Hey guys,
The reason I use 2nd or actually 3rd on 5-speed auto's, is due to a few reasons....
If you dyno your car in 4th gear then you'll find on a stock tune the speed limter setting at 92mph as it will hit it.
With the auto if you punch it 4th, or 3rd on a 4-speed, it can downshift or even worse upshift into overdrive if it reaches a preset limit. This will screw up the HP and TQ readings because it will free up the rear wheels for a split second and cause a huge jump on the dyno sheet.
Now you can get by the above by starting the dyno pull at around 3500 rpm's but when you are only going upto around 5200 that's just too high in my opinion.
I used to think the same way you guys do until I seen it done a few times. The dyno I use,
www.Walkerdynotune.com , has been in service since early 03' and has done it this way on stock to even mildly modified vehicles for along time now. The guy to talk to there is Blake and he can explain it in even further detail if anyone wants to talk to him about it. His email doesn't work anymore so If you have questions just call him from the number on the website.
Now, there are ways around this with tuning. What I like to do these days is to lock the vehicle in 4th gear with the tuning and raise the overdrive shift point to like 155mph. That way I know it is in 4th and I also know that it won't go into 5th if it reaches the set parameters. ALSO, I fully lock the converter from 1mph out and this way you dont get the funny spike at around 4800 rpm's on most automatic cars. This is from the converter quickly locking and unlocking, I have no idea why it does this but on the dyno you dont want it.
I've also attached a dyno sheet of two of my own personal car runs, I have an 05' Roush Automatic GT with the 5R55S transmission. It has 5 gears as well. The first run of the day was just a drive on and pull run and then I finished my tuning for the day and to ensure a proper comparison I always do an final stock dynopull to make sure nothing has went wrong while I was doing something to the vehicle. You can see the major MPH difference at the bottom of the screen, if I put it where it read RPM across the bottom of the screen it would put the runs over the top of each other and you would never know it was in two different gears.
Now I wouldn't go and try to make a pull in first gear but from my tuning experiences with over 100 cars/trucks/SUV's on the dyno now I've learned quite a bit and this is definatly one that helps.
Just thinking about it, I just did a Diesel 05' 6.0 auto on the dyno. This was at a dyno in North Carolina, not my local guy. The dyno operator would try to run it up in 4th and being that a diesel only goes to around 3000 rpm's total it would upshift on him everytime and the rear tires would go crazy on the rollers. Yes, it was even strapped down with 4" straps. I told him to try it in second and we made a full power pull without any problems from the tranny. Made 408rwhp and 580 rwtq with just the street tune
.
Thanks, Doug.