IMSA GTU vintage racer build

[clarkspeed]

   

I have meaning to start up this thread for long time.  In the past 10+ years I have learned quite a bit about building and racing early Z cars.  So I decided to take all my knowledge and build the "ultimate" racing Z.  But of course that is a relative term.  More precisely, an early Z that is built loosely to the IMSA GTU rules of 1978 and legal for vintage racing.   This actually allows quite a bit of modification, so my attempt is to apply lessons learned from 40 years of racing into a car than can compete with muscle cars from the late 60's.  Both HSR and SVRA group the GTU cars in with typical 60's early Trans Am V8 American monsters.  So handling and cornering speeds are critical because HP is a losing fight.  I have a pretty good idea where I want to be in terms of performance, so power to weight is priority #1.  I doubt a Z can ever be the dominate car in this class, but hopefully it can be competitive with the guys that are build to the rule book.  A 800Hp 180mph Duntov Corvette is just another animal all together.

With that in mind, I have become obsessed with weight.  The shell I started with weighed 500 lbs, and I took some more out of that.  I would love to come in at 1800 or 1900 lbs dry, but time will tell.  There is a 3L limit to this class, and I am confident I can get 300+HP to the wheels based on previous builds.  However, the engine is the last thing I will be working on.

I have a library of photos from what I have already completed and have a goal of finishing all this up in the next 12 months. I am starting out this post with pictures of the shell I started with.  It was built up by Chris Leone in Orlando does race car prep for a living and builds a pretty mean cage.  He was going to build a V8 based car and ran out of desire.  I had originally bought the wide body kit he had for this car and have since sold it and decided to go with an unusual kit I purchased from a dude in Tampa. Anyway, he offered it to me for a very reasonable price at a time when I did not have the money.  He also had it for sale on Hybrid forums.  Some kid in Daytona ended up buying it and drug it up there without really knowing what it would take to finish it.  Months later I saw the kid had it listed on Craigslist for the same price he paid.  I ended up buying it for more than I could have paid Chris earlier, but also got an engine and tranny thrown with the purchase.

It is actually a fairly early 70 240 and I will post the info when I can dig it up again.  I think most of its history is just rust.  Even the small amount of original car that is left has multiple patch panels welded in.  I guess it goes to show no matter how bad the rust is, you can still build up a race car. 

Needless to say, this car is not a project for the timid.  It needed practically everything but the cage.  But so far nothing has really thrown me off yet.  

[AydinZ71]

Super excited to follow, and learn a few things! Good luck  👍🏽 

[JMortensen]

Man, what a great place to start from! Can't wait to see it finished. 

[Ironhead]

Very interesting project, I will definitely follow.

 

What year did IMSA allow sections of the GTU cars to be replaced by tube structure?  I used to watch the races as a kid, and in '78/'79 they all still had the full OEM body that was reinforced with tubing.  Sometime in the early-mid '80s, they were suddenly all tube frame "silhouette" cars with little or none of the OEM body remaining.  I'm just not sure exactly when the change occurred.

 

I wanted to also add, that is a huge project.  If you can complete it in 12 months, my hat is off to you...

 

 

Edited April 16, 2021 by Ironhead

[Jeff]

That looks like a very fun project Clark.  I can't remember if we ever met but I was around when Scottie and Mike (Zgad) were drag racing their Z cars.  I work in the SODO area and don't live far from there so I would love to see the project sometime.  I'm heading out this afternoon to go instruct and run TIme Trials with NASA at Sebring this weekend.  I'm currently running my 5th gen Camaro street car but building my 94 Trans Am track car up to be a little more competitive.  My poor 260Z is sitting in a storage unit over in Melbourne wondering if I'm ever going to get back to it again.  Just that time & money thing of course!

 

Looking forward to updates on this.    

[clarkspeed]

13 hours ago, Ironhead said:

Very interesting project, I will definitely follow.

 

What year did IMSA allow sections of the GTU cars to be replaced by tube structure?  I used to watch the races as a kid, and in '78/'79 they all still had the full OEM body that was reinforced with tubing.  Sometime in the early-mid '80s, they were suddenly all tube frame "silhouette" cars with little or none of the OEM body remaining.  I'm just not sure exactly when the change occurred.

 

I wanted to also add, that is a huge project.  If you can complete it in 12 months, my hat is off to you...

 

 

I believe IMSA went to tube frame in 82 or 83.  SVRA rules allow standard tub with tube frame extensions in GTU class which covers FIA rules through 84.  Wheelbase must remain the same.  I am also trying to keep the car legal for SCCA GT3 class at least with minor modifications.  

