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Discussion Starter #1 (Edited)
This story starts with a rainy 25 Hour race, a subsequent phone call, a test day, racewins, a championship then two, love for the midengined no compromises rawness and beauty of the GTM, the awe and excitement over it’s inherent potential, and a crying baby at 5am on a Christmas eve that lead to a 6am plunge into GTM ownership.

So a moderately wrecked GTM got loaded onto a truck at a cold winter yard on the East Coast. The truck trundled through New Years Eve on the vast plains of Nebraska, and after clearing the Sierras, #82 finally arrived to a new home in sunny California, parked up next to the most race winning GTM on planet Earth no less. That particular PDG4 offers almost a decade of lessons learned that focus on performance, serviceability and reliability.

Let the good times begin!

RR46 is not new to the world of GTM's I know he has some experience driving one. I met him a couple times and don't exactly why he chose me to fix it, but I always try to give a hand when I can!
Fast fwd appr 6 months: To start off, Richard has done A LOT more than lend a hand on this one! So even though racing, travel and kids have limited my number of days and nights available to go enjoy this GTM rebuild, magically this car is basically only a brake bleed (and zipties to hold harnesses) from a gocart test drive. Result: Chassis is straight as an arrow, majority of the permanent sheet metal is on, racebred coolant system re-engineering is complete and pressure tested, suspension and steering is in place.

20/20 Given the amount of disassembly needed to replace even ever so slightly bent front frame bits that were discovered as we dug deeper, we established that if we would have just cut loose the whole front frame clip from the cockpit and put on a new clip we’d have ended up with fewer welds in total. But having done the full repair work, it actually feels great to know we had full control over the exact symmetry of the car without any tension in the structure. It also gave full insight and hands on feel for the true strength of this chassis design, and how to best fit some of the front end systems.



I played with a three radiator setup Pcar style to have a proper boot in the front - this will be a great upgrade but a future one since I want to drive this car soon and sort out the handling per experience from the PDG4 racecar.

Coolant pipes on the outside is a relatively straightforward mod with major payoff. Fluid volume, cockpit temp management, and less heat transfer to clutch, brake and AC lines are main benefits. Also positioned the radiator more flat to allow a curved lower part of the radiator box, to flow air upwards through the radiator instead of air hitting it in shear. It should be noted that idle water pump flow was excellent and it bled immediately with no trapped air, we just raised the front while filling and running motor warm.

Currently she sits with stock brake calipers and mix of original and borrowed shocks, with 3-way adjustable shocks going on next: The extraordinary laptime improvement we saw from installing Ridetech 3-way adjustable shocks on the PDG4 car made this an easy decision on where to invest. Great shocks on a street/track car aren’t just for speed but also for chassis compliance with varying quality roads with drops, bumps, split crowns in canyons and more. This actually equates to big improvement to relative comfort, safety, predictability and oh yeah speed again because wheels don’t grip when bouncing away from the surface, and going quick in a well setup car is easy.

Also on that note, a stiffly sprung front isn’t great on cars with light noses, so a front swaybar kit is going in along with the shocks to make it possible to drop front rates yet maintain balance and (adjustable) roll stiffness up front. This is needed along with a bump steer kit to have sweet behavior in canyons, on tracks, and freeways.

Once the car rides like a fast flying carpet with agility and the ability to keep each tire in touch and flowing with any surface shape, fine tuning of caster and camber using parts from ”crash” myraceshop.com will make this a fantastic performer, using the GTM racecar PDG4 as a performance reference and inspiration.

Brakes need a comment, these will have to prove themselves incapable of coping with this low weight car before upgrade is considered. Endless fluid is all I use (also for clutch) and with Ferodo track oriented pads there is no weakness left but rotor size, brake cooling or caliper flex left to look for. I will report whether I can hover near the top of Cali laptime charts with these brakes or not. Note: A brake bias adjuster cable is a requirement on ANY non-ABS car in my opinion.

Now onto some photos from the chassis repair.
 

