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I Saw OJ
Diamond Member
Considering that the whole hotrod, modified cars, drag racing scene pretty much started in California I think we get by just fine with the whole CARB rating.
My 418hp '03 Cobra gets by just fine on 91 octane.
If you own a car that does not need a higher octane gasoline and you use a high octane gas it in, you will most likely see a loss in performance and gas mileage. Higher octane gas is harder to detonate therefor the car that runs on 87 will have a harder time detonating all the fuel which will result in a loss of performance and wasted fuel.
The compression ratio is directly affected by manifold pressure. The more boost you run, the higher your actual ratio in-cylinder. I forget the math, but IIRC #17kbs of boost is going to increase compression quite a bit, up into the low teens I'd imagine. Premium fuel does quite a bit to control detonation compared to regular, but even with bad fuel your anti-knock sensor will save your arse.
It's not exactly the same, since the boost is the manifold pressure over the ambient pressure, not absolute pressure. That's the difference between psia (ambient) and psig (gauge).
If it was exactly the same, turbocharged cars wouldn't lose power with altitude, but they do. Not as much as NA cars, though.
Here's a post I wrote about it on a car forum. The thread was on correction factors, and I was showing them how the same correction factor cannot be applied to a turbo car and a NA car. I used a car running 19 psi boost as an example.
NA car= (atmospheric)
Turbo = (atmospheric+boost)
So at sea level you have:
NA car= (14.7) total of 14.7 psi
Turbo= (14.7 + 19) total of 33.7 psi
We're using sea level as the baseline, so at sea level you both have 100% of your power.
Now in Denver, at 5280 feet, the atmospheric pressure is 12.3 psi. So we have:
NA car= (12.3) total of 12.3 psi, 83.6% of what he had at sea level.
Turbo = (12.3+19) total of 31.3 psi, 92.8% of what you had at sea level.
So if you both had 500 hp at sea level, in Denver the NA car now has 418 hp and you now have 464 hp.
Since the boost level is added on top of the atmospheric pressure and remains constant regardless of altitude, the more boost you run the less altitude will affect you.
Quick comparison at Denver's altitude with a NA car, car running 5 psi, a car running 10 psi, and a car running 20 psi:
NA car = (12.3) total of 12.3 psi, 83.6% of sea level
5 psi turbo =(12.3+5) total of 17.3 psi, 87.8% of sea level
10 psi turbo=(12.3+10) total of 22.3 psi, 90.2% of sea level
20 psi turbo=(12.3+20) total of 32.3 psi, 93% of sea level.
When talking about cars, no it's not, since you're adding a constant pressure (boost) on top of a pressure that varies with altitude (atmospheric pressure). So a car with its boost gauge set to 17 psi will have a higher manifold pressure at sea level than it will at higher altitudes.
I added more to my post above, that will explain what I'm talking about.
I was under the impression that FI cars wastegated the extra pressure they create, thus at higher altitude they would just not waste as much pressure. This actually makes aircraft more effecient and powerful at higher altitudes.
I could understand your reasoning if your induction system was working at maximum capacity at sea level, but if they were there would be no need for wastegates.
They modulate the turbines speed to limit the pressure created in the manifold. If a wastegate failed to function the turbo would create too much pressure in the manifold and cause engine damage. It is a fact that aircraft are more powerful at higher altitude due to higher efficiency of thier turbo, how is this untrue for an automobile?
Edit: I'm realize I'm randomly interchanging supercharger and turbocharger terminology and process, but it really makes no difference in this case.
In your previous post, you said "I was under the impression that FI cars wastegated the extra pressure they create". This is impossible, since the wastegate is on the turbine side, not the compressor side.
Of course the turbo makes cars more powerful at higher altitudes. I'm not arguing against that, and in fact I explained it in a previous post on this thread.
Maybe I'm just misunderstanding what you're trying to say. If so, I didn't mean to offend you.
You said that turbo's create less power at higher altitude, which is not correct. Turbos do wastegate excess pressure, exhaust pressure. Exhaust pressure directly affects intake pressure, so it's all the same whether I phrased it correctly or not, which I admit I did not.
Edit: FWIW, I was thinking of a blow off valve when I said wastegate earlier, which does disperse excess manifold pressure. It's late, I'm tired and my brains not running at 100%. Could be my wastegate's stuck open 🙂
Turbos do create less power at higher altitude. From my earlier post:
NA car= (atmospheric)
Turbo = (atmospheric+boost)
So at sea level you have:
NA car= (14.7) total of 14.7 psi
Turbo= (14.7 + 19) total of 33.7 psi
We're using sea level as the baseline, so at sea level you both have 100% of your power.
Now in Denver, at 5280 feet, the atmospheric pressure is 12.3 psi. So we have:
NA car= (12.3) total of 12.3 psi, 83.6% of what he had at sea level.
