Forced Ind. vs. NA - Altitude's Effect

[Jim 5]

A question involving forced induction theory for those who understand it.

A car is available on the market with two different engine options (for example):

1. 360 hp comparable tq. Normally aspirated V8.. say 4.8L

2. 360 HP (with a simple software mod) turbocharged 6 cyl...say 3.0L

Above being the sea-level HP figures. Let's say just for comparison purposes these two engines had the exact same torque and HP curves. Again, one is normally aspirated, and the other is turbocharged. At sea level they therefore have the same 0-60 time, the same 35-80 sprint, and the same 1/8th and 1/4 mile time. In other words, identical performance.

Now take those cars to the drag-strip at 4,000 ft above sea level. Does this change things? Which one if any comes out on top?

If my suspicion is correct and altitude will affect these engines differently, then I would like to understand how and why.

Bonus points go to those who figure out what theoretical cars I'm talking about (with perhaps what mods). My GPS tells me my house is almost 4,000 ft above sea level.

Thanks for anyone's instruction here!

 

[Shannow]

The turbo car should win well at altitude.

The N.A. car will behave as though the throttle plates are partially closed, as the engine sucks less working fluid (mass) in per revolution.

The turbo could make up for that, depending on whether the wastegate and programming are designed to hold absolute rather than gauge pressures.

 

[MGregoir]

The turbo car would destroy the naturally aspirated one if the turbo was set up correctly.

5-series BMWs with the twin-turbo versus the V8? I love that turbo engine and it does have advantages for you at your altitude, but the V8 will be the more reliable long term engine.

There's a reason the Mexican highway patrol used turbo Stratuses (which is where the engine for the SRT-4 came from) - high altitude horsepower.

 

[brianl703]

On my Saab 93 they are. The service manual states that if the intake air temperature is high or the atmospheric pressure is low, it will increase the PWM ratio to the charge air control solenoid (which controls the wastegate) to compensate.

 

[ms21043]

In the Neon service manual in the section for the 2.4 turbocharged engine in the description and operation for the turbocharger, it says one of it's operating functions is altitude compensation. So in this vehicle at least, manifold pressure is automatically adjusted to keep horsepower at it's rating. Don't ask me up to what altitude.

I once drove a rental Subaru up to the observatory on the Big Island of Hawaii. The altitude up there is about 14,000 ft. The car made it up, but nearing the top I had it at full throttle and we were doing about 10 mph max. At that altitude the air pressure is less than half of what it is at sea level, The engine was receiving much less air than it did at sea level despite the full open throttle, it would have been nice to have a turbocharged car then.

 

[Quattro Pete]

Originally Posted By: Jim 5
Bonus points go to those who figure out what theoretical cars I'm talking about (with perhaps what mods).


550i and 535i with a chip?

 

[BuickGN]

Assuming the same boost, the turbo car will retain most of it's power.

It will lose just a little power because you will have a larger pressure differential at the same absolute boost. Charge air temps can go up a couple degrees.

 

[Drew99GT]

Dead stock turbocharged cars do lose a bit of performance at altitude, even if the engine computer will allow the wastegate to keep the boost pressure the same as at sea level.

I live at 6,000 feet above sea level and go to the 2 drag strips around here often. Dead stock, or very slightly modded STIs for example run about low 14s. Same for Lancer Evolutions. Trap speeds are about 95 to 97 mph. At sea level, they should be in the mid to low 13s with trap speeds above 100 mph.

My guess is even though the ECU can compensate for the altitude and keep the boost pressure the same, the actual amount of airflow through the motor might be less since the ambient air pressure is less. ie, 15 psi with a little tiny turbocharger is going to pump much less airflow then 15 psi with a 100 mm turbocharger. Plus, if the compressor has to spin a lot faster to make the same boost at higher altitude, efficiency can be decreased, which increases charge temps a lot.

 

[BuickGN]

Originally Posted By: Drew99GT
Dead stock turbocharged cars do lose a bit of performance at altitude, even if the engine computer will allow the wastegate to keep the boost pressure the same as at sea level.

I live at 6,000 feet above sea level and go to the 2 drag strips around here often. Dead stock, or very slightly modded STIs for example run about low 14s. Same for Lancer Evolutions. Trap speeds are about 95 to 97 mph. At sea level, they should be in the mid to low 13s with trap speeds above 100 mph.

My guess is even though the ECU can compensate for the altitude and keep the boost pressure the same, the actual amount of airflow through the motor might be less since the ambient air pressure is less. ie, 15 psi with a little tiny turbocharger is going to pump much less airflow then 15 psi with a 100 mm turbocharger. Plus, if the compressor has to spin a lot faster to make the same boost at higher altitude, efficiency can be decreased, which increases charge temps a lot.