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Discussion Starter #2
Before resorting to cutting out bent pieces from the front clip, we made attempts at straightening them using load straps, chains and a portapress. The chassis design is WAY to strong for that, every bar is supported by another which is supported by another so there's no meaningful movement to be had. Going straight to cutting bent pieces would have saved us a day - good reminder/learning.



Richard lending a hand. Looking at the progress, he's got a lot of hands!!!


Assessing the situation after clearing out bent parts, this is a great moment when you can see straight again so to say.
 

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Discussion Starter #3
Pass side coolant routing mockup. I HIGHLY recommend this mod, if you like me plan to drive your GTM a lot. Driver side looks appr the same.

 

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Thanks for updating us! I'm sure I'm not the only one who is jealous you found that insane bargain! Keep us posted! When are we going to get to buy posters of the PDG GTM with the new livery? Or upload some hi res shots?
I also am running coolant outside the cockpit, but from the water pump down, slightly forward, and to the sides just in front of the firewall. Then made insulated covers where it's in the cockpit along the firewall.
I did dual radiators too, but Dave Fraser pioneered it.







 

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Discussion Starter #5
Thanks for updating us! I'm sure I'm not the only one who is jealous you found that insane bargain! Keep us posted! When are we going to get to buy posters of the PDG GTM with the new livery? Or upload some hi res shots?
I also am running coolant outside the cockpit, but from the water pump down, slightly forward, and to the sides just in front of the firewall. Then made insulated covers where it's in the cockpit along the firewall.
I did dual radiators too, but Dave Fraser pioneered it.







That is a cool way to route - no conflict w seat position? This path might be a good idea for offering more room in front of the heads, For example, still trying to see if it is possible to relo either alternator to the top or set a compact AC compressor in front of one of the heads in order to fit a dry sump pump down below, then one coolant line would need to be moved out of the way.
 

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Discussion Starter #6
Gocart drive(s) done! Loving the open convertible feel on a hot Cali day.

Put a Dewitts radiator in for good measure.
Brandwood shifter cables seem to need a little fine adjustment and rerouting away from hot exhaust, slightly inconsistent resistance on shifting, esp after a bump on the road.

Can’t wait to put in the 3-way shocks, running on three kit shocks and a right front raceshock was still good but not ideal. ?

The C5 brakes felt very very solid vs expectation and the Supercar 3R tires delivered massive traction both during hard braking and heavy accel.

Very quick car, didn’t expect that with the LS1 and I was already planning for a LS3. Disclaimer don’t know yet what if any headwork or cam swap that has been done to this LS1 but it propels the GTM very swiftly even compared to the racecar. The Kooks exhaust goes from a civilized idle and cruise rumble to a seriously angry scream above 5krpm. Couldn’t ask for more.

Definitely for a street car of this caliber, ensuring it has good suspension travel and excellent shocks is absolutely vital.

Back to the to do list, still a lot left to do.
 

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Discussion Starter #8 (Edited)
Between family, work, racing schedule and travel, 4 months have gone very quickly but finally with Richard’s help I have gotten a little more done.

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Doors are prepped for primer, going on soon after body has been primered.

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Glassfiber prep is dusty work! This blue is growing on me but probably still won’t be the final color for this car.

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That is indeed a bubble wrapped APR wing.
Wheel offsets were too conservative in the rear and required spacers to fit well.
 

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LOVE that coolant routing... I've long wondered why it isn't done that way from the start! If (and hopefully when!) I ever manage to cobble together the funds for a GTM build, that will be the first mod I make! (The second would be glassing the firewall to the body to seal the engine bay from the cabin-- but this thread isn't about my dreams)
 

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Discussion Starter #10
LOVE that coolant routing... I've long wondered why it isn't done that way from the start! If (and hopefully when!) I ever manage to cobble together the funds for a GTM build, that will be the first mod I make! (The second would be glassing the firewall to the body to seal the engine bay from the cabin-- but this thread isn't about my dreams)
I think this is a place for everyone to dream, and in this case also sharing ideas! The body definitely could have a perpendicular section down from the ceiling to overlap with the firewall. I didn't pay attention to it until spending more work time in that area, while swapping the rear acrylic glass to real glass per Richard's wise advise. So my mind is now passively thinking about how to best seal that rather signifcant gap around the cage.
 

