Last Update: November 14, 2002
Ever wanted to know what kind of power your car puts out after a change, but don't want to pony up the $75-100 for each spin on the dyno? Not sure if your ram air intake is doing any good? Want to learn your 60', 330', 1/8 mile, and 1/4 mile trap times without spending all night waiting in line at the drag strip to make one run?
If you answered yes to any of the above, I have got one hell of a toy for you.
Enter the G-Tech Pro Competition. This little box weighs way less than a cell phone and can provide a wealth of performance data on your car. The G-Tech uses a set of precision accelerometers, electrical noise from the ignition system, and the curb weight of your car to give you a drag strip and dyno in a box.
Using only the internal accelerometers, the G-Tech can accurately hand out anything you want to know about a 1/4 mile. Starting from a dead stop and taking readings at 0.008 second intervals it constantly determines your forward acceleration. Knowing forward acceleration and time makes calculating both current speed and distance easy. Handing out trap times is of course just as trivial.
| Add RPMs, measured via the cigarette lighter power connection, and the vehicle weight (including gas and driver, I got mine at a local truck stop, see right) and you can calculate effective RWHP and Torque. The RWHP and Torque calculations are in many ways more meaningful measured this way because benefits of ram air induction, rolling resistance and air drag on the vehicle all get included in the results. |
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In addition to the basic functionality the G-Tech will give you a bright red strobing shift light at a user selectable RPM and all of the data from each 1/4 mile run is displayed graphically after completion. All trap times, as well as curves plotting MPH against time, and HP/Torque against RPM are also presented for review. Even better, you can download all the data from a run via the serial interface in a format suitable for use in a spread sheet.
Ok, but how accurate is it? The manufacturer claims 5/100 of a second on trap times and 5/1000 of a second repeatability on the same. Horse Power and Torque calculations are reputed to be accurate within 3% and repeatable to within 0.5%.
Well, I don't have trap times to compare against, but I do have real numbers off a Dynojet chassis dyno. I took the car out and made a few runs for comparison.
The three passes were along a level stretch of relatively remote county highway where I didn't think I was likely to get a ticket or cause a traffic incident. All three runs are plotted in second gear since I can't get all the way through 3rd gear inside a 1/4 mile at the moment (New rear end with 3.55 or 3.73 gearing may be in order). With those disclaimers, I managed to get data for three passes recorded and downloaded to a laptop for further study.
Since the instructions for the G-Tech say to average the results over several runs, I did that using a speadsheet. It would seem to be difficult at first inspection, but the G-Tech only records discrete RPM levels so you can match up the columns just fine with a little patience. The G-Tech says about 205-210 HP at the top of 2nd gear. Horse power is calculated as HP = A(G) x V(ft/s) x M(lbw) / 550(ft-lb/s-HP). Where 'A' is in gravities, 'V' is feet per second, and 'M' is pounds weight measured at one gravity. The units are a little muddled due to pounds mass and pounds weight being perhaps the most confusing thing known to man, but trust me. Torque is calculated as Torque=HP X 5252 / RPMs.
How does it compare to the chassis dyno? Well, the numbers track pretty well. The G-Tech values are about 7-8% low due to air drag and rolling resistance. I corrected them and plotted the curves along side the data from the DynoJet chassis test. The combined curves are below.
Mote: The absolute power numbers from this test were taken with the car performing somewhat badly. The ECC performance chip was programmed poorly and things were not right. The comparison is still valid since both the G-Tech testing and the DynoJet tests were recorded under uniformly bad conditions on the same day.
So, my overall impression? An excellent product that delivers what it promises. Accurate performance data in a relatively inexpensive package. If you are looking to tweak your car, I think you could reliably detect changes of as little of 2 horsepower at the tires. In my data the average deviation between the runs was a mere 0.90 HP and 1.05 ft-lbs of torque. This error is slightly more than the 0.5% repeatability advertized, but it's quite good.
Be prepared to spend a few hours with a spread sheet pasting numbers around, and find a nice off highway drag strip to test on. Making good measurements will require at least 3 passes averaged together. 5-6 would be better. More than that will probably be just overkill and make analysing the data too much work. You could do it on public roads, but sooner or later you'll get a ticket or hurt someone, so please, be safe with your lead foot.
