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Old 03-26-2012, 08:50 AM   #1 (permalink)
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Coupe owners: Is a Backbone worthwhile?

Hello Coupe owners!

I've been thinking about the cost / benefit ratio of installing a DDM Race Backbone in my GXP Coupe. It was the first mod I made to my GXP Roadster, but I (wrongly?) assumed the differences in coupe body design stiffness vs. a topless cockpit made it unnecessary.

How many Coupe owners have added a Backbone and is there a noticeable difference in stiffness, handling, suspension control, etc.? Thanks for your opinions on this question.
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Old 03-26-2012, 09:12 AM   #2 (permalink)
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I added the 3/8" thick Backbone as one of my first mods. I definitely notice less cowl shake. And that was after having the stiffer stabilizer bars added at the dealer before picking the car up originally. With the added stiffness the Backbone added, it felt even better in the corners!
I'd say go for it!
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Old 03-26-2012, 09:43 AM   #3 (permalink)
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The coupe is barely (if any) stiffer than the convertible since they didn't join the roof up to the A-pillars. So, yes, the backbone is a good idea.
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Old 03-26-2012, 11:49 AM   #4 (permalink)
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To build on TS's input, the coupe starts life as a roadster. They have different trunk liner, tails, interior plastic, side window glass and then the plastic top is bolted onto the now transformed roadster. The top does not add a whole lot of stifness
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Old 03-26-2012, 04:15 PM   #5 (permalink)
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The coupes do have a support bar that goes side to side at the top
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Old 03-26-2012, 06:11 PM   #6 (permalink)
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Having owned both a GXP non-coupe and now a coupe, I can tell you that the coupe is much much stiffer than the non-coupe. This comes from experience driving in Oklahoma with our renowned terrible roads.
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Old 03-26-2012, 06:45 PM   #7 (permalink)
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That is probably true, but if you want to get the car to handle better, you need to upgrade the suspension.
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Old 03-26-2012, 09:02 PM   #8 (permalink)
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We added the backbone first and the Probeam later The difference is definitely obvious even in the coupe. It tracks better through the turns and has a much more confident feel to it. Highly recommended!!!
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Old 03-26-2012, 10:53 PM   #9 (permalink)
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I'm wondering of those coupe owners that installed the backbone, if anyone noticed a difference/reduction of the squeaking nose when the top is in place.
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Old 03-27-2012, 08:00 AM   #10 (permalink)
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The coupe isn't that "inherently stiff" , really. My friend Stan ( we both retired from Wilmington and Stan worked for a vendor rep for the poeple who made the fascia's and rear couple assembly and targa) was just showing me pictures of the coupe "fastback" installation. Essentially, the fastback just rides on the body and is attached so that it doesn't blow off. A bit of a silly way to put it but that's about how it is, really. The main difference from the convertible was the use of a couple of solid aluminum brackets that were installed to the inner quarter panel structure exactly where the tulip panel would have attached. The fastback would just drop onto those brackets and bolted in place with two bolts each side. The bolts that are used to attach the fastback to the bracket are quite large. From the pictures they look like they are about 16mm. The reason they bracket and hardware is so substantial is to prevent the targa from creaking while the suspension is loading and unloading. In fact, mine creaked a bit before I installed the Backbone and is silent now. The only other attachment point is at the rear under the duck tail spoiler and their are only two small anchors there. If you remove those two stainless screws on each side you can see them. From what I hear they had to make a special tool to secure them during assembly. In fact there is only about 20 pounds difference in weight between the two cars. ( I think the coupe is lighter) And if you think about it the same backbone pan that is under the convertible is under the coupe so there must be similar structural dynamics present in both cars, wouldn't you say?
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Old 03-27-2012, 08:49 AM   #11 (permalink)
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The only way to know is if a coupe rigidity test was performed (pre & post backbone). I admit the coupe feels more solid but feeling vs fact are two different subjects.

For instance; a 3 ton vehicle may "feel" more rigid/solid than a 2 ton vehicle but once you factor in how weight affects feeling that doesn't mean a 3 ton vehicle will perform better than a 2 ton vehicle (all things being equal).

Maybe a better question would be; would a backbone instill more confidence. Thereby giving you more confidence to push it closer to its limit before physics takes over.

Or another way of saying it is; if you believe a backbone will instill more confidence to push the platform to its limit/or enjoy testing it...than chances are you may be rewarded.
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Old 03-28-2012, 07:09 AM   #12 (permalink)
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The Backbone and Probeam in combination make a significant improvement in handling - that's enough for me!

