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The anatomy of the GXP

Written by Darren Parker.

The GXP on display at the Melbourne Hot Rod ShowThe GXP on display at the Melbourne Hot Rod Show

For this article in Just Drag Racing, I thought I would go through some of the differences between racing my high 9 second Super Sedan and my mid 7 second Pontiac GXP in Super Stock.

There are some basic similarities between to the two cars, four wheels, 9" diff and 4 link suspension, and in all honesty that is where it ends. Everything else on the two cars is different. The Commodore was a right hand drive factory steel sedan that had a ½ chassis and roll cage fitted, the GXP is a left hand drive full chrome moly chassis that weighs approximately 100kg bare.

Light weight chassis and body

The steel body of the Commodore was standard, however the doors, bonnet and boot were replaced with fibreglass replicas whereas the GXP has a lightweight carbon fibre body with modified aerodynamics (as per NHRA Pro Stock rules) that means it is much slipperier through the air. The ONLY factory original components on the GXP body are the standard Pontiac G6 head and tail lights, again as per the NHRA rules.

Weight of the two cars is also very different. In the Commodore I was trying to save as much weight as possible and went to many lengths to do so. Without me in the Commodore it weighed in at 2475Lb, or 1125kg. Not bad for a steel sedan however the GXP is out of control light. Without my weight in the driver's seat or ballast in the car the GXP comes in at a featherweight 1936Lb (880kg) and that is with fuel, oil and water in the car! Running in Super Stock I have to weigh a minimum amount based on the engine capacity. In order to reach that minimum we're able to place ballast around the car which assists in getting the correct front to rear and left to right weight distribution for maximum performance.

1000 plus HP small block Chev

400ci Chev making over 1000hp400ci Chev making over 1000hpLet's go onto the driveline. The old Commodore was powered by a Holden V8 stroked to 383ci (6.3lt) and made around 550Hp. The new Small Block Chev in the GXP is 400ci (6.5lt) and has no factory Chev components on it and it revs to over 10,000rpm on the track and makes well over 1000Hp. EFI on the Holden v's twin carburettors on the Chev make tuning a lot different as well.

In between rounds at a race meeting I used to plug the charger into the Commodore and either watch some racing or socialise with other racers. With the GXP there is a bit more between round maintenance to be completed to ensure that everything is ready to go for the next round. First job to do is plug the charger into the battery, then check valve lash and valve spring seat pressure. At 10,000+ rpm we need to know that a valve spring hasn't gone soft and will close the valve before the piston starts to push it back up the bore. The crew then swap the rear tyres which helps even the wear across the tread and extends the life of the tyres. Next job is to download data from the RacePak Data Logger where I can see if there are any improvements to engine tune, chassis setup or my driving that need to be made for the next round.

From the data I can assess fuel mixture, engine and driveshaft rpm, G-force, oil pressure, vacuum and many other items that I log. I can see if there was wheel spin off the line or on the gear changes, I can see bumps in the track and see oil pressure and engine vacuum going down the track. It is easy to change the jets in the carburettors or adjust timing or launch rpm. More time is needed for converter, gearing or suspension changes and I normally do those before I get to the track or at the end of a days racing when there is a bit more time rather than during rounds of qualifying or eliminations.

Thousands of adjustment options

The Ness GMM 4-LinkThe Ness GMM 4-LinkMost drag racers would know what a 4-Link suspension looks like. On these latest technology Pro Stock cars the 4-Link has been taken to the next level that allow finer adjustment and even more instant centres. The average 4-Link chassis plates have ؽ" or Ø3/4" holes pitched up to 1" apart. The Ness GMM 4-Link on the GXP uses titanium plates that are moved in 1/8" increments giving thousands of more adjustment options. While this might sound like a good thing my mind starts ticking when I start looking at adjustments because I now have to choose between 20 or 30 instant centres that are within millimetres of each other.

The transmission is also different. While still an automatic based transmission the Proflite in the GXP is a lot higher maintenance than the Powerglide in the Commodore. All the rotating parts in the Proflite are billet aluminium for light weight and high strength. Putting over 1000Hp through the 3speed transmission makes a lot of demand on the oil, so generally it is replaced at the end of each race meeting, sometimes only 3 or 4 passes, but if you go to the final it can be 7 or 8 passes. Daily maintenance on the transmission involves a band adjustment and clutch packs are checked after every couple of meetings.

The 3rd member in the car is based on the Ford 9" centre and while the Commodore also had a 9" in it there is a lot of difference here too. The Commodore used a factory Ford 9" Housing adapted with 4-Link plates and coil over mounts. On the GXP, Jerry Haas fabricated the housing from chrome moly sheet for more strength and less weight. While I updated the Commodore to a Mark Williams alloy diff centre in the Commodore, I have just fitted a Dewco aluminium centre to the GXP which from the outside doesn't look any different, however on the inside it is a work or art. With custom 9.5" gears (and every tooth modified), ultra lightweight spool, ceramic bearings and aluminium yoke it is the Rolls Royce of diff centres.

Now I mentioned tuning and adjustability of the car above, lets take a quick look at some of the adjustments I have available to me. Starting with the chassis the front struts are adjustable for height, bump and rebound. The rear coil over shocks are also height adjustable, but have pneumatic bump and electric rebound adjustment so I can have it set stiff at the start line for maximum weight transfer to the tyres and soften it progressively going down the track to smooth out the bumps that all tracks have. I previously mentioned being able to move ballast around, this can help with weight transfer and getting a car down a marginal track. The 4-link is adjustable in 1/8" (3mm) increments and you can see even a small change on the data logger! I can adjust engine timing very quickly through the MSD Digital 7 and can adjust everything from rev limits to timing retard on launch, shift light and timing on the gear changes. I carry a couple of converters for the transmission, two diff centres and a second set of tyres to give me gearing options, although the converters and diff centres take longer to change and generally aren't done between rounds.

Drives like a dream

So how about the speed? Well surprisingly the GXP 'feels' smoother and less violent off the line than the Commodore and I think that is partly due to the seating position. In the GXP you are sitting with your bum just over 2" off the ground and the seat is part of the chassis whereas in the Commodore, the seat is bolted in and the base of the seat is probably 8~10" off the ground. A 1.05sec - 60' feels smooth compared to the 1.32 – 60' in the Commodore. The first shift into second gear comes on very quickly and second shift into third gear is 4.7 seconds into the run, just at half track. 295kmh at the finish line feels very fast and when you pull the chutes it is relief to feel your body thrown forward into the belts knowing you only need to use the brake when you get to the turn off and not as the primary brake to stop the car.

I hope you enjoyed a brief look into some of the finer details of running a vehicle in ANDRA Super Stock.

I would like to thank the following people for their on going support:

  • Peter Ridgeway and Arthur Sagiaris (Custom Engines)
  • Neil Hendry at Cryogen Industries
  • Nick Xerakias (ERC Fuel)
  • Paul Rogers Snr and Matt Ramsay

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