[new2the3]

I have an 09 135i with Cobb stage 1 sport tune. Otherwise completely stock, 27k miles. I'm at sea level in San Diego CA. I'm going to be doing a 2500+ mile road trip in the Colorado Rocky Mountains and will spend most of my time between 6500 and 8500 ft but will drive as high as 12,100 feet on independence pass. Fuel will be CO 91 octane.

Is it safe to run my n54 with a tune at such high altitude? If it is safe, should I keep it on the sport map or would it be better to err on the side of caution and use the lowest drive map. Or do you suggest taking the tune completely off for running at those altitudes as the stock ecu will deliver greater boost on it's own to compensate for a certain degree.

Thank you for your thoughts on this!

[jaykayelemenohpee]

It's fine. I've ran my car through co and nm for the last year without issues.

[C.Pop]

It'll be fine but don't be freaked out when it feels like you lose a billion horsepower at elevation.

[ajsalida]

I live at those altitudes in CO (well 8k ft ASL) and run Cobb Stage 2+ on shitty 91 octane. Routinely run up to 12k. You'll be fine.

[new2the3]

Thank you for the replies. Ajslalida, that is very reassuring to hear! I'm pretty sure your CO 91 is better than my CA 91 blend. Would I be boosting a higher psi at altitude compared to sea level? If I currently make my peak around 14, will the tune run much harder in co?

[ajsalida]

Originally Posted by new2the3

Thank you for the replies. Ajslalida, that is very reassuring to hear! I'm pretty sure your CO 91 is better than my CA 91 blend. Would I be boosting a higher psi at altitude compared to sea level? If I currently make my peak around 14, will the tune run much harder in co?

Answer is complicated. The Cobb and OEM ECU do not blindly target either boost levels or power.

You could make same or slightly more boost with Cobb stage 1 sport but since ambient pressure is a lot lower overall you make less power. Total amount of air is what matters, ambient + boost, and also how hot it gets. Lot of safeguards in place to make sure turbos do not work too hard, which they would were they to try to make same total power at altitude. I do not know the exact logic Cobb employs to override some or all of that, my car at Stage 2+ is boosting well over 14 psi, but lower than what I could get at sea level. I see 16.5 a lot here at 8k when others see 18 for example at sea level. IIRC I switch between sport and aggressive every so often with octane booster and 91 octane gas. Bottom line is my car boosts less than eq tune at sea level based on looking at others' logs compared to mine.

It may be that stage 1 sport is limited to below 14 psi, like say 12.5 @ 12k, or it has headroom to stay at 14 I don't know.

It is better than normally aspirated but bottom line you will always make less power at altitude than otherwise all things considered.

Now since the CO shitty 91 octane is a bit better than the CA even shittier 91 ACN octane, you may make more power than you would if you used CA 91 at altitude, but that likely will not matter much compared to the loss of power from the ambient drop at altitude. Just guessing with that though.

Watch oil temps and esp make sure your rad and cooling system is up to it. Thinner air makes everything work harder, including radiators and intercoolers esp when climbing up to 12k like over Independence pass etc. If t were me I'd do a fresh oil change.

Make sure brakes are up to coming down lot of steep grades.

edit: there are places in CO where you can get 93 octane, along the front range. Nothing up in the mountains though.

edit2: I always bring my Cobb unit with me on long trips, just in case I need to check codes or go back to the stock map. You never know.

[new2the3]

Thanks for the awesome reply. I am scheduled for an oil change and brake fluid flush tomorrow before I make the trip. Sounds like I will probably keep my tune on the stage 1 sport map but for independence pass and similar roads will probably flash back to drive map or stock to play it safe.

Also, I know boost isn't everything but at that altitude the stock tune would be pushing around 11 psi correct? As opposed to the standard ~8 at sea level. If my stage 1 sport remains at 14 it would be a pretty big difference compared to sea level since it's more like going from 11 psi stock to 14 as opposed to 8 to 14?

[ajsalida]

Originally Posted by new2the3

Thanks for the awesome reply. I am scheduled for an oil change and brake fluid flush tomorrow before I make the trip. Sounds like I will probably keep my tune on the stage 1 sport map but for independence pass and similar roads will probably flash back to drive map or stock to play it safe.

Also, I know boost isn't everything but at that altitude the stock tune would be pushing around 11 psi correct? As opposed to the standard ~8 at sea level. If my stage 1 sport remains at 14 it would be a pretty big difference compared to sea level since it's more like going from 11 psi stock to 14 as opposed to 8 to 14?