 

It is a huge project, but luckily I am already a good 2 years into it. I wanted to organize and document it so starting this thread will force me to review all the photos, organize them, and hide the stuff I had to rebuild 3 times.  More importantly I wanted to show some of the special stuff I did to make it one of a kind.  Every step of the process has brought unique challenges.  I can't tell you how many nights I have laid in bed and planned what I would do next and how I would do it.  And of course I didn't want to start a thread and then dump it off for another project.  Which is exactly what happened when we bought the ITS car and restored it to the Bob Leitzinger tribute car I have now.  That required putting this thing on hold for a year.  And I have had numerous opportunities to start some other projects, but for the most part I have stayed focused on this thing for some time now and can actually see the end coming.  

[clarkspeed]

4 hours ago, Jeff said:

That looks like a very fun project Clark.  I can't remember if we ever met but I was around when Scottie and Mike (Zgad) were drag racing their Z cars.  I work in the SODO area and don't live far from there so I would love to see the project sometime.  I'm heading out this afternoon to go instruct and run TIme Trials with NASA at Sebring this weekend.  I'm currently running my 5th gen Camaro street car but building my 94 Trans Am track car up to be a little more competitive.  My poor 260Z is sitting in a storage unit over in Melbourne wondering if I'm ever going to get back to it again.  Just that time & money thing of course!

 

Looking forward to updates on this.    

Sounds like I must know you from somewhere.  I was good friends with Mike.  I live in Thornton Park, but my garage is at a private residence in Oviedo.  I haven't been on track since before Covid, so I am getting very itchy.  Yea, time and money.  I either have one of the other, never both.  I hope to do some vintage racing later in the year.

[clarkspeed]

I finally found the picture of me hauling the shell home after purchase.  This picture was dated 3/30/13.  I guess it sat around in the corner of the shop for a little longer than I thought!  Anyway judging by the dates on my pictures, I worked a little bit on it in 2014, and quite a bit in 2015 just before we bought the other car.  Then it sat mostly until 2017 when I put it in the rotisserie. But between racing and keeping up the other car, I didn't really get back into it until 2019.

[clarkspeed]

First up I needed to revise the front end.  The car was originally designed with a V8 in mind and had a cross member designed to hold one.  I know that I will be eventually running a L6, so after a lot of thought, I decided to run the standard Z car cross-member.  This would save some time and I knew with a little modification it could be used for most of the things I want to do.  It also allowed me to set the standard Z car front suspension pickup points without too much trouble.  But it was still not easy.  I had to also relocate the 2x2 square tubing to mimic the Z car frame rails.  So essentially the bottom of the 2x2 channel is located in the same location as the stock frame rails, (note this is a critical measuring datum, I will discuss later).   To accomplish I had to level the entire tub, which was welded down to a 0.100” steel plate sitting on the shop floor.  I used strings tied to the chassis points outlined in the shop manual.   I don’t have any early photos of setting it up but you can see the final result in these 2 pictures.

 I was not really happy with the way the 2x2s tied in with the 2x4 square coming out of the tub.  To get the correct height, they ended up sitting right on top and I really didn’t want to tear into the 2x4’s or relocate those.  It just seems like a frontal impact would drive that front clip right into my lap.  I added a 2x2 brace and a doubler plate to the weld joint. I still plan to add a crush zone in front of the radiator in the future. 

 

You can also see the 2 square tubing rails that are tac welded and located at floor depth and width of the rear measure point located behind the seats. 

 

Next I will post about the strut towers and control arm pick up points.

Edited April 18, 2021 by clarkspeed

[clarkspeed]

After the frame rails and cross member were located, I had my simple front control arms welded up.  I had these tig welded by a friend due to the tricky multi thickness weld.  I took measurements again from the shop manual and stock components.   I used a spherical bearing and holder from UB Machine as the lower ball joint.  This was combined with a monoball pin also from UB.  I used a taper reamer from Speedway Motors to ream out the stock steering knuckle for the taper to fit.  It you try this, be very careful the cast iron steering knuckles cut very fast.  Go slow and measure the depth as you go.   I tapered just enough to have thread engagement so a nut and washer do not bottom out.  I also had to cut just a little clearance for the nut from the TTT bump steer spacers which are roughly like 1.5” long.  The chrome molly tubing has 5/8 welded tube ends.  I built a small tool using aluminum tubing and sheet metal screws to set the rod ends and angle of the control arm exactly as the stock configuration.  I then adjusted the rod ends and locked them down so I could use the control arms as a tool to locate the T/C pickup point and the strut tops.  Also of note, the rear T/C rod end pivots at exactly the same place as a stock one.

So if you are paying attention, I have the roll center located lower by the TTT strut extensions.  In addition, I can lower the outer ball joint.  So no matter how low I go, I can still put the roll center exactly where I want it.  And I also considered that the ball joint pin might actually interfere with the 15” wheels I will be running.  If that is the case, I will slot the cross member and rear mount to move the inner pins upward, but I don’t think I will need to.

