Carpokes Porsche Forum

BennSport wrote: Tue May 19, 2026 8:49 am Hmmm, I might just have to bite the bullet and get that MAF and pull the sensor then. I notice a significant decrease in performance when I take my car up the windy mountain roads.

You can also try adding fuel with the FQS. The altitude switch correction is pretty crude with a big fixed reduction at one particular altitude. It means a car driving at 3400 feet will have 6% less fuel than a car at 3200, despite very modest differences in fuel needs. It also means a car at 3400 feet will get the same fuel as someone at 10k feet, despite significantly different fuel needs.

For what it's worth, the percent fuel reduction once the switch closes is a single cell in the DME chip, and that cell is available in our Tuner Pro files. So if it pulls too much fuel at your altitude, you could adjust that in the chip, but my guess is you can get close to what you need with some combo of the levers you already have -- i.e., keeping the switch connected and adding back fuel via the FQS, or disconnecting it and pulling full via the FQS. I have to wonder what it would be like to live at 3281 feet and having the fueling change by 6+% every time you go up and down a hill.

Keep in mind, you are also fighting physics. At higher altitude, there is just less oxygen available, so power will always be down. That's why turbos became so popular on planes in the early days...

Tom wrote: Tue May 19, 2026 8:00 am Curious, does that MAF come with chips?

The Storch DE MAF does not come with, nor does it need any special chips. I do have a chip in my S2 that I got in 1998 from Steve Russakov on RL. He described it as a "Weltmeister clone." It seems to work well and I haven't had any issues in the last 28 years. He sent me an updated chip that I still have but never got around to installing it.I should pop it in when I install the MAF.

The Lyndsey for the 944 NA does come with a chip board as @NC944ER has said.

Latitude48 wrote: Tue May 19, 2026 9:57 am

Tom wrote: Tue May 19, 2026 8:00 am Curious, does that MAF come with chips?

The Storch DE MAF does not come with, nor does it need any special chips. I do have a chip in my S2 that I got in 1998 from Steve Russakov on RL. He described it as a "Weltmeister clone." It seems to work well and I haven't had any issues in the last 28 years. He sent me an updated chip that I still have but never got around to installing it.I should pop it in when I install the MAF.

The Lyndsey for the 944 NA does come with a chip board as @NC944ER has said.

That would suggest a compromise in terms of the air-flow curve, though if the MAF's curve is close the the AFM's, probably not any more of a compromise than other MAF-chip combinations that don't re-write the DME code to deal with the actual mass air signal (i.e., other than Vitesse and M-tune).

Tom wrote: Tue May 19, 2026 9:03 am

BennSport wrote: Tue May 19, 2026 8:49 am Hmmm, I might just have to bite the bullet and get that MAF and pull the sensor then. I notice a significant decrease in performance when I take my car up the windy mountain roads.

You can also try adding fuel with the FQS. The altitude switch correction is pretty crude with a big fixed reduction at one particular altitude. It means a car driving at 3400 feet will have 6% less fuel than a car at 3200, despite very modest differences in fuel needs. It also means a car at 3400 feet will get the same fuel as someone at 10k feet, despite significantly different fuel needs.

For what it's worth, the percent fuel reduction once the switch closes is a single cell in the DME chip, and that cell is available in our Tuner Pro files. So if it pulls too much fuel at your altitude, you could adjust that in the chip, but my guess is you can get close to what you need with some combo of the levers you already have -- i.e., keeping the switch connected and adding back fuel via the FQS, or disconnecting it and pulling full via the FQS. I have to wonder what it would be like to live at 3281 feet and having the fueling change by 6+% every time you go up and down a hill.

Keep in mind, you are also fighting physics. At higher altitude, there is just less oxygen available, so power will always be down. That's why turbos became so popular on planes in the early days...

I have no clue how to mess with tuning the DME with the files you listed so I’ll probably just tinker with my FQS

Tom wrote: Tue May 19, 2026 8:00 am

BennSport wrote: Mon May 18, 2026 8:48 pm

NC944er wrote: Fri May 15, 2026 3:17 pm The factory AFM unit measures intake air flow and air temperature. It was paired with a separate altitude sensor/switch that activates at 5,000 feet above sea level, changing the DME to a high-altitude fuel map, which only estimates density changes under that specific condition.

