and submit them , they would make it a lot easier for all of us (not saying you haven't done enough , lol , but a final tech article or even a final post in each thread would help eliminate all the if's but's and maybe's).

in any case , thanks for all the info , now I need to decide just what swap to do , lol

RMK

89 Iroc Z

Any updates on this? I'm wondering if this setup will fit behind the 16" 87 IROC rims?

89 Iroc Z

Martin: I have already started a tech article for the C4 and LS1 brake swaps. I will finish it when I finish my brake swap.

Lock & RMK: I have got some measurements done. I very busy because taking summer classes. However, this weekend I should have some good quality time to finish the LS1 bracket design in CAD and get it posted. I’ll keep everyone posted.

In addition, I discovered that the C5 calipers are actually even more different from the LS1 calipers then I originally thought. I guess the internet is not always the best place to go for accurate information. I asked the guy behind the desk in the part store to show me a C5 caliper. The C5 caliper has stiffing ribs the whole length of the caliper and is all around stronger. I have also heard but not confirmed yet that the C5 caliper has a slightly different piston size. I will bring my measuring caliper to the auto parts store next time and get the real measurements. The LS1 and C5 calipers are interchangeable in the caliper carrier or abutment bracket as some call it. So I can upgrade from LS1 calipers to C5 calipers whenever I want without replacing the brake line, caliper carriers (abutment brackets), pads or rotors. For right now, I am sticking with the LS1 calipers.

RMK

Do you have a GM part # for the LS1 calipers?

89 Iroc Z

Front Left #18042491
Front Right #18042492

Rear Left, w/o Pads, Additional parts required. #12455127
Rear Right, w/o Pads, Additional parts required. #12455128

Thats all the info i could get at http://www.gmpartsdirect.com

lock

Any luck with the bracket 89irocz?

SDCal7

I have all the parts needed for the LS brake conversion but also need the actual dimensions for the caliper adapter.



'89 IROC Conv with an '99 LS1 engine, 18" IROC Rims, Moog racing springs and Bilstein Shocks with 275/40-18 Nittos.

89 Iroc Z

I have a rough draft of the bracket and dimensions of the ls1 brakes sketched on some engineering pad. I need to get some more measurements, finalize the design, and have my roommate CAD it out.

I will keep everyone posted on the bracket design. I haven’t had time in the last 2 weeks because I am focusing on school and all my free time is devoted to fixing up my car so I can drive it to Maryland this weekend for the thirdgen nations. I am trying to fix my stock brakes which are totally messed up (I have another post about this) can’t wait to ditch them and go with the LS1 setup.

89 Iroc Z

ah double post

SDCal7

Thanks for the update. I had to order a custom set of brake lines from Earls. Although they list a set for 98-02 brakes, according to Lloyd at Earls tech line, the length on some of the lines in the kit were not long enough. So he assisted me in getting the right lengths and correct ends. Let me know as soon as you figure out the dimensions for the caliper plate. The braided lines should be sent to me by the second week in July, which is when I'll start the job.
Bruce

machinist

RMK,

I just picked up a complete LS1 front brake setup off a wrecked 98 for $50. Also got the aluminum driveshaft for $40
Unfortunately the seats were gone already. Oh well, got what I could.


The insurance co. totaled the car, and he bought it back for the engine and trans. The junker is coming to haul what's left off tomorrow, so I got a smoke'n deal today only Guess I was in the right place at the right time for once in my life.

Anyway, I just put the whole front brake setup in my stock 87 IROC wheel and the caliper clears the wheel by about 1/2". So no problem with wheel clearence at all.

I may just make up a caliper bracket and see how this whole thing fits up for myself. Got a couple of extra spindles I took off my car for the C4 conversion, so why not

Justins86bird

laiky

I have a question and a point.

I understand that the Baer 12" setup will clear a 15 inch wheel. Is this true? If so will the ls1 setup also clear? I run snow tires on 15 z28 rims that i cannot get through the winter without.

Second (the point) I swear i have seen on Napaonline.com that they sell caliper rebuild kits really cheaply. I would go the junkyard route and just rebuild the calipers myself. I managed to get a set of ls1 rear brakes for 50 bucks so i'm going to freshen up the calipers as soon as i find out how to swap them onto my 9 bolt rear.

PS if this swap works out, you guys will rule!!

MikeTTA89

Be careful what rotors you guys are using, the earlier LS1 cars are notourious for warping front rotors. I went through 3 sets under warranty in the first 20K miles in my 98 Trans-Am.

machinist

Well I've got some aftermarket 15" wheels I'm going to check to see if they clear the LS1 stuff, but you could go the C4 setup that is linked to in the top of this thread and that definately clears 15" wheels.

