3.2L Carreras - DME have compensation for altitude?

[GrantG]

I was curious if the 84-89 Carreras have the ability to adjust mixture and timing based on air density (altitude). Specifically, if someone has a chip that is set for max power at sea level, does one need a different custom chip for the same car at 6,000ft?

Also, if a MAF is used, what other complications does this cause?

Many thanks!

[Hendog]

They compensate with a little device under the drivers' seat which senses atmospheric pressure.

[Wil Ferch]

It's the round thing next to the DME box under the seat....

- Wil

[Rot 911]

Someone has posted on here before which each of the settings on the adjuster does.

[Wil Ferch]

no...the DME has a 7-8 positon switch...the round altitude compensator does not have an adjustment...AFIAK..

Wil

[Zef]

All you need to know is there...
http://www.911chips.com/fuelsys.html

[Zeke]

Bentley says it's an on-off switch set for 3000 feet. (Hey, 3 weeks of ownership and I'm gettin' good, eh? )

[bell]

here's the info i found on this bbs a while ago.....from this thread 3.2 dme tunning!

Postion 1 is fully counter clockwise:
Fuel Ignition
1 0 0 stock setting
2 +3% 0
3 +6% 0
4 -3% 0
5 0 -3deg
6 +3% -3deg
7 +6% -3deg
8 -3% -3deg

[Steve W]

As mentioned previously, there is an altitude compensation sensor besides the DME that leans out the mixture by reducing the injection pulse width by 6% at altitudes above 1000 meters, or 3250 ft. The problem is the compensation is crude. In one case study a ROW spec 3.2 precisely tuned for optimal air fuel ratios at an altitude ele' 6500 ft, ran leaner and rougher each time the car descended to ele' 4500 ft. The AFM did not naturally compensate; using the fuel quality switch to richen the overall mixture got it just about right. Of course at both these altitudes, the altitude sensor was still activated. Descending to an altitude of ele' 2000', where the sensor should have switched off and richened the mixture, the car still ran lean, and predictably so. So the FQS was utilized to richen the mixture.

A MAF system does a much more accurate job of measuring the oxygen content in a given mass of air, as it only measures the density and mass of air, unlike the stock flow meter which measures a volume of air. By being able to measure a mass of air, or the exact amount of oxygen passing through, an barometric and temp sensor is not required to compute air mass. A MAF consists of a hot wire or film element which is heated to a specific temp, and the denser the mass and cooler the air that flows over the element to cool it off, the greater the electrical current necessary for the element to maintain the predetermined temp. This measured current is what is used to compute the mass of air. A MAF though brings on a whole different subset of issues or 'complications' unrelated to altitude changes. Primarily they cannot just be a drop in replacement for the stock AFM sensor, but require a completely dedicated and specially developed software package, along with oftentimes a special microprocessing interface box to properly transform the signal to what an AFM Motronic system expects to see. Developing such a system is an extremely complex undertaking as all operating condition factors need to accounted and tested for to ensure smooth and reliable operation if you want to maintain to exceed the factory performance and drivability. Conditions such as cold starts, freezing temps, low part throttle drivability, and temperature dependent ignition trim compensation are all important factors to work out.

No matter how fancy the MAF sensor, the quality and performance of the system will only be as good as the person who developed the MAF software and system and is what ultimately separates a poor or mediocre system from that of a superbly designed one. As you would expect, the quality difference the few systems that have existed are tremendous.

[Lorenfb]

"Descending to an altitude of ele' 2000', where the sensor should have switched off and richened the mixture, the car still ran lean, and predictably so. So the FQS was utilized to richen the mixture."

Let's not over-complicate the situation:

1. No need to "play" with the FQS, just disconnect the altitude sensor, and
2. Then replace the altitude sensor, as it failed to function properly.

To even think (or mention) about a MAF as a solution to a minor problem is incompetence
on the part of any service advisor or technician.

No need to redesign a 20 year old car, just fix the problem!!!!!!

[Wavey]

To rephrase the original question, does this forum have compensation for ********* comments?

[Steve W]

The sensor was tested as functioning. Thank you for your comments.

[RickM]

For the first time since '87 I'm experiencing a crank-no-start condition...out of the blue. While I was down there I took this for reference....

[GrantG]

Thanks for all the info, folks!

[bell]

Originally posted by Wavey
To rephrase the original question, does this forum have compensation for ********* comments?

hey!!! lorens back
and suprisingly posts right after Steve W....i've seen that before....can't remember where......just can't place my finger on it......

yes that was sarcasm