DesR

Hi

Can anyone tell me what (if any) issues exist when mapping cars that run at altitude (1000m - 2000m above sea level). I know that in South Africa, the octane level can be safely reduced by 2-3 points due to the altitude increase, but I was wondering if anyone had ever done any mapping under similar conditions and if there were any particular pitfalls?

Cheers

Jolly Green Monster

iTrader: (4)

if the car is mapped at that altitude then the problems are minimal...

if it were mapped at 0m and then driven at 1000~2000m then you may have problems, due to the air density changes.

Simon

DesR

Hi.

Would the inverse be true as well i.e. mapping at altitude and then going down to the coast? Am I correct in thinking (within reason) that altitude changes would be similar to the issues currently encountered on hot (less oxygen - similar to high altitude) vs cold days (more oxygen - similar to sea level) ? Are there any options that could be employed to create safety margins in the map to accommodate the shifts?

Cheers

Alan Jeffery

The main issues would be mixture and boost related,from the safety viewpoint mapping at sea level would be the better bet. You would have less boost and a richer A/F mixture at altitude. Mapping at that altitude would result in more boost and a leaner mixture the lower you go!
I've been known to fly little aeroplanes, and at around 2km you would normally be expected to lean the mix a tad, although a Cherokee wouldn't have a turbo!

Bob Rawle

The required air fuel ratio to suit the car is/shouldbe the same at whatever altitude, so the only issue is making sure that actually happens, at altitude air is less dense (obvious) and so for the same afr less fuel is needed, note you do not lean the mixture, the mixture (in afr terms) has to stay the same, but you reduce the amount of fuel added to maintain the mixture.

If using a maf based engine management system then its all done for you, providing the ecu has been mapped across its whole load/rev range and not just at wot with assumptions about the rest of it. Mass air flow ... altitude reduces mass air and so the ecu runs at a lower (if correctly mapped) injector pulse width setting to maintain the required afr. Note the ecu is not "clever", garbage in garbage out. The ignition will run more advanced due to the lower engine load (again assuming the car has been fully mapped) and so a compensation map is beneficial to keep ignition at the same value as at lower altitudes, boost maps tend to use absolute values and so the relative boost will be greater in those terms since atmosperic is lower.

If using a speed/density ecu then the problem is different, strictly the way to map this is to use BAP as a reference and differentiate that with MAP, then you will always run the required afr again assuming its been set up correctly in the first place, working this way requires two map sensors to be wired into the ecu and the ecu has to be able to support the function, Motec M800 can, for example.

If you have to map at higher altitude and are not able to use compensation then the required injector pulse widths can be calculated for the lower altitudes and used, the car will run richer than ideal at higher altitude but still be safe when at sea level. The ratio is pressure related.

In any case a compensation map can be calculated and entered by the same method if the facillity exists.

Air temperature, summer/winter conditions, humidity, all have to be factored in when mapping, interesting life we have.

best regards

bob

MikeWood

Thanks for the in depth explanation Bob, which explains rather nicely that you can't map a car in a single session on a dyno and expect it to run optimally in all conditions. The only place it may work properly will be on a dyno in the same atmospheric conditions!!

We get criticised by many for producing a 'generic' map for our PPP which can't possibly be as good as having your car 'mapped' individually but to get it perfect in all conditions just isn't possible in the timeframe that one individual is prepared to pay for. As I've said many times before, there are much bigger variations in atmospheric conditions than in tolerances between individual cars in the same mechanical spec.

Mike

DesR

Hi

Thanks to everyone for their replies (and the explanation Bob). By all accounts it seems I shouldnt have to much to worry about assuming the mapper does a good job.

Cheers

Andy.F

Depending on the year of your car Des, there may already be a considerable amount of atmospheric pressure compensation tables curently active in there.


Unless of course you map in places that don't have winter



Andy

Alan Jeffery

Mike Wood is absolutely correct of course, although the same situation applies for those who travel many miles away to map cars then push off home before the Weather changes, taking the laptop with them!

I'd like to point out that the mixture control on a Cherokee in unbelievably basic, in the form of a sticky out ****, that you shove in an inch or so at around 5000 ft. The other interesting feature is the carb heat control, which if mis-used will result in carb icing, a somewhat reluctant power output, and no lay bys!

DesR

Alan - I think I'll stick to cars... definitely don't want to tell my missus about that before trying to get her to fly in one.

Andy - I suppose I was really trying to get a feel for the type of issues encountered. I might be driving my car at high altitude in the next while and I wanted to know if it was good to go or if I needed to get further attention to the map. Also, as I don't know of any areas in the UK sitting at 1500-1800m above sea level, I wasnt sure if UK mappers could actually do anything (other than best guesstimate).

Andy.F

Not strictly true Bob, at higher altitudes the oxygen content of air reduces as a percentage of total so a MAF equipped car will run richer the higher it goes unless pressure compensations are applied. The MAF just measures the mass of air, not oxygen. As you know, its really the oxygen/fuel ratio that matters.

cheers

Andy

Alan Jeffery

I think if most people knew about the old fashioned technology in the majority of light aircraft they wouldn't go in one either! The problem is the risk of litigation if one crashes, so manufacturers are too scared to come out with something new.

My (simple) take on your trip is that if you map at the bottom and go up it's safer than mapping at the top and going down.
And don't forget to enjoy the journey!

Andy.F

I know what you mean Alan, I was down in Devon mapping a car for a guy last year who was an instructor for pilot training for Brittish Airways, when looking at the Ecutek mapping software he said..."wow that looks high tech" I thought he was joking but he was serious !

lunar tick

iTrader: (2)

Sorry Andy, but your statement is false and Bob is in fact correct. The partial pressure of oxygen does drop as altitude increases, but the % of oxygen in air remains pretty constant. It is true that at very high altitudes (above 100,000ft or so) the concentration of slightly lower molecular weight nitrogen molecules (with higher mean velocities than oxygen molecules) will increase compared to that of oxygen, but I think we can discount this minor effect down here on the planet's surface

Andy.F

If that is the case then why is it that light (piston engined) aircraft lean the mixture with increasing altitude ? They use a carburettor which depends on mass flow to introduce the fuel.
I understood this mixture adjustment was a requirement rather than just an economy device ?

Andy

lunar tick

iTrader: (2)

Hi Andy - I'm sure there are reasons, but I can't tell you them I'm afraid. All I can say is that up to the kind of altitudes you're likely to drive a car (ie 3000m) the % of oxygen in air remains pretty much constant at 21%

DesR

My understanding of the issue:

There is still the same amount of oxygen, nitrogen and argon in the air but the pressure has decreased resulting in the same volume of air containing less of all three gases (i.e. the gas has expanded taking up more room). Similar to the issue of cold and hot air being pulled in by the turbo.

I may be wrong though...