And, by the way the C4 setup is a done deal. No question whether it will work or not. I've been running them now for about 3 weeks without problem one.

I also run 15's on the street because all my 16's have different compound race tires on them, so I'm in the same boat as you are.
That's how I know the C4 setup clears 15" wheels. And, we have checked a stock 1990 5 spoke mag with the C4 setup and they clear those also.

laiky

Thanks

drain89

iTrader: (10)

98-02 F-body brakes are notorious for rotor warpage. Even though they're thicker, they don't seem to be made as good. Everyone on ls1.com and ls1tech.com "who could afford it" switched to baer eradispeed rotors. Other people had good luck with autozone rotors as a low-buck solution. As a former ls1 owner I can honestly say; Avoid the stock GM rotor.

machinist

Ok, I had some time today and looked at this brake setup. It is workable with a 5/16" thick bracket ,and even a 3/8" with a little material removed off the caliper carrier bolt bosses. Since I have access to a mill, I wouldn't even consider a 5/16" bracket since it's so easy to mill off 1/16" the bosses on the caliper carriers. Anyway, the bracket is essentailly the same as the one Luke posted for the C4 calipers with the holes moved in a little.

What is different is the hub diameter. By "hub" I mean what's left of the stock rotor when you machine it down into a hub. You have to go 5.6 diameter, or about 5-5/8" to get the hub to sit flat into the rotor. Wit the C4 mods you only have to go down to 5.9".

Today I had my car up putting in my new $40 aluminum driveshaft and checked to see if my 15" mag cleared the LS1 style caliper. It rubs on both the inner most strengthening ribs on the caliper. It's not much, say about 1/16" taken off one small section of the ribs and it would clear. I know these are supports for the caliper and are critical, but in the area I would need to grind the rib down I don't beleive it would hurt anything.

So there is my LS1 brake update for today

Any questions let me know at alloy@pcez.com

89 Iroc Z

Alloy (machinist),
When is your alloy account going to be fixed? Where you get suck a great deal on the parts? Did your friend buy a wrecked LS1 camaro? According to my measurements the hub can be as big as 5.85.

I was going to use the C4 bracket I designed and move the caliper bolt holes accordingly. I need to find a place around here to put my hubs on a lathe so I can check my LS1 bracket design and post it. What I do is print out the bracket from CAD and glue it to a sheet onto this epoxy/cardboard material I have then drill out the holes.

I am friends with the guys at my local NAPA, hopefully they will let me use there brake lathe and get my hubs done on Monday. I’ll keep you guys posted.

machinist

I wish I knew when the alloy account will be fixed. I've emailed and pm'd both dirk and brad 17 days ago and begged them to fix it, and they still haven't picked up their pm's yet. And all this from me changing my email address on that account. Really sucks.

Anyway, I didn't have a friend with the car I got the parts off of, I was just driving around and saw the wrecked car and stopped and asked about it. The guy wanted lots more for the parts, but early in the conversation he said the junker was coming to get the car the next morning (today), so that gave me leverage to talk his price down. Basically I told him he would have $90, more in his pocket if he sold me the parts. Otherwise if he didn't sell them to me, the car would be gone and no $90 for him. The scrap guy couldn't care less what was off the car. The front end was hit and it wouldn't roll anyway. So, basically I was in the right place at the right time.

On the hub diameter, on my rotors there is a step in the bottom of the rotor and to get past that step and let the hub sit flat in the bottom of the rotor, I had to go down to 5.6". Different rotors may be different, but this is what I had to do to get mine to work correctly.

I'm not sure if a brake lathe will have the proper tooling to cut the rotors. I use a parting tool to plunge in and drop the rotor off. I haven't looked at a brake lathe to close, but I don't think it can do that kind of cutting. If it can post it and then everyone can go get their rotors done that way.

WOOS1

This is a very interesting thread. Please keep us posted. I am looking to build a 3rd gen for road racing/playing and this sounds like the ticket. I have done a C5 conversion on my 96 T/A and it is AWSOME. BUT, I ran into major issues with trying to find a suitable wheel to fit. I ran out of time and wound up buying a couple of sets of ZR1's to get me by until I have time to hash out wheel dimensions with a couple of wheel companies. I don't think I would have as many fitment problems with this LS1 setup.

89 Iroc Z

I wasn’t able to try the brake lathe at NAPA to make my hubs because all the rain we have had flooded the back room with the lathe. I talked to a few guys and some said there is no way a brake lathe can do it and others seemed to think it will work. I am looking for somewhere to get them done around here without paying an arm and a leg.

