The GTM Supercar Build Log

Tuning Progress, Driving Report

Another weekend of tuning in the books.  After getting a handful of public road tests under my belt last week I spent Thursday and Friday night researching how to fix the main thorn in the GTM’s side (and my driving confidence) – stalling during quick stops, and in stop-and-go traffic.  How cool is an exotic supercar that stalls in traffic? During my research I was encouraged by several tuners on the HP Tuners forums with words of advice basically stating that “your cam is a race car cam. It is way too big for reliable drivability on the road. Stalling will always be an issue.”  Maybe this is also why several tuners I contacted via email never responded when I sent them the GTM’s engine specs. Nonetheless, I took this as motivation to get this beast on the road, and in a drivable, safe fashion. Also, fortunately, I met a very encouraging individual on the LS1tech forums that has greatly assisted my research, and has helped reduce the steep tuning learning curve for me. Without getting into boring tuning details, after about 30 VCM flashes, and lots of  middle fingers pointed at the steering wheel after stalling, I had a GTM that I could not force to stall regardless of how hard I tried. Fast stops from 35 to 0, blipping the throttle while backing out of the garage (previously a guaranteed staller), etc. They all passed the test.

With the stalling issues hopefully taken care of, I took the GTM out for the final test – getting on the main highway that passes through the heart of my small town, offering a potential 5 stop lights (after having to pass through 3 stop signs in traffic to get to the on ramp), and then traversing one of my favorite back roads, which leads to a low speed road with more stop signs and more stop lights on the way home. A total round trip of around 20 miles, in 95 degree heat. This is a good test. I fire up the engine, queue up some Slayer, and hit the road. Getting to the highway is uneventful. That’s a good thing – no stalls.  Flying onto the on-ramp, cutting through traffic (the extreme wide-angle backup camera/rear view helps with this quite a bit), I get lucky at the first stop light, then hit a couple in the middle of town. Again, nothing to report. Sitting in traffic, the GTM behaves very well now, that is, if you ignore the lopey cam that sounds like Godzilla laughing at the traffic around you. Sometimes the computer yo-yo’s looking for the perfect idle (minor!). People have asked if others in traffic are gawking at the GTM – I really can’t tell when at a stop light. The car is so low I can only see bumpers and door handles. An occasional chin appears at the top of my sight-line. In motion, though, passengers in cars ahead are usually turned around gawking. Yes I see you with your cell phone camera, and that flash you left on just ruined your picture as it bounced off your rear window. Photography 101. Now and then a vehicle I just passed will speed up to get another look, then drop back again – or they will get ahead and accidentally veer over as they are looking in their side mirror and not at the road. This all happens in the 10 minute drive through town, so its a bit overwhelming. Out of town, I am able to open it up a bit and get away from traffic. Lots of motorcycles out – a handful of them do the signature ‘down low’ motorcyclist wave at me. I guess I am in that special club now. Turning off onto my favorite WOT back road, I find it pleasantly devoid of traffic. I blast through 2nd and set into a cruise, then decide to slowly wind out 3rd to 85 and put the hammer down for a few seconds.  Acceleration from 85+ feels like first gear in my supercharged Z06. Violent. Coolant temps are still reporting at 210. Not bad considering the scorcher of a day. I wind the motor out a few more times, nothing too severe, just getting the carbon out. At the end of the back road I see one car in front of me at the red stop light. A local cop. Cringing, despite actually going under the speed limit at this point, I pull in behind him. He waves at me in his rear view, then puts on his lights and proceeds to pull over a minivan that just blew through the intersection in front of us. Good timing, Mr. Van. At this point I notice the gearbox is getting a bit ‘sludgy’ – as in hard to shift. I’m not surprised, though, I left the trans oil cooler off for a reason to see if it would get hot on this drive.  I cruise home, get lucky at one light, handle the remaining stops fine, and pull the GTM into the garage. I leave it idling and put the fuse back into the trans oil cooler pump so that it fires up. After running a few minutes the gear box seems a bit softer (cooler). The oil pump and cooler should keep it adequately cool when I actually have it running during driving. Coolant temps have dropped to 200 by this point. A very successful test drive.