As for the MAF setup, because the physics of the hot wire sensor automatically account for changes in density on a continuous basis (such as variations in temperature and altitude), it will provide more precise measurements at all elevations.

If you only plan to drive your car at lower elevations, the altitude sensor will have no effect and can remain connected, but certainly disconnect it if you live/operate at 5,000 feet or above, as it will lean out the air-fuel mixture.

Well as someone who lives 4500 feet above sea level, would it be a good idea to disconnect mine?

Curious, does that MAF come with chips? @johnb can confirm hopefully, but I'm fairly sure the altitude correction switch acts as a global modifier on fuel -- like the FQS -- and since the MAF is already accounting for the lower air density at 4500, you would not want to reduce fuel further with that switch. In other words, yes, I'd disconnect. It's up high and to the right behind the glove box, and has a simple connector you can just pull apart to disconnect.

Yes the altitude switch just pulls 6% globally. It's true purpose is not totally clear. It triggers at 1000M, and it's only present in US cars, but obviously people living at 1000M and above is not limited to the US. Figuring out exactly why it's there is complicated by the fact that no one seems to know quite what the stock airflow meter actually measures. People usually say it measures air volume, but how it would manage to be insensitive to air density is not clear. I'm still trying to get to the bottom of that.

If you read the test procedure for the altitude switch in the test plan, it basically says to short out the connector manually and then check CO levels.

The US had stricter CO limits, but also an O2 sensor and closed loop fuel control. The AFM also has a leak - a bypass channel that adds a significant amount of un-metered air at idle. So even if it was an accurate mass air meter for most of its range, it would still be off at idle. Because of all that, my best guess is the altitude switch was only for getting the cold idle CO levels within allowed limits. At warm idle, CO bypass adjustment gets it close and the O2 sensor feedback does the rest. For RoW cars, I'm guessing they just accepted it being less accurate at cold idle. And for RoW part throttle I suspect the AFM just handles it by measuring the mass accurately enough.

That actually makes a lot of sense. The idle bypass air and closed-loop correction would explain why the altitude switch mainly matters during cold idle emissions testing.

Tom wrote: Fri May 15, 2026 7:00 pm The 944 Turbo DME "Test Plan" says the altitude sensor triggers at 1000 meters (~3,281 feet) and I believe the N/A uses the same part (944 606 051 00).

(I see 5,000 feet listed a lot -including chatbots- so suspect that is one of those internet things that gets echo'd around, unless there's something I'm missing?)

The 5,000 feet figure was conveyed to me by Julius of Storch during our testing discussions, but the Porsche document is likely the best source. Thanks for the clarification!

The altitude switch probably leans the mixture out in order to prevent the catalytic converter from overheating. This would explain it's absence on non-US (i.e. non catalytic) cars.

NC944er wrote: Wed May 20, 2026 6:31 am

Tom wrote: Fri May 15, 2026 7:00 pm The 944 Turbo DME "Test Plan" says the altitude sensor triggers at 1000 meters (~3,281 feet) and I believe the N/A uses the same part (944 606 051 00).

(I see 5,000 feet listed a lot -including chatbots- so suspect that is one of those internet things that gets echo'd around, unless there's something I'm missing?)

The 5,000 feet figure was conveyed to me by Julius of Storch during our testing discussions, but the Porsche document is likely the best source. Thanks for the clarification!

He's not the only one who says that, but the Porsche literature is consistent at 1000 meters. I have to believe it's one of those false precision specs -- I bet it varies by 100+/- feet just based on barometric pressure/weather on any given day. I find myself wondering if the hills near my house are 1000 meters high. The world needs more real world 944 data.

On an 8v NA car like my own that doesn’t have a removable DME chip, is there any way aside from the butt-dyno to see if my FQS changes make a difference? I would also love to start tuning my car if I could, but I know the early computers have a lot of restrictions