How much do NA cars struggle at high altitude?

[OppositeLockMT]

I had a run in with an F10 520d, G5 GTi and a W204 C200K in my 328i, safe to say that I was left for dead on all accounts. Honestly thought that with 142kw and 280nm (granted, horses have left the stable over the years) I would fair better. Which leads me to my initial question of how and why do cars with forced induction suffer less from high altitude? Feels like my car suffers the most below 100km/h but after it gets going it seems like it starts to hold its own.

Don't want to go into detail about the runs, will save that for TLGP.

 

[GoCart]

Well covered subject this, it's all about air density / pressure.

---

So at altitude, they struggle a lot, I believe in JHB cars are running at a loss of close to 18%, or thereabouts.

 

[OppositeLockMT]

Indeed it is, however most of what I read are figures on the average percentage loss of power ever 1000ft, facts as to why this happens are a bit inconclusive - mostly because I am a bit of a novice when it comes to how an engine works.

 

[Carbon]

Indeed it is, however most of what I read are figures on the average percentage loss of power ever 1000ft, facts as to why this happens are a bit inconclusive - mostly because I am a bit of a novice when it comes to how an engine works.

Think of it this way. An engine is a pump with a compression ratio eg 10 to 1.

If you pump at sea level (1.013bar) you will have a pressure of 10.13 bar at the top of the piston stroke.

If you pump at Pretoria (0.85bar) then you will have a pressure of 8.5bar at the top od the stroke. This means less air(oxygen) and thus less fuel can be injected. So you have less power.

Turbo cars have pressure regulators the keep the pressure constant, so you will have almost the same power at altitude, but you will have more lag than at sea level.

Most stock supercharged systems also lose power at altitude.

 

[Crash_Nemesis]

18% loss. Air pressure at sea level is 1 atmosphere which is 14.6psi or 101kpa. Up here in Johannesburg it is around 11.3psi, or 0.8 atmospheres or 83 kpa.

142 x 0.18 = 25.5kw loss.
280 x 0.18 = 50,4nM loss.

Your car is only performing at 116.5kw and 229.6nM.

Likewise for M drivers. We lose massive amounts of power. An e46 M3 will lose 45,36kw of power and 63,9nm coming up to Johannesburg.

The bigger the engine the more loss of power.

Power to weight is also a factor. An e92 M3 has a power to weight ratio of 0.111hp for ever kg it weighs down at sea level. But up in JHB, that number drops to 0.098hp for ever Kg.

Turbo cars on the other hand have what is called a wastegate, which work with atmospheric pressure which in turn allows the wastegate to keep a targeted power level to enable the car to deliver similar power between sea level and high altitudes. The car automatically boosts higher to make up for the 3psi difference in air pressure. This is only a simple explanation but in all fairness it is why these cars perform better up here.

---

Most stock supercharged systems also lose power at altitude.

Not true. In fact, all supercharged systems will lose power at altitude. This is because they are fixed gear systems and cannot suddenly spin faster to make up the loss in air pressure, hence, they suffer exactly the same loss than an NA engine would.

For my supercharged m3 to run 5psi up in JHB, I had to use an 8psi pulley wheel.

Hope this helps.

 

[Carbon]

Most stock supercharged systems also lose power at altitude.

Not true. In fact, all supercharged systems will lose power at altitude.

No it is true, it depends on how it is set up, screw typre chargers can produce ridiculous amounts of pressure at low rpm, then you bypass/blowoff some pressure to reduce to the needed pressure with boost controllers.

Such a system is does not lose power at altitude if the decrease in pressure can still be maintained by the charger.

 

[OppositeLockMT]

18% loss. Air pressure at sea level is 1 atmosphere which is 14.6psi or 101kpa. Up here in Johannesburg it is around 11.3psi, or 0.8 atmospheres or 83 kpa.

142 x 0.18 = 25.5kw loss.
280 x 0.18 = 50,4nM loss.

Your car is only performing at 116.5kw and 229.6nM.

Likewise for M drivers. We lose massive amounts of power. An e46 M3 will lose 45,36kw of power and 63,9nm coming up to Johannesburg.

The bigger the engine the more loss of power.

Power to weight is also a factor. An e92 M3 has a power to weight ratio of 0.111hp for ever kg it weighs down at sea level. But up in JHB, that number drops to 0.098hp for ever Kg.

Turbo cars on the other hand have what is called a wastegate, which work with atmospheric pressure which in turn allows the wastegate to keep a targeted power level to enable the car to deliver similar power between sea level and high altitudes. The car automatically boosts higher to make up for the 3psi difference in air pressure. This is only a simple explanation but in all fairness it is why these cars perform better up here.

---

Most stock supercharged systems also lose power at altitude.

Not true. In fact, all supercharged systems will lose power at altitude. This is because they are fixed gear systems and cannot suddenly spin faster to make up the loss in air pressure, hence, they suffer exactly the same loss than an NA engine would.

For my supercharged m3 to run 5psi up in JHB, I had to use an 8psi pulley wheel.

Hope this helps.

Thank you Crash, Carbon.

I assume an NA car in CT or KZN feels completely different. I still prefer the linear power delivery of an NA motor (call me old fashioned), but I appreciate the tech that goes into the newer FI cars, particularly BMW's N57.

 

[cOlDFuSiOn]

Thanks Crash and Carbon for the info, this furthers my knowledge too. Makes sense.

 

[ET was here]

interesting discussion
thanks gents
:coolShake:

 

[Quick///M]

I give E92 M3's a hard time in JHB and they don't like it :rollsmile: but great explanations

 

[Zound]

@OppositeLockMT

I'm in the same boat bud. It is almost criminal what small displacement forced induction cars can do up at altitude, nevermind those with decent displacement, like the X35i platform.

My supposedly 170kW, 300N.m E46 330i competes toe-to-toe, neck and neck with my girlfriends VW Polo 9N3 GTI which only puts out 110kW, 210N.m !!!

Even the frugal F30 320D gives me a hard time!

Impractical and archaic these naturally aspirated old timers might be, I still get chills every morning I start her up and she continues to thrill all the way to redline with the most glorious sound track I could ever ask for from a NA 6 cylinder.

 

[nirvash]

@OppositeLockMT

I'm in the same boat bud. It is almost criminal what small displacement forced induction cars can do up at altitude, nevermind those with decent displacement, like the X35i platform.

My supposedly 170kW, 300N.m E46 330i competes toe-to-toe, neck and neck with my girlfriends VW Polo 9N3 GTI which only puts out 110kW, 210N.m !!!

Even the frugal F30 320D gives me a hard time!

Impractical and archaic these naturally aspirated old timers might be, I still get chills every morning I start her up and she continues to thrill all the way to redline with the most glorious sound track I could ever ask for from a NA 6 cylinder.

even though im at the coast, most modern FI cars are stronger,
but i agree 100% the thrills and chills as well as the sound track from the m54b30 is just to die for, love it regardless of who beats me

 

[Xack]

Nice one guys, biggest mod you can do for an N/A car is coastal air induction,lol