Not sure what you are saying. Let me give you an example how it works. Suppose your sea level tune produces 14 PSI peak. At sea level ambient is 14.7 PSI. At 12k ft ambient is like 9.5 PSI. You just lost 5.2 PSI of total absolute. Power is roughly a function of total air mass assuming everything else is OK. To get the total air mass you got at sea level and 14 PSI you'd need 19.2 PSI to make it up. Make sense?

But the problem is to get to 19.2 PSI over ambient at 12k ft ASL, you need to drive the turbos super hard, things get hot, way beyond their design limit, not to mention all the extra heat and the worse efficiency of radiators and intercoolers at dumping excess into thinner air.

So there are limits built into both OEM and Cobb software that effectively cut back on boost levels as you go up in altitude, but the way they cut back is not simple or linear. It depends on where you're at in the turbo efficiency map, rpm, load, etc and how much heat soak, IAT's, etc is going on. And as you heat soak longer you run under load things get steadily worse.

[new2the3]

I see what you are saying and that does make sense. I guess what I was trying to convey was that with an otherwise stock car like mine it doesn't seem that Stage 1 Sport will make nearly as much power over stock in 7,500+ feet altitude than it does at sea level.

My reason for saying that is if my car is completely stock (no tune) then at 7,500 feet the stock ecu program will raise the ~8psi boost to around ~11 psi to compensate for the loss in stock power.

If my car at sea level hits 14 max, I can't see stage 1 sport at altitude being more than 14-16 psi boosting. So whereas the difference at sea level is going from 8psi to 14psi then at altitude the tune has way less of a delta because the new "stock" psi is 11 and it will probably go up to 14 max still. 3psi increase versus 6psi. I know there are other factors involved besides just boost but all else equal on a stock car with just a tune, it seems that 14 or 15 psi is really the limit that will be thrown at the turbos on stage 1 sport with no DPs, FMIC, etc.

[ajsalida]

Originally Posted by new2the3

I see what you are saying and that does make sense. I guess what I was trying to convey was that with an otherwise stock car like mine it doesn't seem that Stage 1 Sport will make nearly as much power over stock in 7,500+ feet altitude than it does at sea level.

My reason for saying that is if my car is completely stock (no tune) then at 7,500 feet the stock ecu program will raise the ~8psi boost to around ~11 psi to compensate for the loss in stock power.

If my car at sea level hits 14 max, I can't see stage 1 sport at altitude being more than 14-16 psi boosting. So whereas the difference at sea level is going from 8psi to 14psi then at altitude the tune has way less of a delta because the new "stock" psi is 11 and it will probably go up to 14 max still. 3psi increase versus 6psi. I know there are other factors involved besides just boost but all else equal on a stock car with just a tune, it seems that 14 or 15 psi is really the limit that will be thrown at the turbos on stage 1 sport with no DPs, FMIC, etc.

Aha now I see what you mean. Hard to say plus the difference between 7500 and 12000 is substantial regardless. I guess the real issue is do you want to be using 300+ HP with 2000+ ft drop offs around every corner? Penalty for error up here is severe. Your car will be fine, it will be down on power though compared to what you're used to. It is not possible to precisely quantify this, but you will be better off with the tune running.

Very roughly look at any website with altitude vs barometric pressure in PSI. If the drop in PSI due to altitude is MORE than the extra PSI given by the tune over stock, guarantee you'll be down on power with tune at altitude vs stock at sea level. But the tricky part is that is a function of RPM, heat soak, load, ambient temp, humidity etc etc.

Not worth worry about as I said you will not have much chance to go WOT up in the mountains esp if you are gaping at scenery and unfamiliar with the roads. Or I should say you better not.

edit: I am not sure what the stock ECU will do in terms of additional boost to compensate for altitude, if anything, nor do I know how that affects what the Cobb does on top of that. There are limits on the turbos' pressure ratio. So if your tune at sea level safely asked for 14.7 PSI, that is a 2.0 ratio of absolute PSI to ambient. At 12k ft that absolute pressure is close to 3.1 ratio, way outside spec for these turbos. Never gonna happen, maybe get a 2.2 ratio assuming you have mods to support that (so maybe 11.4 boost over 9.5 PSI), depends on what Cobb has targeted with safety margin and a lot of other conditions. But 11.4 PSI at 12k is just 20.9 absolute or sea level + 6.2 PSI (if I did my math right). So pretty lame compared to stock at sea level. But not as lame as stock at 12k ft.

Someone with more knowledge please feel free to chime in.