Altitude Driving

[Stixbnr32]

I was driving my R32 GTR at altitude on the weekend and it ran like a dog up there. Really unresponsive and laggy and when at 5-10km/h it was jerky pretty much a dog.

Once I came out of altitude (3000+ feet) it started to drive fine again.

I know the air is thinner up there but i wasnt expecting such a change. I have had the car up higher before but that was with AFM's and standard computer.

Any ideas?

[blind_elk]

I have had the car up higher before but that was with AFM's and standard computer.

Any ideas?

I think you answered your own question. But until you tell us exactly what is different between now and previously, it would be difficult to speculate on the causes.

[Stixbnr32]

Well not really. Before it was modified it was fine in altitude. Now the AFM's are gone and there is an aftermarket ECU it was not running fine.

So there are alot of changes since then.

[zebra]

probably not tuned for low MAP high tps.

also 5-6% power loss per 1000ft altitude as a rule of thumb

[GTSBoy]

Yuh, people say that pretty much all ECUs are as good as each other, but there is one area where some ECUs don't handle their shit at all well, and that is altitude compensation.

When you get rid of AFMs and rely on MAP sensors, the ABSOLUTE pressure in the inlet manifold is used as the load measurement (coupled with revs of course).

If it is tuned at sea level (101.325 kPa atmospheric pressure) then the pressure drop driving air across the throttle body = 101.325 - MAP. If MAP is 50 kPA, then that driving pressure is 51.325 kPa forcing air across the throttlebody.

But if that same car is taken to 1000m elevation, the atmospheric pressure is only about 90 kPa. For the same MAP of 50 kPa, the driving force across the throttle body is only 90 - 50 = 40. Ie, you only have 80% of the true pressure you used to have, and only get about the square root of that (back to ~90%) as a resulting air flow. Then you factor in the lower density of the air (another 90%) and you lose a little bit more. End result....for the same load signal seen by the ECU, you're only getting ~90% of the air flow you had when it was tuned. Yet the ECU will still shove in 100% of the fuel.

Decent MAP based ECUs need to have a good atmospheric pressure compensation built in to account for this.

Edited April 28, 2015 by GTSBoy

[Stixbnr32]

Thanks for that - tuner finally came back to me and has said the same thing. He said can install a good atmospheric pressure sensor but if you do not go across that road (desert road north island NZ) than not really required. He has seen it before on customers cars and his car also.

Thanks for the help

[GTRPSI]

There are a few people running at the Snowy Mountains 1000 in NSW which is very high altitude, first time they went there the cars ran crap, second time they brought their tuners with them and problems were sorted by pulling fuel out.

Not something most would be thinking about until they went through it......

[Stixbnr32]

Yeah it was a worry when I was driving because its a desert and the middle of no where haha. Thought something was wrong until I went back down.

Was a slow laggy piece of rubbish up there.

[Lithium]

Yuh, people say that pretty much all ECUs are as good as each other, but there is one area where some ECUs don't handle their shit at all well, and that is altitude compensation.

When you get rid of AFMs and rely on MAP sensors, the ABSOLUTE pressure in the inlet manifold is used as the load measurement (coupled with revs of course).

If it is tuned at sea level (101.325 kPa atmospheric pressure) then the pressure drop driving air across the throttle body = 101.325 - MAP. If MAP is 50 kPA, then that driving pressure is 51.325 kPa forcing air across the throttlebody.

But if that same car is taken to 1000m elevation, the atmospheric pressure is only about 90 kPa. For the same MAP of 50 kPa, the driving force across the throttle body is only 90 - 50 = 40. Ie, you only have 80% of the true pressure you used to have, and only get about the square root of that (back to ~90%) as a resulting air flow. Then you factor in the lower density of the air (another 90%) and you lose a little bit more. End result....for the same load signal seen by the ECU, you're only getting ~90% of the air flow you had when it was tuned. Yet the ECU will still shove in 100% of the fuel.

Decent MAP based ECUs need to have a good atmospheric pressure compensation built in to account for this.

(y) Though the ECU is also only as good as the person operating it. A lot of ECUs default (/ force) the tuner to use gauge pressure for the fuel table setup and apply baro correction to it as part of the fuel calculation, though if you are using Alpha-N (as you would with an RB26 with an ITB intake manifold) then the fuel calculation becomes a bit "different" and you need to ensure the ECU has the facility to do baro correction, and you of course need to enable that mode as well

Not something most would be thinking about until they went through it......

Its a bit disappointing how much seems to be overlooked by people willing to take lots of money on the premise they know how to make the car entirely usable.

[Stixbnr32]

Well its not all lost - easy to fix with a new sensor but not required as I have only driven that road 4 times ever.