I wish the coupe HAD been done as a true fixed head, and therefor more rigid.
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Old 03-28-2012, 08:58 PM   #13 (permalink)
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Quote:
Originally Posted by JET JOC View Post
I'm wondering of those coupe owners that installed the backbone, if anyone noticed a difference/reduction of the squeaking nose when the top is in place.
Silicon grease has worked well for me, even on Austin's craptacular city streets.
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Old 04-11-2012, 01:24 PM   #14 (permalink)
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sell me your original backbone

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Old 01-23-2013, 12:21 PM   #15 (permalink)
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Sorry I'm 10 months late with this replay. Somehow this thread slipped through my subscription filter. With that said, I've thrown countless thousands of dollars into Porsche 951's (a.k.a. 944 Turbo) since 1993. I was a tier 1, powertrain electronics Supplier to GM and knew of the Pontiac demise prior to the general public. As the collectors were scrambling for GTO's, I committed to buying one of the 1st 100 (non-sequential vins) Captured fleet evaluation Coupes. It became available in May 2010 and had accumulated 15,000 miles. I'd never driven a Solstice Coupe or vert of any type but my closest friends in GM Powertrain told me the of the GXP's vast, untapped performance potential.

At this time, I was still in possession of my "LAST" Porsche 944T which had over $10k in suspension mods over and above the highly sought M030 option and $15k in the engine to achieve 350bhp. This is arguably the best handling production car ever built. The GXP Coupe was located at a Wisconsin dealership some 700 miles away. Prior to picking up the Coupe, my GM Contacts recommended the GMPP Turbo/SW upgrade so I had the dealer install it prior to picking up the car. So I've got a double disadvantage, I've never driven a GXP Convertible or Coupe with factory HP/TQ and the Coupe came equipped with FE3 Suspension.

Now that you've got a clear understanding of my Solstice background, I'll try and answer your question regarding the backbone and rigidity of my Coupe. When I left the Dealership in Wisconsin, I felt a little claustrophobic and vision confined. The claustrophobia was gone after the 1st 100 miles and the blind spots remedied with a stop at an Autozone and some 1" concave stick on mirrors. Finally feeling comfortable driving the car, I looked for some non interstate options to see how the car felt cornering, braking, etc.. Breaking was good, handling was, at best, marginal.

Once home, I was immediately on the phone with my friends in GM powertrain and performance division. That conversation led to an immediate order for a ZOK Suspension package, a Trifecta Tune and Eibach Springs to lower and stiffen the suspension. I gave Dave a call at DDMWorks and scheduled installation of the suspension upgrades as well as his mandrel bent IC Tubes. By pure luck, I was looking for REAL Tires on the Tirerack site and saw a set of 20"x8.5", one piece aluminium wheels on closeout at $75 each. They had the correct bolt pattern and offset so I made some quick measurements and ordered the wheels and Conti Extreme Contact DW 275/30/20's on all 4 corners. These were installed on receipt with no clearance issues and marked improvements in cornering and noise. There's a lot of negativity on the forum regarding 20" wheels so all I will say is "bring it on, stopping distance, auto-cross, 1/4 mile or full fledged track", I'll let the car speak for itself. I then flashed the Trifecta tune atop the GMPP upgrade. Wow was my only thought.

At DMWorks, Dave and crew made short work of the suspension mods which lowered all 4 corners by 1 1/4" still with no rubbing or Clearance issues. Dave then asked me if I was interested in a backbone. Not knowing what improvement I should expect from the backbone, I drove it before and after. There was a noticeable difference in the cars perceived rigidity and the Coupe was now equal to or better than my "very expensive" 944 Turbo S.

Dave then proceeded to take me for a ride in the 1st GT2871 Turbo upgrade car he'd just finished and once again all I could say was Wow! I placed the Porsche on Auto Trader the next day as I had already accomplished superior performance gains in the Solstice with a ten cent on the dollar investment as compared to the Porsche. With a small percentage of the money from the sale of the Porsche, I called DDMWorks and gave them a deposit on a 2871 upgrade as well as ordered a Solo Free Flow cat and exhaust.

The 2871 with an AEM water/meth kit, Solo Cat/exhaust and DDM's new Latin Venom front cross brace were installed in June of 2012. Depending on the Air Density on any given day, 400 dyno'd whp are achievable. My GXP Coupe now sits in the company of Z06's and pulls over 1.1g on the holy 200ft diameter Motor Trend test setup using a G-Force Monitor. I'm really at a stopping point with HP and TQ until I strengthen the internals (Rods, Pistons and Valve Springs). Getting current power to the road, even with the sticky 275's, is an issue. The rear will support 285's so I may end up with a staggered tire set when I replace the current 275's. A little over 2 years ago I'd have laughed at anyone that told me I'd have to nurse the throttle on a 2 liter turbo through 1st, 2nd and sometimes 3rd gear in order to keep traction.

The remainder of the 944 Turbo proceeds went into the purchase of a 1996 BMW 840 Ci. This wasn't something I was looking for but a close friend found himself in a situation where he needed to part with the BMW. It is an awesome car and I had full intention of making it my daily driver. Now if I can just find a way to walk past my GXP Coupe and actually get in the Bimmer for daily driver purposes, I'll have the best of two worlds. Thus far, I've only been successful about 1 in 4 times of making it to my daily driver.

To get back on topic, buy the DDM Race version of their backbone and the Latin Venom front cross brace. You will not be disappointed.
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