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Barometric Correction on a turbo engine

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So ive been doing datalogs on my way to "work" which is about an hour drive... the last 10 minutes of the run there is up hill and reaches a max elevation of around 250meters above sea level (with the rest of the trip basically at sea level / just above)

And was just wondering how much this would be effecting my tune and my logs in that last 10 minutes...

Once I move to Tasmania shortly the elevation changes a great deal compared to Perth.

So yeh basically just wondering at what point you need to worry about Baro correction and how much it does effect it? ie does your tune go roughly 0.5:1 afr richer for every 500meters? or is there alot of variables?

Is it as important in a turbo car? or maybe more important as the original pressure is multiplied...

If you have o2 control enabled would this correct for it anyway?

This is something I haven't had the opportunity to do much testing with (despite the mountains around our new location). With a turbo, the affect of changing baro can be negated to a point by the boost control system adjusting the wastegate to maintain a constant boost. While this might maintain a constant pressure in the inlet manifold, what it does is change the pressure ratio the turbo is working at and consequently this affects the compressor efficiency. on the other side of the turbo, turbine inlet pressure will also be affected - As the baro pressure drops, the turbo works harder to maintain the same manifold pressure and hence the turbine inlet pressure rises.

So the two net results as baro drops (altitude increases) are:

1. Turbocharger pressure ratio increases, which likely will result in a reduction in compressor efficiency.

2. Turbine inlet pressure will increase.

Ultimately we can expect the airflow through the engine to reduce somewhat then as the barometric pressure increases. This will have some affect on the AFR but the amount would depend on a variety of factors. Over an altitude change of 250 metres I'd be surprised if this resulted in a significant change though and yes, if you're using closed loop fuel control, this should account for any minor error in AFR you see.

Thanks Andre, yeh I didnt think about Compressor efficiency change if using closed loop.

So if you were using open loop you would just expect to see the boost drop which would put you in a different area of the map anyway which would help... I guess its similar when running open loop on a really cold morning the air is denser and boost is typically higher compared to a hot day.

Yes if you're running open loop control I'd expect to see the boost drop off slightly as baro drops. Unless you're seeing significant changes in altitude I'd be surprised if it's really significant though. Open loop should maintain a reasonably consistent pressure ratio across the compressor wheel but you still need to factor in the barometric pressure at the exhaust outlet will be dropping which will have some effect on the engine's ability to breathe.


Usually baro related maps are there to "protect" the turbo from becoming to ineficient or to prevent damage for over speed. We have a shop in mexico that is 8000 ft above sea level, tuning here is quite difficult in some cars. For example Mini´s 1.6 T engines have a baro boost limiter and torquer goes down after 4500 RPM, in some subarus depending on the barometric pressure it will decrease by a percentage the wgdc and boost targets.

When you do logs above sea level take into consideration a lower manifold absolute reading, sometimes at 250 meters boost will be the same as sea level since there is not that much of a difference, but in here we can get as much as 3 to 4 psi difference in boost at same wgdc. Turbo will always try to compensate for higher altitud because the PID will try to reach the desired boost by controling wgdc and other parameters, the baro tables will reduce that target depending on the oem tune.

250m shouldnt make much difference, there is a lot of information about pikes peak tuning on the internet its a good start if you dig deep enough you can find some data logs and maybe baro corrections they did .

P.S. audi s1 1980s is said to loose 200hp out of 800 on the top of pikes peak

yes, also you'll find an interview with a guy driving an EVO at the pikes peaks revival two years back or so. He also stated that they loose like half the power at the top.

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