Let us examine the basics of altitude and engines. In the olden days and for the folks that still like smelly Harleys and such (I live at 7K) there is the issue of mixture and spark advance. You needed to adjust that if you went over 4k. In aircraft, we dynamically adjust the mixture as we climb and cruise. Of course older cars, motorcycles and even lawn mowers, didn't have any way to compensate unless you leaned out your carb and advanced your ignition manually, assuming they had easy adjustments. Now modern cars, they dynamically adjust mixture and spark timing, several times a second. So they can run at an optimal mixture and timing for emissions and fuel consumption or power, depending on the program you select.

So the short answer, your sales rep is full of camel dung on the engine for altitude. The only thing you may lose at very high altitude is total max brake HP, unless your engine is blown by a turbo or supercharger. How often do we request max HP, almost never.

 

I live at 8,000 feet in Utah with a D90S P300 with zero issues in performance. As others have already mentioned, the turbo compensates for the altitude. I traded a normally aspirated 6 cyl BMW X5 that was noticeably affected by the altitude. The only issue that takes some getting used to is there is a fair amount of turbo lag with the small displacement engine. If you plan on towing anything of weight you may want to consider the P400.

 

P300 is turbocharged / P400 is turbocharged, supercharged, and a mild hybrid. P300 should be pretty good for mountains (ie modern turbochargers are), but P400 essentially just has more power in that scenario.

 

Yeah, but is it super-duper charged?

 

It is in fact, super-duper charged, yes.

 

I live in Park City UT and have been completely happy with the p300. Just did a national park road trip and got above 10k ft several times with zero issues. Definitely happy with my choice on the motor

 

I think the only time you might regret the P300 is if you want to really haul up a pass to get by some trucks and need a solid burst of acceleration and power. Obviously if you’re on trails and slower speed dirt roads you’ll be fine. To say that the P300 will have the same performance going up Vail pass, especially if you ever tow, is simply not true. If it’s that kind of power you want, and you’re not doing the kind trail driving that demands 18’s, then I’d test drive the 400 in the mountains before you decide.

 

I'm guessing that's why the sales associate recommended the 400 - power on tap going over passes. Before we moved out here full-time, we'd rent cars in Denver. I've had small 4 cylinders that sounded like they were about to explode going up to the Eisenhower Tunnel - just trying to maintain pace of traffic. That's why I wanted to ask about the 300.

I'm a pretty calm driver, so there won't be any aggressive passing in the mountains. But I certainly don't want to be redlining it just to get over a pass!

After everyone's comments, I think I'll go with the 300. Just don't want the additional burden/cost to get 18's on the 400. And the off-roading ability is pretty important for me.

 

You've got more than enough replies, so just piling on at this point.

But, specific to you, I have been up and over Eisenhower, both ways, several times with no issue whatsoever in my 300. Now could I step on it and pass quickly while climbing up to the pass? Eh, not if I was already going 80, but at 65ish there is still plenty of power to get a burst of speed.

I've also done some very rough mining trails and rock crawling at 11k and never wanted for power. I have absolutely no complaints about my 300. But I also don't tow, nor do I plan to, so it fits me well.

 

That's really the key difference in the engines. There is pretty much endless power with the 400. If you step on it doing 70mph up Vail pass, you'll still have plenty of reserve to get past anything. Towing any decent weight, it's an easy decision. The 400 all day. If you're not towing, and you don't mind giving up some high speed power, 300 will be fine. For me, I'm also a power boat owner, and having more power in reserve is always better than not having it, so it was an easy decision for the 400. I also drive up passes in the Blue Ridge almost every weekend, and passing trucks on uphill dotted line sections is something you want to do quickly for safety. The 400 is amazing for that kind of stuff and I wouldn't trade it.