Need help with ľ mile consistency [Archive] - Forum


View Full Version : Need help with ľ mile consistency

03-14-2004, 08:36 PM
I have been having some serious consistency problems with my current car during drag racing and it has been causing me to loose a lot of rounds of eliminations and I really need some help in figuring this problem out.

For the past year I have been drag racing in the sportsman, sport compact and street classes with a 2003 Pontiac Grand AM SC/T. The car is bone stock, except for a K&N air filter, thatís it. The car is equipped with the 3.4l V6 engine and automatic transmission. For 2 or sometimes 3 runs in a row the car will be fairly consistent, usually within about 5 hundredths of a second in ľ mile ETís, and to me that in itself is quite acceptable for a car like this. And then all of the sudden without warning it will go 2 or 3 full tenths of a second faster or slower than what it had been running, and then on the next run again after that it will revert back to very closely what it had been running consistently before that happened, but then you never know when itís going to happen again and if it will be faster or slower when it does as it goes both ways unpredictably. If it happens during time trials you dodged a bullet but you have no idea what to dial-in at because you donít know ďwhich carĒ is going to show up next time you come to the line with the inconsistency in the ETís. If it happens during eliminations you will either be in danger of braking-out by quite a bit or not even being able to run anywhere near your dial-in time and all you can hope for is that your competition made a bigger mistake somehow, like red lighting, and thatís not likely to happen very often, I know.

I try to always keep the water temperature as exactly the same as possible at the start of each run, on the factory temperature gauge. I do exactly the same burnout procedure every time. I stage the car exactly the same way every time, as shallow as possible. I always leave off idle, from brake peddle to full gas, so that eliminates the launch rpm variable. Traction is not an issue as my 60 foot times are always almost identical, often identical to the hundredth of a second, or usually at least within 1 or 2 hundredths or each other, and I always use the same tire pressure. I normally leave the transmission in ď3Ē all the time and let it shift itself, but this problem has happened on test days where I have shifted the transmission manually at the same rpm all day as well, so itís not the transmission. Itís happened on cold days and on hot days, as well as cool and warm days, on days where the weather and temperature and barometer and adjusted altitude donít change between runs, on sunny, on cloudy, on calm and on windy days, so itís not the weather. I have tried days when I would try to cool the radiator and intake by spraying water on it between runs and days where Iíve never even lifted the hood between runs, and everything in between, with no differences. I have spent many days at the track testing trying to figure this out. I always leave the same amount of time between runs on test days, usually 45 minutes, to try an eliminate the amount of time between runs being a factor. I have tried using premium fuel and regular fuel with absolutely no differences.

I have been bracket racing for about 15 years, up until last year I raced a 1974 Oldsmobile Omega that was also basically bone stock with a 350 Oldsmobile V8 and a 4bbl carburetor, and obviously no factory computer or electronics, and the only thing that would affect itís ET was the changes in the weather, and if there was ever a very drastic change in operating temperatures or track conditions, but at least then it was all very predictable.

Iím sure Iím not the only person to have tried to bracket race with a stock, computer controlled, electronic fuel injected vehicle, so some one out there must have encountered this type of thing before. So whatís going on here? What goes on inside these computer controlled engines to cause these inconsistencies? Are these things hypersensitive to very minor ambient changes or perhaps engine temperatures? Is there something that can be done to the carís computer somehow to help? What can be done to prevent this problem from occurring? As specific as possible and proven tried methods to eliminate this problem would be much preferred, but any and all help will be much appreciated. Thank you.

03-15-2004, 10:34 AM
Perhaps Knock Retard is playing a factor and that is affecting your runs slightly. Most computer controlled cars, including our GA's, have a knock sensor, which tells the computer if there is any spark knock. If there is some knock, the computer will pull back the timing. This could account for some of the inconsistence, some runs may be experiencing more knock than others.

There are other possiblilties too, what you should look into is an autotap, which can tell you what the computer is doing, so you can maybe isolate the issue that is causing problems with consistency.

03-15-2004, 10:54 AM
There has never been any noticeable audible knock condition and I've tried all different grades of pump fuel, but then the sensors are more sensitive than your ears. What do you need to do an autotap? I feel the car's computer must be compensating for some condition that a sensor is picking up somewhere to cause these inconsistent ET's, but how do you know, and what can you do about it? Thanks for the help, keep it coming.

03-15-2004, 11:08 AM
my guess would be is that you are leaving off of Idle instead of loading the convertor and leaving at the stall limit on the convertor

03-15-2004, 11:30 AM
Whether leaving off of idle or not the procedure is identical all the time and the 60 foot times are very consistent, and that hasn't been a factor with the 2 or 3 runs that you would get in a row that do produce consistent ET's, but then you never know when it'll do this. I have the original stock Goodyear tires and I found I have to do the same burnout procedure every time to prevent any wheel slip off the start line to eliminate that inconsistency, and that way they always hook up very good, because without warming them up they just wouldn't grab. I always turn the
traction control off before every run, and I almost always have very good consistent 60 foot times when the top end ET times are all of the sudden way out, but loading the convertor is an idea I'm willing to try out for sure, anything to help solve this problem.

03-15-2004, 12:32 PM
I was also going to suggest leaving the line with a loaded stall, but you said that your 60 foot times are extremely consistent. Then I was going to mention that you might dry manually shifting since the computer isn't always 100% consistent in it's shift points on a stock car. But, you said you get the same results with manually shifting. I'm guessing it is definitely sensor related. It sounds like it's running a different a.f. ratio for different runs which doesn't make sense if the conditions are all the same. I'm wondering if it's possible that the car is sensing a huge loss in fuel economy due to the previous run and it starts tinkering with the a.f. ratio to try to correct it. It might be thinking that it's having a fault or a foul out and wind up trying to correct itself. I don't know how you could test for this other than tapping into your exhaust with an at home air/fuel tester. They're making them alot cheaper now and it might be a reliable way to track that down. A brand new one by Innovate Motorsports just came out last year and it is supposed to be the most accurate and readable for the do-it-yourself price range. They go for around $350. It'll save readings at w.o.t. and you can compare them between runs.

Just an idea. Hope you get it worked out.