[clarkspeed]

So to build up the upper strut mount, I started with a 1” angle assembly that would hold camber plates.  I measured out the Ground Control camber plates I had installed on my other car arranged the angle in such a way to hold another set of them.  Then it was a matter of locating this bracket in relation to the lower control arm pivots.  This was another critical decision, location of the strut tops affects the caster, camber, and shock travel.  So I set up an empty stock strut with a threaded rod substitute.  I attached it to the lower control arm and set the caster right around 6 degrees.  I have successfully ran 5 degrees before using an adjustable t/c rod so I know this will work.  John C on this forum stated 8 degrees was good target, but that seemed a little extreme.  Anyway from 6 nominal I can get to 4 or 8 without too much problem if I want to experiment with it.  So in the end I located the strut top with a 6 degree rake, at a height I thought similar to stock, and at a width a little more narrow than nominal for future camber adjustment.  So essentially a 3 dimensional placement in thin air.  This will not be the first time I will need to locate a  a part 3 dimensionally in thin air.  Fun stuff when all of your reference points are 2, 3, or 4 feet away.

[grannyknot]

Wow, big project, looking forward to the progress.

[Derek]

Man I wish I had the time and energy to do big projects like this. Looks like a really fun project.  

 

 

[JMortensen]

Are you doing this real time, or reporting what happened a year ago? If real time, I have a suggestion: don't use Z camber plates. You've got some freedom to change things, and you can make that strut top whatever size you want. Looks like you've already gone bigger than you need for the Z stuff. I'd use Maximum Motorsports mustang camber plates. Their stuff is camber/caster adjustable, they use larger bearings and have a better setup than Ground Control in terms of centering the spring on the bearing. I know some of the other ones center on the monoball, but they use small 5/8" monoballs. I think MM uses 7/8". Their engineer poo poo'd my GC plates back in the day. He's a good guy, and he wasn't doing it out of malice. Dennis Grant did the same in his book and on his site (towards the bottom of the page): http://www.farnorthracing.com/autocross_secrets5.html.

I showed the MM engineer the updated GC plate and he basically said: "Still trash."  

https://www.maximummotorsports.com/Mustang-Caster-Camber-Plates-1979-1989-P252.aspx

[clarkspeed]

Yes, already done, but never too late to rework. 😀  That is one of the reasons I started this thread, to get input on things done, and help with some future decisions. I ended up reworking the strut tops twice. I will show in a later thread. Instead of Z car camber plates, I ended up with a set of GC universal camber plates in front. They do have caster slotting similar to the Mustang plates you reference, but I chose not to use it for now. I can do some more slotting if needed. 

 

As far as spring support, that has not been designed yet. I only have the basic camber plates, so I will need to use a top hat and thrust washer of my own design.

[clarkspeed]

Jon, I took another look at my camber plates tonight. I seem to remember previous versions of my GC camber plates did incorporate some sort of spring adjustment. Anyway, after reviewing the plates and the Mustang plates, I will add a "floating" spring perch to fit with the GC plates to compensate for spring alignment. 

 

 

Edited May 12, 2021 by clarkspeed

[AydinZ71]

I just picked up another pair of GC bolt-in camber plates for the fronts this time. Price has jumped $50 since the last set I bought in nov. 

 

I do appreciate how they keep the spring concentric with the shock insert. I’m a fan, but they are not inexpensive, and I had to custom make a socket to fit the tight space between the 14MM threaded nut and the surrounding body. 
 

PS: I have four of the original spring perches that come with GC’s coul-over kit anyone needs them. The camber plates come with their own. 

Edited May 12, 2021 by AydinZ71

[clarkspeed]

And on to steering.  All of my OEM rack and pinions on the shelf had a dead spot and I thought this would be a good area to upgrade.  I chose a Coleman Racing square rack in a ratio just slightly quicker than stock.  But to use a circle track rack, I would need to widen the tie rod mounts to get the correct length, which should be equal to the control arm mounts.  I search around the internet and found an adapter bracket design that I decided to copy.  I designed a welded assembly made from waterjet cut mild steel that had the correct length and the slotted adjustment.  Most rack manufacturers will tell you not to do this, but they like to make custom length racks also.  I like the design because it will make eliminating bump steer very easy.  As you can see, I had to do some surgery on the cross member to get access to the rack spline.

[JMortensen]

17 hours ago, clarkspeed said:

Jon, I took another look at my camber plates tonight. I seem to remember previous versions of my GC camber plates did incorporate some sort of spring adjustment. Anyway, after reviewing the plates and the Mustang plates, I will add a "floating" spring perch to fit with the GC plates to compensate for spring alignment. 

Curious to see what you come up with.

[clarkspeed]

3 hours ago, JMortensen said:

Curious to see what you come up with.

I sketched it up last night. This hat will ride in the radius "cup" shown previous so it becomes a hat on a hat. My hat has a generous hole to clearance the rod and I can cut down the outer diameter of the upper GC hat if I choose to make it look better. 

Picture shown is upside down.

 

Now I just need to figure out how to cut it on my homemade mill.