Timing For 72 T at 4000 Feet Altitude

[IAN]

I have a 72 911 T 2.4 MFI Car.

I live just below 4000 feet altitude and I'm wondering if I should adjust the timing to deal with the thinner air.

I do notice a pretty substantial difference between 4000 feet and sea level, mostly in acceleration.

Thanks,
Ian

[2.7RACER]

Altitude will definitely make a difference in engine HP. Here in Albuquerque we're at 5500 ft. Guys around here have been tuning for high altitude forever.
This is the bottom line. You have 29.92 inches of mercury atmospheric pressure at sea level. That's called standard pressure.
The pressure drops right at one inch of mercury for each 1,000 ft over sea level.
At 4,000 ft. the real atmospheric pressure will be 25.92 inches of mercury.
Thats about 13% less air pressure. 13% less dense air. 13% less air in your motor for the fuel to mix with.
So you get about 13% less torque and HP at 4,000 ft. on a standard day.
As for the timing, the rule of thumb is one degree advance for each 1,000 ft. altitude.
The good news is you'll won't need the higher octane fuel at 4,000 ft. that you need at sea level. Why? Because the "effective" compression drops by that same 13%.
If you check compression at 4,000 ft it will be again 13% lower than at sea level.
You TV weatherman reports the barometric pressure for your location corrected for sea level.
The actual pressure will be about 4 inches of mercury less.
If there is a dyno in town, they can tell you the actual pressure. They need it to correct the measured torque to determine HP.
The other deal is if you tune your motor for 4,000 ft and then drive it at sea level, the mixture will be lean. It has caused some folks around here to burn pistons when driving under a load near sea level.
So if you drop down to sea level be sure to retard your timing and richen the mixture.
The folks coming from low altitude to high don't have this problem because their engines go rich and retarded at altitude.
Of course modern engines sense the altitude change and reset timing and mixture.
This is the short explanation. There is more.

[mjw]

Regarding optimum fuel/air balance, doesn't MFI have barometric compensation built in?

[pwd72s]

Ian, I don't think it's really a timing issue. As mjw said, MFI has built in compensations. The bottom line is that the higher you go, the less oxygen there is per cubic foot of air. Bend is close to 4,000 feet? Wow...that's not a lot lower than when I cross the summit of the S. Santiam pass. No wonder my car feels gutless over there...it usually resides at 300 feet above sea level. I remember crossing Colorado's Independence pass...13,000 feet.
Felt like I was driving a VW bus, but I was faster than most...

[IAN]

I used to feel like my 72 2.4T was pretty fast when I lived around Portland but it seems quite a bit slower over here. My understanding is that it is around 10% slower. That's quite a bit.

Not much I can really do it about it I guess...

I did install Pertronix on Gammroth motors advice. So far it works great. It installed in about an hour and I the car runs really smooth and nice.

[pwd72s]

Altitude and horsepower...it all equals out. Ian, I'd suggest that you not worry about it, as long as you think your car is running okay.