High Altitude Boost Retard

co1simba · Post by **co1simba** » Thu Mar 05, 2009 5:48 pm

Hey guys I got a question about high altitude stuff again. eCtune has the feature of retard ign by 1.25*/lb of boost. Since I am higher up, the MAP sensor will read -5 psi when the car is off. So, eCtune wont start retarding boost until the system has built up 5 psi then it will think it is going into boost. What should I do? Thanks co1simba

greasemonkee · Post by **greasemonkee** » Fri Mar 06, 2009 2:18 am

You're 10,000'? Do you have oxygen in your car? Think in terms of absolute pressure. In other words, ambient pressure is 14.7 psia. As far as plenum pressures and map references are concerned it wouldn't matter if you were at 50,000 foot. 5 psig on the tables would still be 19.7 psi absolute in the plenum. The efficiency of the system as a whole is a different story. I'm unsure exactly how much ignition requirements will deviate with those altitudes.

hackish · Post by **hackish** » Fri Mar 06, 2009 12:11 pm

I go to 10k very regularly and the oxygen up there is just fine. Also go to 13,500 and the oxygen up there is OK as well. Last year I went to 16,500 and when I was getting ready to jump I could feel a little hypoxia starting to set in... The boost retard you're thinking of is only a math calculation applied to the ignition tables when you initially set the up. I suspect you won't really need to adjust the timing since you're using the turbo to compress the air and the MAP sensor will be reading the absolute pressure of the manifold it will just act like the turbo is a little less efficient. I know with most turbocharged piston aircraft they are only turbocharged to the sea level pressure to maintain engine output as you climb. I know our 280hp cessna makes much less power at 10k. -Michael

elevation · Post by **elevation** » Fri Mar 06, 2009 12:37 pm

Ignition timing at altitude is a true science! consider this your compresson ratio is differant atmopheric pressure is differant o2 content per cubic foot is differant If youve never done it I got one word DYNO!

co1simba · Post by **co1simba** » Fri Mar 06, 2009 3:15 pm

No, I am asking about when eCtune will start to retard ign. Is it going to retard at -5 psi when it thinks its at correct \"0 atmosphere\" or is it not going to start to correct until it gets to 0 psi atmosphere, therefore already having the system at 5 psi boost.

HiProfile · Post by **HiProfile** » Fri Mar 06, 2009 5:19 pm

FYI Denver's avg pressure is ~830mbar, meaning -3.9psi. It matters only because you'll have less exhaust backpressure than \"us down here\". Your 5psi of boost may only be ~1psi to us, but your turbo is actually working almost as efficiently because less backpressure = better scavenging = more air into the cylinder per absolute PSI. That said, you should actually have your BASE timing map adjusted for that already. Use the variable step retard function of ectune, and/or MANUALLY change the 2 or 3 columns you are so very concerned about. It takes all of 10 seconds to modify an entire column, which you'll do 2-3 times. Select 'modify selection', then input the correction based on the very simple math. Then you can do a regular step retard, or use the variable step retard like I do. A little extra timing at low boost, then 'regular' timing afterwards.

co1simba · Post by **co1simba** » Fri Mar 06, 2009 6:26 pm

Ok maybe my wording is off. I'm not trying to ask about tuning ign at higher altitudes, I'm asking about how eCtune's code is written. IF my baro sensor reads 830 mbar for example. Is eCtune going to take everything higher than 830 mbar to be boost, or is it still going to take everything higher than 1000 to be boost.

elevation · Post by **elevation** » Fri Mar 06, 2009 6:45 pm

pressure is pressure, the baro is going to read how much pressure is on it same with the map.Try to think in mbar.