OT: Turbocharger boost at altitude reduced.

Bill D · Jul 31, 2019

Off Topic: I drove over Carson pass to Nevada the other day. About 8,000 feet reduced my normally aspirated engine by about 24% less horsepower using 3% loss per 1,000 foot altitude increase. The Teslas whip by at full power since batteries are not affected by altitude.
So does a turbocharged car also lose horsepower with altitude? Seems like they would if they limit boost pressure to say 10 PSI Gauge pressure. Or do they do some fancy measurement and limit it to a absolute boost pressure regardless of altitude? In my limited experience all the pressure sensors and guages I have seen give relative pressure and assume STP conditions outside the gauge.
Bill D.

You do not want to be behind a non west coast truck in the mountains or even the hills here since they use much smaller engines.

thermite · Jul 31, 2019

Bill D said:
Off Topic: I drove over Carson pass to Nevada the other day. About 8,000 feet reduced my normally aspirated engine by about 24% less horsepower using 3% loss per 1,000 foot altitude increase. The Teslas whip by at full power since batteries are not affected by altitude.
So does a turbocharged car also lose horsepower with altitude?

Last time I crossed it, the mount was a rented Mustang 5.0 convertible. Didn't notice enough "power loss" that I couldn't still "at least" hold the speed limit.

The air MASS sensor system - most modern IC engines - is the more important player such that any motor, NA or blown, gets a workable F/A ratio.

El Cheapo turbos on daily-driver highway vehicles aren't in the same class as variable-boost ones on Aircraft.

Mind, there IS a slight difference in the altitude range each is expected to deal with.

Best power for either is a cool and also damp, day at low altitude, but convertible tops aren't fun for very long on a submarine.

awake · Jul 31, 2019

So here's a related puzzle, now 32 years old. When I got married, I was driving a VW diesel, NOT turbocharged. We went up in the Colorado mountains for our honeymoon, topping out around 11,000 feet. To my enormous surprise, it seemed like the diesel ran better up there than usual. I've always wondered why ...

Terry Keeley · Jul 31, 2019

awake said:
So here's a related puzzle, now 32 years old. When I got married, I was driving a VW diesel, NOT turbocharged. We went up in the Colorado mountains for our honeymoon, topping out around 11,000 feet. To my enormous surprise, it seemed like the diesel ran better up there than usual. I've always wondered why ...

Good weed at the time?

rat_stomper · Jul 31, 2019

I managed to delete my first post so I'll try again after throughly read the OP.

Basically it depends on the engine management system.

In the most basic system where the waste gate is directly controlled by the intake pressure, yes you would lose some power but less than a naturally aspirated engine. The reason is that the waste gate is still referenced directly to atmospheric pressure...so at higher altitudes you make less boost.

In a more advanced EMS a computer could have a barometer and control of the waste gate. In this case the computer could compensate for the lower atmospheric pressure with a higher boost pressure...which would allow the engine to produce the same amount of power.

Most modern EMS have a barometer but they typically only read it once at startup as far as I know. In aftermarket case, you could easily have on the fly altitude compensation.

strokersix · Jul 31, 2019

thermite said:
Best power for either is a cool and also damp, day at low altitude, but convertible tops aren't fun for very long on a submarine.

You sure about damp air being better? Ingesting water vapor does not add any available oxygen. My understanding.

DDoug · Jul 31, 2019

strokersix said:
You sure about damp air being better? Ingesting water vapor does not add any available oxygen. My understanding.

Bill's got a truck load of the leetle pills they sold at JC Whitney...the ones that
made your car run on water....

DDoug · Jul 31, 2019

FWIW I was visiting the Enstrom Helicopter factory about 28 years ago, and they told me of the turbocharged version ( I think they
just got FAA approval at that time)

I cringed a bit, as adding a dog whistle to a gas airplane engine
shortens the TBO (and reliability, kind of important in rotary wing aircraft eh ?)

The factory person explained the turbo was used very conservatively,
just to guarantee performance up to the ceiling (IIRC 10,000 msl)

On the other hand, the diesel loco to traverse China, has to climb to 17,500 msl. and I was talking with the engineer in charge
about modifications.

A simple internal change on the turbo was all that was needed (not a complete size change), there is no wastegate.

DDoug · Jul 31, 2019

10 Psi ?

My old NPR cabover, I thought the turbo was not working, couldn't hear it, so I
put a gage on it 15psi.

I'm going out on a limb here, but if you could watch the position of the wastegate, I'll bet you would see it open more as needed, to maintain the boost level.

Your altitude limit is when the waste gate is fully closed, and your boost pressure can't stay up.

Would love to hear how the old 8v-92 silver series Detroits do on that climb.....
I'm told when they get all wound up, the turbo(s) are pushing so much, they run up against the positive displacement roots blower, so there is a bypass valve,
to go around it.

sealark37 · Jul 31, 2019

As noted above, there are turbochargers and then, turbochargers. A fixed waste gate improves performance as a % of normal performance as altitude increases. A variable waste gate maintains rated performance of the engine up to a specific altitude. A computer controlled engine system simply makes the process more flexible and efficient.

rat_stomper · Jul 31, 2019

I'm thinking you're right about damp air. In high level racing my understanding is that the tune needs to be leaned out some and have more ignition advance to make the same power...at least theoretically.