Turbo = (12.3+19) total of 31.3 psi, 92.8% of what you had at sea level.
As you can see, the turbo car did lose power due to altitude, although not at the same rate as the NA car.
It's ok dude, I'm tired also.
Zenmervolt
Elite member
Wastegates, at least most modern ones, are controlled by manifold pressure. That is, the wastegate opens at a specified manifold pressure. At higher altitudes, the wastegate will stay closed longer than at lower altitudes, resulting in less exhaust bleed-off from the turbine's hot side.
You're right that they bypass the turbine side and don't vent excess manifold pressure from the compressor, but they are controlled by that manifold pressure and absolutely will maintain the specified manifold pressure until the wastegate stays fully closed and can no longer compensate for the decreasing density of the air.
This is from experience with my 951, which runs a line from the manifold to a control valve on the wastegate. The line from the manifold is a trigger, and the control valve allows the wastegate to open only at a specified manifold pressure. The pressure of exhaust gasses against the valve is not sufficient to open it.
ZV
The boost above atmospheric pressure will remain constant, but the atmospheric pressure itself decreases with altitude. Therefore your boost still won't be as high as it would be at sea level.
I explained it in my post above. I'm not sure why people keeping missing where I clearly explained this. I'll post it again. Example is a car running 19 psi:
NA car= (atmospheric)
Turbo = (atmospheric+boost)
So at sea level you have:
NA car= (14.7) total of 14.7 psi
Turbo= (14.7 + 19) total of 33.7 psi
We're using sea level as the baseline, so at sea level you both have 100% of your power.
Now in Denver, at 5280 feet, the atmospheric pressure is 12.3 psi. So we have:
NA car= (12.3) total of 12.3 psi, 83.6% of what he had at sea level.
Turbo = (12.3+19) total of 31.3 psi, 92.8% of what you had at sea level.
As you can see, while the psig has remained the same, the psia has decreased, therefore reducing the power output of the engine.
I understand what you are posting. I'm not an expert, but it seems to me that, depending on factors such as boost control, car, engine, ECU, etc, the car could compensate for the drop in atmospheric pressure by raising boost levels. For instance...you are at 17PSI at sea level, and it increases the boost to say 20PSI to compensate for the difference in atmospheric pressure at a higher altitude...
You are saying the boost level remains static...
What I'm taking about probably is more turbo specific...
You are saying the boost level remains static...
What I'm taking about probably is more turbo specific...
Zenmervolt
Elite member
Except that the wastegate is controlled by absolute manifold pressure. Which means that, at altitude you have:
NA car: 12.3 psi total.
Turbo: (12.3 + 21.4) for a total of 33.7 PSI since the wastegate is not triggered to open until manifold pressure reaches an absolute pressure of 33.7 PSI.
ZV
This isn't the case for the vast majority of the turbocharged cars on the road here. I think yours may be different, though, since it's European. They tend to use BAR, which measures in absolute terms.
Does your boost gauge read in psi or bar? And is your wastegate controller by the input from the MAP sensor?
One of the problems is that many European cars use Bar instead of psi(g). At sea level with the cars off, the European car will show 1 bar, while the standard gauge will read 0. In Denver, the European Bar gauge will read less than 1 bar, while the standard gauge will still read 0.
From this site
BAR
An absolute scale, where 0.0 BAR is a vacuum and 1.01325 BAR is sea-level. The BAR scale is very common, especially on European cars. A BAR gauge is almost always an electronic instrument driven by the MAP (Manifold Absolute Pressure) sensor in your ECU. The MAP sensor has an absolute reference, usually an aneroid capsule, which allows it to determine the actual ambient pressure just as a barometer does.
If you drive well above sea level and turn an electronic BAR gauge on without starting the engine, you should see a value well below 1 (say about 0.84 in Denver).
psig
Pounds per Square Inch (gauge), the standard English pressure measurement, which is NOT absolute, but is relative to ambient atmospheric pressure. Most aftermarket mechanical boost gauges measure psig, so if you take this gauge to the moon and it will still read 0 when disconnected.
From this site
BAR
An absolute scale, where 0.0 BAR is a vacuum and 1.01325 BAR is sea-level. The BAR scale is very common, especially on European cars. A BAR gauge is almost always an electronic instrument driven by the MAP (Manifold Absolute Pressure) sensor in your ECU. The MAP sensor has an absolute reference, usually an aneroid capsule, which allows it to determine the actual ambient pressure just as a barometer does.
If you drive well above sea level and turn an electronic BAR gauge on without starting the engine, you should see a value well below 1 (say about 0.84 in Denver).
psig
Pounds per Square Inch (gauge), the standard English pressure measurement, which is NOT absolute, but is relative to ambient atmospheric pressure. Most aftermarket mechanical boost gauges measure psig, so if you take this gauge to the moon and it will still read 0 when disconnected.
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