Simon928

I asked this question but it was buried in another thread and I don't want to derail it any further so I thought I would make a new post and ask it again here. I have a '90 S4 and my crank pulley became damaged so I replaced it with a pulley from an '85. I initially thought that they were the same but I then discovered that in 1990 they made the crank pulley for the alternator belt a little larger. The '85 crank arrived in the mail the same day that I discovered this fact so I figured what the heck, I might as well install it and see how it goes. I drove it around the block and I believe that all seems fine. Measured at the battery with the car running is shows 13.75 V at idle, and with AC, headlights & stereo turned on it shows 12.8 V. This corresponds to what the voltmeter on the dash says, and when I rev the engine a bit the voltmeter goes up, but I was alone so I wasn't able to test at the battery for the exact reading. 12.8 is barely good enough to keep the battery charged from my understanding, but I don't know how different that reading is compared to how it was with the larger crank pulley. I assume that this number would be even worse with a heat soaked alternator.

My question is, am I courting disaster by running with this smaller crank pulley for the time being or should I be okay? I was thinking that if the '89 S4 uses the same alternator with the same smaller pulley that I have installed then it *should* be okay, but there might be something that I am missing so I just wanted to get confirmation from those more knowledgeable than myself.

FredR

If you are running a pulley the same diameter as fitted to the 89 then you are no worse off than the 89.

It is not a question of a disaster rather you are not as optimal as you can be and even the 90 setup can struggle at night with the a/c and headlights on round town and the Hi Fi going full blast.

polecat702

I replaced my stock alternator with one that I had rebuilt to put out 200 amps. I put the smaller pulley on the replacement, because it has a keyway for the woodruff key. It charges just fine at idle, with the A/C and headlights on. I'm deaf, so blasting the stereo isn't a concern.

Simon928

Thanks! I didn't think that there would be any major issue, but I appreciate the confirmation.

dr bob

Simon --

Be Aware that the slightly smaller drive pulley on the -'89 cars receives a slightly shorter belt too. Make a note in your logbook of the pulley changes so the right belt can be ordered when needed in the future. And that factory spare belt that came in the spare wheel with the gloves and compressor? It will be wrong too.

Simon928

Thanks for the heads up! I actually already ordered the shorter belt yesterday, so it should hopefully be in my hands by midweek. The longer belt that I already have does kind of fit, but I have to tighten it to the fullest extent of the tensioner, and it should still be a little bit tighter hence me ordering the shorter belt. I'm still going to keep my eyes open for the proper crank pulley so that I can return it to stock at some point. The changes in the pulley and belt are already noted in my service history for future reference.

dr bob

I guess I need to add a '90+ pulley to my Christmas parts planning when 928 International offers a holiday discount.

Rob Edwards

Timely topic as I just finished a round of plating everything in the garage that wasn't tied down. I have 5 S4+ alternators here, I know at least 2 are from '89s, plus a '91 and '93, not sure about the 5th. The pulleys fall into 2 groups, ones with 54 mm OD vs. 55mm OD.


54:



55 mm


When measured on the tops of the grooves, the diameters are 48.7 vs 49.4 mm,

Circumference of 48.7mm diameter = 153mm, if belt wrap is roughly 180 degrees, that's 76.5 mm of belt wrap.

Circumference of 49.4 mm diam = 155mm, only 2mm different.

Doesn't seem like much of a difference. Am I missing something?

huskeric

A smaller alt pulley will create more revolutions per minute, thereby actually creating more charge on the alt. On the other hand, this will also have an effect on the horsepower draw as it creates more power to operate.

A smaller crank pulley will slow down the revs of all the other pulley driven accessories.

This is a popular mod to the Mustang group - using larger 'under drive' pulleys (alt, water pump, etc)/'smaller
crank pulleys, which draw less HP. There are various schools of thought on these mods and I believe the actual HP increase to be miniscule.

I would opt to stay w/the smaller pulley on the alt and the larger on the crank.

dr bob

How myths propagate....

Changing the driven or drive pulley size changes the ratio and therefore the relative speeds of the alternator. It does not change the amount of horsepower needed to make a given amount of charge/energy. It does change the effective mass that the engine needs to spin up when accelerating, and the amount of belt friction, if that's sufficiently important.

huskeric

not propagating any myths here...

as I mentioned above, I noted there may be miniscule gains from the parasitic loss; however, I don't see this to be of any real value, which is why I suggested essentially leaving things as is.

Alan

The other important factor here wrt to the ratios in play is what the idle RPM is set to? if you want better idle performance from your alternator you can change the alternator, change the crank to alternator ratio or change the idle RPM or some combination of these. A smaller alternator pulley (I think you can go down to 52mm) will also have a smaller drive belt contact patch - which can create friction/slip issues - especially if you succeed in pulling more power out of the alternator at idle.

If you need a larger crank to alternator ratio to generate better it may be better to use a larger crank pulley Vs. a smaller alternator pulley (options are limited) but you also have to be careful of the max allowable alternator RPM - you likely won't be able to change the ratio that much. Self destructing alternators are expensive mistakes.

In the end most cars generate OK until they are hot - so a better regulator with a less aggressive temperature de-rating factor may have the best bang for the buck of all these options (e.g. Dave Barton remote regulator). A new ECU EPROM with stock maps but a slightly faster idle may be the next least expensive option (e.g. JDS).

Note idle spec RPM is: 78-86 USA MT/AT @ 750-800, 87-89 MT/AT @ 650-700), 90-91 AT @ 650-700, 90-91 MT @ 700-750, 92-95 GTS @ 650-700, unclear to me if the crank pulley was larger on ALL the nominally 675 RPM cars? - that would make sense.

The Max crank RPM was 6800 on the 90-91 GT(MT) & all GTS cars but was typically 6600 for all earlier US S4 AT/MT cars not quite sure about pre-S4 max RPMs, probably 6300-6500 from what I can see (typically lower end for the manual cars). Good to pay attention to all these items.

Alan