BanzaiGTti

Aquamist has been proven to reduce the onset of possible detonation. While power gains from using Aquamist have been deeply debated, you probably will NOT gain power from using Aquamist itself but from being able to advance ignition timing.

Having said that, a lot of people have been successful in getting noticeable power gains from using 50% water 50% methanol. A bit of a pain the behind when it comes to topping up the methanol though, if you asked me.

One more benefit from using Aquamist is that it keeps your combustion chamber squeaky clean. The effect of direct water injection is similar to steam cleaning. Can't remember the explanation for this.

Adrian

Dear Chip.
The purpose of the O2 sensor is to keep the emissions (exhaust) output within certain limits. It achieves this by adjusting the fuel air mixture. However this is not always the correct fuel air mixture for optimum power.
The O2 sensor is not even used until its temperature has reached a minimum of 600F. The O2 sensor is switched out of the cct when the pedal is at the wide open position. The pedal is often at WOT when climbing a mountain.
Summary: The O2 sensor is an emmission control device not an optimum fuel/air mixture control device. Its signal output is derived solely from the contents of the exhaust gases.
Ciao,
Adrian
964C4

914und993

You are right of course that the best air/fuel mixture for emission control (14.7:1) is not the best for power (12.6:1), and that when the wide open throttle switch is tripped, the DME ignores the O2 sensor input and applies a pre-mapped mixture enrichment.

I would however be surprised if air temperature was not a factor during wide open throttle operation, and equally surprised if the DME couldn't come just as close to the right max power mixture at high ambient temperatures as it could at lower temperatures. Additionally, the power curve when plotted against air/fuel mixtures is relatively flat in the region of 12.6:1, so minor deviations do not have large effects. In short, I do not believe that mixture errors due to higher temperatures in our Motronic equipped cars have any palpable effect on power produced.

There might be additional power loss due to retarded ignition timing to prevent detonation at higher engine and intake air temperatures - that would be much more plausible to me.

Chip

Adrian

Dear Chip,
At WOT with "our" 964 DMEs the air temperature sensor input is not used anymore. By not used I mean that at WOT the adjusted air flow sensor signal (air temp adjusts or tunes this signal) remains at 4.6 volts no matter if the actual signal does change. Air temp adjustments are so rough that they actually make little difference.
I think that you may over estimate the sophistication of these DMEs. They are in reality very basic. In fact they are derated from the systems used on the BMW, Volvo and Jaguar of the same era. Porsche did not really get on board until the 1996 model year 993s.
The sensors used in the Carrera 3.2 and 964 are also pretty rough and much is dependant upon the pre-programmed maps (calculated fuel schedules) which were not very wide in the altitude or temperature ranges.
I can assure you that the 964 loses significant power at very high temperatures. In the very early days of the 964 an altiitude compensation device was installed. However this messed with the O2 sensor and emissions and never made it to the production models except those delivered without emissions control option M150. You can see just from this that Porsche and Bosch were dealing with the issue but decided emissions control was the priority and putting more money into the system for altitude correction (air pressure corrections) was not part of the program at the time. If you knew more about the 964 program, lack of funds etc this would also make more sense.
Ciao,
Adrian
964C4

914und993

Thanks, Adrian, that is very illuminating. I suppose all that leaves me with is surprise that the Motronic used until 1996 wasn't more sophisticated.

I guess I shouldn't be, considering that Porsche was still using mechanical distributors all the way up to 1998.

Chip

Adrian

Dear Chip,
Porsche made a number of cost related decisions in the 80s. The 964 was a real victim of these decisions. I have often been told that the 993 was what the 964 should have been. A lot of the systems used in the 964 are sort of one and a half generation instead of second generation. The ABS is a perfect example along with the hybrid Motronic and L-Jetronic fuel injection system. One factor I did not mention regarding fuel air mixture. The fuel injectors as you know are pulsed. They open and close between each squirt of fuel. The amount or the ability to adjust the fuel being supplied is limited by the physical abilities and the speed at which the fuel injectors can open and close.
Ciao,
Adrian
964C4