964 High Altitude Question

w1clayton · 10-06-2011, 12:12 PM

Greetings,
I am a newbie - just purchased a 1993 C4, which is my first Porsche. Getting sorted out on some things, but can anyone tell me about high altitude issues. I live at almost 8,000 feet and often drive up to 10,000 feet and higher. Rarely go below 5,000 ft.

I see there was a factory option (not on this car) option code 925 for "high altitude areas of the US". I am wondering what this would be. Also, if I were to chip the car maybe that could help? All I know is the car runs well but pulled stronger at an elevation of 1,000 ft. where I bought it. Obviously all cars lose power at elevation, I just would like to manage it as best I can.

Thoughts?

Thanks,
Wilson C.

rsscotty · 10-06-2011, 12:39 PM

It could be a barometric cell that plugs into the DME harness next to the ECU. This would sense altitude and thus lean out the fuel mixture.

RSTarga · 10-06-2011, 12:49 PM

with a normally aspirated engine your going to lose quite a bit of power in high altitudes. I think for example at 7,000ft your about 25% less than at sea level, i'm sure there is a formula somewhere. Normally you want to advance the spark and lean out the mixture. Usually the more advanced computer controlled systems due this automatically but they probably have a lower range than what you are at. Why don't you ask a Porsche factory rep what that option is, my guess is that it moves the high and low range up the scale. One thing is you can get away with lower octane gas.

boba · 10-06-2011, 02:13 PM

What was said above is true, I think the stock ecu is good up to about 5-6,000 ft and then you need to adjust. With these cars it would make sense that it is a programing change. The Dealer should be able to update or add what you will need. A chip might work if no additional sensors are needed and it is programed for your altitude.

w1clayton · 10-06-2011, 03:13 PM

Thanks for the info so far. As an engineer, and a high altitude resident, I sure get the general loss of power. (Pumps and blowers don't work as well up here either.) I will be sure to explore the issue with a local dealer, if I can find one that has experience with it specific to 964's. I am especially interested if anyone has any additional specific input, so keep the comments coming.

Bill Verburg · Bill Verburg's Garage

964 only used the barometric altitude compensation device that plugs into the harness on cars w/o an O sensor & cat. On these vehicles the injection signal is reduced by 5% for each 1k ft of altitude.

It is not used on cars w/ O sensor because of the adaptive nature that the O sensor imparts to the engine management

Flieger · Flieger's Garage

Nitrous Oxide injection + more fuel = more power.

Traveller · Traveller's Garage

Quote:

Originally Posted by w1clayton

Obviously all cars lose power at elevation, I just would like to manage it as best I can.

Not just cars.

Now you know why some piston aircraft have turbo or super chargers.

You would think that a fuel injection system with feedback would at least adjust fuel mixture to the correct ratio (I'm not familiar with your 964, my 993 has an O2 sensor). That will give you as much power as possible without forced induction.

BTW, my carburetted BMW R65 motorcycle would not even idle on any of the higher Alps passes.

w1clayton · 10-08-2011, 07:43 AM

Thanks for the solid info, Bill. Anyone know what option code 925 for "high altitude areas of the US" is all about?

(I have to say nitrous might be the ultimate solution! (haha) Plus you get to have a cool toggle switch in the cockpit. I think chicks would totally dig that.)

-Wilson

Flieger · Flieger's Garage

In all seriousness, it could be a good solution if you can get a small enough shot. Just maybe 40 horsepower or so, and make sure to use a wet setup that injects more fuel with it.

w1clayton · 10-08-2011, 02:48 PM

Nitrous, interesting. OK, so being an enginerd, I had to make a few calcs:

hp loss = (elevation x 0.03 x hp @ sea level)/1000

247 stock hp at sea level
8000 feet elevation
59.3 hp loss
24% percent hp loss
10 typical rate of nitrous (hp/lb-min)

5.9 lbs/min nitrous consumption
20 lbs bottle
3.4 minutes nitrous will last

Wow - nearly 60 hp loss! Yup, that is why my car feels much stronger down below. Nitrous might be an interesting and fun novelty, but by the above calcs show a 20 lb bottle lasts 3 1/2 minutes and that calculation is only to get back to stock spec on hp.

My main issues are: (1) just to make sure the engine management is working as it needs to at this elevation, (2) I am certainly curious what the option code 925 is all about, and (3) wondering if a custom chip might be appropriate.

Still, Fleiger, the nitrous might be a blast!

Thanks,
Wilson

boba · 10-08-2011, 03:34 PM

This will not tell you very much more that you already know, but it is what I was able to find.

Per Adrian Streather's 'Enthusiast's Companion for 964'

Option Code 925 - "Version for high altitude areas in the US"

On page 143 he states -

"Altitude compensation system (normally aspirated models)

In the late 1980s and through the 1990s drivers in many nations did not have access to unleaded fuel. Therefore cars were not fitted with catalytic converters and traditional lambda controls using an oxygen sensor was not available.

In order to provide some control over the system an altitude compensation sensor was installed. The basic function of this sensor was to inform the DME of changes in air pressure."

It reads like a translation.

The shop manual might have some more information but I do not have a copy to reference.

Flieger · Flieger's Garage

It depends on how often you need the extra power. If it is just a street car, the corrected mixture, lower power output may be fine 90% of the time. When you get on a twisty road you would only use full throttle for a few seconds at a time. If you rig the N2O to inject (with fuel) only on full throttle, then you could make 3.4 minutes of nitrous last a good while. I guess it also depends on how easy it is to refill the bottle.

If you track the car, then turbo is the best solution IMO.

Flying Dutchman · 10-12-2011, 04:47 AM

Maybe contact a chip specialist like Steve Wong. he probably knows his way around this issue very good.

Boomtown Rat · Boomtown Rat's Garage

I run my 1991 C4 Coupe regularly through the Alps and Dolomites with elevations of up to 2,600 meters (8,500 ft.). Never ever, have I had a problem or noticeable power drop. Yeah, the car is thirsty, but that may also have something to do with the serpentines, switchbacks and hairpins uphill

Matt Monson · 10-12-2011, 08:46 AM

Here's the answer to what ails you...
Porsche 964 and 993 Supercharger Kit - TPC Racing

Lince · 10-13-2011, 05:13 PM

Quote:

Originally Posted by Matt Monson

Here's the answer to what ails you...
Porsche 964 and 993 Supercharger Kit - TPC Racing

Anyone has done this? I leave 8.500 ft above and would love to have more power

anery · 10-13-2011, 08:35 PM

If it is just a street car, the corrected mixture, lower power output may be fine 90% of the time. When you get on a twisty road you would only use full throttle for a few seconds at a time. If you rig the N2O to inject (with fuel) only on full throttle, then you could make 3.4 minutes of nitrous last a good while. I guess it also depends on how easy it is to refill the bottle.

laptop battery

Matt Monson · 10-14-2011, 06:53 AM

Quote:

Originally Posted by Lince

Anyone has done this? I leave 8.500 ft above and would love to have more power

Where are you located? It helps us get to know you if you completely fill out your profile. If, for example, you are here in Colorado, you could come out to Cars N Coffee and meet a 993 owner with one of those superchargers in his car.

Lince · 10-14-2011, 04:32 PM

Quote:

Originally Posted by Matt Monson

Where are you located? It helps us get to know you if you completely fill out your profile. If, for example, you are here in Colorado, you could come out to Cars N Coffee and meet a 993 owner with one of those superchargers in his car.

Bogota, Colombia