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Old 06-24-2008, 06:23 PM   #1
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Default MiataLink altitude compensation?

New here to the forum, but not new to Miata FI. However, there is a nagging question that has plagued for for some while now...

Does an FMII kit with MiataLink provide a true XXpsi boost regardless of altitude due to the MAP sensor?

I'm trying to understand if the boost is set for, say 15psi, do you get a true 15psi of boost at 7000' altitude, or less boost due to the thinner air?
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Old 06-24-2008, 06:30 PM   #2
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From reading the manual, you would get close to true boost.

Also you set (check) the altitude when you start your tuning and the Link is supose to compinsate for altitude changes.
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Old 06-24-2008, 06:33 PM   #3
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um... yes IF you're using closed loop boost control
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Old 06-24-2008, 06:38 PM   #4
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Actually, you'll get more than 15 psi of boost at 7000'. Assuming you're running electronic boost control and your turbo is capable of it, you'll run whatever manifold pressure you specify. If you're set for 15 psi of boost (aka the increase in manifold pressure relative to ambient) at sea level, you'll have closer to 20 at 7000' and your gauge will reflect that.

That's not a function of the Link, but of the boost control.
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Old 06-24-2008, 07:15 PM   #5
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I'm a bit confused by which gauges are reading "absolute" and which are reading "relative".

Does my analog vacuum/boost gauge read "0" at 7000 feet with the engine off due to it reading "relative" pressure, and therefore will always read "0" at rest regardless of altitude?

As to Keith's point: I had boost set at 12psi sea level, and the analog boost gauge would reflect that--however, at altitude, it also reads 12psi at boost. Am I missing something here?
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Old 06-25-2008, 05:28 AM   #6
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my understanding of this is; while altitude affects turbo cars, the effect is not nearly as noticeable as in an na car. As long as you have not maxed out the turbos efficiency(either by tuning your car that way at sea level, or by going higher up than most of us get to go), you will reach your set psi, but the turbo will be working harder. Chime in please if im wrong.
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Old 06-25-2008, 10:05 AM   #7
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Keith it sounds like you're saying that using closed loop boost control targeted/tuned at 183kpa on the LINK, is going to actually produce 238kpa at 7k ft.?

I think what you're saying is with closed loop, the turbo has to work harder to produce more boost. This means first compensating for less pressure than that of sea level and then adding to that the boost it normally would at sea level. But in the end, all the LINK sees is the absolute pressure in the manifold - so no matter what elevation it will bring the manifold up to the target pressure (to the best of it's ability).

If you then took the target manifold pressure at 7k ft and moved it to sea level it would show as a higher pressure.
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Old 06-25-2008, 11:21 AM   #8
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The Link boost control only does absolute pressures. So if your boost control is set to 183kpa, you'll get 183 kpa.

Boost gauges are relative to ambient. At rest, they show 0. Under boost, they show the difference between ambient and the manifold pressure. I understand "boost" to be the delta over ambient, so 15 psi of boost at sea level is a higher absolute manifold pressure than 15 psi of boost at altitude because it's starting from a higher ambient pressure.

Manual boost controllers will let you set boost, but not the absolute manifold pressure. So a 15 psi MBC at sea level will remain a 15 psi MBC at 7000', while a 15 psi EBC at sea level will show higher on the gauge.

Note that this is about boost control, not the Link specifically.

Turbos do have to work harder at altitude. You need about 3 psi of boost around here just to get to sea level pressures in the intake manifold. This means hotter intake temps, more backpressure and slower spool. But you're right, turbo cars don't take as much of a hit as n/a or supercharged ones, and you can really feel the difference when you're climbing passes.
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Old 06-25-2008, 12:35 PM   #9
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My understanding so far: 15psi boost control setting at sea level will still see 15psi manifold pressure at 7000', but the turbo will be working as if it were pushing 20psi at sea level--therefore potentially moving the turbo out of its efficiency range.

What is MBC/EBC?
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Old 06-25-2008, 12:42 PM   #10
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Manual Boost Ccontroller
Electronic Boost Ccontroller
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Old 06-25-2008, 12:45 PM   #11
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Let's address the "how much HP loss at altitude for turbos":
Common knowledge is "not as much loss as na or S/C, but still some loss".

Would the following logic and calculations be correct:
Sea level atmospheric pressure is 14.7psi.
Atmospheric pressure at 7000' is ~11.5 psi. (21.8% loss of pressure)

Therefore, the power loss for a na engine should be ~21.8%
However, for a turbo at 15psi the loss would be:
Sea level: (atmos. pressure + boost pressure) = 14.7+15 = 29.7
7000' : (atmos. pressure + boost pressure) = 11.5+15 = 26.5
Loss at 7000' vs. sea level = 3.2 psi, which equates to ~10.8% loss.

So in this case of 15psi, the turbo engine only sees losses of about half of what a na car would lose?
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Old 06-25-2008, 12:47 PM   #12
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Quote:
Originally Posted by jayc72 View Post
Manual Boost Ccontroller
Electronic Boost Ccontroller
Thanks!

Interesting--I could swear that my analog boost gauge reads the same at altitude under full boost as when I was at sea level (running Link EBC).
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