The GTM Supercar Build Log

Archive for August, 2010

Tuning Progress, Driving Report

Monday, August 30th, 2010

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.

Quick Vid of the GTM on the Road

Saturday, August 28th, 2010

The Stealth GTM Meets its Brother

Thursday, August 26th, 2010

By sheer coincidence, a friend’s father-in-law acquired a Factory Five ‘33 that was still in need of body work. Said friend completed the hot rod today and since he is shipping it out of state to his father-in-law tomorrow  it was a good day to stop out at his shop and got some pics of the ‘33 with the GTM side-by-side.

Also, I am officially starting a tally of people that think the GTM is a Lamborghini. The tally is at 4.

Vehicle Documents Storage, some Tuning

Thursday, August 26th, 2010

When I sat in the GTM to take it for its first public road test I realized I needed a place to store registration and insurance papers. I just stuffed them in my pocket then, but today I came up with a quick solution. I used a plastic document sleeve, and adhesive backed velcro tape. Now I have a little storage sleeve that velcros behind the passenger dash pod that is within easy reach and can just be ‘ripped’ out, should the need arise.

I also ran the GTM at idle for a half hour to build a desired idle airflow histogram. These values that were produced were then plugged into the RAF  table (base running airflow). The more precise these values are, the smoother the engine will slip into idle. Hopefully this will fix any stalling issues. Until now I just guessed at what the RAF values should be. I was close, but ended up 2 grams/sec too high on the warm temp idle airflow values, so hopefully this will help things quite a bit. I will test soon.

Warm-Start Fixed, Driving Ensues.

Wednesday, August 25th, 2010

Last night I was plodding through my log data for the drives and failed warm-starts I made this past weekend, looking for clues or anything that would help shed some light on the problem at hand, and I found something peculiar. Every failed start, and every warm-start that idled a bit then died, all failed with an IAT of -38. Some quick research shows that the LS computer seeing an IAT of -38 means it thinks the sensor is disconnected/unplugged. The sensor was obviously still in place, so I knew I had a potential problem. In the past, the -38 had shown up during idle tuning, but I assumed it was a software glitch as it only showed up for a few frames here and there. The computer thinking that its -38 out when it’s really 95 poses a problem.

I first tested the IAT sensor by using a multimeter and a heat gun. To test the sensor you just need to measure resistance by putting the multimeter on the 2 prongs on the sensor pins, and heat it with a heatgun. An ambient temperature of 80-85ish will yield a resistance of 2.15 or so. As the sensor gets hotter the resistance drops. I was looking for that ‘-38′ glitch to occur (ie: random jumps in resistance measurements). It never did. My sensor was working flawlessly. Next, I moved on to hooking up the laptop and scanner and monitoring the IAT while I wiggled wires around. I was not confident at all that this would yield anything as the problems only occurred with a warm engine, but…you never know. Within 20 seconds I found the wiring fault that was causing my IAT readings to bottom-out. The IAT sensor signal wire from the engine harness was damaged in the plug that connects to the MAF/IAT sensor harness. I was able to duplicate the -38 reading at will, so I went to the local Chevy dealer and got a new pigtail ordered. In the meantime, being impatient and having hoped to get the car out for its first ‘public appearance’, I snipped the IAT sensor, surgically removed the bad wire segment from the connector pin, soldered a new wire on, put the connector plug back together, spliced it in. Viola – 100% working IAT sensor. This means it’s time to drive and see if anything has changed.

My first drive on trafficked roads took me to my first stop light, introducing the GTM to a busy intersection and testing my nerves hoping it wouldn’t stall out of its idle with 10 cars around it. All was fine, though, and I made it a few miles down the highway to the gas station for the GTM’s first gas station fill-up.  I remember others stating how slowly you have to fill up the tanks, so I took 10 minutes to add 6 gallons of fuel, and explain to another customer why the GTM appeared to not have a steering wheel. Most of the Kwik Trip work staff had vacated the building at this point and were all standing 20 feet away watching and grinning. Wow, really? Perfect time to see if I fixed the warm-start issue by repairing the IAT wiring. No pressure. The GTM fired up, I exhaled, and got out of there. Another stop light  and I hit the highway, then some back roads, which brought me back around and eventually home. Everything ran well, temps stayed below 200, stability was great at speed. Bump steer could use some tweaking, and I learned quickly how sensitive the manual steering is on this beast. I was pleased to find that Slayer could still overpower the 408ci LS2’s drone, via the Blaupunkt thin series components. Initially I reported I hated the rear view mirror; it’s not so bad now that I’m used to the vantage point of the driver’s seat, and it still has its uses, even if I can only see the lower half of the bumper of any vehicle right behind me.

It was my father-in-law’s birthday today, so I invited him out on the second drive, now that I was somewhat confident handling the GTM. The GTM started right up, so it seems the warm-start issue was nailed. 20 miles later after visiting my favorite WOT playground and slow city driving we returned safely. Some choice descriptions of the GTM experience from my father-in-law included ‘that scared the shit out of me’ , ‘that thing is terrifying’, and ‘my legs are still shaking’. His constant grin was the best feedback, though. Good times, and I can say the GTM gets to triple digits awfully fast.

Now the only thing I am really concerned with fixing is the threat of stalling after quick stoppages.  If I stop the GTM quickly the engine comes awfully close to stalling out. Not a huge issue, but it’s not something I want to be worried about every time I come to a stop anywhere – especially in traffic.  I am also researching cameras for shooting in-car video, so hopefully I will get some decent drives to post soon.

