im getting a dyno tune friday. my question is when i get my tune done since its hot out now, will the car start to surge again when it starts getting cold or will the tune be way off or close to what it was? I know its an on going struggle with tuning,and i read some posts in the search section that small changes in humidity,barometer,and temp are not a big deal but all put together can cause quite a change.

Thanks for any help

nathan

 

0 to 100% humidity change makes about 0.9% difference in resulting airmass. the other conditions you mention are much more important, basically airmass is directly proportional to pressure and inversly proportional to temperature in Kelvin. so increase in MAP from 90kpa to 100kpa change will give you 10% more power, and change from 330K to 300K in aircharge temperature will give you 10% more airmass too.

 

I am in the same boat. What you just said is way over my head, but I'm just a newb trying to learn. I understand the whole airmass thing (I think?) It's basically about oxygen content right? IDK. All I know is that my car was tuned in OK City, and I get it here to Alaska where we have perfect air, and the tune sucks. It surges like crazy, and falls on it's face off the line.

 

Red, if I may paraphrase so I may also understand more fully:
Changes in humidity have a minimal effect on the amount of air provided.
If the air pressure increases, you will get more air. An increase of 10% will yield 10% more power.
If the temperature decreases, you will get more air. A decrease of 30 degrees will yield 10% more air.

 

The air temperature is a big factor in the speed
density part of the tune and it's not just the real
IAT, but the false contributions from high underhood
temps and how the PCM tries to play off IAT and
ECT in figuring the "real" air mass. It's messy. Then
you have the IAT and ECT based fuel and spark
adders that mess with open loop fueling and the
whole spark map.

You can't single-point tune it and call it done if
you don't live somewhere like Hawaii where it's a
constant sort of weather.

What it needs is to be tuned straight up for some
"normal" condition, and then for you to adjust the
abnormal using only the adder tables.

There are other issues such as extreme exhaust
cooling that can bother headers cars in closed
loop. Mistrimming makes for surge and stumble off
idle, easily.