Boost and Absolute Pressure at Altitude

[Alvor315]

I live in a mountainous area and it has always made me curious why cars do not use a system that controls the boost pressure as regulated by absolute pressure of the intake manifold. That way no matter the altitude the manifold pressure is always the same, atleast until the turbo can no longer compress the incoming air at altitude. Thus no powerloss is due to altitude. I know that some airplanes have this capability.

Just curious. I don't have a wastegate to dismantle and discover if they are capable of this without much modification. Anybody ever tried this? I would like to know.

Thanks,
Allan

 

[Bort62]

Sure, just change the wastegate reference

But - why?

 

[Anonymous]

If you are familiar with turbocharged/pressurized Cessna twins, then you also know about the VAPC (variable altitude pressure controller)

Has it been done? - YES
On a car? - probably not
it is complex? - YES!
Is it expensive? - YES!

Such a simple question, such a simple answer.

 

[Anonymous]

Hard part would still be maintaining the mixture with a carb. at altitude. EFI w/ a blow-thru turbo would be the best setup IMO.
Maybe CFI like Ford's old 2V unit from the mid `80's?
Can't think of any cars that had a boost controller that compensated for alt. ...maybe Saab or BMW?

 

[Lazy JW]

Sure, just change the wastegate reference

But - why?

Because you can?

Some WW2 fighter pilots had to shift their two-speed superchargers with a manual shifter lever as altitude varied. Shouldn't be too difficult to set up a parallel wastegate reference that you could shift high/low.

The Bf 109 used a boost control that was automatic so the pilot didn't have to mess with it. Not sure just how it worked but the pilots liked it.

It is difficult to postulate a real need for absolute maximum power in an automobile at 14,000+ feet. Not many drag strips up there :roll:
Joe

 

[shmoozo]

It is difficult to postulate a real need for absolute maximum power in an automobile at 14,000+ feet. Not many drag strips up there :roll:
Joe

Not even all that many roads up there. Come to think of it ... there's not even all that much "there" up there.

 

[StrangeRanger]

To point out the obvious: what do they do at the Pike's Peak Int'l. Hillclimb? It would seem that of all the places on earth, that would be the one where they've sussed out the whole business of altitude compensation. I'd almost bet the farm that they have a baro sensor that reads directly into the processor and it ajdusts the fuel curves accordingly.

The Supercharged 3.8 T-birds used a baro sensor as a reference as well as the MAF meter for control, so it would appear that Ford had altitude compensation built in.

 

[wallaka]

Why not just have a wastegate controller? When you get to higher altitudes, turn the boost up. Too easy.

 

[Alvor315]

When you have a drag racer, unless it falls off a cliff, you don't drastically change your altitude. You can build your engine so that it is optomized for a certain altitude, ie; higher compression, more boost, etc.

The only reason you want variable altitude boost control is if your traveling up and down hill all the time. I live right next to Tahoe so I go up and down probably a difference of 2500 feet everyday. I live at 4000ft go up to Tahoe at 6225ft and then every once in a while go into the bay area at 0ft. If you have a naturally aspirated vehicle in my case you would have 3 options. 1) build your engine so that it works well at Sea level, but then it works like crap at home and in Tahoe. 2) build your engine so that it works well at 4000ft, then it works like crap in tahoe, and you're afraid of going down to San Fran becuase of detonation. 3) build your engine so that it works well in Tahoe, and never go home.

Now you loose 3-4% horse power per 1000ft and with a carburetor you're running rich at higher altitude and lean at lower altitude, so if you can maintain a constant manifold pressure you get the same horsepower everywhere, and you don't waste any gas. Say at sea level your engine generates 100hp then at 6225 feet you are left with 75hp. It must suck to live at some of the higher resort destinations of Colorado because your horsepower at times would only be 60hp.

Now, a geared charging system wouldn't make sense becuase the change isn't drastic enough to call for such an aparatus. Those planes used superchargers, since superchargers are belt driven they couldn't continously vary the pressure without an extravegant shifting mechanism. Besides, the shift was meant to give more horsepower at cruising speeds at altitudes higher than any road goes. Because turbochargers are free-floating systems they can spin at speeds that are not merely strict ratios of the engine's speed. This makes them ideal for changing altitude.

The point of a variable boost control for a car is economy. The planes used them as a tactical advantage.

Here is a fun website about the history and practice of variable altitude engines in aircraft. I also used it as a reference.
http://fordsix.com/forum/posting.php?mode=reply&t=42588

Just some Food for thought...

Allan

 

[Lazy JW]

Reckon my simple hillbilly mind is missing something here, but it seems to me that an EFI system with a turbo would be pretty good at altitude. The turbo provides the pressure and the O2 sensor keeps the air/fuel ratio correct. Even without referencing for barometric pressure it's gonna outperform a N/A carby engine big time.

Even a N/A EFI engine gets along pretty well due to the O2 sensor keeping the mixture correct. Yes, you will need to drive a bit deeper into the throttle but so what? Most modern cars have gobs of excess reserve power anyway. I can't imagine anything short of a bona-fide working truck really running short of power now-adays.

The old Volkswagen Bugs had an optional main jet that was barometrically controlled for high altitude useage. I have never seen one but my shop manual has pictures. I would love to get my hands on one though 8)

Some gasoline powered tractors during the 1940's-50's had optional "high altitude" pistons available from the factory. They jumped the the compression all the way up to about 6:1, the LPG engines got WAY up there to 7:1 or even a bit more.

Most of the WW2 era aircraft superchargers were gear driven, some used a turbo to feed into the supercharger (P-47, P-38 ).
Joe