More Driving, Speedo Calibrated

Sunday, August 22nd, 2010

Today I put the stock FFR supplied air filter on, cleaned up some wire loom that had melted from the heat yesterday, and then installed a modified tune. This latest tune is quite a bit better than my previous tune I used yesterday. I then took the GTM out for a few loops around the block to get it warmed up. I then got a little braver and drove 4-5 miles on some side roads by my house – this thing handles incredibly well. It will take some time to get used to the braking – you really have to get on the brakes for them to clamp down, but once they grab…they grab. The tiny turn radius still blows me away.

I shut the GTM down for a half-hour or so, as it had been running quite a while by this point. When I went to restart it didn’t want to drop into idle and kept stalling. Similar to yesterday’s warm start problems with the hot intake.  I got the car started with some throttle work and backed the car out again and ran the previous back-road loop, stopped once for a stop sign, came back home and pulled it back in without any problems. Seems it’s just the start when the temps are warm that gives it trouble.

A little bit later I took out another vehicle and mapped a 2 mile length of road. I’m glad I live in a neighborhood with some good low-traffic roads to do this testing and calibrating on. Calibration is very easy, however. I started the GTM up (still finicky being warm) while holding the speedo trip/reset button until the needle swept to its max, let go, pulled out of the driveway, hit the trip button to start the calibration (the needle drops to half and a tire revolution counter appears on the speedo), and then hit the road. When I hit the 2 mile ‘marker’ I mentally logged I stopped and hit the trip/reset button again. Calibration complete. Much relief ensued after seeing the speedo work properly, matching the computer readout on my laptop perfectly.


First Drive!

Saturday, August 21st, 2010

Finally, after all this time I was able to get the finished GTM on the road for a bit and have a bit of fun.

I got the diffuser mounted again and took the GTM out for the first drive today. I live on a dead end road that has several large cul de sacs 1/4mile down at the dead end so I made 4-5 runs from my driveway to the cul de sacs and back. First impression is GO-KART. I feel like I am driving a large go-kart, plain and simple. The handling feels great, the turning radius is TINY (awesome), and with the 408 it has gobs of go. I really like the feel of the manual steering rack. The G50-52 gearbox is super smooth. I only got on the throttle a little bit a few times and I can tell this car is going to be scary fast. I was concerned with how hard it would be to drive with the limited sightline over the hood, but once you are on the road it all feels ‘right’. The rear view mirror is probably going to be deep-sixed – it blocks too much of the view, imo. My in-dash display can utilize the backup camera as a full time rear view, and it works amazingly well. The rear view mirror lets you see the engine and a few clouds…bout it. I think I will look for a smal led dome light solution I can mount there instead since the wiring is already present.

After my last run around the block I let it idle for a while and then when I went to make another run it started stalling and being hard to start. I pulled it back into the garage and noticed the silicone elbows on my CAI had, from the heat, warmed up quite a bit and sagged onto the exhaust. The elbows are high temperature silicone so they didn’t melt through, but the intake tubing was very, very hot to to the touch. I am guessing that the air just got too hot to keep the car running smoothly as the car had been sitting at 195degrees’ish for 10 minutes at least before it started acting up. I will have to fab up a heat shield to fix that, until then I will throw on the stock FFR air filter. The tuning needs a bit of driveability work on the low end I think, but then again it just might be the large cam that is making the car surge at really low rpms (like when doing a U-turn in the cul de sac).

The feeling of ‘completion’ is immense, though, and makes all of the battles and frustrations worth it over the entire build. I will play with the car some more going around the block with the stock air filter to see if the ‘warmed-up’ drivability issues repeat themselves. I didn’t get any pics or vid today – much too excited for that, but next time I will get something to post up!

Inspection Day!

Friday, August 20th, 2010

The day finally arrived to have the GTM inspected.  After being anxious and nervous it was all for nothing – the inspection took 5 minutes and all the inspector wanted to see was the serial # on the frame, and that it matched my title app serial, etc. Thanks to Josh for hauling the GTM. Here are a few pics in actual day light for the first time since completion.

Weather permitting, driving will commence once the diffuser is reinstalled (just have to rivet the bottom back in place).

New Steering Wheel Installed

Thursday, August 19th, 2010

Got the new steering wheel today so I installed that.  The outer portion of the quick disconnect bolts to the back of the steering wheel, horn is connected to the quick disconnect wiring and pushed/inserted into the quick disconnect piece. I had some troubles getting the quick disconnect to work at first – I could not lock the wheel into place. I realized that one of the bolts that holds the steering wheel onto the disconnect piece was rubbing on the inside of the main disconnect hub so I had to grind that down a bit with a file.  Now it locks/unlocks as it should.

This setup is very nice. The steering wheel can be removed or connected in seconds, and getting in and out of the car is much, much easier. This steering wheel allows for a great view of the gauges, as it follows the curvature of the dash nearly perfectly.

Steering Wheel Quick Release Installed

Tuesday, August 17th, 2010

As I await for my inspection date to arrive I decided to upgrade the steering wheel with a quick release hub from NRG. I am using their SRK-400B ‘thin’ model. The quick release unit consists of two pieces – one bolts to the FFR supplied steering column hub, and the other bolts to the back of the steering wheel. Horn functionality is retained without having to disconnect anything when removing the steering wheel. I installed the column-side portion of the quick release tonight, and will install the new steering wheel when that arrives later this week. Pretty self-explanatory stuff. Not having the steering wheel in the way makes getting in and out much easier.