Alloy (machinist),
Do you have GM LS1 rotors? I have stock GM LS1 rotors and I only need to take my rotors down to 5.85 to get them to fit.

machinist

I have stock 98 camaro rotors and to make them fit flat into the back of the rotor I needed to go down to 5.6. No choice with the rotors I have. There is an aobut .02 step in the rotor back that forced me to go to 5.6.

It's no big deal whether is 5.6 or 5.8. Only a .1 difference per side.

machinist

Some interesting info on vehicle braking requirements I found.

http://www.carcraft.com/techarticles/76680/index1.html

And info on brake torque.

http://www.carcraft.com/techarticles/76680/index2.html

And even more info on master cylinders and braking pressure requirements.

http://www.carcraft.com/techarticles/76680/index3.html

And, brake pedal ratio info.

http://www.carcraft.com/techarticles/76680/index4.html

RMK

Dan,

Lucky you! I'm heaing off to the junk yard tomorrow to see what I can find.

Also, are you making brackets for this setup?

Rob

machinist

Yeah, did get lucky didn't I. First time for everything I guess Boy, I'll tell you I wasn't even to the top of 2nd gear before the car felt smoother with the aluminum drive shaft in it. If I'd have known it would make that much of a difference, I'd have gotten one a lot sooner even if I had to pay more for it. But, still can't argue with $40

On the brackets I was kinda waiting for Luke to see what he was going to do with it. I think he's pretty busy with school type stuff I beleive, and getting an education is way more important that this brake stuff is.

machinist

Ok, got some more info and ideas to throw at everyone here.

I picked up a NEW C5 rotor from Napa yesterday for $56.35 with tax. I've fitted it up on an old spindle and modified rotor (hub) and have found that by using this rotor our bracket thickness problem is solved. The correct thickness for a bracket without shims or spacers is 9/16". But, 9/16" isn't a standard material thickness. So that leaves several options. First is to use 5/8" material and mill it down to 9/16" (not easy and kinda costly way to go). Next option is to use 5/8" material and mill 1/16" off the bolt bosses on the caliper carriers. (fairly easy for me to do). And another option is to use either 3/8" or 1/2" thick material and use spacers (flat washers or something like that) between the bracket and caliper carrier.

Using a 5/8" thick bracket is probably overkill in my opinion. Not to mention this is a BIG bracket and it's going to be pretty heavy out of 5/8" material. And, for that matter even using 1/2" material with spacers it will be heavy. So my thought is to go with 3/8" material and use spacers.

Ok, next problem. Wheel clearance with the C5 rotor and LS1 caliper.

On my stock 87 16" wheel the caliper touches on one part of the wheel, and clears the rest of it about 1/16". (So much for round wheels huh)

Anyway I've been talking to Robert (RMK) and we have come up with an idea to turn the rotor diameter down .200 so we can move the caliper in some to clear 16" wheels. I've looked at the rotor and with the one I got from Napa there is plenty of material to do this without getting into the cooling fins. Probably take me 1/2 hour to do 2 rotors this way.

I guess my thinking on this is we have a bracket thickness problem, and going to the C5 rotors solves that easily with room to spare. Not to mention the larger rotor diameter benefits. Granted the larger rotor benefits are quite small, but it is a measureable improvement over 12" rotors. Not to mention we are only talking around $100 or less to upgrade to the C5 rotors from the LS1 rotors and still use the LS1 calipers.

And if you are going to buy new rotors anyway, the price different per rotor is about $18 more each for the C5 rotors. So $36 more for new C5 rotors over LS1's, and about $30 to turn them down .200. Brackets should run about $80 to $90 per set.


So, let me hear some feedback on my ideas.

Hope everyone has a safe and happy 4th.

Justins86bird

What is the I.D. of the 16" wheel your testing with?

machinist

It measures right at 15 1/2" ID. It's a STOCK 87 IROC wheel.

Zac's92

does someone have pics of this modified hub or can you explain it to me. I thought 4th gens used sealed hubs?

thanks Zac

machinist

Zac's92


Go here: https://www.thirdgen.org/techbb2/sho...hreadid=142115 The hub issue has been covered in great detail.

As a matter of fact, read the frist three sticky threads in this fourm. I know it's gonna be a long read, but necessary for you to get up to speed on this and make a decision as to which way you want to go for front brakes.

Justins86bird

My 17" Firehawk wheel's ID is less then that

machinist

That's interesting that a 17" wheel is smaller that a 16" wheel.

I'm borrowing a 4th gen wheel tomorrow from one of my racer buddies. I think it will be a 16" wheel, but not sure. I have another project I'm working on and need to do some measuring with a 4th gen wheel. When I get the wheel, I'll measure it and let you know what the ID is.

Justins86bird

It has a step in. Can't fit Baer 13" brakes either. It measures slightly over 15". I need to check to see if my LS1 rears will even fit.