I skimmed the 928 owners manual, and did a search on this forum but didn't find much about this... It is in the repair manuals in the wiring diagram as a switch i can hookup on the ignition unit for low-octane fuel (i assume).

I want to think that it retards ignition, but i'm not sure. What does it do?

If it does retard ignition, does anyone know how much? Does it change based on rpm? Is it constant?

Thanks!!

 

It retards the ignition. I did not measured it, but would gestimate 3 degrees. When I applied it to my car, it seems down on power all round, so could be a constant 3 deg retarded.

 

this is only on 85-86 32V engines however

 

Ok, awesome. Thanks for the info guys!

 

what kind of fuel do you guys use in terms of octane. I couldn't understand from the various information i've recieved. Is it OK to use 87 or does it recommend higher?

 

Try this post (click here) and read Wally Ps' post it is the second one, he explains very well the octane rating system.

 

Porsche says to use 87 as the absolute minimum. They highly reccomend 92 octane however.

 

do you think better milage would result in use of 92 or is it strictly a knock setting?
Thanks

 

For our cars we can run regular unleaded without problems and maybe even have a little performance over the 92 octane gas. However when I add a supercharger to my car I'll definitely use the higher octane gas.

 

MY 2 cents on the Mileage VS Octane Rating issue:

As I understand it, lower octane releases slightly more energy than an equivalent amount of High octane gasoline. But lower octane gas cannot handle the high performance requirements of full throttle engine loads.

SO, when cruising at 65 down the highway (SF to LA), the lower octane gas will give you slightly better mileage. HOWEVER, once you factor in acceleration such as in city driving, or the higher engine loads of high speed cruising, then the engine must operate at a much lower efficiency (due to the low octane) to keep from pinging. The lower efficiency means you must push your foot farther to get the same performance which in turn offsets the gains from using the lower octane gas while in the city.

SO, if you baby your car, you drive it conservatively, and never push the gas pedal to the floor then low octane gas is perfectly fine. BUT, if you occasionally drive your car the way it was designed to be driven.... high octane gas is the only way to go.

A few years ago when I tested this theory on my Volvo 850R (2.3L turbo) I saw 1.5 mpg better mileage and a big performance difference when using the 91 octane with my agressive driving style.

Now back to your regularly scheduled program.......

 

Nathon, it's FAR more complex than that.

It depends on how your car's engine was created, in several ways.

Early 928 motors, were built dureing the end of the gas crunch, when regular gasoline was sometimes the only gasoline avalible. As a result, Porsche tuned the engine for use of 87 octane gasoline at peak effency. The engine is a reletivly low compression, without any of the electrical stuff of the S4 cars. As a result, the engine will not know what type of gasoline you put in the car, (no electronics) and won't adjust anything to compensate. This means that extra octane in a 84 or earler US 928 is just empting your wallet faster.

For the 85-86 US cars, the world was getting out of the gas crunch, AND the new gasoline addtive plants were comming online allowing higher grades of gasoline to be more readaly avalible. Porsche still was not useing the electronic advancements of knock sensors, but wanted a bit more power and effency that higher conpression would give them. Thus Porsche recomends a minimum of 91 octane fuel. Useing less than that would, IMHO be dangerous for the engine. This is also why Porsche added the "low octane loop" to the engine control system. There are still places in the US were 87 octane fuel is the ONLY fuel avalible. This loop causes the computer to retard the ignition timeing enough to prevent detonation with 87 octane fuel. The retarded timeing prevents max tempetures and pressures being reaching inside the engine. This prevents detonation, but it also prevents the engine from makeing maximum power. Again, useing 93 octane fuel in the car will only be throwing your money away.

Finaly, in 87, Porsche introduced the S4. The S4 has the higher compression ratio of the 85-86, BUT with some electronic equipment as well. The S4's engine control unit has a sensor that tells it if the engine is knocking or not. If the engine is knocking, it automaticly retards the timeing, and prevents the engine from destorying itself. Again, the retareded timeing prevents maximum power.

Now, also, knocking is only an issue when the engine is at or near WOT. Knocking only happens when the tempertures and pressures in the engine combustion chamber reach above a certain tempeture and pressure. This is very case specific. That temperature and pressure only happen at or near WOT. Clearly nobody is going to use WOT cruiseing down the high way. So unless you drive like your speed racer, you should not notice a diffrence in your fuel economey. If you like to go a good bit faster, then maybe you will.

 

Basically, higher octane gas burns slower. On early cars, it's true that with timing set at factory spec there is little benefit from using higher octane fuel. However, if you plan to always run high octane, you can advance the timing a few degrees.

I do this by ear. On a hot day, advance the timing a degree at a time and do some WOT runs between each change. When you begin to hear the faintest hint of detonation, back off a degree or two. If you do this on a cold day, then you will have detonation when you operate at WOT on a hot day. The few extra degrees of advance will result in extra HP on just about any car... enough to feel in the seat of your pants even on a 1.6l 4-banger.

If you're planning to try this and are at all unsure of yourself, I highly recommend that you try it first on another car that is not so precious to you as your shark. As others have implied above, this would be a useless exercise on any vehicle that electronically adjusts the timing in a closed loop manner. Such vehicles do the same thing many times per second on an ongoing basis.

D

 

Probably a dumb question, but...

How easy is it to advance/retard the timing on these beasts? Oil changes seem to be the only simple things to do on the 928.

Seems like a hundred years ago and might as well have been with the advances in technology, but it was cake adjusting timing on a '72 Toyota. I haven't purchased manuals yet for my 928 and do not know the ins and outs enough to know where to go, how to tweak (in what direction?) and what to be careful of.

Any simple instructions or pics or should I wait to get dangerously into the shop manuals?

Thanks!

 

Up to 84 in the US. REally simple. You adjust the ignition timeing by losseing a allen bolt, adjusting the distributer, and then tightening the allen bolt.

Other than putting in a new set of chips or the low octane loop into the 85+ US cars, it's impossible.

That said, I can't imagion why you would want to adjust the timeing of a S4. It is controled by a feed back loop to be perfect all the time. They sell a new set of chips for the 85-86 models that will give youa bit more timeing advance. Also, once you set even the early cars, there is never a need to reset it, there are no wearable componets in the system that would affect the timeing.

Maybe if you ran alought of boost, or something?

 

To clarify... I mean cars that do not have the capability to adjust their own timing, for example my 78 with CIS. I guess that means pre-85... VU, I'll take your word for it.

All of my cars to date have been able to benefit from tweaking the timing. But none of my cars have had EFI, though my girlfriend's Saturn does.

D