PatrickP

Are plugs, wires, distributor caps and rotors for an '85 928 so "unique" that they can'tbe/shouldn't be purchased for AutoZone, Pep Boys, etc.? I'm sure that all of the same, except the plugs, on mine have at least 100k on them. Replacing w/i the next 2 weeks.

Thanks in advance for the guidance.

SQLGuy

On the 32V '85 engines, at least, caps and plug wire set are quite specific to the 928. Autozone etc may be able to special order them, but they'll almost definitely not stock them. Not positive how specific the rotors are, but I think they're pretty special as well.

Even a "generic" V-8 plug wire set would do you no good, as the plug ends of those sets are not meant for OHC engines.

Caps and rotors can be had 3rd party from places like rockauto. Best bet, though, is to talk with the usual suspects (928srus, 928 International, 928 Specialists, etc).

GregBBRD

Even if they had them...they might not be of good enough quality to use.

I just replaced a set of plug wires that had about 6 months of use that were absolute crap...causing lots of problems.

Make sure the caps and rotors are Bosch and the plug wires are Beru.

dr bob

Also, save some money by avoiding any of the 'gimick' plugs. The car willl run best with Bosch copper resistor plugs as supplied from the factory.

PatrickP

Thanks to all. BTW, I did check with Autozone about the wires and, sure enough, they had to be special ordered and we just as expensive as those on 928Intl. Appreciate all the help and insight.

JHowell37

On the '85+ the caps are used on some different vehicles. I know they're used on the '88-'89 Volvo 740 (turbo and non-turbo.) My local Autozone keeps them in stock, and I've used them before. They're made in Italy but other than that, there are no other identifying marks. The only difference I can detect from the Autozone cap versus a Bosch cap is the contact in the center of the cap that is in constant contact with the rotor is made of a different material. The Autozone caps are the same ones that NAPA sells, only in a different box. That center contact that I mentioned also doesn't seem to last as long on the Autozone/NAPA cap. I had to replace one last year (with only about 45K miles), and this year I'll probably be due for the other. You have to reuse the flimsy black covers from your old caps.

The autozone part #: RB928.

dr bob

Wires are a tricky issue, since the lengths vary so much. The factory Beru wires have resistors in the plug ends only, and none in the wires themselves. Most aftermarket wires are resistance in the wires and metal ends. Th firing voltages are therefore different if you choose an average aftermarket wires set.

SQLGuy

Well... sort of.

Resistance in the ignition system is to reduce the intensity of the spike at firing time, to reduce noise/interference in things like the radio and ECU, and, more significantly, to set the burn time of the spark. The resistance doesn't change much about the spark ignition event itself, as the voltage is going to be high enough to arc over regardless, but, once the arc is ignited, and current is flowing, the total resistance of the plug, wire, and plug connector, regulates how long it will take to exhaust the charge of the coil's magnetic field, and therefore how long the spark will burn. Correct burn time is pretty important for complete combustion and proper power and emissions.

The stock 32V system has copper (non-resistance) 7mm wires, resistor plug connectors, resistor rotors (I believe), and resistor plugs. If you were to use resistor wires with non-resistor ends, where the resistance of the wires was similar to that of the stock Beru ends, the overall result would be the pretty much the same as stock. The tougher problem would be finding ends that would correctly fit the spark plug access holes of a 928 engine.

PatrickP

Thanks for all of the advice. Seems as if the best course of action is to not be "penny wise and pound foolinsh" and get my parts from a source that knows 928s and their specific requirements.

dr bob

I won't beat you to death on the technicalities of how the differences in firing voltage between plugs seriously affects the way the car runs and particularly how emissions are formed.

The challenge with such varying plug wire lengths comes when you use resistor wires, with the large variation in resistance among them. Firing voltages would vary among the plugs, and the ignition timing at the cylinders varies as a result. These are characteristics of the coil and the ignition module(s) that are driven by the physics of the way the magnetic field collapses in the coil.

SQLGuy

Again, I agee, and disagree, with what I think you're saying. Firing voltage of the system depends almost entirely on the plug gap. The resistance of the wires, plugs, ends, whatever, is peanuts compared to the gap resistance before firing. Once the arc is ignited, the ionized air in the plug gap is an effective short, and the resistance of the system is the controlling factor in burn time. The inductances of the coils (primary and secondary windings) combined with the dwell time and dwell voltage, can be used to calculate the stored energy of the coil. That, now combined with resistance of the system, will tell you how long the spark will burn before the charge in the collapsing magnetic field is exhausted.

All told, you have probably 8K Ohms of secondary resistance in the coil itself, 2 to 3 KOhms in the rotors of a 32V engine, can't find a spec on the WR7DC, but I'd guess about 5K Ohms there, and the Beru ends (another 5K)? Let's say 20K Ohms as a likely system resistance for a firing coil. If you pick resistor wires so that the average length ones have 5K Ohms of resistance, then even if the shortest ones are 2.5K and the longest ones are 7.5K, you're still looking at burn times that will all be within 12.5% of the ideal.

Again, the resistance, as long as you don't have a bad (i.e. open) resistor, has pretty much nothing to do with firing voltage or initial spark timing. It has plenty to do with burn time; so, if your timing to so far over-advanced that you can't get good ignition from the leading edge of the spark, then resistance may make the difference between a long burn that still leads to (apparently) normal ignition, and a short burn that leads to a miss.

----------------------Edit----------------------
And by the way, as noted before, this discussion is pretty much purely rhetorical, since I still don't think there's much chance of finding suitable, non-resistor, spark plug boots for this car. :-) However, I do think you bring up a good point about why resistor ends can be better than resistor wires.