I can see the small turbo cars losing a little more power due to the reasons you stated. The guys I've seen that take their cars to a high altitude meet usually lose 1-3mph and that's it. But, most of those cars are running fairly large turbos and intercoolers.

I would like to see some of the class cars that are limited to a 70mm turbo that are using all the available airflow take their cars to altitude to see if they slow down like a NA car.

 

[oilyriser]

Also, intercooler won't work as well at altitude, so the turbo won't perfectly compensate for altitude, but it'll be way better than n.a.

 

[Jim 5]

Originally Posted By: Quattro Pete
Originally Posted By: Jim 5
Bonus points go to those who figure out what theoretical cars I'm talking about (with perhaps what mods).


550i and 535i with a chip?


bingo!

 

[Jim 5]

We lose a great deal of horsepower at 4,000ft. Based on what I am reading, the FI 535 is going to close the small gap that exists between it and the 550i even without a chip.

Thanks for everyone's advice. Looks like the 535xi might be the way to go given we've had around a foot and a half of snow on the ground for the last two months.

Of course the 550i is going to be a 4.4 twin turbo when the body style changes. Maybe I'll wait for that!

 

[eljefino]

Originally Posted By: ms21043


I once drove a rental Subaru up to the observatory on the Big Island of Hawaii. The altitude up there is about 14,000 ft. The car made it up, but nearing the top I had it at full throttle and we were doing about 10 mph max.


Some cars can compensate for altitude (somewhat) but they only measure it when the car is being started. So if you go on a long road trip up a mountain a "reboot" may help.

 

[Jim 5]

Originally Posted By: eljefino
Originally Posted By: ms21043


I once drove a rental Subaru up to the observatory on the Big Island of Hawaii. The altitude up there is about 14,000 ft. The car made it up, but nearing the top I had it at full throttle and we were doing about 10 mph max.


Some cars can compensate for altitude (somewhat) but they only measure it when the car is being started. So if you go on a long road trip up a mountain a "reboot" may help.

Doesn't a mass air sensor typically measure the amount of air molecules passing it on the fly?

 

[Tornado Red]

Originally Posted By: eljefino
Some cars can compensate for altitude (somewhat) but they only measure it when the car is being started. So if you go on a long road trip up a mountain a "reboot" may help.

I think Drew and BuickGN have the right idea. The ECM or ECU of every modern turbocharged car should have a built-in barometric altimeter, allowing for constant fuel-air mixture adjustments. Rebooting should not be necessary.

 

[brianl703]

Originally Posted By: eljefino

Some cars can compensate for altitude (somewhat) but they only measure it when the car is being started. So if you go on a long road trip up a mountain a "reboot" may help.

Sometime between 1989 and 1995 Ford engineers figured out how to calculate the barometer reading from the mass air flow sensor data. The older mass-air Ford vehicles had a barometric pressure sensor. The newer ones did away with it, but still somehow are able to calculate barometric pressure from MAF data.

Service manuals suggest that a baro reading that's out of whack, on a Ford vehicle without baro sensor, indicates a bad MAF.

 

[OVERKILL]

Yup, all the MAF Fox cars had a BAP. The funny part was that the BAP was nothing more than the earlier SD MAP without a vacuum line going to it

 

[Doug Hillary]

Hi,
oilyriser - You said this:
"Also, intercooler won't work as well at altitude,........."

IMHO the intercooler (air to air) is really more dependent on the ambient temperature than the altitude

 

[eljefino]

Originally Posted By: Jim 5
Originally Posted By: eljefino
Originally Posted By: ms21043


I once drove a rental Subaru up to the observatory on the Big Island of Hawaii. The altitude up there is about 14,000 ft. The car made it up, but nearing the top I had it at full throttle and we were doing about 10 mph max.


Some cars can compensate for altitude (somewhat) but they only measure it when the car is being started. So if you go on a long road trip up a mountain a "reboot" may help.

Doesn't a mass air sensor typically measure the amount of air molecules passing it on the fly?


Exactly but a lot of cheaper cars still run speed density with a MAP sensor, computer logic uses same MAP sensor data before start to determine altitude.

 

[Tornado Red]

Originally Posted By: Doug Hillary
Hi,
oilyriser - You said this:
"Also, intercooler won't work as well at altitude,........."

IMHO the intercooler (air to air) is really more dependent on the ambient temperature than the altitude

I think you are correct, sir.

As for the MAF sensor... if you were at the top of Mount Washington in New Hampshire, and the wind was blowing 100 mph, it would strike you with the same force as a 100 mph wind at sea level. The oxygen content would be lower, however, so a barometric altimeter would help the ECU adjust fuel flow and turbo boost to compensate for altitude. The MAF sensor could not do this by itself.

Not sure, but I think maybe an intercooler would be less efficient at high altitude, all else being equal. Same with a radiator.