| DustenT | 02-26-2007 10:54 PM |
Here is some interesting reading about boost in higher altitudes. I'm planning on moving to Colorado soon, so I've been researching this myself. Quote:
This has come up a number of times. And I think this is important to understand. What I'm going to say... well, I have no problem if Pace finds something wrong and changes it or argues with it.
The first thing to understand is what your engine power really is. I once heard someone say you could tune a car solely on cylinder pressure. The basis for this was that cylinder pressure is the source of all power, and nothing else matters so long as knock and EGTs are not causing damage. Well, that may be close enough to true for a n/a car, but not a turbo. See, the power you get out of your car is not the power your engine produces. What you get is (The power your engine makes) minus (The power your engine consumes). That's a big difference. If you take a 5hp hit to the power your engine produces but make it spend 10 hp less doing so, you come out ahead, right?
Now, for an N/A car, there's not a lot you can do. You can open up the intake and exhaust and make the engine work less hard to suck air in and expel the exhaust. A short ram intake will make the engine expend the least amount of energy sucking the air through the intake, so the power your engine consumes will be minimized. A CAI will be a bit of a longer straw for your engine, but it might put more dense air into your engine, which will increase the power your engine produces by raising total cylinder pressure.
Now, let's look at a turbo car. Sure, the bigger the MAP, so long as it is sufficiently cooled, it will always produce more power. More air in, spark it, burn it, the cylinder pressure is insanely high. Great, right? Here's the problem - exhaust. Your valves open, and the engine has to work to push that exhaust out. Now, without a turbo that's fairly easy. With a turbo, the engine is pushing the exhaust through a turbine. If it's way outside of the efficiency range of that turbo, it's like it you walked up to a turbine at a power plant and tried to spin it. It's hard! So, sure, you can keep turning up the boost, but there's no point. Your engine will continue producing power, but it will consume that power and more. What that means is you're running the life out of your turbo and your engine for the purpose of moving slower.
You can still do things to reduce the amount of energy your car consumes for a turbo car. When we do exhaust, uppipe, and silencer removal, that's what we're doing. Smoother pipes, etc. Even the lighter pulleys and whatnot. Your engine isn't producing more power, it's consuming less energy in the process. You don't start making more power until you increase boost, engine managment, and that sort of thing.
Now, the turbo. I'm going to try to explain this without many numbers. A turbo is a very simple device. It doesn't know gauge pressure. It doesn't know absolute pressure. It only does ratios. Imagine if you ask a guy to make $10 for you. If you give him $10 seed money, that's kinda hard. If you give him $1000, that's easy, right? A turbo works hard based on the pressure that is coming out of it divided by the pressure going into it. So a turbo at sea level (14.7psia) would work the same amount to produce 14.7psig as a turbo up here (12.2psia) would to produce 12.2psig. It would be a pressure ratio of 2.0 in both cases. Keep in mind the absolute pressure coming out of the turbo is atmospheric pressure plus gauge pressure. So that means with a 2.0 pressure ratio you'd have 14.7+14.7=29.4psi at sea level. At high altitude that would be 24.4psi... sucks, don't it? That's what sucks for us. But even so, THE TURBO DID THE SAME AMOUNT OF WORK TO DO 12.2PSIG UP HERE AS IT WOULD TO DO 14.7PSIG AT SEA LEVEL!!!
Now, that's not being entirely honest with you. We measure pressure after the intercooler, and there is a drop there. We also are neglecting the intake, and there is a pressure drop there. You need to account for those things, and they usually make it even worse for us altitude folks. I'm not going to get into the details of that.
So here's what I'm saying - you can't run the same boost pressure as people at sea level for any given turbo. There is a little uncertainty because compressor maps are generally shown in volume flow rate, and we're dealing with 20% less dense air... but I don't want to pretend to guess at that math. Well, maybe I could, but I couldn't explain it without words like "enthalpy", and I don't think that would accomplish anything for anyone who doesn't already understand it.
If you want to get a little extra power from a MBC, that's fine. Maybe 14.5psig up here with some better exhaust. But any more than that and you're just consuming too much energy to spin the turbine.
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