Forced Ind. vs. NA - Altitude's Effect

[BuickGN]

Originally Posted By: 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.


As far as I'm aware, a hot wire style MAF sensor reads density, not speed so it should compensate for altitude.

You don't *need* a computer to maintain a constant boost at altitude. Most purely mechanical wastegates will keep boost the same under all conditions.

 

[artificialist]

The wastegate will close more to boost pressure, and as it does that, exhaust backpressure will rise, causing power loss at high altitude versus low altitude.

That in mind, I don't know exactly how far exhaust backpressure will rise, and how much power it will take away.

Still, the turbo car will win the race at an altitude of 4000 feet.

And about MAP sensors and barometric pressure. I was told by friends in the car business that when you turn the ignition to "ON" that the ECU uses the MAP reading to determine barometric pressure. Then, once then engine is started, that reading is saved. That would mean that shutting off and restarting a MAP would change the way it performs.

The heated wire or heated film MAF compensates for changes in altitude because the less dense the air, the less ability it has to cool heated wire or film. That is similar to a radiator losing cooling capacity at higher altitudes.

Manufacturers don't always decide between mass air and speed density solely by price. My 2003 Saturn ION is a speed density engine, but a 2003 Ford Focus had a Mass air engine. The Saturn was $1000 more than the Focus.

Other airflow sensors that involved pushing a vane back couldn't compensate for altitude. Those cars would either get a MAP or BARO sensor to help the vane sensor do its job. Too bad those vane sensors cause serious intake restriction.

 

[OVERKILL]

Ford slowly converted all their vehicles to MAF. This begun with the Mustang in California in 1988 IIRC. It's a superior system. Especially when dealing with modified engines.

 

[artificialist]

Originally Posted By: OVERK1LL
Ford slowly converted all their vehicles to MAF. This begun with the Mustang in California in 1988 IIRC. It's a superior system. Especially when dealing with modified engines.

Yes, it certainly is better for modded engines, since lower exhaust backpressure causes the engine to pull more vacuum, and cause the MAP to signal less fuel.

However, for non modified engines, I'm not sure how much better it is. Many current Chrysler SUV engines are speed density, even though smog tests get tougher every year.

 

[XS650]

Originally Posted By: Tornado Red


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.


Not so. The force from the wind will be directly proportional to the air density.

Drag = 1/2(air density)x(velocity^2)xCdxArea

They are called the laws of physics, not the suggestions of physics for good reason

 

[OVERKILL]

Originally Posted By: artificialist
Originally Posted By: OVERK1LL
Ford slowly converted all their vehicles to MAF. This begun with the Mustang in California in 1988 IIRC. It's a superior system. Especially when dealing with modified engines.

Yes, it certainly is better for modded engines, since lower exhaust backpressure causes the engine to pull more vacuum, and cause the MAP to signal less fuel.

However, for non modified engines, I'm not sure how much better it is. Many current Chrysler SUV engines are speed density, even though smog tests get tougher every year.


It is more accurate. It doesn't mean it's necessary or justified in all applications though. SD is much less expensive to implement and works just fine for most scenarios.

 

[Lazy JW]

Even the high-performance piston engined WW2 aircraft engines eventually ran out of power at high altitude; some of those used two-speed, two-stage mechanically driven superchargers and they didn't have to meet any emissions standards, just pure performance.

Any earthbound engine will also experience some loss at altitude but it sure beats naturally aspirated!
Joe

 

[Shannow]

Yep, they ran out of puff at very high altitudes, but were still fully compensated up to any height that you'll be driving a car.

Eventually Compressor efficiency bites you and the sized compressor that you need at extreme altitudes is so big and inefficient that it takes too much power to drive.

 

[Jim 5]

Originally Posted By: Lazy JW
Even the high-performance piston engined WW2 aircraft engines eventually ran out of power at high altitude; some of those used two-speed, two-stage mechanically driven superchargers and they didn't have to meet any emissions standards, just pure performance.

Any earthbound engine will also experience some loss at altitude but it sure beats naturally aspirated!
Joe


As I was typing my original post, I was thinking of the P-38 Lightning. I thought this point would come up sooner.

 

[Drew99GT]

Originally Posted By: Jim 5
Originally Posted By: Lazy JW
Even the high-performance piston engined WW2 aircraft engines eventually ran out of power at high altitude; some of those used two-speed, two-stage mechanically driven superchargers and they didn't have to meet any emissions standards, just pure performance.

Any earthbound engine will also experience some loss at altitude but it sure beats naturally aspirated!
Joe


As I was typing my original post, I was thinking of the P-38 Lightning. I thought this point would come up sooner.


I was thinking of the F8F Bearcat.