At this point I have 68 miles on the odo and am able to offer a bit of feedback now as to how it drives, handles, feels, smells, etc. I feel very confident in the cooling system right now – running at 210 in 95 degree heat, while flogging it on the back roads at speed is pretty remarkable to me with this monster motor lurking over my shoulder. Actually seeing it cool down during stops and idling is something I am not used to. The Z06, if I were ballsy enough to take it out in this weather, would idle in traffic at 235+ and throw a fit. It does not like idling in traffic at all even in cool temps.

The handling as it is, is very nice. The manual steering is very precise and it gives so much feedback – love it. There is a bit of body roll, and the bump steer (as I’ve mentioned previously) still needs to be tweaked, but it’s not so bad – it just keeps you alert.  At 120 the car gets a bit jittery. It certainly is not a Corvette at this point at speed. Once the bump steer is corrected that should greatly alleviate some of the high speed jitters and I will report back. I also plan on stiffening up the spring rates, lifting the rear end a bit more, and possibly getting stiffer shocks. The GTM as it is, rides very smooth. While that is nice for most, I like the jarring ‘feel the road with your ass’ sensation that a race-prepped suspension gives. I want (need) to figure out how to implement sway bars. For the casual driver, and especially for the driver who is not used to high-performance vehicles, the handing ‘out of the box’ will be awe inspiring. It really is incredible and only needs some slight fine-tuning for those that want/demand a bit more.

Comfort is a surprising point of review. While the interior of the car on a 95 day with no AC (I have a leak somewhere so I haven’t had it charged yet – winter project) can be warm, the GTM is not excessively hot. I was never much of an AC user anyways – I always prefer open windows. The Dynamat and heat barrier I applied to all of the panels separating the cabin from the engine bay do a great job. The engine cover, which is inches from the motor, gets warm, but not hot or uncomfortable. The tunnel does not get hot, either. Being 5’10, 245, getting into the GTM can be a tight fit, but the seat fits me well, its comfortable, and I like how ergonomic everything is.  Wind barely travels into the cabin of the car at speed. Due to the insanely angled windshield I have never wanted or even thought about needing sun visors. Another aspect of the GTM’s comfort – noise levels – can be taken as a pro or con. This thing is loud, even with all the sound proofing. In 1st through 4th Godzilla lets me know what it is up to back there. 5th gear, though, is pretty quiet and at cruising, I can listen to the radio without blasting it.  Conversation can be had. One very positive aspect of the noise levels, though, is that road noise is very minimal. You can hear gaps in pavement and road noise a bit louder than you can in a production car (then again, I am on slicks, which are loud to begin with), but I do not hear rocks and dirt ticking off of the wheel wells or underside of the car. You hear things that you need to hear, and there really isn’t anything that annoys me at this point.

The interior smells good. Not joking, one thing I was worried about was smelling oil, gas, and other ‘industrial’ odors from the engine sitting right behind me. I run the fuel tank vent hosing to the back of the car, and I have air filters on the trans oil overflow tank and on the engine itself. I can look in my rear view and see steam billowing out of the air filter mounted on the oil fill cap on the motor, but I never smell that stuff.

If you drive past the tennis courts while the high school girl’s team is practicing returning volley shots, they will all simultaneously turn and look at you and you will see tennis balls fly past them into the fencing.

Getting back to tuning, another thing that I addressed was the startup procedure. Previously to get it running I had to dump loads of air into the  motor to get it to start, and then I had to VERY slowly decay it out to keep it running and to set into idle. I figured out how to determine the running air flow requirements for the motor using my VCM scanner this week, and was able to dial in the running air flow tables precisely. As  a result I was able to re-tune the startup procedure so that it fires right up and decays out startup airflow instantly. Why is this a point of concern? With all that extra startup air, if I were to start driving before  it decayed out (which takes around 40 seconds), throttle response would be very erratic, and often caused stalls. Now I can just jump in and go. Very convenient.

If there are any aspects of the ‘driving experience’ that I left out and you readers want to see reviewed or talked about, post a comment or message me.

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