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Discussion Starter #11 (Edited)
A bit of a sprint has taken place over the last few weeks. I keep going back to cooling, but it's so important. I put a Moroso reservoir in the back, above the heads near the rear hatch pass side hinge. Took a lot of hose out of the car, reservoir is the highest point, directly above where it "fills" to the side of the water pump housing, and the steam vent goes right to it from the heads, making the engine self-bleed the way it should.

Outside of that, it's been mostly about installing many parts that needed to go back onto the car, and an increasing frequency of longer and longer test drives. :)

A key aspect to a car like this is seating position, seating rigidity and steering wheel position. The car has gobs of legroom, but is low, which is cool, but low. I had bought a pair of Braum Advan seats that are a good looking option for these cars. Some say driver side requires cutting the tunnel side seat belt bracket out. Anyway, I want this as a track toy so I picked out a Sparco QRT-R seat as final choice for driver side, with special low mounts from Sparco that I bolted directly to the chassis (not only to the sheet metal). On the passenger side I have the Braum seat, I just set it in and zero cutting is needed, it goes perfectly into place. The Braum seat looks really good in the car, still have to bolt it down.

The Sparco QRT-R driver seat is pretty narrow on the outside which facilitates correct positioning, close to the tunnel in the GTM, while tall. It has a drop at the bottom to maximize how low the driver sits, and it fits very comfortable yet supportive at 195 lb and 6'2". I removed the seat padding for max headroom during helmet use. The height of the seat gives support at the right height for body core and shoulders. Many smaller seats discussed here and elsewhere for small cars just don't offer good height. QRT-R does.

Love how the seating position turned out. I can't stress enough that a very dialled in seating position is key to feeling a car, this being in control of it. A GTM is a proper beast which makes it even more important.

The car is in primer. A track day is planned in about 3 weeks, fingers crossed this timing will hold.
Engine and trans coolers must go in before then + pan baffles from Improved Racing (good for 1.4G).
Splitter and APR wing going on too.
Ferodo street/track pads.
Endless extreme temp brake fluid (also for the clutch since that gets very hot at the slave cylinder)

Very excited to see what's solid on the car and what will still need work.

This photo shows a nice addition that Richard had done to the front: fender lips/diverters. Together with a splitter that has diffusers in the areas beneath the headlights, it adds both downforce and natural brake cooling. The diffusers act to pull up air up into the wheel housings on the inside of the wheels, where it gets extracted out through the wheel by the lower pressure that those fender lips generate. Downforce and cooling without dedicated brake ducting.

More photos and notes coming in about 3 weeks or so!

356971
 

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The diverters and internal ducting are cool (pun intended!)
Its amazing the difference that is made by little things.
I've often wondered if it was practical to run air ducting through the center tunnel (with the cooling tubes now moved to the sides of the vehicle) and routing the air towards the exhaust manifolds for forced air cooling (and an exit out the back to provide through-flow reduce drag, as well) or to an airbox to provide ram-air to the engine from the nose.

Again, my dreams intruding. Sorry.
 

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Discussion Starter #13
I like that idea Taz. I've seen naca ducts on the underside to pull up air - but thinking through what you just said, there's room at the sides of the radiator which I haven't allocated yet, had them closed off for potential future brake ducting if more turns out to be needed. However, with nothing in the way over the steering rack and though the tunnel it is possible to have two large diameter lightweight tubes from the grille all the way through the car that can come all the way back to the motor. The rule of thumb is still to evacuate more air than you're letting in, but not with a ratio that creates too much drag. Worth a thought and I'll do some assessment of temps this year. Thanks for sharing that idea!
 

