The other day I read a very interesting article in "American Iron" (motorcycle mag.) about dyno tuning by a guy that goes by Dr. Dyno.

in summary, the article pointed out numerous methods some can use to show gains simply by not establishing a proper baseline prior to the 'tuning' process.

one method he mentioned, that appears to be common practice, was the limited pulls process.

basically, he showed how one can make it appear that gains were obtained simply by making a limited number of pullls. he showed that unless one does enough pulls to 'establish a proper baseline' (when the vehicle in question reports the same power pull after pull) that subsequent pulls will more than likely show gains.

 

 

i tried to google for the article but so far i'm unable to find it, however, i did find this article that explains basically the same thing.

"Variations in Dyno Runs

The dyno can be a very important tool for the performance enthusiast. It has the ability to report what your car is putting to the ground over the whole RPM range. However, some people and manufacturers “twist” dyno results for their purposes. They can do this because there is quite a bit of variability from one dyno run to the next even with the same car in the same configuration and even taken just minutes apart.
....
For example, people usually do 2 to 3 dyno runs at a time. Take a look at the dyno runs of this BMW M3 in the Dynoperformance data base. There was a huge difference of 15 HP at 6250 RPM between its first and third run. I remember when I had gone to dyno an automatic 98 vette (LS1 power pack and corsa exhaust mods). I did three pulls with run1 = 285 RWHP, run2 = 297 RWHP, run3 = 301 RWHP. Why is there so much variation? The car needs to be warmed up correctly and running in its optimum configuration.
...
The three runs of that 98 vette did produce those numbers. It is technically not a lie to say that it pulls 285 RWHP, because during that run it did. However, let’s say I had done only one run with that car in stock configuration and got 275 RWHP after waiting for a few hours on that dyno day for my turn (so the car was cold). Then I drove away and put exhaust on and went back to the dyno and did a pull as soon as I got there with the car nice and warm. Now I pull 290 RWHP. Did I really gain 15 HP from the exhaust? No. But now you can see how easy it is for a manufacturer to make some huge claim for their product. Most people will not dyno their car enough times before and after a mod to remove this variation.
...."
http://www.dynoperformance.com/artic...?article_id=14

 

This is why... in my experience.... a car's second pull is usually the better and more accurate one. It is very true that a car is typically pulled on the rollers when cooled off below optimum (trans and rear diff too cool). In my dyno experience, run 3 is usually pretty close to run 2 so long as no changes are made. Beyond pull 3 or 4 heat soak starts and power starts to drop off. This is where it is great to let things cool for 30 minutes to an hour and go at it again for another 2-3 pulls.

Letting a car idle on the rollers for a long time isn't helpful either. That definitely lets you get well into heatsoak for the "before" modding dyno pulls. I like to jump on the dyno, do a nice easy run through the gears up to 4th (assuming a manual car) and then nail it. Then immediately do a second one as soon as the rollers are slowed back down. Then do run 3 just as soon as the rollers are slowed down again. That seems to work for me personally.

That's why a real dyno tune is so hard to do. you're always stopping and starting to do it well. No more than 3-4 runs in a set and I'd want to cool back down again. How can a person host a dyno tune day and do 8-10 cars properly in a business day? I don't think you can dyno tune more than 2-3 cars in a day.