Terry Keeley · Jul 31, 2019

Tbartoletta said:
I'm thinking you're right about damp air. In high level racing my understanding is that the tune needs to be leaned out some and have more ignition advance to make the same power...at least theoretically.

Yup, in drag racing the standard is gpp or grains of water per pound of air. The water displaces the air and therefore makes less power.

Bill D · Jul 31, 2019

I used to have a Datsun 810 wagon. it had a 240Z straight six engine in it. I cobbled a little water systm that sucked water out of the windshield washer tank into the manifold. I set it to about one quart per tank of gas using aquarium air valves.
Before I had to retard timing a bit at altitudes over 3,500 feet or the engine would ping. the water system allowed me to advance the timing and not get pinging so I had more horsepower at altitude. I shut it off for flat land driving because I thought all that water can not be good for the engine parts that can rust.
Bill D

thermite · Jul 31, 2019

Bill D said:
I used to have a Datsun 810 wagon. it had a 240Z straight six engine in it. I cobbled a little water systm that sucked water out of the windshield washer tank into the manifold. I set it to about one quart per tank of gas using aquarium air valves.
Before I had to retard timing a bit at altitudes over 3,500 feet or the engine would ping. the water system allowed me to advance the timing and not get pinging so I had more horsepower at altitude. I shut it off for flat land driving because I thought all that water can not be good for the engine parts that can rust.
Bill D

Datsun? The effing engine wasn't what rusted. It was everything else.

Pre-rusted on the production line, were they not?

rat_stomper · Jul 31, 2019

Terry Keeley said:
Yup, in drag racing the standard is gpp or grains of water per pound of air. The water displaces the air and therefore makes less power.

Thanks Terry, I didn't know the technical term. I remember the tune guys would basically have a weather station and make small adjustments every round.

DDoug · Aug 1, 2019

thermite said:
Datsun? The effing engine wasn't what rusted. It was everything else.

Pre-rusted on the production line, were they not?

It used "Powder Metallurgy"....it was made from compressed rust.

Scottl · Aug 1, 2019

strokersix said:
You sure about damp air being better? Ingesting water vapor does not add any available oxygen. My understanding.

What water vapor does is reduce detonation. Even in a vehicle without ECM there can be a slight horsepower loss as detonation even at minor levels can rob power. At low levels detonation is not obvious to the driver.

With an ECM, and especially with a turbo or roots blower the ECM will either dial back on timing or reduce boost if detonation is detected by the sensor(s). Either will reduce seat of the pants feel re power.

Back when lead was first removed from gasoline owners of 60s musclecars faced a dilemma. They could either retard the timing and reduce horsepower, add an octane booster, or add water injection. I know because at one time or another I tried all three. Retarding the timing made the engine a "dog" and commercial octane boosters were expensive. A well designed water injection system using distilled water was cheap to run and worked fine, at least when temperatures were above freezing. I eventually went to adding 2% acetone with a slight reduction in timing.

Buick has had 2 recalls for potential engine fires in their 90s supercharged vehicles. Oil can leak from a valve cover gasket onto the hot exhaust. Those in the know say that all of the vehicles that had fires were run on low octane fuel despite what the owners manual said about not doing so. What happened was the ECM dialed back the timing to compensate for the inadequate fuel, raising the exhaust temperature.

As to the effect of altitude leaner mixtures can increase detonation.

DDoug · Aug 1, 2019

...and then there's:
Lean-burn - Wikipedia

:popcorn:

strokersix · Aug 1, 2019

Yup, I agree.

Water injection can help make more power if it is covering up another problem such as low octane fuel.

Heat capacity of water and water vapor absorbs heat and is also essentially inert. Both effects tend to reduce spark knock tendency by reducing peak temps and pressures. This effect can be used to develop more power from an engine. But again, it's a crutch to cover up something else, sometimes emissions regulation.

Scottl · Aug 1, 2019

One reason I think water injection got a bad rap was the poor quality of many of the rigs, especially some of the home brew ones.

What I ended up using was a commercially made one that used an aluminum carburetor base plate with adjustable nozzles. A solenoid energized by the ignition shut off the water supply when off to prevent any chance of dripping.

Water has been used by mechanics for ages to remove carbon deposits. I suspect that one of the added benefits of a water injection system was keeping the engine clean of detonation causing carbon particles. One of the best additions I made was an aftermarket capacitor discharge ignition. The unit I used had a bypass switch used for troubleshooting, setting dwell, etc. Tests with the switch in both positions showed the engine ran smoother and had more power when the unit was activated. It undoubtedly made the engine run cleaner.

Fortunately the octane deprivation only lasted a few years. Sunoco came to the rescue when they reintroduced higher octane fuel along with the blending pump.

OT: Turbocharger boost at altitude reduced.

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