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Discussion Starter #14
Oil coolers are hung up in front of the rear wheels, with pump for the trans cooler. Battery mount done, rear wing mount anchored to the chassis almost there, also supports and centers the body. Additional braces for the rear toe links set up to eliminate flex. Testing will tell how much if any rear bump steer adjustment will be needed. Only a couple of weeks to go!
 

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Discussion Starter #15
Should also note that for track duty, or spirited canyon carving , the camber shim kit from MyRaceShop.com is a huge benefit to lock down the suspension geometry.
 

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Discussion Starter #16 (Edited)
Can officially state that re-build has been successful. ?????? Insanely fun to drive.

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What was intended to be a gentle shakedown on track started as such, then ended up being a 60-lap fantastic track day, with the last 30 laps consisting of me completely hammering the car to its limits. Outcome;

Temps:
Zero overheating: Prototype Development Group external coolant pipe routing + rear mounted Moroso coolant reservoir + DeWitts radiator. Gen2 nose with unmodified grille and hood outlets. Using a Setrab 25-row oil cooler. Needle was pinned at ~183 deg F all day, 72 ambient. Long sessions were 25-30 min.

Brakes:
Zero brake issues: Stock C5 brakes with Ferodo DS2500 brake pads, Wilwood cable brake bias adjuster to get the correct balance, vented discs of unknown origin marked E1872. Very good feel, zero fade. Putting front brakes on the rear axle would improve pedal stiffness a little bit but would also add unnecessary weight.

Suspension:
Extremely enjoyable handling, responds superbly to weight shift by minute throttle changes to rotate or stabilize the car. Also the 3-way adjustable suspension setup was simply perfect, minor changes throughout the day to low speed compression polished corner entry and exit behaviors to be perfect. It just rolls a little bit more than ideal, but swaybars will be installed soon. I do need to get more front camber out of it - lower mount fab work or offset bushings will fix that. Drop link kit front, reinforced rear toe links and clocked rear spindles all have contributed to reduction of bumpsteer, no instability present granted the speeds were never over 130mph on this twisty track.

Aero:
Didn't install splitter or diffuser, both will get specific design modifications. The rear wing from APR does a great job. Started with no wicker due to time constraints and had a mildly loose rear at high speed with 0 angle of attack. Added wicker between sessions and it balanced out perfectly.

Motor:
I still haven't had the time to take a proper look yet but at over 50% throttle it seemed like it dropped 1-2 cylinders and ran on 6 or 7 cylinders. Power at 50% throttle was enough to keep it useful, and with 101 octane fuel replacing the original pump gas throughout the day, the behavior got better and the later in the day I could put in a bit more throttle and it would still run cleanly. I honestly could have checked plugs / gaps and plug wires at the track, but I was just too focused first on trying to put as much distance/time on the car as possible to find any robustness issues. The motor still pulls great from 5500 to the revlimiter. The limiter surprised me at 6100rpm, very low. Definitely need to upgrade pushrods and springs to open up for some more rpm. Then again, this car should get another 150-200hp within the next 12 months.

Tires:
Ran 305 front and 325 rear which is a great track setup, on 19" wheels.

Interior:
Sparco D-shaped alcantara/leather steering wheel and Sparco QRP-R seat were perfect.

All in all, a huge injection of fun. The GTM is an extremely fun track toy.

357282
 

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Discussion Starter #17
Oh and in the same timeframe CHP + DMV gave their blessing so it is now CA registered and road legal, although on a 3-month temporary tag until DMV HQ in Sacramento decides whether or not it is already smog exempt, or if it will need to see the BAR for an exemption certificate.
 

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Discussion Starter #20
Sounds like fun!! I do have a question on your hood. You say your running the "factory hood outlets". Most of the photos do not look like those are installed?....are you just running the cut-outs in the hood wide open or do you have the fiberglass vent inserts installed?...like this car has:

Great observation, to clarify I meant factory size cutouts, not enlarged. There are grilles instead of louvers. From aero simulations by a PDG member hood louvers significantly streamline the airflow compared to grilles or open holes, which helps the rear wing so I will be going to louvers.
 
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