modifying motronic system
 

Does anyone have any info on how to modify a Motronic system out of any 85 Carrera? I'm trying to see what would be my best option for fuel delivery. Most people say to go with carbs. but I've also heard of modifying the present system to perform even better than carbs.

Thanks All

The Motronic system is set up to provide the "right" amount of fuel under pretty much all circumstances. It should run rich at full throttle, and right at stoichiometric (14.7:1 air-to-fuel ratio by mass) at idle and cruise for good behavior and good economy. Carbs will "dis-improve" this significantly, usually by making the car run rich under all circumstances.

What the carbs will do for you is twofold. They are very modification-tolerant, you can for instance change out the cams and upsize the valves and port the heck out of everything and you can re-jet the carbs and they should work. (Though a 3.2 is supposedly on the border between "40s are too small" and "46es are too large", so choosing which carbs can be problemmatical.) The Motronic system is much less tolerant of engine modifications, particularly aggressive cam grinds.

The other thing they do is they remove the flow restriction of the stock intake manifolding and air flow meter. The early Motronic system measures incoming air by having it push on a spring-loaded flap, which is somewhat of an impediment to getting air into the engine. Carbs place much less restriction on the air flow, most especially at high flow rates (e.g., high RPM and wide-open throttle). Note that more-modern systems, like the VarioRam 993 intakes, actually flow better than the carbs at most RPM and load combinations. But I do not believe this is the case with the early Motronic in the 85 Carrera.

You can get rid of some of the intake restriction by converting to a MAF (mass air-flow) sensor instead of the "flapper". One of the 914-6 conversion guys did this swap and is ga-ga over it. I don't know how that compares to a properly tweaked set of carbs on an otherwise-stock motor, though.

You may get some (very small, usually) gains by "chipping" the DME. The gains can be larger if you have a modified motor (intake, exhaust, cams, etc.) than stock. The 914-6 conversion guy above chipped his motor at the same time as putting the MAF on, so that may be part of it. (Oh, and note that a 914-6 conversion has to have headers on it, rather than the roundy-round exhaust of the mid-range 911.) On a stock motor, a chip will usually advance the ignition timing (requiring higher-octane fuel) and enrich the mixture slightly--theoretically to the "max power" point, but perhaps past that to where power starts dropping off again. The fuel mixture is a very very small effect; the timing is the larger one and it ain't gonna be a whole lot.

In general, Porsche doesn't leave a whole lot of "free and easy HP" on the table.

--DD

Does anyone have any experience with a company called Modern Performance that advertises in Grassroots Motorsports? They make a MAF system and chips for Motronic cars. Just curious

Armando, thry these guys, they seem to be the guru's of the Motronic systems!

http://www.systemsc.com/background.htm

I want to make my question a little clearer. I will be building a race 3.4 (originally 3.2). The standard is usually 46mm PMOs. But I've also heard stories of modified Motronic systems which run even better than carbs. So far some of the mods I've heard were:

Port, Polish and match intake
remove flap in the MAF
larger throttle body
alter resistance in air temp sensor???
and of course modify and dyno computer.

Does anyone know anything else. Not only am I woundering if it would work, but also if it would help my wallet.

Thanks everyone for responding.

Armando, call ANDIAL, they are experts at making RACE engines from 3.2 motors 714-957-3900

Thanks bigrubberjeep will do.

No problem - we have the same goal, my budget is just not as cooporative!